O.C. BERKELEY ENGINEERING LIBRARY ; E Donald Ifcnilton ilo&iughlin VOL. I A thoBis i>re8nted in partial fulf lllaont of th rtfairaents f c - the decree of 3)ootor of Philoeophy. Hi"rv rd University. 1917. i. CONTENTS Part I. General Introduction. Importance of the subject. Statement of the problems. Treatment of the subject. Field and laboratory work, and acknowledgments. Part II. Mineralogy of Bornite. History. Physical properties. Crystal forms. Other properties. Cherrical properties. Composition of bornite. ) P....-S 1-14. 1-3. 3-6. 7-10. 10-14 . 15-40. 15-36. 27-39. 27-28. 28-29. 30-31. 33-40. Part III. Descriptions of deposits. Magiratio-pneun.atolytio deposits. Ookiep, Little Namaqualand, South Africa. La Fleur Mt., Danville District, Washington, Introduction Geological relations. Mineralization. Ores in the syenite dikes. Mineralization in the vail rocks. Ore-minerals. 41-98. 41-143, 41-44 45-57 45-46 46-48 43-52 43-49 49-50 50-52 il Contents Continued. Pages Oxidation and Enrioiuceut. 52-53. Discussion. 53-57. Diagram of mineral sequence. 55. Summary. 56-57. Evergreen Mine, Gilpin Co., Colorado. 58-87. Introduction. 58-63. Situation. 5e - Development . 58-59 . Literature. 59-60. Physiographic feature!. 61-63. General geologic features. 62-63. Description of the deposit. 63-76. Associated rooks. 63-67. Prirrary mineralization and rook alteration. 67-73. Secondary products. 73-76. Summary and Discussion. 76-86. Mineral list and diagram of sequence. arid 77-82. discussion. The origin of the deposit. 82-85. The origin of the ohaloooite. 35-86. Conclusions. 86-87. Copper Kt., Similkameen District, British Columbia. 8 8 "9 8 Introduction. 8 " 91 * Situation. 88 ' iii. Contents Continued. Pages Summary of previous work. 88-91. Rooks associated with the ores. 91-93. Ores and rock-alteration. 93-95. Oxi lation and enrichment. 95-97. Sumrcary. 97-98. En^ele, Plumas Co., California. 99-143. Introduction and conclusions. 99-103. General descriptive treatment 102-.125. country rock, ores, and alteration. Magmatio period. 103-104. Solidification of norite. 102-104. Pneutnatolytio period. 104-110. Alteration of norite. 104-105. Formation of segregations. 105-106. Formation of pegmatites. 105-107. Dynamic changes. 107-108. Development of iron oxides. 108-109. Development of pneutoatolytic sulphides. 109-110. Intense hydrothermal period. 110-116. Development of chlorite, serioite, andepidote J.10-113. Development of hydro thermal sulphides. 113-116. Late hydrothermal period. 116-118. Development of zeolites and carbonates. 116-118. Period of oxidation and enrichment. 118-123. iv. Contents Continued^ Pages Zone of coaplete oxidation. 119. Zone of sulphide enrichment . 119-123, Discussion. 133-141. Diagram of mineral sequence. 133. Genet io classification of the deposit. 133-136. Origin of the ohalcooite. 126 ' Chalcooite clearly of replacement origin. 1 6-134. Chaloocite in the graphic structure. 134-135. The Superior Mine. 135-141. Summary. 141-143. Contaot-rretarcorphio deposits. 144- Seven Devils, Idaho. ^~ White Horee, Yukon Territory. 149-153. Marble Bay Mine, Texada Island, British Columbia. 154-170. Introduction. 154-157. 1 KA_ Situation. Development. 154-155. Literature. General geology. 156-157. Rocka and ores at the Marble Bay Mine. 157-164. Linestone. 157 Porphyry. 158-161 : Primary ore -minerals. 161-163, Secondary sulphides. 183-164. Contents Continue.!. Distribution ~ni origin of the ore-bodies. Mineral sequence. Origin of secondary sulphides. Summary. Biabee, Arizona. Primary features of the ores. Secondary alteration. Summary. Deposits of hydr thermal origin. Magma Mine, Superior, Arizona. Introduction. Situation. Mines. Literature. Acknowledgments . General geology. Sadic.entary column. Igneous rooks. Structure. Primary mineralization. Fora of ore -body. Distribution of ore -minerals. Mineral sequence. Pages 164-169. <. 164-165. 165-168. 168-169. 169-170. 171-173. 173- 173-177. 177-178. 179-311. 179-307. 179-181. ' 179-180. 180-181. 181- 181. 182-187. 182. 183-185. 185-187. 187rl93 . 187- 187-189, 189-193. vi. Contents Continued. Oxidation and enrichment. Water level. Oxidized ores. Secondary sulphides. Discussion ar.d suMr.ury. Diagrarc of mineral sequence. Origin of the priirary ores. Origin of the secondary ores. Conclusions. The Kenneoott district, Alaska. Introduction. Situation. Vines. Literature. Aoknowle dgments . General geology. Nikolai greenstone. Chitistone limestone. McCarthy shale. Post-Triuasio deforn.ation. Keanecott formation. Porphyries. Wrangell voloanios. Structures. .-.-... -3 3 193-197. 193- 193-195. 195-197. 198-207. 158- 199-203. 303-207 . 207 T 208-311 . 208-213. 208-209. 209-210. 211-213. 213- 213-218. 213-216. 216-218. 318- 218- 218-220. 220-223. 223-224. 224-226. . vli. Contents Continued. Pages Physiography. 237-334. Pro-Chitistone surface. 337-338. Pre-Kenneoott surface. 338- Pre-Tertiary voloanio surface . 338-230. Pro-Glacial surface. 330-331. Glaoiatlon and the present surface. 331-234. Mineralization in the greenstone. 234-343. Form of deposits. 234-335. Gangue minerals. 235-236. Rook alteration. 236- Primary ore -minerals. 236- Seoondary alteration. 237-338. Deposits near the Bonanza Mine. 238-239. Kuskulana District. 239-241. Kotsina District. 241-242. Suiroary of important features. 343- Ore-deposits in the Chitistone limestone. 243-379. Geologic situation of the mines. 243- Struotural relations. 244-347. Gangue minerals and rook alteration. 247-248. Ore -mineral s. 248-266. Relative abundance. 248-349. Chaloooite. 249-255. viii, Con tent 8 "r Continued. Covellite. 255-357. Bornite. 257-263. Enargite. 263-364. Chaloopyrite. 264 ~ Luzon! te. 264-265. Tennantite. 265-266. Pyrite. 366 - Sphalerite and galena. Conoentrio and banded etruoturee. 367-268. Oxidation. Dietribution. 269-370 Underground temperatures. Water level. 371 ~ Minerals due to oxidation. 271-279. The origin of the oopper deposits in the 279-265. greenstone . Similarity to other regions. 279- Source of the netala. 279-380. Theories of concentration of oopper in 280-285. basic lavas. Resume . 285- The origin of the oopper -deposit a in the 286-294. limestone. Summary of significant features. 286-288. Origin of the primary ore. 289-294. is. Contents - Continual. The nature of the ohaloooite. 294-307. Evidence fron- the studies of the 294-296. Geophysical Laboratory. Evidence that the ohaloooite is a 297-299. replacement of bornite. Secondary versus primary origin 9-307. for the ohaloocite. Table of arguments concerning the origin of the ohaloooite. Summary arid conclusions. 07-311. Part IV. General Discussion. 312-401. General features of bornite deposition. 312-334. Bornite in deposits of magnetic or s - pneumatolytic origin. Some properties of inagtratio sulphide ores. 312-313. Pyrrhotite-chaloopyrite -bornite ores. -316. Pneumatolytio ores. 316-319. Bornite in deposits of oontaot-retansorphio 319-322. origin. Bornite in deposits of hydrotherical origin. 322-325. Bornite in replacement ores in unaltered litres tone. Bornite deposited from cold meteoric solutions. ' 330< Ores of the "Red Beds" type. 326-328. Secondary bornite. 328-330. Sutr,niary of conditions under which bornite '-334. has bean forired. X. Contents - Continued. Pages The alteration of bornite. 335-353. Field relations. 335-339. Selective enrichment. 339-343. e Uicroaoopio relations. 343-355. Sinrple structures. 344-345. The lattice structure. 345-353. Residual structures related to the 353-355. lattice structure. Chaloooite derived from bornite. 355-358. Tiie graphic structure. 360-401. Description. 360-361. minerals in the graphic structure 351-363. with bornite. Chaloooite in the graphic structure. 363-398. Li terature . 364-366 . Summary of evidence. 367-368. Argument favoring a contemporaneous origin. 369-375. Arguments favoring a replacement origin. 375-382. The hypothesis of contemporaneous origin 283-388. modified by replacement. The hypothesis of selective replacement. 383-398. Summary and oonelusions. 398-401. 4 S T I X 1 1 L I fl I ii D U 3? I O.fl GENERAL INTRODUCTION Importance of the Subject In dealing with the numerous problems connected with a comprehensive geologic study of the sulphide ores of copper, it has been found that some of the most intricate and puzzling questions encountered are those associated with the mineral bornite. In the case of the other ore-minerals, the relations observed either in the field or under the microscope are at' a rule relatively simple and constant, and generally yield a definite but limited amount of information. Bornite, however, exhibits under diverse conditions a variety of features and a complexity of relationships which are difficult of inter- pretation, but which are of great significance when understood. The questions which arise concerning this mineral range from broad problems demanding knowledge of the field relations to intimate queries concerning the details of intricate micro- scopic structures, and an appreciation of the critical evidence which is offered by associations and properties of bornite is often necessary before a satisfactory understanding of the na- ture of certain ore-bo lies can be obtained. A clear concep- tion of the genesis of the ore can often be obtained from the relations of the bornite to other primary sulphides and to the gangue minsrals, and the effects of superficial agencies are invariably recorded in the bornite with far greater delicacy . CX? " .3 and completeness than in the other sulphides. Apart from the scientific value of detailed knowledge of the position occupied by bornite in the processes of primary mineralization, the richness of bornite as an ore of copper makes any information especially welcome which would lead to a more accurate com- prehension of the habits and distribution of the mineral and which would direct prospecting and development work most in- telligently. The study of secondary ores derived from bor- nite presents another field which offers many open problems of both scientific and practical worth. The economic value of a correct judgment concerning the secondary or primary nature of a chcxloocite ore makes it of great importance to seek all means by which the necessary information oan be gained. In most deposits, ohaloocite ores are believed to be of secondary nature, and all plans for their development are based upon the expectation that they will pass into lean- er primary material at relatively shallow depths. In the deposits in which the nature of the chalcocite is in question, the mineral Has beeii found almost invariably to be in close association with bornite, and the problems of its origin have resolved themselves largely into studies of its relations to this mineral. 1. Throughout this paper the terms primary and secondary are used only in their geologic sense. The former is re- served for processes ani products associated with the orig- inal mineralization; the lutter for processes and products dependent on superficial alteration. - . . , ' It has therefore been considered desirable to as- certain by all available means the facts concerning the oc- currence and relations of bornite, and to gain an understand- ing of their significance. To do this hae been the object of this investigation. Although many of the broad features of bornite ores have been recognized for rrany years, the reoent applica- tion of the metallographic microscope to the study of opaque minerals has revealed so many unsuspected relations that very little of the older literature concerning the paragen- esis of bornite can be accepted without reservation. Conse- quently, with the exception of the work of a few recant in- vestigators, the study of the relations of bornite by modern microscopical methods opens an almost new field in which thorough exploration is fully Justified by the promise of results of both scientific and economic value. Statement of the Problems From both a scientific and an economic standpoint, the chief problems associated with the deposits studied may be divided into two groups: (1) those concerned with the nature of the primary mineralization arid (2) those concerned with the character of the second- ary alteration. In the first group, the establishment of the range of condi- tions under which bornite may form is the most general prob- , . . ; - 30. . ' . . lem, and all others are units leading to its solution. In tbe oase of each deposit, knowledge must be obtained con- cerning the form of the ore-bodies, their relation to the neighboring rocks and structural features, and the position of bornite in the sequence of gangue and ore -minerals. From the interpretation of the evidence afforded by these points and others, the genesis of the deposit may be inferred in most cases with reasonable certainty. The second group of problems, viz. those related to the secondary changes due to the superficial alteration of an ore -body brings up a greater number of queries. The determination of the importance of bornite as a product of surface processes is one of the most definite problems which has been encountered. The suspicion in many mining camps that the bornite ores are of secondary origin emphasizes the value of positive information on this point and it is be- lieved that evidence of a final nature is presented in these studies. The distribution of the secondary sulphides, the depth to which enrichment can extend in bornite ores, com- parisons of similar influences on pyritio and other ores, and the influence on the alteration of bornite of numerous factors such as climate, physiography, wall rock, rook alteration, and structure all offer problems depending largely upon field observations for solution. On the other hand, the degree of replacement by secondary sulphides which the bornite has suffered, the relative ease of bornite en- 5. richment compared with the other sulphides, and the rela- tions between the various ore and gangue -minerals are questions which are roost definitely settled in the labora- tory. The microscopic relations of chaloocite to bornite offer a great number of important problems, among which the most important are the interpretation of the "lattice structure", in which the chaloooite penetrates the bornite as a regular grill of intersecting lines or the "graphic structure", in which the two minerals are associated in eutectic like patterns on the polished surface. In seeking the meaning of these difficult relations, numerous minor /i / features, such as residual "spines" of bornite in chaloooite, rims of bornite about partially altered grains, ""haloa". and "hazy boundaries" all demand their share of attention, and add the weight of their evidence to the general attack on the problems of bornite alteration. The important question of primary chaloocite ia closely bound up with the interpretation of the various relations between chaloocite and bornite. The deep enrich- ment in bornite ores, the inheritance of bornite structure by ohalcooite, the two etoh patterns in chalcooite, and the origin of the lattice and of the graphic structures are points which must be understood before a final verdict can be given. The economic importance of the problem ie ob- vious. Experience in many camps has shown that the rich chalcocite ores are usually of secondary origin, i.e. . ' - , 6. formed by the action of meteoric waters descending through the deposit from the surface. Consequently they are ex- pected to give way at relatively shallow depths to leaner primary sulphides, which may possibly be too low in copper to be worked profitably. If, however, it can be established from a study of the relations between the sulphides on the upper levels, that the ohaloooite is a primary constituent of the ore, its continuance with depth would be expected, and all plans for the development of the deposit would be influenced by this knowledge. If the chaloocite is second- ary, it becomes of economic importance to be able to pre- dict the nature of the change where the superficial sul- phides yield to the primary ores. If it can be determined that the chaloocite is largely a replacement of bornite, the decrease in copper per centage of the ore will be relatively small, and will ordinarily permit profitable operations to be carried into the primary ore; if, however, the ohaloooite is a replacement in part at least, of pyritic material, the decline in value is much greater and may not allow the profitable mining of the deeper ore. Although evidence on these points may be gained by drilling or other exploratory methods, it is believed that in many cases reli- able conclusions may be gained at less expense from micro- scopical studies of samples from the upper parts of the de- posits. Treatment of the Subject The aoouraoy of any conclusions of general nature whioh may be drawn, depanda directly upon the degree to V which tiia material selected as a fair sample of all occur- rences, and consequently the attempt has been made to in- clude among the deposits studied, examples from all types of ores in -thioh bornite is an important 7 mineral. The de- posits chosen for detailed investigation include several in whioh the bornite is believed to be of either magmatio or pneumatolytic origin, others considered to be typical examples of oontact-rcetarrorphic ores, and certain deposits regarded as products of various phases of hydrothermal action. In addition the camps selected are situated in widely separated regions, consequently their ores have been subjected to sufficiently diverse climatic and physio- graphic influences to offer as great a range in conditions affecting the development of secondary sulphides as is necessary to insure the general application of the rela- tions observed. Before the main part of the paper is presented, a short discussion is given cf some features of general mineralogical interest in connection with bornite. A brief historical account of the mineral is presented with a summary of older analytic and synthetic work, whioh al- though of little present value, is of some interest as a record of the difficulties caused by unsuspected impurities 8, in the materials available for study* The descriptions of a series of ore-deposits in which bornite plays an important part, either as an ore-min- eral or as a sigriigioant factor in their history, form an important part of paper. Where definite problems can be handled in the case of individual deposits, they will be considered there; if more general evidence is required for their solution, they will be reserved for the later chapters in which knowledge from the whole field may be brought to bear upon them* The different deposits are treated with various degrees of thoroughness. In oases in which the field work and collections offered new and more definite information than had been previously published, the details are given at greater length than for those deposits in which the investigation is limited to a study of older collections, or merely to the literature. A few especially interesting deposits, such as those at Engels, California and at Kennecott, Alaska, have been described more fully than many of the others, but their unusual features are be- lieved to justify the prominence given them. Other impor- tant deposits, however, such as t; oae at Butte, Montana, and Bisbee, Arizona, have been dealt with very briefly, even though their ores present many of the most intricate problems associated with bornite. The chief features of the great ore -bodies, however, have already been given a prominent place in the literature, and the time available was too short to make a study of the field conditions which would add any additional knowledge of value. The handicap of being forced by lack of time to slight these important deposits is greatly offset, however, by reliable information that is available from recent work by other investigations. A de- tailed paper on the Bisbee ores, based on an intimate knowl- edge of both field and microscopical relations supplies valuable data which may be accepted without question, and intensive studies of Butte material which are being carried on in this laboratory constantly add trustworthy evidence on the problems presented by those deposits. The order in which the camps are described is based in a broad way upon the interpretation of their origin. The ores which are believed to be rrost closely related to magmatio conditions are discussed first; they are followed by the contaot-metamorphio deposits, and later by descrip- tions of ores formed under various stages of hydrothermal action. The descriptions are thus arranged according to decreasing intensity of primary conditions of origin, but on account of the complex and transitional nature of some of the deposits, their grouping must be somewhat arbitrary. For example, the Engels ore -body furnishes evidence of sulphides formed under varying conditions from pneumatolytio to late hydrothermal, but on account of the greater interest attached to the earlier products, it is presented with the pneumatolytic deposits, although the larger part of the 10, commercial ere was foriwd under hydrothermal conditions. In the fourth section of the paper, the impor- tant primary relations of the bornite ores are discussed, and the common properties and general range of genetic conditions favorable for the production of bornite are emphasized. A second divison of this section is concerned with the problems associated with this alteration of bornite, such as the relative ease of replacement by ohaloooite, the field distribution of the chalcocite, and the significance of the different types of replacement structures. The final divison of the section is devoted to the much debated ques- tion of the primary or secondary origin of the chalcocite in the graphic structure with bornite. The evidence and the various arguments are summarized, and two explanations are proposed. The paper closes with a summary of the results and conclusions in individual cases and in general. Fisld and Laboratory Work, and Acknowledge merits The work of this investigation of bornite ores has extended over a period of about two years, but the time has been somewhat diminished by other smaller problems and academic duties. During two summers in the field, attention has been largely directed toward the occurrences of bornite. A large fraction of the other months have been consumed in laboratory studies of the material and information gathered. The work embodied in this paper is part of an ex- W c 11, tensive study of the sulphide ores of copper which is being carried on by the Secondary Enrichment Investigation, and consequently a wider range of information wa8 available upon which arguments and conclusions could be based than the special field emphasized in direct connection with the problems of bornite. The deposits near Kenneoott, Alaska, were visited in the summer of 1915, in the company of Messrs. L. C. Graton and A. M. Bate man. The larger part of the time was spent in studying the great ohalcocite ore -bodies of the region in which bornite is a small but very significant constituent, and a short time was available for a study of the smaller ohaloopyrite -bornite deposits in the greenstone of the neighborhood. In addition to material collected in 1915, specimens and information from parts of the region I was unable to visit have been placed at my disposal by Mr. Alfred Wandtke to whom I owe my sincere thanks. A broader range of country was covered in the summer of 1916, and a valuable collection of material was obtained from most of the important camps in western United States in which bornite is^oteworthy ore-mineral. The list n of mines and districts in which the occurrence of bornite was especially studied in 1916 includes: Engels, Plumas Co., California; Marble Bay Mine, Texada Island, B. C., the Similkameen district, British Columbia; the Boundary District, British Columbia and Washington; Butte, Montana; the Ever- ^ . 12, green Mine, Gilpin Co., Colorado, and Biebee, Superior, and Morenci, Arizona. Visits were also wade to the following camps in which bornite is of minor importance: Rossland, B. C., Bingham arid Tintic, Utah; Ely, Nevada) Tularoaa, Tyrone and Santa Rita, New Mexico; and Jerome, Ray, Miami, Globe, and Morenci, Arizona. A considerable part of the expenses of the work were covered by a Sheldon Traveling Fellowship from Harvard University. In several of the most important camps, the field observations were made in the company of Messrs. L. C. Graton and A. Locke, and from their intimate knowledge from the results of previous work of the Investigation, I gained a far clearer conception of the field conditions than coull have possibly been obtained in any other way. In all places the hospitality, character- istic of the mining profession, expressed itself in the kind efforts that were made to give me all opportunities to study the mine workings, and to acquaint me with the local features of geologic importance. Especial thanks are due to Messrs. Stephen Birch, W. H. Seagrave and H. D. Smith of the Kenne- cott Copper Corporation for many courtesies extended dur- ing our work in Alaska in 1915; to Mr. John Reinmiller at the Engela Mine, Calfornia; to Mr. A. F. Eastman of the Taooma Steel Co., to whom I owe the privilege of visiting the Marble Bay Mine on Texada Island; to Messrs. Patrick Crane and H. E. Doelle of the British Columbia Copper Com- pany, whose guest I was for several days on Copper Moun- 13, near Prinoeton, 3. C., arid at the Mother Lode Mine; to Messrs. C. F. Martin and Nelson at the Granby Companies' properties at Phoenix, B. C.; to Mr. 0. S. Trombley, whose cabin I shared on the slopes of La Fleur Mt. in the Boundary District, a few mi lea from the line in Washington; to Mosars. Arthur Linforth, D. A. Hall and other members of the geolog-- ical staffs at Butts, Montana; to Professor H. B. Patton of the Colorado School of Mines at Golden, with whom I apsnt two enjoyable and instructive days on a trip to the Ever- green Mine in Gilpin Co., Colorado; to Messrs. Y. S. Bonillas and Leon Fenohere of the Copper Queen Mining Co., at Biabee arid Mr. Ira B. Joralemon of the Calumet and Arizona Mining Co., at Warren, Arizona, who kindly showed me the aoat inter- esting parts of their ore-bodies in connection with my problems; and to Messrs. W. C. Browning and I. A. Ettlinger of the Magma Copper Company at Superior, Arizona. To many others at the various camps visited, I am indebted for many courtesies, for which I wish to express cy thanks* I am indebted to Mr. Joseph Murdoch for many excel- lent photographs of bornite structures, which were made by him during his work on the staff of the Secondary Enrichment Investigation, and also to Mr. W. L. Whitehead for several photographs of material from the Engels Mine . My thanks are especially due to Professor L. C. Graton for the thoughtful and valuable criticism that he has given all parts of this work, both in the field and in the 14. laboratory, and in connection with the preparation of the manuscript, for which I ans very grateful. P A It 2 MIBBEALOGY 0? B R M I g JS 15, PAST ii. :.:i:i:HALoaY o? History The existence of "oopper pyrites" has probably "been known since the metal was first won from sulphide ores, but the similarity in color between bornite and ohalcopyrite when they are tarnished made their separation into distinct mineral species uncertain almost until modern times. There is evidence, however, that the Identity of bornite was suspected as early as 1546, for Georgius Agrioola in his famous TO rk . Natura Fossilium printed in that year, mentioned two minerals, "Pyrites aurel colore" and "Pyrites aerosus" , which may be correlated fairly certainly with ch&lcopyrite and bornite. The distinction of the tarnished variety of "oopper pyrites' 1 by a separate name is significant, and warrants the addition of bornite to the long list of minerals first recognized by Agrioola.^ Appreciation of the distinct character of bornite is probably found in Johann Friedrioh Henokel's "Pyritologia' f or "Kies-Historie" , published in Leipzig in 172.5, but the vagueness of his statements and the variable application of the names he uses, makes his contribution even less definite than the des- criptive terms employed by Agrioola. The sentences containing 1. The source of information concerning Arrioola and his v.orks most easily available is the excellent translation of le Re :.ietallloa. (1556) by Herbert Clark Hoover and Lou Henry Hoover (London, 1912 ). On page 109, a summary of minerals in De Natura Fossilium is given. 16. the probable reference to bornite are moat satisfactorily given in hia own language, and are as follows: - - Xupffer- glass iat noon dunokler [than ?ahlerz, previously described] und neiget sioh gar zur Sohwartze, aus Ursachen, weil as sehr iisenschussig 1st, dergleichen mir von den awfllff Sohluaseln bekannt; und Kupf f er- ,asur nimmt sioh nebst seiner dunckeln Farbe ait seinen stahlblauen tfarben aus, wiewohl der I.Ii-sbrauch auoh eingefuhret hat, ein sonst gelb-grunlichtes Xupffer-iSrz, ao nur auf Klufften mit blauen Farben spielet, Kupffer-Lasur zu nennen, und es scheinet dass Kupffergiass bei den Alten keine andere Saohe als Kupffor-uasur bedeutet habe, sondern gleichwie aus Glaaur Lasur, als aus Glasur auoh Glass naoh der alien Spraohen ge.Tieinen von Geschwindigkeit in ausspreohen entstehenden Worter Verkurtzungen, und 2uaammenziehungen ( entsprungon sei. ' She mineral described by Henokel is 1. Translation; Kupffer-glaaaCohaloooitet] is still darker than Pahlerz, tetrahedrito, previously referred to] and even tends toward blackness, because it has a high iron content due to causes which ar9 known to me from the twelve keysf ?3; and Kupf fer- Gasur C probably bornitej has in addition to its dark color a steel blue shade, although the misuse his arisen of calling a usually yellow-groen copper-ore, Kupffer-lasur which sparkles with a blue color only along fractures and it seems the Kupf f erglass C chaloocite} among tho ancients meant nothing less than Kupfer-ijusur, just as from Glasur, Lasur and from Glasur, Gla 8 originated from abbreviations and contractions common to all languages re- sulting from rapidity in .speech. ._Johann Priedrich Henckel, Pyritologia, Leipzig, 1VE5, oage 451. 17. classified as a member of the "^less-Holla" , and Is therefore a metallic sulphide or arsenide, according to his definition. It is doubtful, however, whether the name Xupffer-Lasur was confined to this usage, for it la applied to the mineral azurite by slightly later writers. The earliest publication in which names are given which clearly indicate that the mineral was commonly recognized at the time, is the Mineralogy by Johann tfottskalk Wallerius, printed in 1725. A description of 1 bornlte appears in the Latin edition of 1778 under the heading, Cuprum, pyrite fuso mine rali sat urn, minera flavo fuaoa. and the following popular names are added : - Swedish, Le fever slag, Gulbrun kopparmalm; French, Mine de ouivre hepatioue; and German, Braunes kupfererz, or Leberschlag. In the French translation of Wallerius, by Paul Thiry, published In 1759, the mineral is referred to under the names:- "Mine de ouivre hepatique ou de la oouleur du foye," vrith the latin terms, "cuprum sulphure, ferro mineral! satua, minera pyriticosa fulva; Minera oupri hepatic a. Tho description is as follows:- "Elle est d'um jaune tirant sur le brun on d'une oouleur pale, 1. Page 285. Oolore est flavo fusoo, reipsa pyrites fuscus (spec. 277} cuprum solutum oontinans parva quantitate; proprletatas pyrltae fusci nonlmatl utplurimum retinens; a quibus haeo minera oognosi et agnosoi potost* 18. et etroitement unie avec du soufre et du fer; la substance est pyriteuse: 11 y a meme lea Haturalistea qul la mettent au rang dea pyrites; frappee aveo 1'acier, elle ne donne que peu ou point d'etinoellea. On a (1) La mine de ouivre hepatique brune (Minera cupri hepatioa fulva) Slle est de la couleur du foye; riche en cuivre, et d'une oonsistenoe tantot aerre'e tantot pen compacte. (2) La mine de cuivre hepatique pale. (Minera oupri hepatica livida.) Bile eat preaque blanche et pale comme de 1'etain, d'une couleur fonoee, et d'un brun tirant aur le bleu; intorieure- ment elle parot composee de grains, mala extorieureraent elle , 1 aomble feuilletee. The various tarnished colors of the bornite are probably the cause for the subdivision of the species, but another mineral may be meant by the aeoond description. 1. Translation. It is of a yellow color, tending toward brown, or a pale color, and intimately united with sulphur and iron; the material is pyritio: there are even some Naturalists who place it in the group of pyrites; struck with steel it gives only a few sparka or none at all. We have (1) the liver brown copper mineral. It is the color of liver, rich in copper, and of a dense to slightly compact texture. () The pale liver-colored copper mineral. It is almost white and as pale as tin, of a warm color, and of a brown, turning toward blue; internally it appears granular, but on the outside it seems foliated. 19, Cronstedt in his Mineralogy, published in 1758 definitely recognises the mineral, terrain? it "Pyrites oupri hepatious" according to a quotation in the later Latin edition of Wallerius. In the Knglish translation by Jin- 1 geet6m in 1770, bornite is classified in a subdivision of the main group of copper-minerals, headed "With sul- phurated iron, rainera cupri pyritaoea," and Is described ae follows:- "Reddish yellow, or liver brown, with a blue coat on tho surface, Ulnera cupri le^urea. This ore yields between 40 and 50 % of copper, and is commonly said to be blue, though it is as red whon freeh broke, es a rioh oopper regulus." The term" liver-ore" was probably the earliest name in common use in England. It is given by Forster in his 2 "Introduction to Liineralogy, published in 1768. He writes, "Liver-ore is a rich copper-ore consisting of a good deal of oopper mixed with iron, of a yellow brownish colour, "but mixed with some green spots, which distinguish them from the iron ores." The value of the mineral as an ore of copper was undoubtedly well jcnownat this time. 1. Axel Predrik Gronstedt. An Essay towards a SyBten of Mineralogy. Translated from the original Swedish, with notes, by oustaf Engestrom. London, 1770, page 193. (Original published in 1758; translated into German in 1760.) 2. John Eeinhold Forster. An Introduction to L.ineralogy, or an Accurate Classification of Fossils and Minerals, viz., Earths, otonos, S;.lte, Inflammable and Metallic Substances* London 1768; page 46. 20, The gradual assembling of knowledge concerning bornite ie easily traced in the increasing numbers of Mineralogies published. Kioherd Kirwan, who first approached the problems of mineralogical determination from the ohemioal 1 side states in his Elements of Mineralogy (London 1784) that the species termed Azure Copper Ore, Kupfer lazur, or Kupfer Malm is "cine rail zed v/ith sulphur and with iron. Its color consists in various shades of blue; it contains 40 - 6O/ of copper, 20 - 30$ of iron, and the remainder sulphur; the poorer it is in iron, the richer in copper. It has been by many confounded with indurated mountain- blue." In an edition of 1810, he adds that the mineral "effervesces v/ith nitrous acid importing to It a green colour; does not immediately give a blue tinge to oaustio volalkall. Its specific gravity is 4.956 - 4.983 and effervescence >vith acids prevents all possibility of mistaking it -for the tarnished ores of the former family Ccopper pyrites}." The first distinct analytical results to be pub- lished, appeared in 1797 and were the work of the mineral - 2 ogist Klaproth. Material from Hitterdahl in Horway and from the Friederike Juliane Mine in Eudelstadt, Silesia, yielded the follOvvlng re suits :- 1. Richard Kirwan, Elements of Mineralogy - London ,1784, 2. liartin Heinrioh Xlaproth, Boitrage zur Chemischen Kenntniss der Mineralkbrper, Vol. II ,~p. 281, (1797). 21, Hitterdahl Budelstadt Copper 69.50 $ 58. % Sulphur 19. 18 Iron 7.50 19 Oxygen 4. 5 % The lo . summation of the copper, iron and sulphur led Klaproth to the conclusion that the variegated colors of the mineral were due to oxygen r which was believed to be present in sufficient quantity to account for the observed discrepancy. The peraistanoe of the deep color of the bornite was believed to be due to the thorough abeorbtion of oxygen, similar to but more intensive than the manner in which the action of air commonly produces an iridescent tarnish on chaloopyrite . For such early work, the results are remarkably oloeo to the usual values of modern analyses especially if the "oxygen" Is added to the sulphur percentages* The work is of distinct historical interest for it marks the beginning of a long and varied series of analyses and their accompanying controversies. The name B m unt kupf e re rz , which IB commonly used in Germany to-day, was piven the mineral by Werner according to 1 statements in the books of several of his followers. 1. Emmerling, Uineralogy, 179G; E. Jameson, ilineralogy, 1805, and others. Hintze states eroneouyly that the name Euntkupfererz was given by Hoffman, (Min. 1816,36,110). 22, The term Buntkupfererz was translated Into English 1 as variegated Copper Ore, "by Jameson who gave a full de- scription of the properties of the mineral in his Mineralogy published in 1805. Ho stated that it was found in "beds, veins, and disseminated In rocks of different formations, and concluded that it occurred in greatest quantity in primative mineral "beds, according to the tfernerlan hypothesis of the formation of the rooks of the earth's crust. He re- garded the mineral as an intermediate species between oopper- glanoe and copper pyrites, and "believed that it occurred as abundantly as the former, but not in as great quantities as the latter. 2 Hauy gave a description of bornite under the name Cuivre pyriteux hepetloue, and stated that bornite was re- garded by many distinguished mineralogists to have originated from chaleopyrite. The bright and varied colors often ob- served on the surface of the "cuivre pyriteux" were regarded as the first products of the change. 3 The name Philllpsite was given the mineral by Beudant in 183T, in honor of the mineralogist and chemist liR. Phillips, who had described crystal forms (cubes and octahedrons) and analysed Bpme specimens from Boss Island 1. Eobert Jameson, System of Mineralogy, Edinburgh, 1803. Vol. II., page 189. 2. C. Hauy, Traite de Mineralogie, Paris, 1801. 3. Beudant, Traite de Lllneralogie , Paris 183 Vol-II page 411 . c S3. 1 In Killarney, Ireland. The name is oho oldest of the modern terminology, nut owing- to the prior usage of phillipsits for a member of the zeolite group, it has not been retainer, for . the sulphiae, ^looker in 1839 created tha term poikilopyrite t (from ~n~oi/iAos variegated,) "but even though it may justly olaim priority over others of tha modern type, it is rarely if 3ver encountered in the literature. 3 Dana in the first edition of his mineralogy (1837) called the mineral "Pyrites erubesoens", alluding to Its liability to tarnish and to assume a reddish hue. In the edition of 1850, the name was shortened to e rube site, but the term was discarded in the later editions, due to the earlier usage of the name nornite. The oornmonly accepted namo, bornite, was given the 4 mineral by W, Haidinger in 1845 in honor of Ignatius von Born 5 (1742-1791) . In a letter to his father, Haidinger wrote; "The species was first definitely separated from chalao- 1. Phillips, ;,iinerelogy , 1819, page 22 . 2. Glooker, ilinaralogie , 1839, page 328. 3. J. D. Dana, System of Mineralogy, 1837, page 408. 4. '". Haidinger, Handbuoh der bestioimenden Mineralogie, .-n, 1845, page 562. 5. Quoted in Hintse's Handbuch der Mineralofle, Bd.I,(1901), page 905 in a foot note. 24. cite and ohalcoi^yrite in the classification of the Royal Mineral collection, which is "being newly arranged under the direction of von Born." Tha claim that this was the earliest recognition of tha distinot character of the species may be ouestioned, "but "by general usage the name bornite has been firmly established. Boll shed surfaces 07" "hornite were first made by H. 1 Baumhauer in 1895, to study the properties and relations of the mineral in some spoo linens from Chloride, Hew Mexico. He observer 7 that the grain of the material ?as revealed by etching with nitric acid. Products believed to be bornite have been obtainedartif ioiallyb: various workers, "but as the homogeneity of thair material was established only in a rourh v/ay, the results have little more than historic value. One of the earliest experiments was by Booking in 1855. 36 g. of copper, 10 g. of iron (re.-uoed from the oxide by hydrogen) and an excess of sulphur were melted together under a cover of cr.lt, and a brittle rerrulus was obtained, .vhich appeared like bornite on the fractured surface, and tarnished in moist air in c. i/irnil&r way. By analysis it contained: Oopper 55.77., iron 15.9> t sulphur 25.9$ 1. H. Baumhauer, Uober die mikroskopishe Beschaffenheit einer Euntkupfererzeu von Chloride, flew Mexico. Zeitsohr. fur Kryjt., eto. Vol. 10, 1885, pa^o 447. Inaug. Diss. GrOttingen,1855, 29. Elntze Handbuch der or .10,- MO, Bd. I. ittne 25. 1 Marigny in 1864 obtained a crystalline aggregate of material resembling borriite by melting 39 parts of pyrito; 45 parts of oopper shavings, and 20 parts of sulphur under a cover of borax. 2 Doalter produced an aggregate of small cubes correspond- ing to the formula C\*3 Pe 3^ by passing hydrogen sulphido over a mixture of Cug 0, Cu end Fer^Og at a low temperature. (100-200 C) without melting the oxides. Tho material agroed with natural bornite in oolor, specif io gravity, analysis, and crystal form, '^ith a mixture richer in oopper, some oovallite was obtained with the bornite. 3 Beuss observed bornite and chalcopyrite irregularly intergrown in a blaok slag at Hermannseifen near Trautonan in Bohemia. In tho hot springs of Bourbon I'Arohambault In the Department Allier, rornan coins were observed altered to bornite and ohaloopyrite; at Bourbonne-les-Bains in the De- partment Haute - Marne, email crystals of hornlte were found 4 on old coins in an artificial water course. The forms (100) 1. Me.ripny, Compt. rand., 1864,58,967. Hintse, ibid. 0. Doelter,Ueber d. kunstl. Darstell. oiniger Mineral. &d. -rruppe d. Sulphide, u. Sulphosalze, Zeit. f.Krist. u. Llineral., Vol. 11, (18S5-86),pp 36-38. 3. Labor?, Marz I860, 10, 40. Hint^e, ibid. 4, Daubroe, Corapt. rend., 1875,j30, 461, 604; and 81, 16-, 834, 1008. LaoroiXf ilin. de Pranoe, 1897, _^ ; 677. Mntze.ibid, 27, rropertJ33 Crystal forne- Bornite crystallizes in the Isometric system. Good crystals are rare, "hut descriptions of material from many widely separated localities have established its forms with fair certainty. The cube and octahedron, alone or in combination, and the trapezohedron are the forms most commonly described. Dana notes the following forr.is and com- binations:- (100) (110); (111) (110) (100); and (111) (110) 1 (211). E. H. Kraus and J. P. Goldaberry observed the following additional forms in material from Bristol, Conn.:- (211), (322), (433), (411), (522), (533), (833). Only 411 is considered doubtful. Crystals of bornito several centimeters in diameter have been found in the Tyrol. The descriptions of the deposits in which they occur are very brief, but they are probably of 2 the Alpine type . iVeinschenk describes crystals 3 1/2 cm. in diameter, on which the form (211) dominates with (100) less prominent. G. Gasser obtained crystals from the Prospnitzeralpe which measured 5.6 X M-.^ X 3 cm. Sealenoheirons of oalcite and nodular particles of native gold, about 1. ::jr.. in diameter, adherer to one side. The crystal forms (211) and (322) were observed. In the Hof museum in Vienna, there is a 1. The Chemical Conipositi on of bornite and its relation to other eulpho -minerals, A.<7.S. r Vol. 37, p. 539. (1914). 2. (J. Gnsser, Dio llineralien Tirolq, einschliesslich Vorarlbergs und der Hohen Tauern, Innsbruck, 1913, p. 112. 28. specimen of bornite from the same locality which is a regularly formed trapezohedron about 4.3 cm in diameter. Its measurements yielded a new form for bornite, viz: (553), With it is associated native gold, calcite, and albite. Other physical properties. Bornite possesses an 1 octahedral cleavage. The cleavage is rarely seen in the hand specimen, however, but may be easily developed on a microacopic scale by pressure with a blunt point. In the plane of the polished surface, series of straight cracks, usually parallel to two or three directions, or less commonly to four directions, which form in the bornite around the edges of crushed areas resulting from this pressure are undoubtedly due to the oc- tahedral cleavage of the mineral, (fig. ) The fracture on a coarse scale is uneven, but under the microscope it may be seen to be finely conch oidal. Bornite varies notably in brittleness. Material from certain localities breaks so readily that it is difficult to polish, while that from other deposits may be almost ductile. The hardness of bornite is 3. According to A. Sella, the specific heat is 0.1177 (calculated, .1 0.1195) It is a good conductor of electricity; the resistance 2 increases with the temperature. 1. Breithaupt, Berg and Huttenm. ztg., 1859 ,18,322. Hintze, Loc. cit. n 2. Beitrag. zur. Kenntniss der sp. Warrae der Min., Hach d.k. Gas. d. ysriss. zu Gottingen, 1891, 10, 211-322; also Zeit. f. Kryst.u.min., 22, 180. 3. Beijerinck, H. Jahrb.,1897, Beil. Bd.,11, 437; Hintzi,ibid. so 9, Bornlte occupies a place "below marcasite, chaloopyrlte, enargite, oovellite, and pyrite, and above galena, chalcocite, hematite, oUprite, metallic copper, and sphalerite in the eleotro -chemical ueries of the sulphides and oxides determined 1 ty V. H. Gottsohalk and H. A. Euehler. Measured against copper v/ire in distilled water, bornite v/as found by them to assume a charge of 0.17 volts. 1. Oxidation of Sulphides (Second Paper) ? Loon. Geol., Vol. 7, p. 31. (1912), so. 1 Chemical Properties. Before the blow-pipe on oharcoal In tho reducing flame, bornite melts to a little raagnetlo bead with a gray- red fracture ; with soda to a oopper bead. In the open tube, sulphurous fumes but no sublimate are given off; in a closed tube, a weak sublimate of sulphur, Bornite dissolves with 2 efforvoooonoe In nitric acid, dilute or concentrated with the oeparation of sulphur. It IK soluble in potassium cyanide * solution, whereby it may be separated from pyrite, chal- oopyrite, uagnetite and lollingltc. It precipitates silver, often in crystals, froia cool acid silver sulphate solution* On the polished surface, nitrlo aoid changes the normal pin&ish brown color of the mineral to a golden yellow. It reacts with effervescence, leaving a surface etched in fin39 from an analysis of crystalline bornite from Cornwall. This was fol- lowed in the same year by in inalyfiis of another Cornish spec- imen by Varrentrap 1 , while a third by Chodney, appeared in the same journal in 1&3M-. Subsequent studies have confirmed their analytical results In a rough way, but have shown that similar material from Cornwall is extremely impure, usually with chal- 5 oopyrite. 1. Joseph Murdoch* Op. o It., Page 36 and Frontispiece, Pig. 1. 2. Plattner, Pogg. Ann, 14-7 1 351. 1839. 3. Varrentrap, ibid. , p. 372. )L lfV> /%<*>,<* 4 , -I A . : ' . [ ' rtB CIC- . lie: I IB t 35, The analyses are as follows: I II III IV V VI Cu 56.76 5S.20 57.S9 57.71 57.65 55. 5 Pe lif.M. 1M..S5 1M-.94- 13. 89 15.11 16.36 S 2S.2H- 26.9* 26. K^ 27*17 26.^6 2S.06 99.ft- 100.02 99.67 9S.77 99.25 100.00 I. Condona Uine, Cornwall. Plattner. II. Cornwall. Varrentrap. III. Redruth, Cornwall. Chodney. IV. &V. Cornwall. Harrlngton.( Bornite oontralning ^haloopyrite, visible with a hand lens). VI. Calculated for CUz Fe Sz. As Harrington points out the oomViinatlon of CUB Pe 83 and Ou F.e Sp yields 2CU*. Pe S^ , which is a mixture containing 73,2$ bornite and only 26. 8fi chaloopyrite , assuming the first formula to be correct for bornite. In 1S75 Cleve recognized that few published analyses of bornite agreed with the formula Cuj Pe 83 . Results of several analyses done under his direction lod to the for- mula Cu;j ?e 84. , but from others more complicated formulae were derived which forced hin to the conclusion that there were several forms of the mineral , 34, The variouf? speculations of the following years are of little value, as the analytical evidence on which they were based is questionable, A long lisjc of analyses is 1 given in Hintze's Handbook fur Mineralogie. 2 In 1903, 8. S. Harrington published a series of analyses of bornite from various Canadian localities, and round that his results agreed closely with the formula Pe Sjj.. The uniformity of his results indicates that the material was carefully selected, but it was examined only with the hand ions, which oan hardly be depended upon in general. Hi;: results are as follows: I II III IV V VI CU 63.55 6^.75 62.73 63.34- 63.15 63.27 P0 10.92 11.25 11,05 10.53 11.25 11.15 s 25.63 25.39 25.79 25.5* 24-. 55 25.55 Insoluble .30 .3* .24- 100.10 99.75 99.57 100.09 99.55 100.00 S.6 at 15 5.055 5.055 5.090 5.029 I. Harvey Hill, P. Q. II. Prince Mine, Ontario. III. Dean Channel, Howe Sound, B. C. IV. Copper lit., S inilfcameen District, B. C. V. Texada Island, B. C. VI. Calculated for Cu,- Fe 84. , for coraparison. 1. . 914-916. 2. Loo. cit. . ..: ' ft* 35, Ory:;t/uil::o''. Somite, (partly nasHl^o, partly in rhonbio dod'ioahodrons ) i"Vo~i Bristol, Conn, was analyzed as follow?.: Ou 63.21J- Fe 11.20 3 25. p 1 *- 99.9* Harrington concludes very justly from these data that tho composition oi'bornite is correctly expressed by the formula Cu^ ?e Sq., and that the divergence of analyses in the pant vas due to the impure nature of the material, The question however was reopened by H, Kraus J, P. Goldsberry in 191*4-. Two analyses wers made of crystalline bornlte from Bristol, vrhich are as follows; Average Gu 65.M-2 65.91 65.665 Pe 9*74- 9,67 9.705 s 2M-.79 2*J-5.1. 2^.656 99.95 100.09 100.020 S.o. at ordinary temperatures 5.0S6 Their two analyses yield the fori.iula Cu-j^ Peg Sc,. Pieisos of the sane material from Bristol as was analyzed by Harrington vero obtained ann his results checked. Based on thiB evidence, and nuinoroua diver.c'inc 'ui'ilyBes culled from tho literature, the authors construct an elaborate series of minerals v/ith the general formula Cu x ?&Q S y , v/here 36, y - X/2 f 3. The series ranges from Cu Pe S 2 ( ohalcopyrite ) through CUy^ Peg 8^ with ohalcocite as the final limit* Each member differs from the preceding by the radical Qu^ S. The bornite analyzed was examined by ordinary methods and considered pure, A metallographlc examination of the material actually analyzed was not made, but material from the same group of crystals was studied In this way later and found to be pure. Similar specimens from Bristol, however, hav n found to contain large amounts of ohalooolte, consequently thoir high copper values are not above suspicion* This morpho tropic series postulated by Kraus and Goldoberry has been criticized by A* P. Rogers} who suggests that the variation In composition of bornite may be ex- plained as the result of solid solutions of ohalooolte i . bornite of the formula Ou* Pe 8* . The two analyses of Kraus an' Goldsberryare accepted, but no new evidence is brought forward except a triangular diagram upon which nu- merous -analyoeB givon in Hintze are plotted. The dotu form an irregular oval, the longer axis of which is roughly parallel to the line Cu x Pe S( x f^, but the coincidence is not striking. The most significant point brought out by the diagram is the clustering of dots around the point corres- : ending to tho formula Ou^ Pe 814.. The obvious significance of tho fact IB disregarded by Rogers, who merely states that it may be explained as the value for the average solubility of chalcooite in bornite, 1. Science, New Series, Vol. 4-2, p. 3#6, 1915* 37. 1 3, T. Wherry in a discussion of Rogers' paper differs from this interpretation, and shows that the variation in the composition of bornite nay toe explained equally well by assuming Cu^ ?e 8^ as the formula of "normal" bornite, with the variation in one direction caused by the presence of chalcopyrite or pyrite in solid solution, and in the other direction by the presence of chalcoolte. He does not favor the idea of solid solutions, however, but believes that the variations in apparently pure material are due to eubmicrosoopio inclusions of ohalcopyrite, pyrite or ohalcoolte. The limit of microscopic visibility ( about 0*001 ma, ) is determined by the wave length of light, and there is no reason to assume that the size of inclusions stops at this point. The suggestion is a valuable one for there is evidence which will be presented lator, that submicrosooplo ohalcopyrite may develop in bornite. Even In such cases, hov/ever, the Impure nature of the boraits w is revealed by the abnormal color on the polished surface, and it is vary probable that the variations in the analyses pre- viously mentioned hare been largely due to Included material which could have been easily detected under the microscope. It la vory improbable that mibralorosoopic pyrite is of Im- portance, The difference in hardness of bornite and pyrite ^9 even small grains of the latter very apparent when a bornite specimen is poliwhed. 1, Science, Vol. M-2, II, s., p, 570, 1915. 38. The controversy over the composition of bomlte, however, may bo regarded as settled by malysea of the mineral 1 in the Geophysical Laboratory in Washington, by Dr. E. T. Allen* Polishod surface a of the specimens were carefully studied in all cases, and the material found to bo practically free fron impurities, with the exception of one specimen from North Carolina which contained a little ohalcocite. Applying a correction in this one case, tho results are renar&able uni- form, and confirm Harrington's analyses closely. The for- mula Cu^ pe 814. is indicated with a high degree of certainty. Dr. Allen 1 analyses are given below. 2 Analyses of Natural Bornltee Local- ity Superior, Arizona Urn- Costa Rloa Bristol Conn. , Oullf ord Co. ,N.C. Messina Trans- vaal Cal Cu^ for re s^ Cu rk s Pb 62.99 11.23 25. 5S .10 63.19 11.31 none 63.08 11.22 25.5M- none 63.26 63.90 10.79 25.17 none 62.2M- 11.12 25.54- 63. 11. 25. 33 12 55 A/ none .02 none '. none 99.90 99.96 99. 99.6 99.90 100.00 at 25 Ifater at 25 5.076 5.076 5.052 5.079 5.103 5.09H- Mineral at 25 fater at M- 5.061 5.061 5.037 5.061J- 5.079 1. Composition of natural bornlte, A. J. S., Vol. 1916, Pp. M-09-^13. 2. E. T. Allen, Loc. oit., p. 39. Dr. Allen's paper concludes as follows; For the sake of oompletoness, we may Include here a recent analysis of the bornito from Virgilina, Virginia, by Ohase Palmer. Palmer's material was examined metallograph- ically and it la noteworthy that the principal impurity was Pound Cal. for Gu^ Pe Sij. CT! 62.50 63.33 Fe 11,64- 11.12 S 25. '10 : 25*55 99.5^ 100.00 ohaloopyrite. Though the results are in fair agreement with CUc Fe 54., 'the relatively high proportion of iron suggests the presence of a small quantity of ohaloopyrite r/hich in preparing the sarjple for n-tudy has escaped detection. ' "Finally, observations of two physical properties of 2 bornite confirn the chemical evidence. Murdoch states that the raineralographic examination of polished surfaces of bornlte from at least 30 different localities has revealed only an exceedingly slight variation in color and practically none in niorochemical behavior. While reli-ince on density alono as a criterion of purity is unsafe, the determinations in the table [p. 3S ire oonf imatory. All the determinations aro in good accord ( 506l to 5.079) except that on the Costa Rica Bp^ctoen; and while it may be possible that we have here 1. J. Wash. Acad. Sol., 5, 351, 1915. 2. Loc. ^it., p. 35. ' 40, -mother crystalline form of the sane composition; the low value for this bornite is reasonably accounted for by Dr. Morwin's observations, viz., that under the microscope it has a porous appearance, Aside, then, from these slight variations in con- position vhioh are so common throughout the mineral kingdom, wl vrhich are 'lue to foreign admixtures or to solid solution, there IB, in my opinion, no satisfactory evidence that natural bornite la variable in compos it ion, or that it is even of any other composition than that expressed by the formula Cu Pe Sij..* Still later investigations now about ready for pub- lication by the chemists of the Geophysical Laboratory, show that Hynthetio products of the composition CU5 Pe S^. agree in all respects with natural bornite whereas synthetic product* of other compositions differ more or less notably from natural bornite. PAR T _ II I DiSSCEIPITIOBS 09 DEPOSITS 41. PART III DESCRIPTION? OF DEPOSIT MAGMATIC-PNEUMATOLYTIC DEPOSITS Qokiep. Little Namaaualand. South Afrio^ The ohaloopyrite and bornite deposits in the vicin- ity of the town of Ookiep in Little Kamaqualand, Cape Colony, South Africa have been considered by many writers to be of magmatio origin* f This interpretation is supported by 0. P. Tolman, Jr. and A. F. Rogers 1 who hare recently published the results 1. A study of the Magmatio Sulfid Ores, Leland Stanford Junior University Publications, University Series, 1916, pp. 59-60. The following bibliography is given in Tolman and Rogers 1 paper: Delesae, U. Sur lee mines de cuivre du Cap de Bonne e, Ann. des Mines, eerie, 8, pp. 186-313, (1855). Wiley, A. Report on the mineral and geological struo- h Hamaqu Land, Parliamentary Report, Cape Town, Zerrener, C. Reise des Ingenieurs A. Thiers nach den 3ergwerken Namaqualande in Sud Africa, Berg, und Hut- tenm. Zeitung, 1860, pp. 41-44 and 53-54. Knopf, A. Uber die Kupfererzlagerstatten von Klein aaquaxand and Demaraland, Neues Jahrh. f. Min. Geol. u. Pal., 1861, pp. 613-550. Schenk, A. Die Kupferenslagerstatten von Ookiep in Hamaqualand, Zeit. der deutsoh. Geol. Ges. Verhand. Ges., pp. 53, 64-65, (1903). Kunta, J. Copper ore in Southwest Africa, Trans. Geol. Soo. South Africa, 7, pp. 70-76 (1904). Kupfererzvorkommen in Sudweatafrika, Zeit. f. prakt. Geol., 12, pp. 199-303, (1904). Ronalds on, J. H. Notes on the copper deposits of Lit- ind, Trans. Geol. Soc. South Africa, 8, pp. Io8-167, (1905). * Stutaer, 0. Magmatische Ausecheidungen von Bornit, t. f. prakt. Geol., 15, p. 371 , (1907). Rogers, A. W. The nature of the copper deposits of lamaqualand, Proceed. Geol. Soo. Southwest* Africa, 191o, pp. 21-34. 42, of a microscopical study of a suite of specimens from the district* Specimens from this district have not been available for study in connection with this work, but ae descriptions indicate that the magmatio origin of the ores is more definite- ly shown than in the case of any of the bornito deposits of this continent a brief summary of their important features will be given. The ore-minerals ocour in a great variety of coarse grained basic intrusivee, among which norite, mica diorite, augite diorite, diorite, hypersthenite and anorthoeite have been described* The rooks may be uniform within a single body, or two or more varieties may be associated* The contacts in these oases, however, are sharp, and interpreted as evidence of differentiation before intrusion. Dikes and sill* are the commonest forms, but horizontal sheets, stocks, pipes and irregular branching bodies have been described, and according to A* W. Rogers 344 of them have been mapped* (1916)* The ore occurs in lenticular shoots, usually with the greatest dimension in a horizontal position* The ore-bodies ocour in various parts of the intrusives and in some oases break into the wall-rock, which is the fundamental gneiss of South Africa* The ore-minerals are magnetite, ilmenite, hematite, pyrrhotite, bornite and chalcopyrite. *'rom the microscopical studies of Tolman and Rogers, it is established that the sul- 43, phides are without question later than the rook silicates. A small amount of euhedral magnetite occurs in the earlier rock minerals, but it is believed by the authors to be of replacement origin. The feldspars appear to be more easily replaced than the pyroxene, and the ores are more closely associated with them. Biotite is very susceptible to the attack of the sulphides, and is usually replaced by them a- long its cleavages. The rocks from many of the mines show almost no traces of the alteration products which commonly accompany sulphide ores. Anthophyllite, olinozoieite and chlorite are mentioned, but are not of general occurrence. Anthophyl- lite, cutting bornite and chalcopyrite in a vein-like band, and penetrating the sulphides from the borders of grains, is described by Tolman and Rogers, and the relations are clearly illustrated by their excellent photographs. 1 Chlo- rite is said to be later than the sulphides. One of their p photographs shows a ve inlet of chlorite breaking opaque minerals. The sequence of the ore-minerals is not very clearly shown. From the shape* of grains, Tolman and Rogers state that it is probable that the bornite was formed in part by the replacement of magnetite. There is good evidence that ohalcopyrite is later than pyrrhotite 3 , and a suggestion 1. Op. cit., Plate XIII, Fig. 55 and Plate XV, Fig. 63-63. 2. [bid,, Plate XVI, Fi. 65. 3. Tolman and Rogers, Op. cit., Plate XVI, Fig. 64. 44, that pyrrhotite has in part been formed by the replacement 1 of magnetite* The relation between the bornite and the ohalcopyrite is not stated* Chaloopyrite gashes cutting the bornite are described, and are believed to be the products of post-magmatic readjustment a. The Ookiep ores for tha most part are evidently of the same origin aa many sulphide ores elsewhere which are commonly classified as magmatio* The ore-minerals are later than the original rock silicates, but this is commonly found to be the case in ores of this sort* Alteration products characteristic of hydrothermal ores are lacking or only slightly developed* The conditions under which the bornite in the Ookiep ores was produced may be regarded as more nearly magmatic than in the case of any other bornite deposit known* If the Ookiep ores are not strictly magmatic, they are at least the closest approach to magmatic bornite ores that have been described. 1. Ibid., Plate XVI, Fi. 67, 45. La Plour Mountain. Danville District t Ferry Co.. i?aahington INTRODUCTION Bornlte oros of unusual interest are exposed in several pros- pects on the slopes of La Fleur ',rt,, Washington, a few miles south 3f the international boundary and about 10 miles southwest of the bown of Grand Forks, B. C. The ores have not been described pre- viously. The only information available concerning the general ge- ologic relations is to be found in the reports of the Canadian Ge- ological Survey , which deal with the Boundary District immediate- ly to the north. The nearest mine, the Lone Star, a small proper- ty of the British Columbia Copper Company, is about 3 miles distant, md the large deposits of pyritio ore near Phoenix, worked by the Jranby Co. , are ten miles or more across the boundary. The miner- ilization of these deposits, however, is distinctly different in jharacter from that of the bornite deposits, and there is probably 10 genetic relation between them. The claims on La fleur Mt. have been developed only to a very 1. E. ff. Brook, Summary reports of the Director of the Geolog- ical Survey of Canada for 1901 and 1902. R. A. Daly, Seol. of the North American Cordillera at the 19th Parallel, Part I, p. 277, (1915). 0. 3. LeRoy, The Geology of the Phoenix IHning District, iemoir 21, C.G.S., 1912. . i 46, slight extent "by a few shallow shafts or short adits, A little ore has been mined, and shipped, but no noteworthy work has been done. OEOLQaiCAL DELATIONS. She older rocka of the mountain form a aeries of vari- ous sediments, of which limestone beds and some altered por- phyries are prominent members. ?rom the mapping on the Cana- dian side of the line, it is possible that the sedimentary rooks may be members of the Attwood series, argillites, quartzites and limestones, stated to be of Carboniferous (?) age. A diorite porphyry, now altered in places to a somewhat sohistose rook containing abundant biotite and shreddy araphi- bole, is associated with the sediments, but neither its exact relations to them nor the form of its bodies could be detect- ed. The original rock possessed small phenocrysts of andeslne, oligoelase and biotite, set in a microgranitic ground mass of the same mineral, in which there is also a little orthoolase and fine shreds of amphibole. The older rocks of the mountain are intruded by an ir- regular mass of gabbro. The rock varies from dark varieties, in which hornblende becomes prominent, to lighter types in which feldspar predominates . In the commonest form, the rock possesses a medium to fine grained texture, and is composed chiefly of acid labradorite (Ab 5 A^) and colorless to pale . 47 green augite. Hornblende is usually present, apparently formed In part at the expense of the pyroxene. In places, the rook becomes of dioritic character. The mass of the rock is cut by numerous small seams of pegraatitic material, nearly always feldspathio, with little or no quartz. They are usually less than one inch in thickness. The gabbro is out by dikes of coarse syenite, averag- ing about four feet in width, but occasionally much wider. In places, the rock becomes distinctly pegmatitic in charac- ter, with the orthoclase in crystals two or three inches long. A sub-parallel orientation of the feldspar prisana is usually prominent. It suggests a flow structure. The chief mineral of the syenite or the syenite-pegmatite is orthoclase Under the microscope, albite is found to be common also, usually in smaller grains around the margins of the coarser orthoclase, and to a slight extent in mi- croperthitio intergrowths. Muscovite is the only other mineral prominent in the syenite* It occurs as medium grained flakes of slightly greenish tinge when seen with the unaided eye* The mica is closely associated with the albite, and is usually a constituent of the zone of finer grains surrounding the coarse orthoclase crystals. Both the albite and musoovite are in part later than the or- thoclase. Other constituents are of little quantitative importance* Titanite in sharp crystals is not uncommon. 48, and there is a little apatite. 7/here in contact with the altered porphyries of the older aeries, a little augite sometimes developes in the dike rock. Other modifications with respect to the wall rocks are closely related to the processes oi' the mineralization and will be described in that connection. The dikes are in a north-south zone and may bo traced about a mile, although no single dike is con- tinuoiis for that distance. The latest igneous rock is a gray diorlte por- phyry, which forms a broad dike cutting all other formations. It is fresh and unmineralized. and in striking contrast to the older and more or less altered rooks of the mountain. \ m Aj&*^ \ V ]fj^^ MUTSRALI ZATIOH Ores in the Syenite Dikes. The mineralization is closely associated with the syenite or syenite-pegmatite dikes. The sulphides, bornite and chaloopyrite^ are most abundant in the dikes themselves, but where the dikes out the gabbro, the sulphides penetrate the wall-rock along fine seams or occur disseminated through it to a slight ex- tent. The mineralization in the dikes however is not con- tinuous and it is only locally that the sulphides are suf- ficiently abundant to constitute an ore. The ore-minerals corrode the orthoclase, albite and Muscovite, but with the exception of an albite rim which . ' 49, often ia present between the ore-minerals and the orthoolaae, there la no other important alteration of the rook-minerals. A little fine-grained musoovite or serioite is sometimes de- veloped in the albite, and later oaloite replaces the feld- spars to a slight degree, but the absence of alteration pro- ducts which usually accompany sulphide mineralization is an outstanding feature. Mineralization in the Wall-rocks* Where the dikes cut the limestone members of the sedimentary aeries, a garnet rook is formed composed chiefly of ondradite, albite and oal- cite, with a little diopside, titanite and apatite, The rock contains numerous small cavities between the crystals of its constituents, and a scattering of chalcopyrite grains. Gar- net is also abundant in ono part of the syenite dike on one of the claims on the east end of the mountain, and is probably to be attributed to the influence of limestone on the magma* The garnet is poikilitio with albite, and probably crystal- lized later than the feldspars. The mica in this part of the dike is biotite (deep green parallel to c , pale yellowish green to colorless perpendicular to c ) , and the colorless muscovite, abundant elsewhere, is in small flakes and quite subordinate. Where the mineralization has penetrated ths gabbro, there is more pronounced rock-alt oration. There is a notable Increase in hornblende near the dikea, and also along the small 50, seams of feldspathic pegraatitio material which cut the rook* The sequence of minerals is well shown in one thin section which includes a portion of the edge of one of these string- ers. The pegmatitic material is largely orthoclase and al- blte, in inicroperthitic relations for the most part; near the margins garnet is slightly developed. In the rock nearest th seam, hornblende is most abundant. In places it possesses a rim of deeper green than the heart of the grain; (pleocroism: deep green parallel to b; yellow or pale brown parallel to a; light yellow or yellowish green parallel to c). farther into the rock, pyroxene (colorless augite) becomes of greater im- portance and io the chief femic constituent. Kpidote and chlorite are abundant in the rock, as replacements of horn- blende, pyroxene and the feldspars. The older plagioclase of the rock contains abundant sericite and calcite. Apatite and titanite are somewhat more abundant in the rock than is usual- ly the case. The chief opaque mineral in this particular slide ia magnetite, which is later than the pyroxene and feldspars, and in part at leaat, later than the hornblende, for all the preceding minerals are clearly corroded by it. It frequently includes small apatite grains. Ore-:.!inorala. The ore-minerals are chiefly chalcopyrite and bornite. The former is slightly nore abundant. Magnetite ia largely confined to the gabbro; it was not observed in ores from the syenite. The ore-minerals in the dikes are later ae of;! 51. than the rock-minerals, as has been stated, although probably not very much later .for there are so few alteration products associated with then. In the gabbro, they are probably con- temporaneous with the epidote and chlorite. The chalcopyrite and bornite are usually in close asso- ciation, and in part probably of contemporaneous origin. The contacts between the two minerals, as shown on the polished surface, are smooth and even, with few irregularities such as tongues or veinlets which would definitely show sequence. In many oases, their relations to each other would be little al- tered, if the two minerals were interchanged. This relation i i has been termed the mutual b oundargr , and is believed to be particularly characteristic between minerals of the primary sequence (figure 67 ). There is a little evidence, however, of corrosion of the ohalcopyrite by the bornite in some places, and it is probable that conditions favoring the formation of bornite continued after the deposition of chalcopyrite had ceased, which resulted in a slight replacement of the chalco- pyrite by the bornite. This interpretation is shown more clearly on the diagram of mineral sequence. Magnetite, and a small amount of specularite are probab- ly earlier than the sulphides, but their relations could not be determined with final satisfaction in this deposit. A little sphalerite, galena, and tetrahedrite were ob- 1. L. C. Graton and I). H. McLaughlin, Ore deposition and enrichment at Engels, California* tioon. Geol., Vol. XII, 1917, p 17, 52, served as small spooks on the polished surfaces in the midst of the more abundant sulphides. OXIDA2IC1I A1TD SilKICHI,!; Tho deposits occur in a mountainous region of mature topography (Fig. 6 } and possess no noticable gossan, a fact whioh nay be attributed to the present vigorous erosion and preceding glacial scour to whioh they have been subjected* A small amount of linonite and malachite occur, but the sulphides outcrop directly at the surface in a fairly fresh condition* A small amount of chalcoeite, covellite and chalcopyrite occur in the bornite, and are undoubtedly to be attributed to the processes of secondary enrichment, but as far as their total copper content is concerned, they are of little Importance. She chalcocite is in fine veinlets or rims associated with the bornite. It follows the easy channel-ways afforded by the contacts of bornite and the gaugue or rock minerals, or seeks out grain boundaries, seams or other obvious lines of attack. Covellite is for the most part the first stage of the alteration of chalcocite to malachite and usually occupies an intermediate position between the two minerals. It dovel- opes directly from bornite, however, to some extent. 2he veinlets of chalcooite and covellite penetrate the primary ohalcopyrite, but the latter is clearly more resistant to al- teration, and they do not develop as easily as in the bornite. The bornite in the neighborhood of the chalcocite and covellite veinlets commonly contains a small amount of chal- I* $ -, - ' - tat 53. oopyrite, which is closoly related in origin to the secondary sulphides. The chaloopyrito in this association developes as plates in the bornite, apparently oriented parallel to throe or rarely four crystallographic directions, which on the pol- ished surface yield the reticulate pattern known as the lattice structure. In some grains of bornite, the plates of secon- dary chalcopyrite are so thin and so finely spaced that they are scarcely visible under the highest available magnifica- tions, and it is justifiable to assuao that the peculiar yellow oolor of certain areas of bornite in the same vicinity is due to subraicrosoopic chalcopyrite of this sort. DISCUSSIOH The possibility that the syenite and syenite-pegmatite dikes are late products of the same magma which produced the gabbro offers a problem of great interest. v The small seams of pegraatitio material which are common in the gabbro, and which would ordinarily be accepted es its pegmatitic phases, are closely related mineralogically to the syenite, and are probably of the same origin. The greater alteration and more abundant mineralization of the gabbro along the ore-bearing portions of the syenite dikes than it* the case where other rocks are encountered might be interpreted as an indication that the gabbrp was still heated at the time of the intru- 54. sion of the syenite, and in a receptive state to react with // the vapors given off. There is no evidence against the view, ^ and although the arguments in its favor are not of direct or compelling character, the suggestion that the syenite dikes and the ores are final products of the same magma from which the gabbro crystallised remains a rather attractive hypothe- sis. ' s=* i tfe The syenite contains numerous miarolitic cavities, which may be regarded as a strong indication that gases played an important r3le both in the formation of the differentiate and in the determination of the character of the crystallization. The abundance of hornblende in the gabbro near the edges of the dike also testifies to the importance of mineralizers. The concentration and formation of the various ore-bodies was very probably the work of the same agencies. The ores are magmatic in the sense that they are associa- ted in close relationship with the feldspar of the syenite dikes. The ore-minerals, however, are later than the rook minerals, and were probably formed either during the closing phases of the magmatic period or under pneumatolytic condi- tions. The abnormal character of the dikes (the variable tex- ture, the coarse grain usually with the ores, and the abundant miarolitic cavities) indicates that the concentration both of the syenite and the ore was a pneumatolytic process. Conse- quently it seems nearer the truth to consider the ores to be of pneumatolytic rather than of direct magmatic origin. The 55. Diagram of Mineral Sequence Minerals Magmatic Period Pneurcato- lytic Pe- riod Hydro- thermal Period Period of Ox- idation Lab r ado rite (in gabbro) Orthoclase (in syenite) Augite (in gabbro) Albite Muscovite Biotite Apatite Titanite Hornblende Garnet Epidote Sericite Chlorite Chalcopyrite Bornite Galena Tetrahedrite Calcite Chalcopyrite Chalcocite Covellite Malachite . J at . srti - eJiic 93 i e. ' 56, almost complete absence in the syenite of the usual alteration products which, commonly accompany ores of hydrother-nal origin indicates that the sulphides in the dikes were probably formed under earlier and probably pneumatolytic conditions. The period of ore formation in the wall-rock, however, con- tinued through the transition from gaseous to hydrothermal con- ditions, as is shown by the association of epidote and chlorite with the sulphide stringers in the gabbro. SUMMAP.Y The chalcopyrite-bornite ores of La Fleur Mt occur in unaltered syenite and syenite-pegmatite dikes, and to a slight extent associated with minerals of hydrothermal origin in ad- joining gabbro. The syenite and syenite-pegmatite may have been late extracts fron the magma from which the gabbro crys- tallized, but there is no positive evidence to establish this point. The formation of the syenitio magma and the concen- tration of the sulphides are believed to have been caused largely by pneumatolytic agencies. The sulphides are later than the rock minerals of the dike, but were not accompanied by changes of hydrothermal nature. In the gabbro, the occur- rence of epidote, chlorite and sericite, associated with the sulphides, indicates that the mineralization in the wall rock took place under milder conditions than in the dikes. The alteration of the ores by surface agencies is slight, 57, but affords a clear oxanplc of bornite altering to chaloo- cite and covellite with the development of secondary chal- copyrite in fine lattice structures in parts of the bornite protected fro.-n direct attack by the oxidizing agencies. 58, The Evergreen Mine. Gilpin Co.. Colorado. INTRODUCTION The copper ores of the Evergreen Mine have attracted the attention of geologists during the last few years on account of the intimate association shown to exist there between the sulphides of copper and the apophyses of a neighboring stock of monzonite. The copper minerals, chiefly bornite and chaloopyrite, occur largely in two small igneous dikes, and in the matrix of an igneous breccia formed by the complex intrusion of the molten mass into the walls of shattered schist and pegmatite. Situation* The little town of Apex, near which the Evergreen Mine is situated, is well in the heart of the Front Range of the Rooky Mountains, in the high region seven miles east of the Continental Divide. The nearest railway point is Central City, about 6 miles to the south- east, which is the terminus of a narrow gauge line of the Colorado and Southern Railway, which ascends the rugged canons of Clear Creek and its northern tributary from the town of Golden. The topography is mapped on the Central City sheet of the U.S.C.S. Development. The mine has been operated in a small way for nine or ten years. A shaft about 350 feet deep has been sunk, and levels run r.t 100', 200' and 350' 59, respectively. A little ore has also been mined from a tunnel penetrating the deposit from a point on the surface north of the shaft. A email mill has been erected recently, and at the time of my visit some experimental work with flotation methods was being done. Very little ore, however, has been shipped. Literature. In the first published description of the deposit, a paper by E. A. Hitter 1 , the opinion was expressed that the ores are primary constituents of the intrusive rock, and crystallized directly from the magm^. with the rook minerals* The unusual concentration of the sulphides is regarded as a segregation caused by sublimations from the magma* The ore-bearing intrusive is described as a rook composed "of quarts, alkali feldspars, orthoolass and albite, (often interlocked as mioroperthitio), with augite of the aegirine variety and long needles of enstatite and diallage", and the name Evergreenite is given it* Phases termed miorogranitio and porphyritic are mentioned, the latter being regarded as a more advanced stage of the segre- gation as it is characterized by a greater abundance of enstatite and diallage, or of quarts. The mention of micro- pegmatite in the "microgranitic" phase is of interest, and the structure ie well shown in a photograph of a thin section. Two types of alteration of the country rock (a biotite 1. E. A. Rittsr, The 'Evergreen Copper Deposit, Colorado, T.A.I.E.E., Vol. 38; pp. >,(1907). 60, schist) were observed by Mr. Hitter, viz., (1) an introduction of quartz and aegirite forming pseudo-schists, and (3) the formation of pseudo-gneisses by the development of alkali feldspars and aegi- rite, the biotite remaining little al- tered* Covellite la described as an alteration of bornite and ohalco- pyrite due to descending waters, but it is not common, and is believed to be confined to the ore adjacent to the schist and not in the massive intrusive. Baetin and Hill in a later article 1 add new data concerning the mineralogy of the ore, and come to some interesting conclusions regarding Its genesis* The colorless pyroxene in the Intrusive is shown to be wollaaton- ite, and not enstatlte or dlallage as determined by Hitter. The occurrence of garnet in the rook with the wollaston- ite is regarded as suggestive of the digestion of oal- oareous wall rook by the magma, probably before it reached its present position. The authors support Hitter's view that the sulphides are primary constituents of the dike rook, but th~y believe them to have be^n derived either from the wal} rocks at depth by absorption or from the m&gma by differentiation. Their conclusion Is: "The deposit under this view represents an endomorphic effect produced by contact metamorphism at the border of a large intrusion of monzonite." 1. E. 8. Bastin and J. M. Hill. The Evergreen Cppper Mine. Colorado. Econ. Geol. Vol 6." p. 465-4 (1911). 61, Physiographic Features. The Evergreen Mine is at an altitude of 9800, in a region of rather subdued re- lief, slightly above the line of the preoipitouo elopes of the deep canons. If the great relief due to the youth- ful V-shaped gorges of the present roaster streams, is ig- nored, the topography above the 8000 foot contour is rather subdued; the a lopes are gentle, the ridges well rounded and in places flat-topped, and the valleys open and with fre- quent meadows. The highest peaks of the range rise 3000 or 4000 feet above this general surface, but with surpris- ingly gentle and regular elopes for such high altitude's. Glacial cirques interrupt the smoothness of their contours for the last thousand feet or more, but the glacial erosion is far from mature, and has failed to destroy the older surfaces completely. From the neighborhood of the mine, I saw few peaks which could not be ascended on horseback. The high surface of relatively gentle relief ie sharply incised by the master streams of the region, and the cutting has worked back alonjr the tributaries in varying degrees. Between Black Hawk and Golden, the North Fork and main stream of Clear Creek have cut a canon SOOO feet deep, in places with high cliffs, and many of the smaller valleys of the same ^nage basin present strikingly abrupt slopes, as is shown in/figures. The three distinct topographic features, which have been described above, may be summarised as follows: 61A PLATE I. The Svergr een Mine. Gilpin Co.. Coloradp Fig. 1. The Evergreen Mine from the aouth east. Fig. ?.. The Evergreen Mine and the valley of Pine Creek from the eouth. 613, PLATE II. The Region near the Ever green Mine. Gilpin Co.. Colorado. Fit? * The valley of Clear Creek looking south east frorr, a point 212 miles northwest of Central City. Fig. The valley of Pine Creek, looking south from a point near the Evergreen Mine. 62, (1) the old surface of moderate relief, () the sharp V-shaped valleys dissecting it, and (3) the glacial cirques modifying the slopes of the higher peaks. 1 The Evergreen ore-body io in a shallow valley of the older cycle of erosion in the region of moderate topographic relief. It ia still above the reach of the actively cutting youthful streams. The climate is severe, the snow las tin-; for seven months, but the under-ground workings are rarely below freezing. General Geologic Features. The district is in the wide area of pre-Canbrian rocks, which form the greater part of the Front Range, the oldest formation ie a fine- grained biotite schist, believed to be of sedimentary ori- gin. Into it are intruded several plutonic rocks also of pre-Cambrian age, showing various degrees of metamorphism. Pegmatitio intrusions associated with the granites are very prominent, in places cleaving the fissile schists as broad dike e tens of feet In width, elsewhere forming numerous fine laminae in the earlier rock. The only rocks younger than the pre-Cambrian are various early Tertiary intrusivee, chiefly monzonitic in character, which form many small stocks and dikes. The longer axis of these later bodies is usually roughly par- allel to the schistosity of the metamorphic rocks, although in some cases the intrusions are quite irregular. The 1. Spear and Carrey also recognize similar topographic features in the Georgetown Quadrangle. (Prof. Paper 63, U.S. G.S., The Geol. of the Georgetovm Quadrangle, Colorado.) 63. chief ore-bodies of the region, the gold-silver veins and the tungsten veins, are clearly related to the Tertiary intrusivee, and no claim in thoroughly satisfactory to the prospector which does not show an outcrop of porphyry. DESCRIPTION OF THE DEPOSIT The occurrence of copper in workable amounts la rare in the district and the ore-body on the Evergreen Claim is the only one which supports a mine. The dependence of the ore on the dike rocke is unusually definitely shown, and the evidence read from the relations there, greatly strengthens the general belief that the "porphyries" were the mineralizing agents throughout the region* Associated Rocks. The wall rock of the deposit is the pre-Oambrian biotlte schist, with numerous tabular v lenticular intrusions of a siliceous pegmatite. Although nor. the thin sections show the schist without some altera- tion due to the mineralizing fluids, the original rock may be readily inferred to be composed of fine flakes of a deep brown-green biotlte (deep brownish-green parallel to c; pale green perpendicular to c) , and a fine-grained aggregate Of orthoclase and quarts. The pe;iatitic material is chiefly quartz and orthoclase, in coarse irregular masses* To the east of the mine, a stock of monzonite outcrops, and the two dik-:s with which the ore is associated, are very probably apcphyees of this stock. The dikes strike 64. a little west of north, and dip steeply (60-70 ) to the east, Leinc approximately parallel both in strike and dip to the structure of the schist* The dikes are about twenty feet apart. The one to the eaat is the larger, averaging 13-15 feet thick, while the one to the west varies from a few inches to about three feet* In places the wall rocks are penetrated irregu- larly by the intrusion of the igneous material, producing masses of igneous breccia often of notable extent* The rooks between the two dikes have suffered this breociation particularly. Places were observed in which the identity of tha dikes themselves becomes loet in a confused mass of this shattered schist* All gradations exist from the ratio of wall rook to intrusive in which the schist fragments are properly described as inclusions, to the conditions in which the wall rock predominates, and the igneous rook fills ir- regular fractures in it. The schist and pegmatite appear as if shattered by violent explosions, and then oemented by tha intrusive rock. The fragments of the breccia vary from small chips, a fraction of an inch in thickness to blocks a couple of feet wide* The normal dike rock is a fine-grained hypidi- omorphic granular rock, of a medium gray color, with small feldspars and pyroxenes visible with the hand-lens. For the most part it is even-grained, but in places there is a sub-porphyritic development of the orthoclase. Dnder 65. the microscope, the unmineralized rook is seen to contain orthoclase as the chief constituent (in part mioroperthitic with alblte) with somewhat less abundant quarts. A small amount of oligoolase is sometimes present. Green augite (non-pleocroic or only faintly pleocroic) is the most abundant dark mineral, but a little pale blue-green amp- hibole was observed in one elide, and a few laths of deep green biotite, (deep green parallel to c, light yell. green perpendicular to c). Titanite crystals and small prisms of apatite and zircon are common accessories* The orthoolase grains average about .5- .75 mm. in longest diameter, with a maximum of abovt 1..5 mm; the quartz usually about .5 mm, and the pyroxene .5 m, with a maxi- mum of 1 mm. 'rom the Microscopic work, the rook seems more nearly a granite in mineral composition than a mon- aonite. Approaching the ore-bodies, the dik rook alters to a peculiar alkalins granite, with aegirite-augite, and fine laths of wollastonlte usually in great abundance. In the hand-specimen, it ie a llrht colored, fine-grained rock with a peculiar pearly luster, distinctly different from a normal feldapathio rock. In some cases, the l&ths of wollaetonita are sub-parallel; in others they form an int-rlaoing mass, orthoclaae with some miorooline, quartz, wollaetonite and the deep green aegirite-aupjite are the chiff components of the rock. A little nlagioclase (oli^oolase - 66 alblte?), good sphenes of titanits, and email crystals of airoon are accessory. . Apatite is abundant, in some cases as large grains. The aesirlte-augita forms small stubby crystals, usually with a pale green, non-pleocroic core which is in striking contrast to the deep green tints of the outer portions of the grains. The extinction angles of both portions of the crystal are high, but di ffer by a few degrees, the outer portion having a slightly lower angle of extinction than the inner. The feldspars and quartz are In the usual hypidiomorphio relations, and of about the same grain as described for the normal rook. The wollastonite, which is the most unusual feature, is In long laths, (max. about 75 mm.), commonly in sub- parallel orientation. It is most ordinarily grouped along the contacts of grains, and tends to avoid the quarts, although a few prisms penetrating the latter were observed* It projects into the feldspars in some caeea, but the re- lations may be explained equally well by assuming a re- placement ori in for it, as by assuming it to be a primary rock mineral. The parallel structure shown by the wollaston- ite is not shown by the other rock minerals except to a slight degree. Bastln and Hill mention finding abund- ant garnet in the dike-rock, and in the breccia, but in the thin sections of my specimens it is not an important mineral. The unusual abundance of apatite does not seem 67, to have been emphasised by the previous writers. The dike rook containing the sulphides, and the rook forming the iprneous breccia is the aegirite-augit* and wollastonite bearing phase of the granite. For the most part, the thin western dike (the main Evergreen dike) consists entirely of the vvollastonlte bearing rook, but on the 300* level, where a drift has been pushed several hun- dred feet to the northwest along it, it changes without any distinct boundary into the normal gray granite. In one place the change required about 10 feet, - in another it was apparent within the dimensions of a hand-specimen. The wollastonite- bear ing rock la probably the rock named "Ever c r*enite" by Hitter, but as it has been proved by later work*, to be of different mineralogioal com- position and as it is merely a Modification of an alkaline granite, it seems hardly worth while to burden the already bewildering list of rock types with another name. The name, however, eervee the local needs admirably, and will be used to refer to the wollaetonite bearing phase of the alka- lian granite associated with the ores. Primary Mineralization and Rook Alteration. The common ore-minerals are ohaloopyrite and bornite, present in about equal amounts. The largset masses of the two sulphides occur in the "evergreen it e, usually not in the dikes th-em- 1. Baatin and Hill, Loo. oit. 68. selves but in the irregular intrusions in the breooiated wall rook. The inclusions of schist or rgatite contain fine seams of ohaloopyrlte, or disseminated specks of ohaico- pyrite or bornite, but no large masses of the ore-minerals were observed except in the igneous matrix itself. Thd dike rock associated with the ore contains numerous mlaro- litic cavities, and in places develops a coarser structure, suggesting pegmatitic tendencies. The presence of micro- pegmatite in these rocks mentioned by Ritter is in accord with this observation. In the miarolitic cavities crystals of orthoolase, with crocidolite, and imperfectly crystallized grains of bornite and chaloopyrlte between them, were ob- served in a few cases. Fluorlte occurs in small grains in the intrusive, usually in the miarolitic portions. A few particles believed to be tourmaline were observed, but were not positively identified. Hydrothermal minerals are not common, but epidote is associated with the bornite in many oases. It fringes the bornite grains, replaces the feldspars and pyroxenes, or cuts across the rook in veinlete. Serioite is somewhat developed in the feldspars, but on the whole it is not very abundant. In one of two oases, laths included in th? outer portions of bornite graina were observed, and interpreted to indicate the extension of the period of bornite formation into the period of conditions favorable for sericite (I.e. hydrothermal). 69, Calcite ie common, replacing both the earlier rook minerals, and thf vollaatonite. The latter ie particularly susceptible to the attack oi' the caloit?, and in many specimens, its previous existence in the rock can be inferred only from the lath-like forma assumed by the oalcite. The caloite seems a little later than the epidote, and is moat probably a product of the closing phases 01 tn.; enan&tione which formed the ores* AB in the field and in the hand-specimens, the achiets present fairly sharp boundaries against the invading dike-rock, but in a few places, intimate injections p-lor. , the cleav.--5j.es cf small fragments have reduced them to mere bands oi the dark constituents (chiefly biotite) floating in coarser grained bands of the intrusive. Under the microscope, the schist is saan to contain in many cases, a blue-green amphibole in broad, rather irregular grains, up^to r5 to ,75 mm. in length, which commonly con- tains abundant inclusions of fine flakes of the biotite of the schist* (The microscopic properties fail to identify the amphibole with any of the ordinary varieties. They may be summarized as follows: Index and birefringence some- what 1&3& than augite; amphibole cleavage; oblique extinc- tion to c, max* =ua ;j le 41; pleocroic, light blue-green para- llel to A; optio&lly poeitive(?). Pale cores with more deeply colored rims are common* The change is accompanied by a slight incree^e in the angle of extinction). The longer 70. axle of the amphibole is usually parallel to the achistosity. Its similarity to the amphibole observed in the normal gran- ite, and the inclusions of biotite favor the view that it is a product of emanations from the intrusive, reacting with the min-srala of the schist. The borders of the schist inclusions may be seen with the aid of the microscope to be usually lined with fine grains of deep green aegirite-augite. Fine grains of augite are abundant throughout the inclusions, but as they occur along veinlats in a few places, and seem closely related to the aegirite-augite near the margins, they were probably introduced from the magma. The augite grains do not occur as inclusions in the amphibole in the echist, and are prob- ably later. The feldepathic portions of the schist are partially reorystallized, especially near the edges of the inclusions. Large grains of orthoolaee, surrounded by fine-grained aggre- gates of biotite and augite, although united in one fairly definite crystal, still show by their wavy extinction, the irregular interlocking boundaries of the grains from which they were welded. Irregular, elongated grains of dirty looking titanite are common, frequently associated with a peppery aggregate of small magnetite specks. Both minerals are probably original constituents, but thay seem to have suffered many hardships in their severe experiences of suc- cessive intrusions. 71. The pegmatitic material la little altered by the dike-rook. The occurrence of coarse quartz in certain breo- oiated ones is rather puzzling to interpret, but it is most probably pre-Cambri?n material for very siliceous phases of the pegmatites were observed. The ooareer types of the ore- bearing rock become almost pegmatitio in small areas, but rarely approach the grain of the older material* Epldote and sericite are developed to varying de- grees in both types of the wall rook. The fine-grained feld- spars of the sohists are especially susceptible to the attack of the sericite, but they and the other minerals of the schists usually show less alteration to epidote than the minerals of the Igneous matrix. The bornite and ohaloopyrite are present in approximately equal amounts. The contacts between the two minerals are commonly sharp and definite, with smooth un- broken lines, and as commonly showing blunt penetrations of the bornite by chaloopyrite, ae of the ohalcopyrite by the bornite; in brief the minerals exhibit mutual boundaries! toward each other. In a number of specimens, the graphic structure is developed, usually with the chaloopyrite as the host mineral, and with bornite as the smoothly irregular blebs in it. (Figures 45,46. ) When these structures are studied, it is difficult to avoid forming the opinion that 1. Page 51 S the two sulphid so are contemporaneous in origin. In a fair number 01 casee, however, bornite was observed to form broad margins alorv, gangue veinlets, now altered to calcite, which out chalcopyrite areas. (Figure 42.) The deduction that thie bornite is later than the chalcopyrite follows inevitably, and it is -: : 17. Bornit* _ - _ 18. Sphalerite 19. Tetrahsdrite 20 . Galena 21. Epidote _ 22 Sarioita 23. Chlorite 24. Calaite 25. Chaloooita 26. Covellite 27. Malachite 28. Azurite 29. Lirconite 30. Kaolin ^^ 78. The minerals grouped as magmatio are those which are believed to have crystallized directly from the igneous melt. Those termed pneumatolytic, are believed to have been formed at the period in which the concentration of gases was great, and *hen conditions of equilibrium were governed largely by the presence of mineralizers. No sharp boundary between the magraatio period and the pneumatolytic period oan be drawn, as probably conditions favoring simple crystallization from the melt grade into the more complex conditions of pneu- matolytic alteration with the increasing concentration of gases and their accompanying elements. The final and complete consolidation of the rook probably took place during the pneumatolytic period. With constantly decreasing temperatures, pneumatolytic conditions gave way to those predominently hydrothermal in character, which mark the closing phases of the mineralization connected with the intrusion. The whole presents an unbroken sequence from the initial magmatio con- ditions to the final feeble hydrothermal effects. Periods in which one or another set of conditions dominates may be recog- nized, and their products described, but definite breaks be- tween them do not exist. The minerals placed in the magmatic group require little discussion. Their sequence is that commonly observed in igneous rooks. Both the feldspar and quartz, however, con- tinued to form under the early etag'es of pneumatolytio con- ditions, if the occurrence of micropegraatite noted by Hit- 79 may be ao interpreted. The large crystals of ortho- olaee in miarolltio cavities *ith the sulphides and cro- cidolite are further indication of the formation of the feldspar under the later conditions. The aegerite-augite occurs as rims around cores of audits, and is believed to be the result of the shifting conditions brought about by the increasing concentration of volatile constituents* The amphibole, which is developed chiefly in the inclusions and wall rock is probably also a product of pneumatolytic action. Its formation is known to be favored by the presence of hydrous vapors. The orocldollte , which ia meet commonly formed in cavities of pegmatite dikes may be placed in this group without question. The abundant development or vratite, both in the evergreenite and in the schist, is a further indication of gas concentration, for the exist ance of notable quantities of the mineral demands the presence of ei "her chlorine or fluorine. Fluor ite, which was observed in small grains in miarolitlc material, al*?o empha- sizes the importance of the pneuaiatolytic period in the history of the deposit. Tourmaline was doubtfully identified and if present it belongs in this group. As has been discussed previously, there is reason to believe that the wollastonlte is later than the feldspar or the quartz of the evergreenite. Its rarity as a product of direct magmatic crystallization argues against grouping 1. Loc. cit. 80 it with the earlier minerals. It is chiefly found as a contact-met amorphic mineral in limestones, where the abund- ance of Ca is undoubtedly a controlling chemical factor, but where at the same time, the physical conditions of forma- tion are controlled by the heated emanatione from the in- trusion, and are comparable to the pneumatolytic conditions postulated in the genesis of the Evergreen rocks and ores. For these reasons, it seems better to place the mineral in the pneumatolytic group in this case, although the micro- scopic evidence is not absolutely conclusive that it belongs there Baatin and Kill describe the ore-body as an endo-. morphic effect due to contact raetamorphism as quoted on page 60 The interpretation of the wollastonite as a pneuma- tolytic product is in agreement with their conclusion, al- though elsewhere in their paper, the impression ia given that they regard the mineral as a product of direct crystallization from the melt. The sulphides are clearly later than the feldspar, quart a and pyroxene, for they unmistakably corrode these rock minerals. The occurrence of the sulphidec in the rocks show- ing greatest concentration of aegerite-augite, wollastonite and apatite, and in rocks with abundant miarolitic cavities, points to their formation under pneumatolytic conditions. The microscopic relations show the sequence of ohalcopyrite and bornite to be approximately as indicated by the diagram. (The evidence is given on pages 71-73.) The other 81, sulphides are hardly abundant enough to give conclusive evidence of their age relations, but they are approximately contemporaneous *ith the bornite, although probably somewhat icor? abundant toward the closing phases of its period of formation. It is of int?rf.*t that the amount of magnetite and pyrite As very small. The little magnetite observed the was originally present in schists. The pyrite occurs only A a.a a few scattered grains, apparently in the schist, and may be the product of other mineralizing conditions, unaeso- elated with the- chief mass of th ore-body. Baetin and Hill, on what seems to be good evidence believe another p-riod of mineralization has introduced galena and sphalerite, but neither the field nor microeoopic results of my work, givo any further information on thie subject. The small amount of sphalerite and galena to which reference has previously been made, is probably not what they had in mind. Epidote, eericite, chlorite, and calcite fto-faydr^- aiiiuiulB have been classified ee hydrotheraml in agreement with the general interpretation of the conditions under which these rrinersle form. The cloee association of bornita and epidote in several caees indicates the continuation of the formation of bornite under hydrothermal conditions, although thrre le lese evidence of its association with th other minerals ol the group. In the rich ores, the wollaetonite of the evergreenite is invariably rpplaceci by calcite; this special relationship between the sulphides and caloitee sug- 82. gests that the latter IB probably a late product of the mineralizing agencies, rather than a result of descending carbonates. The ehallowneos of unquestioned surface altera- tion except along narrow Beams and veinlete aleo favors this view. The oalcite la later than the aerioite or chlorite and may be regarded as the closing product of the mineralising processes. The reaiitte of oxidation are limited to & mall amount of secondary chalcopyrit* and veinlete of chalcocite, and to a few fet of malachite and liEonit"'-et*.Ined capping. The Origin of th~ Deposit. Baetin and Hill state the that the dike of evergre conclusion that the deposit is pre- dominantly of pneumatolytic origin, although the mineralization continued srith declining strength under hydrothermal conditions. Ths unusual occurrence of wollastonite, however, is less satisfactorily explained by such an hypothesis. Its rarity as a const it\ient of an igneous rock, or as a pegmatitic mineral suggests that thsre la probably some peculiar con- dition governing its formation in this case. Baetin and Hill believe that the high content of Ca in the magma is dua to ths reaction of tha melt on caicareouo country roc 1 *. As the boundaries between the inolxided aahiet fragment* and 1. S. Arrheniusj Zur Physik der Vulkaniswusj Geoi. Foren> Forh., 22, p. 247, 1900. 84. ths intrusive exclude the possibility of any notable rr^ having been absorbed from the wall rocke at the present sur- face, tli*y conclude aft- ing the v-rlour, possibilities that the mac^a probably absorbed the Ca from calcareous rocke encountered .-vt ^;r-- t r depths. The explanation of the origin of the depooit advanced by Bastin and Hill IE in no way opposed to the conception of a pn-ivur.atclytic origin for the oree, and may be enlarged with- out difficulty to strengthen the idea. According to a theory advanced by R. A. Daly 1 , alkaline rocks are the acid pol? of the differentiation or & eyntectlc formed by the reaction between s, basic magma and a calcareous rock. A mechanism, chemical and mechanical by *hich the introduced Ca CCXj may react with the inagiaa to fora and concentrate a differentiate Uigh in the alkalies la daecribed by Daly 2 , and will not be di-scuased here except thoee phases of it which may be of direct application* The increased concentration cf Ca is believed to r---iult in the formation of ths comparatively denee li?T.e-irori- ii;-'iuE silicates, which tend to concentrate toward the lower portions of the magoa, leaving the remaining material rela- tively richer in the alkalies. An ejcoecs of Ce. 0, which might conceivably separate as the simple silicate, wollaatonite, McGraw-Hill Co. . 1. R. A. Daly, Igneous Rocks and Their Origin, II. Y., 1914, p. 430. '. Ibid., p. 430. (Ca 31 0-^) would have little tendency to settle away from the acid pole, as its speoifio gravity of 2.g (at normal temperature) la but little greater than that of a monr.onitic rook. The Importance of ths resurgent CO, is emphasised by Daly as a condition aiding the differentiation of the magma, and evidence la given illustrating the manner in which the alkalian elements are transported by this gas. Applied to the conditions at the Evergreen Mine, it is evident that the theory offers a common explanation for the alkalian character of the granite dike, the presence of wollastonite, and the pneumatolytic nature of the deposit. The concentration of sulphides of copper and subordinate quantities of other metals may be attributed to resurgent gases, chiefly carbon dioxide and water, provided It Is assumed that caloareoua rocks have been encountered at depths, Origin of the Chalcooite. Although th amount of oxidation is slight, it is entirely sufficient to have furnished the very small quantity of copper necessary to have formed the thin rims or veinlets of chaloooite in the bornite. The similarity between the character of the replacement of the bornite, and the association between the chaloocite and the spines of chalcopyrite la very similar indeed to the conditions found in other ore-bodies in connection with chalcocite of known secondary origin, end there is no reason for believing this case to be exceptional. A. F. Rogers , in an article on seoon- ~TI Secondary Sulphide Enrichment of Copper Ores with Special Reference to Microscopic Study, liin. and Sol. Press, Oct. 31 191^-, p. 6S6. 86, dary sulphide enrichment, suggests an origin by ascending solutions for the chalcocite at the Evergreen Mine, and publishes a photograph to support this view. It is true that there is a small amount of ohalcooite in blebs in the bornite, which is difficult to interpret as an ordinary replacement of the bornita, and which is possibly of an earlier age than \ the chaloooite forming the veinlets and rima. In one or two cases associated blebs of chalcocite and tetrahedrite in the bornite show a faint tendency toward the graphic structure. If limited to these rather indefinite forms, which surely constitute less than 5 % of the ohaloocite present, Rogers' suggestion carries weight, and can not be disregarded. There is nothing in the evidence to eliminate the possibility that this small portion of the chalcocite is primary, but it is fairly certain that the larger part of the chaloocite and the associated ohalcopyrite which replaces the bornite la due to the action of descending surface waters. Bastin, in a later article* sees no reason for attributing the ohalcooite to ascending solutions, and prefers the commoner explanation for it. Conclusions. The conclusion that the ore-deposit is the result of the differentiation of apopbyses of a monzonltio stock, 1. The ore-deposits of Gilpin Co.. Colorado, EC on. Geol.. Vol. I0l (T9l377~ P- 876. 87 brought about chiefly by pneumc.tolytio agencies, follows with fair certainty from the evidence. The presence of wollaston- ite, according to Baetin and Hill, suggests the reaction be- tween limestone and the magma at depth. This conception, en- larged by Daly's theory or the origin of alkallan rooks, offers an explanation for all the observed facts, but it rests upon the assumption of the existance of calcareous rooks in depth, which is impossible to prove or disprove* The conoluaion that the ores and associated dike rooks are of pneumatolytic oiigin rests on a direct inter- pretation of definite evidence, and ie equally applicably whether the gases, are of juvenile or resurgent origin, or both* The small amount of chaloocite, which is developed entirely in the bornite, is believed to be the product of secondary processes. The origin of an almost negligible amount in small blebs in the bornite cannot be determined. 88. Copper Mountain, Yale District, British Columbia. ItfTRODUCTIOfl Situation. - Copper Mountain in southern British Columbia is a north - south ridge rising about 1500 feet above the Similkameen River about 13 miles south of Princeton, the chief town of the Similkameen Mining Division of the Yale District. The mineralized portion of the mountain is largely the proper- ty of the British Columbia Copper Company, which has thorough- ly prospected the deposits by trenches and diamond drills, and is rapidly pushing development work. An attractive camp has been established on the upper slopes of the mountain in the de- lightful open forest of yellow pine and lodge-pole pine charac- teristic of this climatic belt. (Pig. 5 ) . Summary of_ Previous ffork. - The deposits on Copper Mt. were first brought to the attention of geologists by a note published by J. Jf. Kemp in 1901, in which the bornite was described as a primary constituent of pegmatite dikes, and was stated to be of direct raagmatic origin. A microscopic study of the material collected by Prof. Kemp was made somewhat later 2 by Jules Catherinet , who describes Copper Mountain as a mass of gabbro, with other eruptives, cut by ore-bearing seams and peg- 1. Trans. A. I. M.S., Vol. 31, p. 182, (1901). 2. Eng. and Min. Jour., Vol. 79, p. 125-127, (1905) . - ' 88A. PLATE III 5 View looking wast from Copper Mountain, Similkameen District, British Columbia. Fig 6. View looking weet from La Fleur Mountain, Danville Dietriot, Washington. * 89, matite dilres in the mineralized area. He wrote that the ore occurs in two ways, contrasted but genetically akin. In one, the sulphides form coatings along small crevices, accompanied by little pegmatite seams of the same magnitude as ths bornite veinlets. In the other, the bornito occurs in large, typical pegmatite veins exposed on the Copper Cliff Claim. Bornite in masses up to 1/2 cubic ft. were described scattered through the pegmatite. The bbrnita is accompanied 'by smaller amounts of chalcopyrite. Orthoclase and biotlte are the chief minerals, with smaller amounts of oligoclase, tourmaline, fluorite and calcite and a very little primary quarta. It is stated that the bornite is partially altered to chalcopyrite. 3ketches and descriptions make it quite evident that some of the chal- copyrite is of secondary nature, but it seems to me to be very doubtful if this is true for tho larger part. The chief ar- gument favoring the magraatic origin of the bornite is its oc- currence with unaltered orthoclase and biotite in pegmatite. Calcite is said to be the only mineral present which seems to be an alteration product. It was not observed by Catherinet to have any genetic relations with the bornite. Gold and platinum (as sperrylite) occurring in flakes in bornite and in orthoclase ara described. Sperrylite in biotite is also mentioned. Both are believed to be magmatic. The ores have been described also by D. N. Scott, who 1. Jour. Canadian Mining Inst., Vol. 5, pp. 493-508, 1902. 90. emphasises some structural features. The geology of the region la treat od in a very general way by Chas. Carasell in a report of the Canadian Geological Survey. The geology of the mineralized region on the surface of the mountain has been worked up very thoroughly by S. H. Ball for tho British Columbia Copper Company, and certain rock determinations have been made by C. P. Berkoy. Their reports however have not been made available for-.public uso. The geologic sequence according to Hr. Ball's report is as follows: Pleistocene (1} Glacial drift; Eocene (2) porphyry dikes (moiizonites, quartz, porphyry, andesites and others) ; i'ertiary lavas (nearest a couple of miles away); Jurassic (?) (3) Pegmatite and granitoid equivalents; (4) Granodiorite; (5) Augite monzonite porphyry, and a gneissic augite monzonite porphyry. Pre-Jurassic (6) Altered rocks, in part probably altered sediments. he sulphides occur in the pegmatites and in various rocks of earlier age. The Tertiary dikes, which tiro very abundant, are entirely barren, and clearly post-ore. They are far more extensive than the mineralized area, and show no traces of the 1. Report 986, C.G.3., (The Similkameen District'* 1906. 91. alteration associated with the sulphides. Hy information concerning the ores was gained chiefly from an examination along numerous trenches across the moun- tain and from material on the dumps of shallow shafts. There were no underground workings of importance at the time of my visit, and most of the prospect pits were flooded and inacces- sible. I v;as given the privilege of examining the drill records and the sections drawn from them, and also gained much valuable information from :.Er. Patrick Crane, in charge of the property at the time, and from Mr. James Tremlett, an engineer of the company. ROCKS ASSOCIAIV.J 7ITH TH3 ORES. \ The sulphides occur most abundantly in a dark medium- grained gabbro. The prevailing rock of the region is grano- diorite, and it ie probable that the rock associated with the ore is merely a more basic phase of the larger mass. A speci- men of the rock remote from ore was found to contain basic anAesine and orthoclase with the latter slightly less abundant than tha former, and a subordinate amount of quartz. The pla- giocle.se in some grains ranges from a core near acid labrador- ite in composition to a rim near oligoclase. The chief dark constituents are pale green augite and a somewhat smaller amount of dark brovra biotite. Apatite and magnetite are pres- ent as accessory minerals. The rock is apparently more basic than the usual granodiorite. Near the ore, the orthoclase be- 92. cornea subordinate or is entirely lacking, and there is usual- ly no quartz. The brown biotite of the more remote rock is lacking, but its place is taken by a green variety. The py- roxene remains abundant as before. The more basic types of this rock would be properly termed gabbro, but in places it seems closer to an augite diorite in composition. The plutoiiic rocks are cut by seams of pegmatitic mater- ial, usually less than half and inch in thickness but occa- sionally in small dikes, sis: inches or aore across. In them, pink orthoclase is the chief constituent, usually- with a little alb it e associated with it. Quartz becomes of importance in places, and apatite is usually common. JOeep green biotite is rather abundant. . Calcite is usually present, bat it is ap- parently an alteration product. Small irregular segregations of similar material, often very rich in the biotite, occur in the heart of the rock and are probably related to the peg- matites in origin. The older porphyry of the mountain, termed augite-mon- zonite porphyry, is somewhat similar in general composition to the associated plutonic rocks, and may be merely an earlier product of the same intrusion. Although the rock is greatly altered by the mineralizing processes /\ all my specimens, it can be seen that the phenoorysts are plagioclase (oligoclase- andesine (?) ) and augite in a microgranitic ground-mass con- taining abundant orthoclase and a. little quartz. The latter may have been due in part to the mineralization. The green- 93. biotite common in the diorite or gabbro occurs in these rocks also, The presence of the pale augite and the green biotite suggests a relationship to the plutonlc rocks. The dikes later than the ores were not studied. They are fresh, unmineralized fine-grained porphyries for the most part, among which a quartz porphyry is a prominent member. OSES AUD ROCK-ALTERATIOlf. The ores of Copper Mountain are chiefly bornite and chal- copyrite in approximately equal amounts, The chalcopyrite and bornite are contemporaneous in part, but there is occasionally a little evidence of corrosion by the bornite which suggests that its period of formation lagged a little after the deposi- tion of the phalcopyrite had ceased. A small amount of pyrr- hotite was noted in ores from the Silver Dollar claim on the northern end of the mineralized zone. Pyrite is noticably ab- sent. 'Jagnetite occurs but is not abundant. In ores from Voigts 1 Camp a mile or so from the mountain, specularite is a prominent mineral, but it does not occur in notable amounts with the bornite ores of the larger deposit. A few small blebs of galena were detected under the microscope, but the mineral is of no quantitative importance. The sulphides are disseminated through the gabbro or diorite, or occur as thin seams along slight fractures. The latter are prominent in broken ore, for the rock commonly splits along- such cracks, leaving the bornite as a thin film on the face of the block. 94, In the older altered porphyries and sediments, the ore- minerals appeared to me to be less abundantly distributed and more cloaaly confined to partings or sea^a. She sul- phides also occur in the small seams or dikes of pegroatitic material, and in the mica rich segregations. Although fair- ly coarse grains of bornite or chalcopyrite are sometimes seen in these associations, tho sulphides in the pegrnatitic materi- al are of little importance from a quantitative standpoint and constitxite only a small fraction of the ore. The copper values lie ohiei'ly in the mineralized gabbro and older rocks. There is little rock alteration associated with the sul- phides in the pegmatite. The ore-minerals are later than the feldspar and biotite, uhich they corrode or cut in veins, but their introduction is not accompanied by sericite, chlorite or epidote, w minerals which are commonly associated with sulphides of hydrothermal origin. Calcite is often abundant, but appears to be later than the ores. In the plutonic rocks and the older porphyries, however, there is fairly pronounced alteration where the sulphides are abundant. Th> ins of oornite or chalcopyrite are commonly bordered by rims of epidote, which is often developed in small amounts elsewhere through the rock. The plagioclase, especial- ly the basic cores of the grains, are usually somewhat serici- tised; the orthoclase on the other hand is little altered. Chlorite ia not abundant, although it occurs to some extent in the basic constituents. Calcite is wide-spread, cutting the '95, rock in many small seams, or scattered as replacements of the rook-minerals. It is probably a product of the closing phases of the mineralization. Bornite in small amounts was observed in one oalcite seam, which indicates that it contin- ued to form in a feeble way even under these late milder con- ditions. OZIDATIOH AND MRICHMEHT The surface alteration is very slight. There is a little carbonate staining, but it usually does not penetrate many feet below the surface. The mountain has been severely glacia- ted, and all traces of an earlier gossan, if such existed, have been removed. The ground-water level is between ten and twenty feet below the surface in most places, and the secondary sul- phides are only feebly developed. Little of the bornite exposed to view is entirely free from traces of the secondary sulphides, but in no place ia the enrichment of especial value. The development of veinlets of chaloocite and covellite through the bornite is the chief manifestation of surface changes. These minerals, however, are also accompanied by chalcopyrite, which has developed as fine lattices of intersecting plates, closely similar to the rela- tions described in the ores from La Pleur Mountain. The plates of secondary chalcopyrite range from fairly coarse plates, scattered here and there along veinlets which are easily detec- . line erifr 96. ted under low magnifications, to the finest grills of thin intersecting lines on the polished surface which can hardly be detected under the oil-immersion lens. As in the other case described, it seems very probable that submicroscoplo ohalcopyrite exists in this form which may account for the yellowish tints of some of the bornite grains. The secon- dary origin of the visible chalcopyrite plates is convincing- ly shown by the dependence in distribution upon veinlets of ohalcocite and oovellite. Chalcooite in the graphic structure with bornite. Be- sides the chalcocite of easily recognized replacement origin derived by secondary processes, just described, the mineral also occurs in small amounts associated with the bornite in groups of irregular, curved and divided blebs and lobes, which possess a rather definite pattern not unlike that of certain metallic eutectios or of mioropegmatitic intergrowths of quartz and feldspar. The relation has been termed the graphic structure. In the bornite from Copper Mountain, the chaleocite in these structures is differentiated from the secondary sulphides previously mentioned by the disregard it shows for the obvious channel-ways which would be sought by replacing solutions, such as grain boundaries, gangue con- tacts, or small cracks. For the most part, it shows little' regularity which would suggest the influence of orystallo- 1. L. C. Graton & D. H. McLaughlin, Econ. Geol.,vol. 12, p. 23, 1917. OltJOOB 97. graphic forces in its formation, but in a few examples, there is a rough allignment which may possibly indicate such ten- dencies. These graphic structures in the Copper Mountain ores are usually somewhat modified by chalcocite replacement, but the latter appears to be related to the chalcooite of the vein- lets, and superimposed on an older structure. The chalcocite in both structures is bluish and no line can be drawn between them where they intersect. However, the existance of graphic ohalcocite in some blebs unassociated with the material clear- ly of replacement origin, suggests that it is not dependent on the feeble veinlets of chalcocite which represent enrichment from the present surface. The origin of the chalcocite in these graphic structures is one of the most difficult problems encountered in this in- vestigation. Further discussion will be postponed until later pages, where wider evidence will be available and the many ar- guments bearing upon its solution can be presented in full. Discussion and Summary. The sulphides in the pegmatite dikes may be considered primary constituents, but as they are later than the silicates with which they are associated, it is probable that they were formed under pneumatolytio rather than strictly magmatic conditions. The occurrence of tourmaline and fluorite in these ores as mentioned by Catherinet, streng- thens this interpretation of origin. The more widely developed ores in the gabbro and older rocks may be attributed with a fair degree of certainty to the same general source, but the . ,j ' got 98, close association of epi dote and sericite with the sulphides in most parts of the deposit indicates that the sulphides continued to form and were deposited in greatest abundance under later hydrothermal conditions, A small amount of ohalcocite occurs in graphic structures, and a little in veinlets of undoubted secondary origin. With the latter is associated a small amount of chalcopyrite in lattice forms, some exceedingly fine-grained, and a littlo covellite. eeJ 99 1 -Bngels. California* INTRODUCTION AND CONCLUSIONS The Engels copper mine, located in northern Plumas Coun- ty, California, 27 miles northeast of Xeddie on the Western Pacific Railroad, lies at an elevation of about 5,200 feet on a gentle, forested slope that descends toward the southwest from the crest of one of the eastern ridges of the Sierra Ne- vada. The property has been worked more or less steadily for many years, but has suffered because of ramoteness from rail transportation and of difficulty in either smelting or concen- trating the bornite-chalcopyrite ore which it chiefly yields. The installation of an oil-flotation plant and a vigorous cam- paign of underground development have recently brought produc- tion to a higher level than previously attained. Preceding these recent developments, a geological examin- ation of the deposit was made by Mr. H. W. Turner, and speci- mens which he collected were studied microscopically by Prof. A. P. Rogers, of Stanford University. The results of their 2 investigations, published as a joint paper, merit particular 1. Taken with little change from an article entitled Ore Deposition and Enrichment at Bngels, California, by ! C. Qraton and D. H. McLaughlin, published in Kcon. Geol., Vol. XII, pp. 1-38. Jan. 1917. 2. Scon. Geol., Vol. 9, pp. 359-391 ,( June, 1914). 100 attention because of the two principal conclusions which the authors reach, viz.: (1) that the ores are of direct magmatio origin, and (2) that the development of ohalcooite and some oovellite by replacement of bornite is the work of ascending, heated alkaline waters and is thus to be regarded as an exam- ple of "upward secondary enrichment." The accelerating tendency in recent years to ascribe the origin of many ore deposits to igneous influences must be ad- mitted to rest more on reasonable inference and on elimination of objections than upon direct and positive evidence. Any in- stance in which sulphide ores have been formed in syngenetio relation to the enclosing igneous rook therefore assumes spec- ial interest and significance. The importance thus given to the Engels occurrence was only increased by a paper by T T. 1 Head, who, after a brief visit to the deposit, concluded that the magmatic origin of the ores is not certain* Still more interest, if possible, attaches to the Sngels deposit in connection with the origin of the chalocite. The idea of "upward secondary enrichment," though it had been ear- lier hinted at or somewhat vaguely invoked to explain certain observations, was proposed as a definite hypothesis for copper 2 sulphide ores in an earlier paper by Professor Rogers. That paper was virtually an announcement of the hypothesis and an 1. Mining and Scientific Press, July 31, 1915, pp. 167- 171. 2. Eoon. Geol., Vol. 8, pp. 781-794, Dec., 1913. . .? '; 100A. PLATE IV Fig. 7. The Engela Mine California, showing the various tunnel antranoee. Fig. t. The outcrop of the Engela Vine California, above upper tunnel. lo; application of it to the deposits of Butte, in advance of the complete description of an unnamed occurrence which, in Professor Rogers 'a opinion, afforded the most convincing proof of the hypothesis. This hypothesis was immediately ac- cepted and adopted by the corps of able and assiduous Inves- tigators at Stanford and has already been given by them a con- spicuous place in the literature. Among interested students elsewhere, the expected article by Professor Rogers that should deal with specific evidence of upward secondary enrichment in a carefully studied example was awaited with interest; it proved to be the paper on the Engels mine. In a later article, Professor Eogers had somewhat revised his first ideas, especially in recognizing that part of the Sngels chalcocite is due to the normal process of descending enrichment. In consequence, some of our evidence upon this important point is merely confirmatory, as is indeed the case with regard to wmy of the other matters discussed. There re- mains, however, sufficient divergence of views as in our opini- on to warrant presentation of all our evidence. With regard to the two principal questions involved in the JSngels occurrence, we conclude as follows: 1, She ores, instead of being magmatic in the sense that they were initial constituents of the dioritic rock in which they occur, were introduced after the rook had solidi- fied and had suffered notable dynamic and chemical changes, 102 and constitute replacements formed under pneumatolytio and hydrothermal conditions. The deposition of the sulphides was an intermediate episode in a complex and unusually com- plete sequence of magraatio phenomena and after-effects that began with crystallization of the parent rock and ended with the formation of zeolites and carbonates. 2. Although the possibility of formation of a small amount of chaloocite from ascending solutions cannot be ab- solutely excluded, no satisfactory evidence of chalcocite of replacement origin formed in this way, i.e., by "upward secon- dary enrichment," has come under our observation. Most of th chalcocite and all of the covellite at Engels unquestionably result from replacement of earlier sulphides through the agen- cy of descending metorio waters, and a competent explanation for all of the ohalcocite is to be found in normal downward enrichment. OEHHRAL DESCRIPTIVE TREATMENT - COUNTY. ROCK, ORES, AND ALTERATION, Magmatio Period. Solidification of Horite. - The Engels ore-deposit occurs in a body of rather dark, medium-grained plutonic rock, which is probably a basic differentiate of the great Sierra Nevada 103 batholith of granodiorite. The rock was nowhere seen in its original condition, but it may confidently be inferred from its altered phases that it was noritio in character, being composed of plagioolase and slightly subordinate am- ounts of orthorhombic and monoclinic pyroxenes, and biotite. The feldspar varies somewhat in character, but usually it is near an acid labradorite in composition (Ab^ An^). Bronzite, hypersthene, and diopside are present in quantity that approx- imately equals the feldspar, while the biotite is usually subordinate in amount, although its small laths are occasion- ally plentiful. Zircon, apatite and euhedral magnetite are common accessories. The form of the body of no rite is not definitely known,, as surface croppings are scarce, and its contacts with the surrounding rook of the batholith can not be traced. Under- ground, en several levels, in passing out of the ore, sili- ceous rocks are encountered, which possibly mark the limits of the norite body in those directions. In a cross-cut off Tunnel 4, one of these light-colored, acidic rocks is en- countered, and proves to be a granodiorite porphyry. The rock is characterized by large feldspar phenocrysts, set in a medium to fine grained ground-mass, consisting chiefly of unstriated feldspar and biotite. Under the microscope, the "phenocrysts" are seen to be composite, consisting of aggre- gates of smaller laths of andesine, of about tie same grain as the constituents of the ground-mass (Pigs. 53 and5 1 +) . . ii- r cJOiiiGxfo ei " The ground -mass is ohiefly orthoelase, quartz and blot it e with an accessory amount of plagioclase* This somewhat acid rock may reasonably be interpreted as another variant of the granodiorlte batholith bat of subordinate importance. Pneumatolytic Period, Alteration of Mo-rite* - After the crystallization of the norite, a period ensued in which conditions of equilib- rium were apparently disturbed, probably by increasing con- centration of water and other mineralizers; this may be termed the pneumatolytlc period* The magmatio products were partially altered into minerals more stable under the new conditions; chief among these is amphibole, formed in part at the expense of the pyroxenes, while the labradorite under- went partial recryatallization with attendant cracking and straining, and the production of rims of more acidic varie- ties, oligoclase and alb it e. The resulting rock may be re- garded as a peculiar type of diorite, or perhaps more proper- ly as a meta-norite. This derivative of the original norite is more extensive than the ore-bodies and constitutes the principal country rock of the Sngels mine. The amphibole a chiefly a light green hornblende, but locally, along seams and in segregations, laths of actinolite were formed* The hornblende is widely developed throughout the rock, but it is more abundant and in largest crystals in the ore-bearing portions. It forms large, strikingly poikilitic crystals. si . . , . 105 frequently containing corroded cores of pyroxene, but ex- tending far beyond the original limits of the pyroxene crys- tals, and including numerous rounded grains of labradorite and small particles of the accessory minerals (apatite and magnetite) Slightly corroded laths of biotite are fre- quently included in the large crystals* In many cases, the inclusions are so abundant that their total volume la con- siderably greater than that of the amphibole host* Formation f Segregations* - The formation of small lens-shaped segregations in the norite (or diorite) is clear- ly related to this period of pneumatolytic alteration. They probably represent the products of the late crystallisation of small localized concentrations or residues after the crys- tallization of the normal magmatic minerals* In these segre- gations, actinolite and albite are the chief minerals, al- though in a few cases, biotite assumes an equal importance. In one interesting occurrence, a lenticular mass, over eight inches in diameter and approximately two inches thick, occurs In the fine-grained diorltic rook. The outer layer of this segregation consists of stubby, green laths of aotinolite, and it Is succeeded Inward by a well-marked zone of albite, in coarse anhedra. This in turn surrounds an inner core which was originally an aggregate of actinolite, albite and large crystals of titanite. In this and in most of the other segregations studied, the cores and frequently the outer layers are largely replaced by chlorite, epidote and oaloite, with associated copper ores, but these changes are clearly 106 subsequent to the formation of the segregations themselves. The amphibole is always earlier than the alblte, the latter being molded about the well- formed prisms of the former in many oases, but it is later than the biotite, except that locally, where the biotite is especially abundant in a segre- gation, the order may be reversed. Formation of Pegmatites. - In a few places underground, the workings expose narrow pegmatite dikes cutting the diorite. They are composed chiefly of acid feldspar and subordinate quartz, with a small amount of actinolite, usually concentra- ted near the edges or developed in the adjoining wall rook. Chalcopyrite, but rarely bornite, is associated with the peg- matites, as intermittent central bands, or as disseminated grains in the neighboring diorite* In one specimen from a one-inch dike exposed in a drift to the northeast off Tunnel 4, microscopic examination shows the feldspar to be albite- oligoclase, with small cores of a more basic type. With it are pale green laths of actinolite, with subordinate amounts of quartz, titanite, and apatite. A persistent band of chal- oopyrite along the center of the dike is the only sulphide in this case. In another specimen from a larger dike, the peg- matite was found to be composed of quartz, microcline, and small amounts of labradorite, titanite, and a deeply pleoch- roic hornblende, with a few crystals of tourmaline. Patches of ohalcopyrite and bornite occur in it, and appear to be original constituents. v "?-.- Dynamic Changes - A pronounced straining and shearing of the feldspars, accompanied by recrystallization in various degrees, is notable throughout the rocks studied. In many cases, no traces of these changes are found in the large amphibole crystals, although the surrounding and included feldspars show wavy extinction (see Fig. 52) and partial granulation* In other oases, the amphibole itself may lose its fresh, coarse-grained character, and become reduced to shreddy masses of variously oriented fibers, often distinct- ly uralitic in appearance. It may therefore be inferred that the straining and accompanying reerystallization took place within the pneumatolytic period, although locally it may have continued longer, or even have started somewhat later. The reerystallization is not confined to the diorite, but is also well shown in the neighboring siliceous rocks. An excellent example is the breaking dov/n of the phenocrysts of the granodiorite porphyry into an aggregate of andesine crystals, as is clearly shown in the two photographs (Figs. 53 and 54-) In several of the specimens, a distinct schistose structure has been produced by the grouping of the recrys- tallisod biotite laths in bands. 2he feldspar assumes a crystalloblastic texture in these cases, but the grains of the pyroxenes show little change. Where the recrystalliza- tion is most intense, apatite is abundant, forming elonga- ted clusters of rounded grains of distinctly different habit from the ordinary niasrraatic apatite. In one or two slides, a subparallel arrangement of plagioclase laths may be attribu- 10* tod to the flowage of the partially crystallized magma, but superimposed on this structure are changes similar to those previously described, which can be satisfactorily explained only as the result of stresses acting on the rigid rock. The fractured apatite crystal in ]?ig,52 is striking evidence of dynamic action. Development of Iron Oxides - Associated with the more in- tense phases of the development of the amphibole, but for the most part a. littlo later, is magnetite, in large masses with abundant incluaiona of apatite. This iron ore is clearly later than the small euhedral grains of magnetite, accessory in the norite, which were included by the growing crystals of the pneuraatolytic hornblende. The later magnetite occurs commonly molded about the aaiphibole forms, occasionally slightly corrod- ing them, and in a few oases it is so abundant that the crys- tals of hornblende appear as if set in a cement of magnet it* and apatite, with a relatively subordinate amount of rounded feldspar grains and a little spinel (see Pig.59). The pneu- matolytic origin of this magnetite is indicated by its close relationship with the development of the amphibole, and thia view ia confirmed by the abundant apatite and by the occur- rence in several instances of small crystals of tourmaline in close association with the magnetite. With the magnetite, and perhaps in part a little later in origin, ocour ilmonite and hematite. Intergrowths of all throe minerals have boon observed, ?;hile those of ilraenite and hematite are very common, occurring in nearly all of the 109 polished chips studied. In theae intergrowtha the hematite id usually auoordinate, and appears moat commonly on the polished surface as fine white linea, in parallel orientation in one, or more rarely, two directions. Other sections show the hematite us elongated, narrow ovals or aa dots scattered with a rough regularity through the ilmenite* 2he magnetite occasionally contains platen of hematite, and now and then occurs in rather patchy inclusions in ilmenite -hematite areas, the fine white lines of hematite passing uninterruptedly through the magnetite as well as the ilmenite* (These minerals were determined by mineralographic methods, and the presence of ilnenite was confirmed by obtaining titanium tests from gravity concentrates from the crushed specimens* Development of Pneumat oly t ic Sulphides* - Che most impor- tant phase of mineralization, as far as the value of the ore io ccnoorned, is that which is characterized by the develop- ment of chalcopyrite and bornite. In most oases, these sul- phide ores are accompanied by notable hydrothermal alteration of their host and are therefore described under a subsequent heading, but in a few specimens, they occur in reruarkably fresh feldspathic rock. U?hey are clearly later than the rook minerals, which they surround and slightly corrode, and are in places so abundant that the rounded grains of plagioclas* and laths of biotite appear as if set in an opaque cement (Fig5l), In relative smoothness of outline, the sulphide 1. J. Murdoch, Loc. olt. 110 grains rosemble those of magnetite. These sulphides seam to be analogous to the ores in the pegmatite, previouoly men- tioned, and are regarded as the earliest in origin whioh w have observed. The structure might readily suggest a mag- matio origin for the chalcopyrite and bornite, but, on more careful examination, occasional laths of chlorite and seri- cite may be seen included in the sulphide grains, and, where the sulphides are in contact with the plagioolase, thin bor- ders of alb it e are found. In the material whioh best exhib- its these relations, the bornite occurs as definite seams, near whioh the impregnation of the rook with ore is greatest. Along these seams the rock readily breaks apart, yielding fracture faces completely coated with bornite. This pneumatolytic stage is the nearest approach to mag- matic conditions that is shown by the development of the sul- phides, for the earlier minerals such as the labradorite and biotite remain fairly resistant to the introduced material, but the presence of the hydrothermal minerals, and the frac- turing of the rock, indicate that strictly magmatic conditions had passed. BJTEHSE HYDROTHEHMAJi PERIOD, Development f Chlorite, Serioite and ffpidote. - The commonest type of sulphide ore, however, is indicative of slightly milder conditions than those of the pheumatolytic period just described. Chlorite, sericite, epidote and cal- Ill cits become more und ;uore important. The chlorite is especial- ly characteristic, occurring as blunt latha or slivers included in the bornite or chalcopyrito, or occasionally as complicated tangles of small laths, with the sulphides both as acutely angn- lar areas between them and as thin wedges extending along the cleavage of the chlorite. Lowers first regarded the chlorite as earlier than the bornite but in his later paper he states that the chlorite is younger than the bornito and replaces it. Our evidence ia in essential accord with the first view and we are unable to follow Hogers's argument in favor of the second. In some cases, the bornite distinctly corrodes the chlorite laths (Pig. 66). Moreover, it is generally true that the di- rection of the chlorite laths is not in accord with the struc- txire of the bornite as rovealed by natural alteration or by etching. On the other hand, many of the laths appear unattacked, and probably nre virtually contemporaneous with the bornite. The chlorite was frequently found between the magnetite indiri- duuls, somo of vrhich are well crystallised (Jigs. 57 and5#); it was never obcerved as inclusions in the magnetite. The biotite is partially or completely replaced by chlorite, especially whore the latter is well developed v/ith the ores. She amphibole also suffered a partial alteration to chlorite, and in many cases amphibole crystals are fringed with ragged borders of chlorite and bornito, or are penetrated by voinlcte of these two minerals. 2he largo hornblende crystals in i'ig^y- II il- lustrate this type of alteration. 112 Tho sorioite ia largely confined to the basic plagioolase, but occasionally it is more widely developed, and bears a re- lation to the bornlte very similar to that shown by the chlor- ite. Fine laths and slivera of sericite are included in the bornite or chalcopyrite, usually near the edges, less commonly in the heart of the grains* In both of Rogers 's papers, the sericite laths penetrating the bornite are regarded as having replaced the sulphide, but wo have failed to find evidence sup- porting this view. It seems to be much more in accord with the U3ial criteria of sequence to regard then, like the chlor- ite, as inclusions in the bornite. Apparently the bornite and the sericite developed at about the same time; the bornite com- monly started first, accounting for the usual absence of seri- cite in the cores, but toward the edges of the grains, the se- ricite formed as laths a little ahead of the last outermost parts of the bornite, in which they are therefore included. Soricite ie not abundant in specimens which on other grounds aro regarded as examples of the earliest sulphides, and this is in accord with the view just expressed. 3Jho basic feld- spars are most thoroughly altered to sericite, vrtiile the more acid rims or rocrystallized portions usually remain untouched* The biotite is slightly attacked by the soricite, but to a greater extent it is altered to chlorite, ae is conclusively shown by the presence of residual sagenite webs in certain large areas of chlorite. In a few instances, the lath-shaped inclusions of chlorite and soricite in the ores may likewise \ 113 have bean derived from corroded biotite or remnants of amphi- bole, but this explanation will not hold for the majority of ciiBes since the inclusions are in general distinctly different in habit from the original brown biotite or the amphibole of the rock. Although It iu impossible to roach a positive con- clusion concerning the relative ages of the chlorite and seri- cite, there is a suggestion that part of the chlorite is a little older than the soricite, for the kind of corrosion by bornite shown by chlorite laths has not boon certainly ob- served in tho caoo of the sericite; also the chlorite is less noticeably confined to the outer margins of the bornite grains than is the sorioite. Epidote is also a common aaaooiate of the sulphides* In 3one cases voinlets of it out chlorite (i?ig.63), "^ f r the most part the two minerals aro apparently contemporaneous. She epidote aiay form fringes about grains of bornite and cnalcopy- rite; elsewhere it occurs in euhedral forms about whioh the sulphides are molded (j?ig.56). In aiany places, the amphibole i.3 greatly altered to opidote, and veinleta or irregular pat- ches of the lattor are coirmon as roplaceraents in the feldspars. A few veinleta have boon observed cutting magnetite. Kagged areas of fine-grainod epidote, thickly filled with s/uall grains of bornite or chalcopyrito, and with more or less chlo- rite are found here and there in greatly altered speciiae:. . gevelop.'aent of Hydrother7::al 3ulphidoa - The coniraerciul oro at Kngels is distributed in oomowhat indefinitely -bounded shoots, of which several have bean opened and v/hich collective- ly constitute a deposit of roughly elliptical plan within the 11M- modified no rite* As a rule the ore merges into the country rook and all gradations exist from scattered minute sulphide grains to heavy masses of ore with subordinate gangue, The bornite and ohaloopyrite, which are the most abun- dant ore minerals of the Bngels deposit, were formed mainly under hydrothermal conditions, together with a small amount of enargite, tetrahedrite and sphalerite. Magnetite, il- menite and hematite, which apparently had continued to devel- op during the early portion of the period of bornite forma- tion, ceased to form as conditions became milder, and became surrounded or rarely veined by the bornite and ohaloopyrite which continued to increase* The bornite is about three or four times as abundant as the ohaloopyrite* For the most part, the contacts between the chalcopy- rite and bornite offer little evidence of the sequence of the two minerals. Broad bays of ohaloopyrite extend into the bornite or smooth projections of the bornite penetrate the chaloopyrite, but in many oases if the forms of the two miner- als wero interchanged, their relations to each other would be little altered* The term mutual boundary has been found use- ful in referring to the line of contact of minerals in this association (Fig.67). There is evidence, however, that the ohalcopyrite slightly preceded the bornite, as inclusions of the former are commoner in the latter than the reverse, even where the two sulphides occur in equal amounts, and in a few oases the chalcopyrite shows slight corrosion by the bornite* Nevertheless, for the greater part of their periods of forma- jfcns 115 tion, they were probably contemporaneous. Aa the proportion of the chlorite and epidote grows, in- dicating increased departure from magmatio conditions, the ores become more and more ragged and irregular, and in the thin section are easily distinguished by their form along from the more regular magnetite grains. They seem also to hare exerted a greater corrosive action on the rock minerals, the amphibole especially suffering, although the other constituents are not spared. The enargite occurs sparingly with the other sulphides, and is apparently of about the same age. Tetrahedrite is per- haps a little commoner and its presence may account for the slight silver value of the ore. It occurs in the same rela- tionship to the bornite and ohaloopyrite as the enargite. A few minute grains ooouring in hornblende were identified as galena. Calcite is abundant in most of the specimens. It clear- ly replaces amphibole and feldspar, and is associated with the epidote in an intimate way, veinlets of each cutting the other. The oaloite had a longer range, however, extending into the period of later hydrothermal conditions. Rogers mentions finding calcite as a late magmatic mineral. In our thin sections, the calcite is clearly a replacement of earli- (Fig. 61) er minerals, although in certain areas the replacement is so A complete that, if evidence from other portions of the slide had not revealed its true character, it might have been mis- 116 taken for an earlier constituent of the rook. In no case, however, was there the least doubt of its distinctly later character* LATE HYDROTHISRitAL PERIOD. Development _of Zeolites and Carbonates* - One of the most in- teresting phases of the rock alteration is the development with the carbonates of a series of zeolites, as a late hydrothermal product* The following members of the group were definitely determined, by means both of thin sections and of index liquids: soolesite, analoite, heulandite, natrolite, and laumontite* Thin radial aggregates of thomsonite and small patches probably of chabazite and phillipsite were identified in thin sections only, but the minerals are not present in sufficient quantities to permit of their determination with absolute certainty. A little prehnite, found in only one section, is probably a pro- duct of the same conditions as the zeolites* The minerals of the zeolite group are fairly widespread, occurring in over one third of the specimens studied* In one specimen, soolesite has completely replaced the plagioolase, whose former presence is indicated only by the zeolite-filled molds of magnetite, which had shaped themselves about the original feldspar laths* A surprising feature is the presence of unaltered hypersthene, bronzite and diopside in this greatly altered rook; having apparently escaped the alteration to amphibole in the pneumatolytio period, the pyrox- 117 enes remained resistant to the zeolitizing conditions. The sooleaite also forms a veinlet half an inch wide of finely radial, white fibers, and in it are a few grains of bornite, which show the feeble extension of the bornite forming condi- tions into the period of the zeolites. The commonest type of zeolitio alteration is the replace- ment of feldspar by heulandite, and in about half of the cases, the subsequent replacement of the heulandite by natrolite. The analcite in the rocks was observed by Rogers, who regarded it as a primary constituent. In one or two instances, it oc- curs as shapeless grains, which give no clue to their origin, but in other cases it seems unquestionably to be a replacement of feldspar, and its association with the other zeolites con- firms this view. Caloite or siderite is usually present, re- placing even the latest zeolites, although occasionally they are themselves out by veinlets of natrolite* Our evidence is not sufficient to permit the entire list of zeolites observed to be grouped in the precise order of their formation, but the analcite and heulandite are clearly earliest, with natrolite, laumontite, and thomsonite forming a later group. The relative positions of the others are in doubt. The carbonates are persistent, and in many places the completeness with which they replace the preceding minerals has greatly obscured the evidence of the earlier relationships, Siderite is less common than calcite, but it is frequently found as a border about caleite grains, being readily distin- .iofr. t IIS guished by its much higher relief, better crystal form, and slight iron staining where exposed to oxidation. It is not established with certainty, however, that the siderite is & member of this mineral group formed under late hydrothermal conditions; the possibility that it is a product of descend ing oxidizing solutions will be considered later. PERIOD OP OXIDATIOH A3D S Between the period of the formation of the zeolites and the exposure of the ore to the influence of oxidation, the ore-bearing rock was again broken by series of roughly parallel cracks, which became filled with fine veinlets of siderite and calcite. The breaks are abundant in certain zones, and almost lacking in others* They show no relation to the earlier shearing and straining of the rock* All minerals and structures previously described are broken by them, except that they show a marked avoidance of bornite grains which lie in their paths. Occasional carbonate vein- lets out through the bornite, but far more commonly they axe diverted, and pass around the border of the sulphide grains* On the other hand, the magnetite and ilmenite-hematite inter- growths are greatly cracked, as if they were more brittle, and are penetrated by the veinlets wherever these are notably developed. The earlier shearing and recrystallization of the rook, and the later fractures occupied by voinlets were of great importance in facilitating the downward movement of sur- too e;r.f 119 face solutions by which oxidation and descending secondary enrichment were accomplished. ?one of Complete Oxidation. - The outcrop of the iingels deposit, which lies at an elevation of about 5,500 feet, is (Figs. 7 and 8). wholly inconspicuous. A A mantle of granular soil conceals the rook except at a few places where slightly copper-stained croppings are exposed but do not project noticeably above their surroundings. The orebody is capped by a moderate but Irregular thickness of oxidized and leached rook. Plainly thio material once contained sulphides for it is stained by limonite. malachite and ohrysocolla, and is out by banded seams and small veins of these minerals. In these vainleta, the silicate aeems to have been the last to form, frequently breaking aotoas the carbonate bands, and building out as knobby, botryoidal forms into open spaces presumably formed by removal of the sulphides. Zone f Sulphide Enrichment. - According to Mr. Juessen, who was in charge when the upper parts of the orebodies were developed, the zone of complete oxidation changes irregularly into a zone of mixed carbonate and chalcocite ore, rich por- tions of which were mined, but caving has made these upper- most stopes now inaccessible. This was succeeded immedi- ately below by a much more definite and regular zone of chal- cooite ore, of which much the largest and richest portions were discovered subsequent to the examination made by Mr. Tur- ner. Ore of this kind was found in a locus averaging about 120 twenty-five feet thick, and dipping approximately parallel to the surface, which slopes rather gently to the southwest. A very considerable tonnage of such chaloocite ore was mined out, with values up to 16 per cent, copper, and an average grade materially higher than that of the ore at greater depth. In general, this ore gradually fingered out at depths of about 90 to 130 feet below the outcrop into ore consisting essen- tially of bornite, but occasional stringers and bunches of chal- oocite ore extend considerably deeper. In nearly all the chips studied, chalcocite was found in varying amounts, commonly as a replacement of bornite and to a very small extent a replacement of chalcopyrite. Its moat striking development is in connection with the veinlets of carbonate previously discussed, and this relationship persists to the deepest v/orklngs of the mine (June, 1916), a winze Iii5 feet below the level of Ho. 4 Tunnel or about 375 feet from the surface. In many cases chalcooite occurs only where th bornite is broken or bounded by such veinlets. The abrupt manner in which laths of sericito are cut by the voinlets with which tho chalcocite is associated, leaves no doubt of the inde- pendence of orisrin of the ohaloocite from the sericite. Vein- lets of chalcooito are frequently observed to take advantage of the presence of included laths of chlorite or sericite along their course, breaking from one to another, while other laths nearby, but not intercepted by the veinlets, may be sharply bounded by the bornite without a trace of chalcocito. 7iftiere 121 the bornite occurs in small ragged slivers in aggregates of epi-lote, chlorite or sericite, the enrichment is moat com- plete, due both to the greater permeability of the rock at such places, and to the relatively greater surface of bornite exposed by the finer grains. Chalcocite ia commonly wall developed alao where the bornite is penetrated by laths of chlorite or aericito, although occasionally no enrichment is observed in connection with these minerals. Our studies of chalcocite formation in many districts indicate that enrich- .ent favors the contacts between primary sulphides and gangue minerals (Pig. 74 ), because of the greater local permeability along these junctions, and that micaceous gangue minerals, aa aericite or chlorite, notably promote both local and general enrichment, as Beeson has ao clearly shown in the case of 1 Bingham. In addition to the replacement of bornite along irregu- lar ve inlets and grain boundaries, the ohalcocite alao re- places that mineral along crystallographio planes, '.yhere this type of replacement is well developed, the polished surface reveals a strikingly regular pattern, consisting of three in- tersecting systems of narrow strips of chalcocite, separated by triangular and rhombic residues of the bornite; the well- known relationship which is termed the lattice structure. 1. Beeson, J. J. T. j..l. I. ., Vol.54, pp. 402-441. 1915- . I 122 Associations of bornite and ohaloocite in graphic struc- tures are present sparingly throughout the deposit aa now de- veloped* except where the bornite has been completely con- verted into ohalcocite (Pigs. 69, 70, 75 and79> These graphic structures at 3ngels occur in the rafdst of bornite masses away from fractures and grain boundaries, and they also oc- cur in the marginal portions of bornite areas as well as ad- jacent to veinlets of chalcocite from which the graphic ohal- oocite is not separ ,ted by any visible demarcation. The ques- tion of the origin and significance of these graphic struc- tures, which is one of considerable complexity, will be given attention on a subsequent page. Covellite is occasionally produced in subordinate amounts by partial marginal replacement of bornite and chalcopyrite. In a few places, either in ohalcooite near bornite or in the bornite itself.it forms fine lines on the polished surface, which show the same structural control by the bornite as has beon described for the chalcocite and bornite in the lattice structure. Examples are clearly illustraded by sketches and photographs in Turner and Roger's paper. Secondary chalcopyrite is found to a relatively small ex- tent. It ia commonly asaociated with chalcocito, but appears to prefer small cracks or nooks in the bornite somewhat removed from the paths of strongest ohalcocite development. The re- placement of bornite by chalcocite involves the removal of iron as well as the addition of copper, and the small isolated veinlets or spines of ohalcopyrite probably represent local oonoentration of iron small eddies, as it were, in the main stream of enrichment where conditions of equilibrium were temporarily reversed, and the iron-rich sulphide formed. The ohaloopyrite frequently penetrates the bornite along def- inite structural lines, producing in some cases a lattioe pat- tern similar to that described for chaloocite and for covel- lite in bornite (Pig. 77) Inasmuch as various stages have been observed from a few isolated spines penetrating the born- ite from the edge of a ve inlet of ohaloocite due to downward enrichment to the well-developed lattice-work, there oan be no doubt that these ohalcopyrite spines are of the same origin as the ohalcocite. In the specimen which shows the moat striking development of secondary ohalcopyrite, the high con- centration of iron and the influence of surficial conditions are indicated by the development of a strong seam of limonite, to which the distribution of the chalcopyrite shows a defin- ite relationship. DISCUSSION. Genetic Classification of the Deposit* The grouping of the minerals (p.!23a)is merely a summary of the interpretation of the genesis of the deposit presented on the preceding pages, and so needs little further discussion. The magmatio group includes only the minerals which crystal- lized directly from the differentiated magma. *.s the separa- 123 -a DIAGRAM 0? MINERAL SEQUENCE. ^_ Period Mineral Mag- natio Pneunat olytic Intense Hydro- thermal Late Hydro- ther- gal Ox Ida" tion & enrioh merit * 1. Zircon 2. Hutlls 3. nronzite 4. Hypers thene 5. Dlopside o. Labradorite 7. Biotite ff. Apatite y. Magnetite 10. Spinel 11. Hornblende 12. Actinolite 13. Albite I 1 *-. Oli^oclase 15- Orthoclase 16. i'lcrocline 17. Iliaenlte 1#. Hematite 19. Tourmaline HO. Titanite ( as leucoxene ) HI. Quartz _^_ HH. Chlorite H3. Sericite HM-. Epidote 25. -ioisite 26. Chalcopyrite 27. Bornite 2flf. Enargite H^. Tetrahedrite 30. Galena ?1. Sphalerite ^2. Anilcite 33- Heulandite 3^-. Scolesite 35. Prehenite 36. Thonusonite 37. dhabazite ( ? ) 3*. Phillipsite ( ? ) 39. Natrolite '4-0. Lausionite *H. Calcite ^^-^ 7 ^2. Siderite U-3. Covellite U-M-. Chalcocite 4-5. Liala'-hite ^. Llnonite ^7. Chncaooolla -^ sequence given under tho j^*^ * WA tion ai;l enrichment are intended to show trv. progressive ar- rival of various influences at any given horizon. In reality, probably all tho minerals of the period have been forming, at one part or another of the deposit, from the beginning of oxida- tlon of the normal rook-forming constituents became more and more complete, the concentration of volatile constituents in the residual magmatic material steadily increased, and condi- tions of normal magmatio crystallization gradually changed to those in which pneumatolytic processes played the dominant part. The widespread alteration of pyroxene to amphibole, the formation of tourmaline, the small lenticular segregations and dikes of pegmatitio nature are attributed to this phase. Following this type of alteration, changes of distinctly hy- drothormal character were imposed upon the rock, differenti- ated from tho preceding pneumatolytic conditions by the gradu- ally increasing abundance of chlorite and epidote in particu- lar. The closing relatively mild phases of hydrothermal ac- tion are characterized by the series of zeolites and by car- bonates. The amphibole and the major amount of the iron oxides associated with it are to be regarded as products of pneuma- tolysis, both on account of their position in the general min- eral sequence, and from the abundance of apatite and occa- sional, inclusions of tourmaline associated with them. The period of magnetite formation may have continued, however, into the early intense phases of the hydrothermal period. The principal coppor ores, ohalcopyrite and bornite, commenced to form under pneumatolytic conditions, as indi- cated by their occurrence in the pegmatite, but they attained * * y > * * : :-i 125 their maximum development when accompanied by minerals of ac- cepted hydrothermal origin, and even continued in a feeble way into the period characterized by the zeolites. The percent- age of the ores connected with the pegmatites, or as occasion- al amall inclusiona in the magnetite, is small, and there seems to be good evidence that the larger part of the sulphides the part which makes the deposit a commercial orebody - is of hydrothermal origin. Turner and Rogers regard the ore as a magmatic segregation, From field evidence. Turner considers the sulphides to be the final products of the crystallization of the magma, analagous to the quartz in a granite, and to have been formed before the development of the pegmatites, ^s a matter of fact, such a view is justified by the general appearance of much of the ore and by many of the broad field relations, though it is contra- dicted by certain megascopic features, subordinate in distribu- tion but significant in character, such as the schistose struc- ture of some of the ore-bearing rock, and the presence of sul- phides both along fractures, and in seams of pegmatite and rarely of zeolites. Rogers concludes, from microscopic examin- ation of Turner's specimens, that the ores are products of gaa- eous solutions, rioh in mineraliaers, and evidently assigns to them, a later origin than did Turner. Although pointing out that deLaunay groups deposits of such origin under the heading 126 Gritea de depart imme'diat , Rogera retains the term "magmatic segregation," because of the close analogy he believes to exist between the orea at Bngols and those at Sudbury, Insizwa and other deposits that are commonly regarded as magmatic segrega- tions. Che phase in the geologic history of tho angels deposit which seems to us to correspond most closely to the conditions of origin postulated by deLaunay for the deposits classified by him as Sites de depart imm^diat , is what we have termed the pneumatolytic period. The iron oxides at Engels and a small proportion of the bornite and chalcopyrite were formed at this stage* They might possibly be classified as magmatic segrega- tions provided the term may properly be expanded to such an ex- tent* But the close association of the greatest part of the primary copper sulphides with hydrothermal minerals is distinct- ly different from the conditions as described for Sudbury or other deposits regarded as raagmatic, and unquestionably classi- fies the Engels orebody as a hydrothermal deposit. ORIQIE OP TH3 CHALCOCITE. Ghaloocite Clearly of_ Replacement Origin. - Rogers first attributed the formation of chalcocite at Sngels entirely to the agency of ascending alkaline waters and considered it a hydrothermal effect. In his later publication he recognizes that chalcooite due to descending waters is present, but main- tains that "the evidence in favor of 'upward enrichment 1 is even stronger than before." The absence of kaolin, the occur- 5" i e .JbolTW oi*ilJo*amirr tw erf* r :o riolJfiDrcoi ".3 127 rence of chalcocite in massive ore-bearing rocks, and the re- lation of the chalcocite to the sericite and chlorite are points he has urged to support the thesis of "upward secondary enrich- ment." They may be considered in order. The absence of kaolin was advanced in Rogers f s first paper as a proof that the chalcocite is not the product of de- scending solutions. Confirming his observations, we find that it is not a common product at iilngels, although small amounts were found in a few oases. The formation of kaolin depends primari- ly on the presence of sulphuric acid, which in turn is largely derived from pyrite. As no pyrite has been observed at Engels, the lack of kaolin with the oxidized products is not surprising. The solutions descending from the oxidized zone are probably at moot only mildly acid, as they do not noticeably attack the car- bonate present in the rook or even in the veinlets with which chaloocite is associated, but there is no reason to believe that such solutions are incapable of producing chalcocite en- richment. Our observations in many districts indicate that the enrichment of bornite to chalcocite by descending waters is accomplished so readily, and with the production of so little sulphuric acid, that it may take place without the ap- preciable development of kaolin. 1. The generalizations in this paragraph regarding kaolin have been drawn in large part from investigations being carried on by our associate, Mward H. Perry. 2. Zies, E. G. Allen, E. I'., and Herwin, H. !., "Some Reac- tions Involved in Secondary copper Sulphide Enrichment," Boon. Geol., Vol. 11, p. 500, 1916. 12S Chalcocite la frequently developed from bornite at Engels in hard, dense rooks beyond the limits of the oxidized materials, but notwithstanding the great ease with which born- ite ia enriched, this transformation is even here dependent upon the degree to which chlorite and sericite, or microscopic fractures have materially augmented the normal permeability of the ore due chiefly to minute channelways along gangue bounda- ries* In his first paper, Rogers assigned an earlier age to the chlorite than to the bornite and chalcopyrite, or the chalcocito, but stated that the sericite is younger than the bornite and chalcopyrite, and that it developed simultaneously and in inti- mate genetic association with the ohaloocite* In hia later ar- ticle, the chalcocite which he attributes to ascending solu- tions is considered older than both the chlorite and the seri- cite, while the chalcocite which he ascribes to descending in- fluences ia regarded as younger than these gangue minerals. Both kinds of chalcocite, he adds, were formed independent of the process of sericitization, and he now accepts Beeson's ex- planation of greater permeability to account for the close po- sitional relation observed between the sericite and chlorite and some of the chalcooite. As we interpret the evidence, the chlorite and sericite laths are to be regarded as inclusions in the bornite-ohalcopy- rite matrix, yet formed at practically the same time as their host, and perhaps developed with more perfect outline chiefly 129 because of their greater power of crystallization. We find no satisfactory foundation for Rogers 's view that the latha of aericite and chlorite have been formed by later replacement of the bornite and ohalcopyrite. We are convinced that this is not true for the chlorite. As for the sericite, however, the evidences (pp. 112 and 113) controverting his interpretation are not so abundant or so forceful, and our dissent from it arises as much from its incompatibility with the tendencies of se- quence in sulphide ores in general and from the notable simi- larity of habit and occurrence between the sericite and the chlorite, as from any compelling evidence against it in this particular case. i"ith respect to the chalcocite, except for the compara- tively small amounts in the graphic areas which will be dis- cussed later, the evidence is conclusive that it is later than the serioite or the chlorite. The greater general de- velopment of chalcocite in those parts of the ore containing abundant sericite and chlorite is readily explained as a re- sult of the greater permeability caused by the presence of these micaceous minerals, as already noted* Between the chalcocite in the lattice structure on the one hand, which Rogers regards as due to ascending altera- tion, and on the other hand the chalcocite of admittedly de- scending origin which constitutes more or less regular rims 1. See Fig. 8 of Rogers 's article in Kcon. Geol., Vol. 11, p. 582, 1916. 1 about laths of chlo'rite and serioite and grains of other & a gangue .minerals or forms veinlets with or without accom- panying siderite or limonite, we find it impossible to dravr any important distinctions as to gcmesis, and we aro forced to conclude that both kinds of chalcocito were formed in essentially the same manner and "by essentially the same means. Examination of Rogers 's Fig. 2, to which reference has been made, and of our Jig. 74- reveals the presence of numerous narrow tongues or spines of chalcocite extending from the chal- cocite rims into the bornite cores and arranged in several sys- tems of directions. This feature, which is very common at Engels, we have found in many districts to be simply an early stage of the development of the lattice structure. Moreover, this formation of lattice chalcocite in bornite, the develop- ment of narrow gashes or spines of chalcopyrite, with or with- out ohalcocite or covellite (Pig. J8 ) in bornite near where it is being altered to chalcocite, and the similar production of either felted aggregates or sharp plates of covellite along boraite-chulcocite boundaries are features very common in con- nection with the alteration of bornite to chalcocite by the normal process of downward enrichment, as is exhibited in such unquestionable examples as Bisbee, Uorenci, Globe, A jo, Bing- ham, and Ely. Che last two of these features, furthermore, 1. See Fig. 5 of same paper by Rogers. 2. See Fig. 2 of Rogers 's paper and Fig. J4- of this paper. 3. See Fig. 73 of this paper. 131 have been produced artificially in the Geophysical Laboratory by the action of solutions containing copper sulphate and sul- phuric acid on bornite. Added to these evidences favoring a descending origin for the chalcocite are several others . The chalcocite, though de- veloped in part by replacement of chaloopyrite, is yielded main- ly by bornite; in other words the bornite is much more easily enriched than the chalcopyrite, This is entirely in accord with repeated observations in examples of unquestioned downward en- .richment and with the results of experiments in artificial en- richment* In this connection it may be noted that in the con- version of bornite to chalcocite, only a relatively small addi- tion of copper is required to produce a result of important magnitude, for the ono mineral contains 63.33 per cent, copper or almost 4/5 as much as the other, viz., 79.86 per cent. .?ur- thermore, the oxidation and leaching of a zone of solid born- ite ore would liberate and make available for enrichment at greater depth more than twice as much copper as would a solid chalcopyrite ore of equal thickness. Thus, as regards quan- tity of chalcocite produced, bornite is doubly at an advantage over any of the sulphides poorer in copper. In the particular case of the Engels deposit, all the 1. Sies, Allen, and Menvin, Loo. cit., pp. 498-499. 2* Allen, &. T., "She Composition of Natural Bornite," Am. Jour. 3oi., Vol. 41, pp. 409-413, 1916. 132 chalcocite thus far encountered could have been produced, through enrichment of tho original sulphides, by a decidedly smaller amount of copper than has plainly been removed from the tipper, oxidized zone, notwithstanding the incompleteness of leaching; and although quantitative data obviously are un- available, it is not improbable that this source was suffi- cient, even thoxigh the enrichment process may have been waste- ful of copper. In any event, it is evident that the Engels deposit once continued above the present surface and, as ap- pears later, topogra lie conditions were favorable for enrich- ment during the erosion that cut down to the present outcrop* The copper furnished by leaching of a relatively small thick- ness of the ore now eroded away, added to that removed from the present leached zone, would unquestionably suffice to ac- count both for any reasonable degree of waste in the enrichment process and for an extension of enriching chaloocite in gradu- ally decreasing abundance to a very considerable depth below the horizon to which the mine workings have yet been carried. As a matter of fact, if all the ehalcocite at Kngels be regarded as due to descending waters, the amount of enrichment la moderate or slight as compared with that in other bornite- rich deposits of comparable physiographic history. Finally in the upper parts of the mine, there are asso- ciated with the chalcocite, veinlets of liraonite and of sider- ite (with perhaps a little quartz), while at greater depth, tho limonite disappears but the siderite persists. These pro- dticta thus disposed are what might be expected to result from iron-bearing solutions at different distances from the source of oxygon supply, i. e., the surface, and of course the fixation of iron in a partly or completely oxidized con- dition in the circulation channel-ways at either horizon ac- corda with the idea of replacement of a copper-iron sulphide by a copper sulphide under conditions of descending, oxidi- 1 zing enrichment. Siderite, associated v/ith a little chal- 2 cocite, has recently been described at Bisbee aa the deepest prodiict of oxidising influences. In the Bisbee occurrence, the siderite is largely a replacement of oaloite (limestone). At Engels, the aiderite in the veinlets is pretty certainly a product of surface influences; but as to the siderite that appears to replace areas of caloite in some of the aeolite- bearing rocks, we are somewhat in doubt whether it likewise was formed by meteoric waters or is a final product of hy- drothermal action* Whatever its origin, the total amount of sidorite at Sngels is insignificant. Covellite, which at Engels is wholly subordinate in qitantity to chalcocite, is a replacement of earlier sulphides, and, like the replacement chalcocite, is competently explained 1. See Spencer, A. C,, Jour. Washington Acad. 8ci, Vol. 3. pp. 70-75, 1915. Also Qraton and Murdoch, loc. cit., p. 65. 2. Bonillas, Y. S., Tenney, J. B., and Feuchere, Leon, "Geology of the Warren Mining District," Bull. A.I.M.E. , Sept., 1916, pp. 1451 and 1457. ori on the ground that it is of superficial origin. We are un- able to follow Rogers 'a argument that chlorite replaces covel- lite, but find that covellite, like chaloocite,inay replace bornite and chaloopyrito along the margins of chlorite laths and along gangue boundaries in general. In short, we find at lintels that all the clearly enriching chalcocite - that plainly formed by replacement .of earlier, leaner sulphides - is to be explained as a result of the action of superficial solutions, and that there is no need or justifi- cation for resorting to the idea of "ascending secondary enrioh- raont . " ChalcQcito in the Graphic Struct tire, - The discussion under tho preceding section applies to the chalcooite which gives ready evidence of having been derived by replacement of bornite (and a little ohalcopyrite) , and only when all tho ohalcocite of the deposit has been mentioned has reference been intended to the chalcocite that occurs in graphic relation to bornite. This graphic ohalcocite is present in amount decidedly subor- dinate to the chalcooite clearly of replacement origin. In OMT opinion, the validity of the hypothesis of ascend- ing origin for part of the 3ngels chalcooite hinges on the source and character of the ohalcocite in the graphic structures. Concerning the origin of the graphic relations between bornite and ohalcocito, which have boon observed in sevoral districts, a marked diversity of opinion results from the difficulty of the problem and the conflicting nature of much of the evidence. 135 The jHPObloia problem of their origin however will be con- sidered in detail on later pages, whore more general evi- dence will be presented than that offered by the Angela de- posit, alone* If the replacement origin of the graphic ohalcocite at Engela is to be accepted, as urged by Rogers, w find no reason whatever for differentiating it in any essential re- spect as to source and age from the chaloocite of clearly re- placement origin which we believe is unquestionably due to secondary enrichment of the orthodox kind. But if the graphic chalcocite is a result of descending enrichment, we believe it represents the feeblest effects of that process and is an exceedingly delicate ;uanifostution of bornite replacement, Possible light on this idea is afforded by conditions at tho Superior deposit, THE SUIPiir.IOR iJIHJS. An instructive parallel to the occurrence of the ore at the 2ngels Mine is offered by the neighboring deposit known as the Superior. The hydrothermal character of the ore is very definitely shown in the latter, and the idea that the angels ora is to a large extent of pneunatolytic and hydrothermal ori- gin, is greatly strengthened by the many points of similarity between the two deposits. The Superior orebody is situated about two miles to the southwest of the Angela Mine, and approximately 1,200 feet lev . 136 lower in elevation. The ore outcrop ca the steep eaatern side of a canon, several hundred feet above the stream near the lower camp at the foot of the aerial tramway from >the Engels mill. Aside from the diamond drill holes, the only development on the property in June, 1916, was a shaft about forty foot deep, but a tunnel was being started lov/er on the slope to intersect the ledge. The orobody consists of veins of massive bornite and magnetite, ranging from thin seams up to a width of six inches or aore. They are spaced a foot or so apart in a fracture zone 10-12 faet wide. The lode as a whole seems to have a high dip, but the individual veins are more or less irregular. The surrounding rock is a diorite, and is part of the same large plutonio body in which the 3ngels ore occurs. It is however lighter in color than the rock near the larger mine, and examination shows it to be of more acidic charac- ter. The feldspar is andesino with rims of oligoclase or oligocluse-albite, and the hornblende is lass prominent than in the darker rock. The veins are accompanied by intense rock alteration. The development of amphibole, chiefly ac- tinolito, at tho exponae of the feldspar or the original hornblondo, is tho earliest phase of the mineralisation. Apa- tite in large crystals and titanito in numerous irregular grains are abundant, and were probably produced at this time. ..exe i Quartz in varying amounts is common under the microscope, but is not prominent in the hand- specimen. It probably com- menced to form with the amphibole. The acidic rims of the feldspars may also be products of this early phase of the mineralization. The feldspar, some of the quartz and the ara- phibole all show the effects of intense stresses. The amphi- bole, especially, is fractured and sheared. The bornite and its associated minerals are clearly later and take advantage of the channelways thus afforded. The formation of the sul- phides was accompanied by the abundant development of small laths of a green mica, and of epidote, chlorite and serioite. The green mica is commonly a replacement of feldspar or of amphibole, and the serioite, the least important from a quan- titative standpoint, is largely confined to the feldspar. The latest mineral of the primary sequence is heulandite, which occurs abundantly as a replacement of the plagioclase. Bornite and magnetite are by far the most abundant ore minerals* The magnetite appears as large grains sometimes boldly euhedral, surrounded by a cement of bornite. The lat- ter also occurs as fine blebs or irregular veinlets in the magnetite; it is clearly of later age and in part a replace- ment of the iron oxide. The association between the two min- erals is more intimate than in the Engels ore, and the mag- netite seems to have been more generally attacked by the bornite than is the case in the larger deposit (Pigs .^5 and 2he absence of specularite and ilmenite is noteworthy. f i ;Ii 19 rA*fn el J" ' tvy. j eoc 13* Chaloopyrite is less plentiful than at Engels. A few small blebs of galena were observed. Chalcooite Is present in subordinate amount intimately associated in all oases with the bornite. In small part, it occurs as veinlets and tongues which are clearly replacements. In a few oases, imperfect lattice structures are developed. Chiefly, however, it is in graphic areas or in irregular patch- es with mutual boundaries, and in these forms, it commonly shows a complete independence of the contacts between the born- ite and magnetite (Pigs. 86, 88) Where this chalcocite inter- sects laths of chlorite included in the bornite, the line of contact is in many cases likewise ignored {Fig.S6 ). In some oases, however, no break can be discerned between ohalcocite of the graphic areas and adjacent ohalcocite of the replace- ment type. Clearly of later age than the more abundant chaloooite Just described is a bluish variety of the mineral in tiny veinlets cutting the bornite. Where these cut bornite-chal- cocite graphic areas, it is sometimes found that the veinlet appears in the bornite lobes but is not distinguishable in the chalcocite blebs. With the chalcocite in these veinlets, there ia a small amount of oovellite, malachite, and limonite. A little kaolin and limonite are the only products of surface alteration in the wall rock. The slope upon which the ore- body outcrops is very steep, and almost devoid of soil. There is no zone of oxidized material, although faint limonite and Si 7.' ~ ' 139 malachite stains extend to the bottom of the shaft. The bornite occurs directly at the surface, with only the merest traces of alteration. The form of the deposit (viz., a definite lode in the diorite) and the nature of the associated minerals (including aotinolite, epidote, chlorite, serioite, and heulandite) in- dicate that the ore is epigenetic, as pointed out by Turner, and of hydrothermal origin. The early formation of amphibole, the dynamic modification of the rock, the subsequent introduc- tion of chlorite, epidote and sericite accompanied by bornite, and the development of heulandite in the closing phases afford, together with the absence of pyrite, a striking similarity to the sequence and character of the minerals at Engels. The oc- currence of the ore in a well-defined lode, the greater abun- dance of the green mica, the closer relationship between the magnetite and bornite, the slightly greater rock alteration, and the unimportance of products of oxidation and enrichment at Superior are the chief points of difference. Conclusions as to the nature of most of the chalcocite at Superior rest on the interpretation of the same kind of graphic structures between bornite and chalcocite as are shown at Engels, and the discussion of this subject on pages ]3>4-' i mdl35 Applies to both deposits. At Superior, however, it is ol v . that the chalcocite in these forms is not the product of enrichment from the present surface. The small . Jjt "'4 , . c- amount of oxidation under the active mechanical erosion to which the deposit is now subjected may be measured by the feebleness of the veinlets of blue chaloocite and covellite and the alight development of limonite and malachite. Most of the chalcocite is clearly of an earlier age than the vein- lets, and, if a product of descending secondary enrichment, it must have been produced under previous topographic condi- tions. The surface of gentle relief in this immediate neigh- borhood upon which were deposited the extensive Tertiary gravels of the Jura Eiver appears to have been not far above the present surface at Engels, while perhaps as much as a thousand feet above the Superior outcrop of to-day. There is distinct evidence of at least two later stages in the topo- graphic development, respectively shown by the relatively gen- tle slopes and open valleys of the higher levels, and the deep V-sliaped canons of the larger streams. Down at the Superior deposit where the deep incision of the country by an active stream permits only the feeblest present enrichment, most of the chalcocite may be interpreted as the deep roots of an older enrichment formed at a time of less violent degredation. At Engels, where the ore has not yet been reached by the severe erosion of the present cycle, 1. Diller, J.S., Bull. U.3.G.3., Wo. 353, Plate II. .leve LB ^ there is a moderately thick cover of oxidized and impover- ished material, underlain by a layer of rich chalcocite, and still deeper, by plentiful chaloocite that clearly replaces bornite* Although the vertical range of the Bngels mine workings is too small to afford conclusive evidence, there is a distinct indication that with depth, the chalcocite of undoubtedly replacement origin decreases in proportion to that in the graphic structures. It may be expected with a fair degree of certainty from the topographic and microscopic evidence that with increasing depth at Bngels, the character of the chalcocite will become more and more similar to that observed in the Superior ores. In other words, it is believed that the relations of the chalcocite and bornite exposed in the Superior deposit are similar to the relations between th two minerals in the deeper unexposed portions of the Bngels orebody. It is to be noted, however, that this inference is not necessarily dependent upon the interpretation given the graphic structure. SUMMARY. The Bngels deposit presents an unusually complete record of the varied conditions to which the rock has been subjected from its initial crystallization to the last feeble hydrother- raal changes and to the subsequent alterations by surface agen- cies. The history of the deposit may be summarized as follows: JJtlfiT o 142 1. Crystallization of a basic differentiate, of noritio character, from the batholith of the Sierra Hevada. The granodiorite porphyry also probably appeared at this time* 2. Iiocal development of pegmatites; also pneumato- lytic alteration of the rook, generally widespread, but oc- casionally emphasized along seams, producing amphibole, al- bite, tourmaline, magnetite, and some sulphides, accompanied and perhaps followed by straining, and partial recrystalli- zation. 3. and 4. More localized mineralization and altera- tion produced under hydrothermal conditions, intense at first, but gradually diminishing, characterized by chlorite, sericite, / epidote, and sulphides in the earlier stages, and by zeolites in the closing phases, with increasing dependence upon frac- tures. 5. Subsequent cracking of the rocks and ores* 6. Exposure of the deposit by erosion to oxidation, with the production of an impoverished oxidized zone and the development of secondary sulphides. The first four stages, covering the time from initial magmatic conditions to the close of primary mineralization, are merely convenient divisions of one uninterrupted se- quence, while the fifth and sixth were separated from the others, and probably from each other by indefinite time in- tervals. xe&stf J>eo In the primary sequence, the presence of rainoralizers such as boron and either chlorine or fluorine is indicated at an early stage by the occurrence of tourmaline and apatite, but the chief mineralizer from the beginning to the end was undoubtedly water. Its influence in the early stages is sug- gested by the presence of amphibole, and the development of the zeolites is conclusive proof of its presence at the close. Much of the rock alteration is probably merely the readjust- ment of the materials of the rock into forms more stable un- der the influence of the water vapor or water at high tempera- tures, without notable additions of new elements, except the iron, copper, titanium and sulphur to form the ore minerals. The orebody is in our opinion a direct result of igneous ac- tion, but was formed as a final concentration following the crystallization of the rock, and not as a direct magmatic segregation. Secondary enrichment, which followed the exposure of r the deposit to oxidizing influences, accounts for all the ohaloocite present, with the possible exception of the rela- tively small amount in the graphic structure, which also we are inclined to attribute to downward enrichment. These conclusions find support in the conditions ex- hibited at the nearby Superior deposit and in certain physio- graphic features of the region. 144, CONTACT METAMORPHIC DEPOSITS Seven Devils, Idaho. The ores of the Seven Devils district, Idaho, of far definite evidence of the position of bornite with respeot to the silicates formed during contact metaraorphism, and present good examples of several important structures Of bornite and the secondary sulphides. The district was not visited in connection with this investigation, and the field relations given in the following paragraphs were summarised frora descrip- 1 2 tionsby Waldemar Lindgren and by J. B. Umpleby. The rocfcs of the Seven Devils district consist of elate, quartz ite, limestone and large amounts of aBoniated greenstone, intruded and altered by a quarts dlorlte phase of the Idaho granite. The series is capped locally by Columbia basalt. The ore-deposits occur along the contact of the quartz -iiorite and limestone in plaoe, or about the narglns of included blocks of limestone. The most abundant gangue-minerals are garnet, epidote and quartz. Bornite and chalcopyrite are the chief ore- minerals. In three mines the sulphides occur on the limestone side of the garnet zone. They are connected with the in- trusive by means of veins dipping at a low angle, which cut 1. w. Lindgren. c:0th Anr.. Rep., U.S.a.S., Pt. 3, pp. ^4-y- 2. J. B. Umpleby, The occurrence of ore on the limestone side of garnet zones, Univ. of California Publication, Depart. of Geology v . Vol. 10, pp. 2^-37, (1915). aorotss the altered Intervening rock. The veins have been followed by Quartz Jiorite limestone JO feet Fig. y. Transverse section shoving relation of ore to r-net zone, Seven Devils, Idaho. 1 operations only to the diorite contact, but they are known to penetrate the igneous roci. for at least 25 or feet without diminution. At the Peacock iiine, the ore occurs in the central part of a large garnet-epidote area bordered on two aides by diorite and. on the south by an engulfed block of greenstone. A drill-core chows the centre of the mineralized area to con- tain unaltered limestone at depth, thus affording another example of the occurrence of sulphides between the limestone ir.l the contact silicate rooks. rrom the microscopical study, the sequence of ganque and ore-minerals may be established in a general way as follows. Orthoclase, amphibole, pyroxene (diopside), garnet aiio :i|.itite ooristituts a ^roup fornod. under the early in- tense phases of the mineral iz at ion. Zpidote and titanite, 1. J. B. Umpleby, Ibid 146. are probably somewhat later In origin, and chlorite lags still further behind. Quartz and oalcite were formed from the be- /. ginning to the end in various amounts. Oalcite, however, is most prominent as a late product, and is common as a re- placement of epidote, the feldspars and other earlier min- erals. The sulphides, bornite and chaloopyrite, are later than the first group of minerals, and later than the ep- idote, but they are probably in part contemporaneous and in part earlier than the chlorite, quartz and oalcite. These relations, :mp^ortsd by the field distribution of the ore- minerals, indicate that the sulphides were formed during milder phases of the mineralization, than those under which the high- temperature contact silicates were deposited. The amount of secondary alteration which the ore has suffered is not great, but there IB a small amount of chal- ooc ite, qovellite and ohalcopyrlte of secondary nature in nearly all the specimens of bornite studied. The chalooclte occurs for the most part in snail irregular velnlets or rlns about bornite grains and in thes<3 forms can be attributed without henitation to the agency of descending surface solutions It also occurs, however, to a minor extent in graphic struct- ures with the bornite and in a few larger patches in massive bornite in which it ia less certainly of replacement origin. In these forme, the chalooclte apparently possesses the ortho- 147 horabie structure, for in a few oases the character let ic pattern of straight lines oriented parallel to one direction ie shown by the etch-cleavage and by malachite veinlets. The structure of the chalcocite in the riiaa and velnletn is less definitely shorn by etching, but lattice patterns are frequently formed in it by the development of covellite and oxidized products. The replacement of bornite by chaloocite yields Imperfect lattice structures in a few planes, but it in not a common relation between the two minerals in thesr? ores. Chalcopyrlte ie abundant as fine plates in the bornite, vrhich form usually well-developed lattice patterns on the polished surface. The chalocpyrite is more widely distributed than the chalcocite, but it is clearly related to the same ohannel-waya and is without doubt the product of the same solutions. The relations are very similar to thoae previously described in the ores from Engels, La Fleur Mountain, and Copper Mountain in the Slrnilfcaiaeen region. In some grains, the plates of ohalcopyrlte are BO fine and eo cloaely spaced that they can be detected only with difficulty under the highest magnifications, and it IB very probable that the peculiar yellow tint of certain spots in the bornite from Seven Devils is due to sub-microeoople chalcopyrite of this character. Plates of covellite aeouna lattice orientation in both the bornite and in the derived chaloocite. In places, 148, the oovellite In an intermediate product in the replace- ment of ohalcocite by malachite. In summary, the deposits at Seven Devils are believed to offer very convincing evidence both from the field and . i'.roscopic relations that the bornite is later than the high- temperature contact silicates. The development of chaloo- pyrite, in exceedingly minute lattices, and its dependence on the development of secondary chalcooite and oovellite is exceptionally well shown, and the inheritance of structure of ths bornite by the chalcocite is strongly suggested by the formation of covellite in the lattice structure in the chal- cocite. The origin of the small amount of chalcocite in the graphic structure can not be settled from the evidence in this case alone, and will be licousssd in a nore general way in a later part of the pr.per. 149. The ffhitehorr.e District. Yukon Territory.. In the fiontact-zaetamorphlc ores of the Whitehors District, YuKon Territory, bornite is the most abundant sulphide. The district was not studied In the field in connection with thip \vorfc, but from information gained from R. Q. McCormell^s excellent descriptions, and from an exam- arainntlon of material in the collection of the Massachusetts Institute of Technology the occurrence is believed to be of sufficient value ut vTlth the exception of the Marble Bay ::ine, the irregularity in form and distribution of the ore -bodies has made the results distinctly discouraging. In the liarble Bay i^ine, however, niuoh larger masses of sulphides have been encountered with depth than ,-ould have been expected from the scanty surfftQe ohowings, and until the last few years the annual shipments have ranged from 12000 to 15000 tons of ore, varying from 3 to 11 percent copper with 1-5 oz* iae'C 155, 1 of silver and .1 to .7 oz. of gold per ton. The mine is opened by meane of a shaft about 1300 ft. deep. The ore occurs in a zone of discontinuous irregular bodies pitching to the northwest, which carries them from the neighborhood of the shaft on the upper levels to several hundred feet to the north and west in the lower v/orkings. The deepest level (July, 1916) Is termed the 15th, and is about 1360 ft. below the collar of the shaft, and a little over 1300 ft .below sea-level Literature. The most complete description of the geology and ore -deposit s of Texada Island is a report by B. 6. MoConnell, published as a memoir of the Canadian Geological Survey. In it, 2 3 previous work of the Survey by James Biohardson, G. M. Dawson, 4 end 0. E. LeBoy is summarized. The bornite ores of the island have also been described by ff. M. Brewer in a general article on the subject of bornlte-chaloopyrlte deposits of the ooast- 5 region. Le Boy's views of the ore-deposit at the Marble Bay G :.!ine are published in a later article as well as in the earlier 4 survey report (1906) and they are essentially the same as those 7 expressed in greater detail in McDonnell's memoir. 1. R. C. MoConnell, Teradft,Island B.C., Mornoir 58,C.G.S.,1914. H. Jaraes Hiohardson, Ann. Ttoport C.G. 3., 1873-74, pp 99-100. 3. G.M. Dawson, (1885) Annual Beport C.G.S.,Vol. Il.pp. 36-37-B- 4. }] . Le3oy, Preliminary Report on a Portion of the ^l Coast of B.C. and Adjacent Islands, C.G. 3., 1909. 5. .7. :i. Brewer, Jour. Can. Mine, Inst., 6. R. G. MoConnell, Loo. oit. 156, 1 GKHERAL )QT A series of voloanios and interbedded limestones known as the Anderson Buy formation are the oldest rooks recognized on tho island. They are overlain "by crystalline limestones of Trlassio age termed the Marble Bay formation, which is of distinct economlo importance as it is the chief ore-bearer of the copper region. Both formations are broken by extensive intrusions of variable nature called porphyrites, v;hloh are the most widely distribute, rooks of tho inland. These are in turn Intruded by quartz diorite, diorite and diorlte porphyry which are believed to be local manifestations of the great igneous invasion at the close of the Jurassic that formed the Ooast Ranpe bathollth. The quartz diorite and diorite form email stocks, but are not known In larger bodies on tho island* V A number of small intrusions of the diorite occur near the karble Bay -line, and are clearly the source of the mineralization* Diorite porphry dikes, either ae direct apophyses of tho stocks, or more widely distributed, are very common. They are fre- quently encountered in the workings of the mine and apparently increase in abundance with depth* The mineralization is be- lieved by MoConnell to have followed the period of dike intrusion* Ho record, of igneous activity leter than the Jurassic Is found. In the Cretaceous, a aeries of conglomerates, sandstones 1. Summarized from II. G. MoConnell 's Memoir. 157 and shales were formed, but only remnants remain In protected depressions. The Tertiary is entirely a period of degradation- At the time of maximum glaclatlon in the Pleistocene the island was completely covered "by southeastwardly moving io. During pert of this period tho island was submerged, and interplaoial clays, sands and silts were deposited along its coast. Glacial deposits containing narine fossils have been found on the island at an altitude of 500 ft., and are be- lieved to indicate a post -glacial uplift of that magnitude. Since the retreat of the ice, and the rise of the land, con- ditions have been stationary for t sufficient time to permit occasional wide rock beaches and low cliffs to have b >en out in the resistant quartz diorlte or limestone. The southern part of tho island composed ohiafly of porphy rites, has been left ^ region of notable rolief , .vith peaks rising to a maximum elevation of 892 ft., while the northern part, com- posed of the softer sediments, has been worn by the long con- tinued erosion to a milder country of basins and rough sea- ward sloping plains. ROCK3 AMD ORSS AT THE MABBLE BAY MINE Limestone. The Marblo Bay limestone IB an obscurely bedded rook, grey or white in color, and in plaoes of sufficient purity to be of commercial value. In the viainity of the ore- deposits, however, it becomes marmorized, and impure with various contact-metarnorphic silicates. Near the mine, no trac^ of bedding can be detected in it, although in plaoes there is a curious fine banding. 158. Porphyry* Three small stocks of rather basic diorite and dlorlte porphyry outorop in the neighborhood of the mine* numerous apophyses from these "bodies are associated with the ore, and below the 13th level, masses of stock- like proportions are encountered by the mine workings* ::icro8oopical examination of cpeoiaiens from the 15tn level, i show the rook to be composed of andeeine and hornblende phanoorysts in a microrranltio ground-mass of andeeine " ,vith some oligoolase and orthoclase* The feldspar rhenooryets occur in sharpl^ ^uheflral forms, and in many oases are strikingly zoned, ranging from cores as basic as medium labradorite to rime as acid ae oligoolase. Tho horn- blonde io oommonly in small prisms* Some dikes are distinctly dark* due to a reator abundance of hornblende* A small amount of biotite occasionally accompanies the hornblende. Small crystals of apatite, a little titanite and some quartz are usually present as accessories. The Igneous rooks are fairly fresh, "but show a slight development of the contact -metamorphio minerals* SerioitA in the feldspars, and oalolte and chlorite as replacements of the horiblende ho\vv" - common, even when the rook is apparently freeh. In a few instances, the alteration of the dikes ie intense and accompanied by the introduction of sulphides* In these cases ch&lcopyrite usually predominates; bornite is rare. 159, Alteration of the limestone. The development of garnet, diopside and vesuvianlte in the limestone was the earliest phase of the mineralization. The exact age relationships of the minerals are difficult to decipher, but it is fairly probable that the vesuviar.lte was earliest, and the garnet and the diopside i'ougrhly contemporaneous. The vesuvlanlte is less abundant than the others, but it Is prominent in certain parts of the ore, and occurs often in largo well- formed prisms, set in a cement of the later sulphides. Tha garnet is the yellow-bro andradite , usually in granular masses, but In places well-crystallized in small rhombic dodecahedrons. The diopside is much finer in grain, and la t usually observed megascoploally only as dull green compact material, making up a notablo part of the altered limestone. Under the microscope, it is commonly seen to be In email but in many cases, euiiedral, crystals, readily identified by their pyroxene cleavage, or in very fine shapeless grains comprising the chief mass of the rook. It is probably of eoual quantita- tive importance <7lth the garnet, "but as It Is lass easily recognized, the garnet appears far more abundant in the field. Large masses of coarse laths of wollastonlte are occasionally found, and fine laths of the mineral are fairly common under the microscope. Tremolite occurs -vith It but It is much less abundant. The two minerals are closely associated with diopside end garnet, und are probably of an eerly age. 160, Epldote, chlorite, sorlcito, ;;nd quartz form a seoond group clearly Inter than the minerals just described, since thoy are found chiefly as replacements of tho earlier products. With these later minerals, tha sulphides.! seem most closely associated, although in part the oros follov; them, for euhedral or corroded inclusions of even these gangue minerals have been observed in the bornite and chalcopyrito. The epidote for the most part IE the colorless variety low in iron. It is optically both positive and negative, but still beyond the range 01' clinozoieite. It fonas broad bands in the altered limestone, and in places clearly replaces tht earlier garnet, and vesuvianite. Sericite and chlorite are not abundant, but occur as replacements of feldspars and hornblende In the porphyries, or as seams in the other metamorphio minerals* Both are abundant in certain bornite and ohaloopyrite grains, usually near the margins. Quartz is uncommon but occurs in small amounts in various relationships. Calcite is abundant, both as rocrystallized grains of the limestone and as vein lots and masses of replacement origin, developed at tho expense of the contact minerals. One of tho most unusual features is the occurrence of the zeolites, heulandite and laumonite, in grains vvith caloite and metaaorphio minerals, or as fine seame cutting all other con- stituents. Their total amount is ouite small, but they seem fairly v.-idely distributed. The lauaionite was present in sufficient quantity in one specimen from a garnet zono on the 15th level to permit the identification in the thin section > to be checked by means of index liquids. Certain other minerals of low relief were observed which, may be other zeolites, but a satisfactory determination was not possible on account of the small amount of material, Ore Minerals* Chalcopyrite and bornite &ro the chief sulphides. Prom the impression gained underground, uhalcopy- rite is more abundant at present, but from descriptionaof the larger ore-bodies, now mined out, it ie probable that bornite was the most important ore-mineral. There is a small amount of pyrite, but usually not in the bornite ores. The pyrite, in the few polished chips where it was observed, is the earliest sulphide. Galena, steinmannite , sphalerite, tetrahecirite , and klaprotholite all occur in subordinate amounts* A little pyrrhotito was reported near a dike on the 1 7th level, but it hao not been recognized elsewhere. Hone was observed under the microscope. Molybdenite occurs in small amounts from the cropping to the lowest levels. Its relation to the copper minerals was not detected. A few grains of a hard, somewhat pinkish mineral were observed under the microscope, but it could not be identified by microchemioal means. Native silver is said to be common in various parts of the mine. It occurred most abundantly in the large ore-body above the 13th level. Native gold has been reported but it is very rare. The cheloopyrlte and bornite are intimately associated; their boundaries are commonly of the mutual type, but as usual 1. R.C.iloConnell. Loc. oit. ID. 51. there is a littlo evidence that the bornite was slightly later, or at least continued to fora longer and corroded tho ohalcopy- rite. Thero are few definite bornite veinlets in tho ohaloopy- rite, nut inoro numerous torifuoe end embayments of bornite tend to surround and isolate chaloopyrite grains than the reverse, and in general the secuence is fairly clear. (Pig. 50 ) In a far; instances, tho relations between the two minerals suggest the graphic: structure* Sphalerite in rather large grains is .'. not uncommon. It seems contemporaneous v.ith the ohaloopyrite ae some fine grained intergrowths of tho two minerals wore observed and it is slightly aorroded by the bornite* In addition to the forms Just described email specks and spines of chalcopyrite occur in the bornite, in some oases In fine lines of small blebs apparently along the contacts of grains. Selena and tetr&hedrito are also common In this relationship, "hut in addition both of these minerals form minute end intricate ^rephlc structures v/Jth the bornite, which are most readily interpreted ae contemporaneous intergrowths, t Aj-eocie-ted vith the galena, and not readily distinguishable from it except chemically is the arsenical antitnonial variety, stoinmannite. It is abundant in galana fror, the l?th level. The small amount of a mineral, somewhat doubtfully identified 1 as klaprotholite- .VBS observed in the bornite. 1. Klaprotholite '.vat- first determined in OOT>J er ores by PB. Laney in tnatorial fro.a Butte. It has al-^o been obsorvec b,y A.F.r.orrers in ores i'rora several different localities. (see Koon.Oeol. Vol.;T,p.5i3 5 ,>n the policho- surface the nlncral in the Taxada ores has a bright creainy, silver- .-hite color, and is distinctly softer than the sur- rounding bornite. Its microchemical properties are as follov/s:- with dilute HiJO^i brownish black, poreistant : ;;ith dilute or co-nc, H Cl, brown thoa black, rubs to a gray surface: ,vith ?.C3, no re- 163 The mineral ir. in round or elongated blebs for the most part, "but in a few instances its vein-like form makee it very probable that it is a replacement of the bornite. In some cases, it forms eriali irregular foathery masses alonp the margins of bornite areas. The specimens in vhioh it was observed were from a large ore-body on the 10th level. ( ?lg. '4-9). Secondary Sulphides. Chaloocite and oovelllte occur in very small amounts as deep as the 10th level. They are usually present only as vo inlets of thread-like dimensions, "hut in one specimen noteworthy rima of ohalco- cite folio-, the ed^es of bornito grains. Jhe bornite is the only sulphide in v.-hioh the veinlets are at all prominent. Fuzzy rosettes of oovellite penetrate the bornite from certain ragged patches of included >?angue minerals, and are identical in form, and appearance with oovollite known to have been +'ound by descending proceeees. In the chalcorite rims, in the case mentioned above, there is a notable amount of the klaprotholite, in ragged laoy forms, partially altered to a dull, blue-rray product, which in turn is surrounded by the ohalcoclte. There it- a distinct suggestion that the klcr>rotholite former un inter- mittant rim about the bornito arer.e, and that it became include*^ in the ch&lcooite v/hich replaced the Bornite. The chalooolte ae -"or as observed does not develop the lattice structure with the bornite. .( ' - -IJ-B 164. The ores on the? uppor levels have been wor .ut, and the stopee ero not acoeuaible, but there was eaid to be no marked zone of secondary oroB or of complete oxidation. Primary sulphides o.sour within a few feet of the surface. On tho other hand a small amount of azurite oceure with the chalcooito in bornito in a specimen eald to have como from the 10th level* It replaces the bornite in otaall peculiarly anpular patches and :rrcine. Although 1500 ft. below sea-level, the lower workings of the mine are quits dry. Surface waters enter the mine along tn open fracture termed the Llud-ellp, but eecept during ex- ceptionally wet seasons, the flov is caught on the upper levels .v.nd pumper back* r-ucussioa Distribution and origin of the ore -bodies* The relations between the sones of altered limestone and tho ore-bodies are complex, and difficult to state in a genorei way* There is, however, a distinct suggestion that the ores occur most between garnotized limestone and pure recrystallized or marinorized limestone, i'his is shown by Mc.Connell's maps 1 of the levels* In many jasos, however, tho sulphides apparently occur in unaltere, marble, but in most of these ti'-noea, ;;arnetized rook would probably be found in contact .vith the oro on tho sides "Solow tho section shown on the maps* Prom the promising results in similar occurrences studied by 2 A* Locke and ii . ri. Perry, and from the valuable generalizations 1. Op* cit* 2, Oral cor. .unioation. 165 1 by J. B. Uiapleby on the distribution of sulphidas, j limestone and iinaltereo limestone in deposits of this sort, it is very prooable that a thorough study of the distribution of ores v/ith respect to altered limestone would yield results of definite commercial valuo in directing further prospecting for new ore-bodies. There seems to be little significance in the relations of the ore-bodies and the numerous dikes. Both are probably ' manifestations of the same igneous tiCtivity, but it seems probable that the ores arc later, as the dikes themselves become mineralizes and altered. They may have had a directing action on the ;aineruli;:ing solutions however, which v.ould be .vorthy of study. McConncll suggests that the emanations which produced the ores were derived from deeper portions of the magma tna not directly from the rocks exposed at present. The rielng solutions were probably directed to a certain extent by the limestone - prophyry contacts. ::inerr.l se cue nee. The seouenoe of the minerals of the deposit, based on evidence obtained by the microscope for the most part, ie expresses graphically on page (167). ihe details of the grouping are of course only a rough approximation, but certair broad features may be regarded v/ith certainty. The sulphides are vithout ruestion later than the garnet, vesuv- ianite, diopside, v/ollastonite and tremolite, and are more 1. Loc. cit.,pp. 25-37. 166 nearly contemporaneous v;lth the epidote, chlorite and serioite. The zeolites (laumontite and heulandite) and the calcite in part are later than the sulphides. The occurrence of the zeolites in association with hi:h temperature contact - metamorphic ninerals is interesting: testimony of the change from intense conditions of temperature in the early stages of the mineralization to mild hydrothermal conditions toward the close. Ho. 8 Lerel 6 BO feet '/& No. II Level -feet 'inze . 10 Level Sfaft Sc-A? o so' too' Crystalline. Limestone repbceJ Ore limestone. by contact silicates stapes Fig. 10, Plans of ore-bodies on different levels of the Ifirble Bay Mine, Texada Island. 1 1. R. C. MoConnell, Loc. cit. 'i 167 Diagram of Mineral Sequence Intense Mild Oxidation Vesuvianite Garnet Diopeide Cordierite Titanite Wollastonite Trenollte Zpidote Chlorite Serioite Quartz Calclte Pyrite Chalcopyrlte Sphalerite Pornlte Galena ( Stelnrnannlte ) Tetrahedrite Klaprotholite Launontite Heulandite Molybdenite Chalooolte Covellite Azurite Malachite \ r 168, Pyrite at; usual io the earliest sulphide. chalcopyrite e.nd borr.ite are represented to be of essentially contemporaneous origin, "out by the form of the curves, it is rented that in the early phases, the formation of the chalco. -, 'ito preco ruinate a, while in the later, the formation of bornito ;ti. i.iore important. Sphalerite accompanied the chal copy rite i-.\ its period of greatest development; galena &nd tetrahedritg acooaip&nied the bornite. Tho fclaprotholite is somewhat luter than most of the "bornito. )rigij of the seoor.dary eulphidos* The ooourrenoes of chaloooite eni3 c'ovellite even in am&ll t^ounts as rsplaoements of bornite at a depth of about 1000 ft. bolow sea-level is surprising. The minerals are In forms characteristic of the zor.e of secondary sulphides, In the speoinaen in whioh the ohaloosite is moet &bund^.nt, azurite alao occurs, so there oan be no doubt that oxidizing conditions had penetrated to this depth The rocks ho-.vever are tight and fairly dry, and although there ie flo.v nlong certain prominent fractures as ntioned previously, there iu practically no movement of water through tho rooke. Aocordin. to MoGonnall, the most recent change vith relation to the soa-level ,vas uplift, so a higher position in tho iuuieJiato past can not be postulated. In pro ;!;: oifil times the present ore ,vus undoubtedly more do for thare is abundant eviuenco that glacial orosior. in thi;- re^jo,. ry r.evore* 169, nevertheless in spite of these unfavorable conditions the ohalooolte is there, and in forms which impel bolief that it is secondary* faters may retain their oxidizing powers even after peno- trating to great depths in barren limestone, for the rook contains few minerals susceptible to chemical alteration under oxidi",inp conditions. It is possible that slow lateral migration of waters of this sort may have produced the slight readjustments indicated by the chalcocite and oovelllte in the "bornlte. The azurlta associated with the ohalcoclte may b formed by mildly o;:ldi-in ; T solutions rich in carbonate ions acting on bornite and the excess of copper from the reaction would be available for the production of ohaloocite and oovellite in cracks beyond the reach of the feeble oxidation. The tightness of the rooks, and the relation of the deposit to present and past aea-levels and to older topography are all unfavorable for the extension of secondary changes to notable depths. The occurrence of small amounts of asurite, ohalooclte and oovellite so deep below sea-level indicates the grebt ease with which the alteration of bornite to secondary sulphides takes place. Summary. The ores of the Marble Bay Mine are of oonteot-metamorphic origin, and occur in limestone near intrusions of diorite porphyry. The sulphides replaoe the llmeetone and earlier hifrh temperature silicates. They are closely associate*' .;ith epidote, chlorite and serioite, 'it . ' ' . 6J-.' ' ' 170, The rich ore "bodies are usually situate^ between garnetized limestone on one side, and pure marmorized limestone on the other. The sulphides are "believed to have been 'ormed under conditions of intermediate intensity, .milder than those .vhloh produced the gurnet, ve^uvienite and diop^lde, "but more intense than those .viii'Xi .produced the zeolites. Bornite is the chief oro-naineral, but ohalcopyrite is also abundant. The two minerals are olosely associated for the most part but the bornite is slightly later* A mineral doubtfully identified ao fclaprotholite was observed as a replacement of bornite. ?jrite ie preotioally absent in the ores in the altered limestone. The great- ease of the alteration of bornite to secondary sulphides is sho.'/n by the development of a small amount of chaloocite and oovellito on the 10th. level, under conditions particularly unfavorable for deep enrichment. 171, Blebeo. Arizona. The time has not been available either in the field or in the laboratory for ae thorough a study of the complex problems connected with the Bisbee ore-deposits as has been given many of the other camps. The abundance of reliable 1 information in accessible form, however, makes It unnecessary to do more than merely outline the most Important features re- lated to the distribution of the bornite in the primary ores 1. James Douglas, The Copper Queen Kin, Tran. A.I.M.E., Vol. 29, PP. 511-51-6, (1900). P. L. Raneone; Copper deposits of Bisbee, Ariz., U.3.(J.3. f Bull. 213, 1903, pp. 14-9-157. The geology and ore deposits of the Bisbee Quadrangle, Ariz., U.S. a. 8., Prof. Paper 21, 1904-. Bisbee folio, U.S.O.S., No. 112, (190M-). Weed, w. H.. The copper mines of the United States in 1905: U.S.O.S., Bull. 233, 1906, pp. 99-100. Arthur Notinan, The Copper Queen nines and Works, Part II, Trans. Inst. Mining and Metallurgy, Vol. 22, PP. 550-562, ( 1913 ) W. L. Tovote, Bisbee, a geological sketch, Mining and Scientific Press, vol. 102, pp. 203-20K, (Feb. 1911). Y. S. Bonlllas, J. B. Tenney and Leon Pouchere, Geology of the Warren Mining District, Trans. A.I.M.E, (Bull. 117, pp. 1397- 14-65; Sept. 1916). A detailed description of the ore-bodies, based on intimate knowledge of both the field and micro- scopic relations. oK iw 172. and the character of Its alteration to chalcocite. ;jy in- formation of the field relations was gained largely from iiesBrs. Y. S. Bonillae find Leon Feuchere of the Copper Queen Co. and Mr. Ira Joralsmon of the Calumet and Arizona Co. during the five days spent with them in the district. Primary features of the bornite ores. Bornite is found in all parts of the Biabee ores, but in general it is lees abundant than ohaloopyrite. In the disseminated ores of Sacramento Hill, however, ohiloopyrite is lacking and the primary ore consists only of pyrite and bornite. The porphyry in which the bornite is associated is serloitlzed In general, and locally silioified. Elsewhere, as in the contact- raetaiaorphio zone or in many of the ore-bodlee in relatively unaltered limestone, chalcopyrlte predominates although bornite is usually present. High temperature silicates such as garnet, dlopside and wollastonite occur in the contact zone, but only in snail amounts. The development of pyrite is believed to be later than these minerals, and closely associated with the sericitlzatlon of the porphyry and sed- iments, while the chalcopyrite and bornite are somewhat later ani accompany the silicification. The relations indicate that ohilcopyrite and bornite were produced under the same con- ditions, but that the formation of bornite lagged somewhat, and in part was a little later than the chalcopyrite. Sphaler- ite in small ^uounts is usually associated with chaloopyrlte v:hile galena is later n x related more closely to the development of the bornite. 173. Secondary alteration. The distribution of oxidation and enrichment la well described and Illustrated in the recent 1 literature. Oxidation and the development of chalcooite are deepest in the ores in little altered limestone, although in these cases an actual reduction of values per unit vol- ume ins taken place rather than an enrichment. In the serlclt- ized porphyry ores, oxidation is slower and a distinct zone of sulphide enrichment is found. In the silicified contact breccia, where the ores are least permeable, the zone of ox- idation and secondary sulphides is relatively thin. 2 According tc Bonlllas, Tenney and Feuehdre, the mine workings have now penetrated to a depth at which the rocks as well as the ores show no effects of surface changes, iul continue downward without the vertical variations oom- non in the upper portions. "Chalcocite is very abundant in many forms in the zone of enrichment but has not been seen in the undoubted primary zone, though this has been consider- ably explored. " Consequently the geologints of the Copper Queen Company conclude that it is highly probable that the ohaloocite was not formed by ascending solutions in this dis- trict. The force of this conclusion is slightly weakened by the fact that the zone of enrichment is so deep that primary variations in the ore could conceivably be confined to it. The greater abundance of galena on the upper levels offers an example of such a change, for this mineral is surely of primary origin. 1. Y. S. Bonillas, J. B. Tenney, and Leon Feuchere. Loo. oit. 174, The chalcoolte Is to a large extent a replacement of bornite. The ratio of pyrite to chalooclte in many of the enriched oros is approximately the sane as the ratio of pyrite to bornit in the primary ores, and there can be little doubt that In these the pyrite remained fairly resistant to the en- riching processes. However, in some of the richer ohal- cooite ores, especially those mined some years ago, enrich- ment was sufficiently intense to have largely replaced the pyrite as well as the other sulphides. Chaloopyrite, as usual, alters less readily than the bornite, but in many parts of the ores it has been converted completely to ohaloo- cite. Chalcocite has been observed associated with bornite in blebs in galena, which suggests that the latter is more resistant to chalcocite alteration than the former. Jiicroscopical study of the bornite-ohalcocite ores, however, reveals a great variety of relations between the two minerals. In most cases veinlets or rims afford clear ev- idence that the chalcocite in part at least is of replacement origin. In such relations, the distribution of the chalco- clte if* clearly dependent on obvious channel-ways, such as the surface of contact between sulphide and gangue minerals, or along graifc boundaries or definite fractures in the bornite. In the deeper ores, or in places Where the development of chalcocite is less intense, the alteration of bornite yields the lattice-structure. In it all degrees of replacement are shown from the initial strips of chalcocite penetrating bornite fields to the final complete stages where bornite 175. remains only In bluish residues in broad areas of chaico- cite. The triangular or rhombic pattern of the bornlte chalcocite lattice is inherited by the chalooeite. It is shown either in the blue and white tints of the polished sur- surface or by the etch-patterns developed with nitrio acid or potassium cyanide. The white spines, which correspond to the first invasions of the chalcocite into the bornite etch more readily, while the blue areas between then are more resistant, and the lattice pattern is thus emphasized. Certain of the linss produced by etching, however, are not necessarily parallel to the lattice directions. The blue patches frequently etch with only one set of parallel lines, usually making a small angle to one of the directions of the lattice pattern. The replacement origin of the chaloocite in the lattice-structure can not be doubted, but its attack on the bornlte takes place in a selective manner which is difficult to explain. Certain grains or parts of grains nay be largely reduced to rhombic or triangular residues of bornite between Intersecting bands of chalcocite, while the adjacent bornite maintains a Rmooth resistant boundary against the corroding agents. The distribution of chaicocite in the lattice structure does not show the same dependence on obvious channel- ways for solutions such as grain boundaries and fractures that is commonly exhibited by secondary sulphides. The graphic structure between bornite and chalcocite is only slightly developed in the Bistoee ores, but irregular 176. blebs and specks of bornite scattered through chalcocit fields often assume shapes very similar to It and are common. In modified form it has been observed associated with the lattice pattern, in relations which suggest that the chalcocite between tho irregular bornite blebs was derived by replace- ment. Numerous small grains of bornite, with sharp clean outlines, often remain mattered through fields of ohalcocite which are clearly products of lattice replacements. The distribution of these grains in many cases greatly resembles certain types of graphic patterns. The bornite grains are Clearly isolated residues in chaloooite of replacement origin. It must be noted, however, that the lattice replacement in the final stages normally does not yield products of this sort, but sub-angular patches of bornite with hazy boundaries. The discussion of these significant relations will be postponed until the subject can be treated in a more general way on later pages. Bornite residues or inclusions in chaloocite assume various peculiar forms. Bands of bornite near the margins of ohalcoolte areas are not uncommon. ( Fig. ). Spines of bornite remain apparently resistant in areas of well advanced lattice replacements. In other places, bornite occurs as a fine peppery sprinkling of aharply-^ef ined specks in chal- cocite. This association has been termed by J. Murdoch the "drop-erjulsion structure," which is a very descriptive name, 177, but as the term "emulsion" suggests a contemporary origin for the two minerals, a lees definite name, such as "drop- structure 11 is perhaps to be preferred. Fine bornite stringers in coarse caloite veins have been observed almost completely replaced by chalcoclte, which indicates that the enrichment of bornite to chalcocite is possible in the presence of calcium carbonate, and that the acid liberated by the reaction is not sufficient to cor- rode the limestone to any notable extent. Secondary bornite is uncommon, although a snail amount exists in the Bisbee ores. Fine veinlets of chalcocite cutting bornite areas have been observed to change into vein- lets of bornite when chalcopyrlte was encountered. Chaloo pyrite residues in chalcoclte in many cases possess complete or intermittent rims of bornite, which suggests that the alteration of chalcopyrite to chalcoclte may taJce place through bornite. A more complete sequence la shown by successive rims of chalcopyrite and bornite, which may be termed "haloe", around pyrite grains in a chalcoclte field. The secondary origin of the bornite in these cases IB probable, but the quantity of the mineral involved is very small. ( Fig. 153 ). Suimary. The bornits at Bisbee is a replacement of porphyry or limestone under milder conditions than those under which the contact metamorphic silicates, the pyrite, or the sericite were developed. The bornite is accompanied by sil- 178. icification usually, but in Boni? places replaces limestone v;ithout the development of other minerals . It occurs both in pyritio ores and in those poor in pyrite. Froir. the fiold evidence it is probable that the ehaloocite associated with the bornite is all of secondary origin. The microscopical evidence indicates that the chalcocite is to a large extent a replacement of bornite. The chalcocite in the lattice structure apparently shows a se- lect tive action in its attack on the bornite, and there is evidence vrhioh suggests that the chalcoclte-bornite graphic structure nay be produced by a somewhat similar selective replacement. 179. DEPOSITS OF HYDRO-THERMAL ORIGIN The Matnsa Mine. Superior. Final Co.. Arizona INTRODUCTION The Magma Mine is worthy of particular attention in connection with the study of th? problems associated with bornite for a large percentage of its cojper output is derived frost this mineral. From a commercial standpoint, it is at present the most important bornite deposit in the United States, with the exception of certain ore-bodies at Butte and Bisbee. Situation. The Pioneer Mining District of Final Co. is situated about 65 miles east of Phoenix, in the re- gion of indefinite rugged ridges and peaks south of the Salt River. The chief town in the district is Superior, which is supported almost entirely by the Queen Mine of the Magma Copper Company. The actively producing camp of Ray is only 16 miles to the east, but across a rough ridge. A narrow gauge railroad extends to the south, connecting with the Arizona and Eastern Railroad near Florence. In addition to the railroad, an automobile stage service between Ray and Phoenix passes through Superior and makes the camp readily accessible. The town of Superior is built on a gentle slope of . JIOZl - -* . . ag^i atfjtn lo o . leiric silT .Tff- :i fct". . : .. Q i I 180. unconsoli dated desert gravels on the northern aide of an irregular structural valley, drained chiefly by Queen Creek. Immediately to the east, an Imposing precipitous ridge known as Apache Leap rises about 1500 feet above the town, and pre- sents to the geologist a fascinating section of the rooks of the district, exposed with almost diagramatio clearness. Queen Creek, which fringes the town on the south, breaks the escarpment with a deep gorge, along which the eastward! y dip- ping strata are out in succession, as one ascends the stream. Mines. The only ore -body in the vicinity of Su- perior which supports a r.ine of importance ia property of the Magma Copper Company. The ore is almost entirely confined to a porphyry dike, which varies from a few feet to over 40 feet in width. It is worked by a 1500 foot shaft, and levels at the usual intervals. At the time of my visit, (August, 1S16), the deepest workings were on the 1200 level, but since then the main cross cut from the shaft at a depth of 1500 feet; has encountered the dike and found good bornite ore. The ores are rather complex, and in the deeper ore- bodies silver, lead ani zinc values are sufficient to make their recovery worth vrhila. Flotation has proved to be the most satisfactory method of concentration. The Silver King Mine, five miles to the north, has been a famous silver producer in the past but it has not been in operation for many years. The ruins of the old mill, surrounded by crumbling adobe walls of the small town it ..' 38. 181. supported, may be seen from the Phoenix stage road, ten miles or so east of Superior, anl still testify to the former great- ness of the property. The Lake Superior and Arizona Mine, immediately north of the town of Superior possesses exten- sive drifts along the strike of the upper surface of the Cambrian (?) quartzite, but it has developed little ore. Literature. Some notes on the geology and the ore- 1 bodies of the district have been published by F. L. Raneome, & but no detailed paper has yet appeared. Earlier mine reports furnish a little information, ohiefly of value for the de- scription of the older and now exhausted silver ores. An ex- cellent topographic and geologic map of the ridges near the mine has been oa.de by the engineers of the Magma Copper Co., but the v?ork has not been published. Acknowledgements. My thanks are lue to Mr. W. C. Browning, the superintendent, for the privilege, of visiting the mine, and for several rare specimens. A large part of my information concerning features of geological interest in the mine and in the immediate neighborhood was obtained from Mr. I. A. Ettlinger, the ohief engineer, who very kindly devoted many hours to showing roe all important parts of the deposit. 1. Bulletin 540 D. U.S.G.S., Copper Deposits near Super ior,Ari 20. , (1913), 3. One on the Silver King Mine, quoted by Ransome, is by W. P. Blake, Description of the Silver King Mine of Arizona, New Haven, 1838, 48 ,p. with illustrations. 181A. .A 13 1 PLATE V Super i of | 'rizcna. 11 View to the south from the Magma Mine. CKrlM(T) quartzite outcrops on he lower slopes, with Carton! feroua limestone above, capped by the great daoite oliffs of Apache Leap. 12 Aoaohe Leap from the west. The town of Su- SSrior at the base of the oliffs may be seen on the right. The deposit of the 1 :agma Mine is situated along the fault shown by the dis- placement of the "white limestone beds juat to the left of centre. 181B. PLATE VI Superior, Arizona^ Fig. in. The Magma Mine from the south. Fl?. 1-'.. Outcrop of the Magma deposit. The prominent cropping in the foreground ia quartzite. isa. General Geology Sedimentary Column. The geologic column is ex- posed with satisfactory thoroughness on the bcld cliffs of Apaohe Leap, -And in the canon dissecting it. The lowest of the sedimentary beds, and the oldest rocks exposed, are massive quartzite a, averaging about 200 feat in thickness. They are of pro -Devonian age, and are probably to be cor- related with the Cairbrian(?) formations described in the 1 neighboring Globe and Ray Districts. The quartzite is over- lain conformably by a thousand feet or more of limestone, ranging froir. Devonian into the Carboniferous. There are several well marked variations in the character of the lime- stone, which have enabled the local geologists to irake seven or more subdivisions of the series. Igneous Rooks. The igneous rocks noted are diabase, monzonite. porphyry, and iacite. Thi diabase forms large sills, the only one of importance at the mine being intruded into the quartzite. It is at least 1100 feet thick, as shown by the rrdr.s workings, and the floor has not been reached as yet. There are said to be exposures in the neigh- borhood, however, where the diabase sill may be seen rest- ing on atill lower quartzite beds, but the short time avail- able did not allow roe to see them. A few irregular trap dikes in the region are probably of the same general period 1. F. L. Ransome, Some Paleozoic sections in Arizor their correlations, Prof. Paper, 38-K, US. 6.S., 1916. 183. of intrusion. certon/f^i^ ^ For P h yy J''fa limestone ISA. North -south section through the Ma 3 n, a Mi ne . Scale I in = IDoo ft. Figure. 15 B West-east section through Acache Leap^ near Magma Mine. The porphyry, whioh ie locally termed granite porphyry, occurs in a few dikes, the most important being the large dike in whioh the ores of the Queen Mine occur. The rook has a light gray color, with a slightly greenish tinge. Feldspar phenooryats ooour sparsely scattered in an aphanitio ground case. Under the microscope, the orig- inal constituents in the specimens studied were found to be all greatly altered by the mineralizing processes, but the 184. original character of the rock may be reconstructed with little doubt. The phenocrysts are chiefly orthooiase, acid plagioolase, and thin laths. of hornblende. The ground mass ie miorogranitio, and although now highly silioified it probably contained little quartz originally. The rook, aa far as may be inferred from microscopical study, is more properly termed a monzonite porphyry than a granite porphyry* Variations in the petrographic character of the dike are said to occur, however, Mr. Fred B. Ely, an engineer of the com- pany, believes that the dike it composite in character but during my short time at the mine I saw only one type of in- trusive. Ransome states that the rook is probably a quarts diorite porphyry, but adds that the determination is not cer- tain on account of its greatly altered condition. It is quite possible that there is a notable variation in the char- acter of the rook but several specimens collected from dif- ferent parts of the mine all indicate a monzonite composition for the chief mass of the dike. The most prominent igneous rook in the vicinity of Superior is daoite, which occurs as thick flows resting on a fairly even erosional surface out roughly parallel to the bedding of the Carboniferous limestone. The rook is of Tertiary age, and later than the dikes previously described. The Tertiary sediments which occur with the lavas in the Globe and Hay Districts are not exposed in the immediate vicinity of the mine. Coarse desert wash slightly cemented "io 185. fan glomerate, fills the valley to the south to an indefinite thickness, and nearly completely mantles the older rooks. Structure. The sedimentary beds and the parallel igneous formations strike roughly north and south, and dip 30-40 to the east. The deformation is chiefly due to the faulting of post-iaoite age. In the vicinity of Superior, the beds are broken by a strong north-south fault, which extends along the base of the steep eastern slope of Apache Leap. The boldness of the escarpment testifies to the \ \ \ ^ g 1 ^ 1 ^ ' "1 *- \ Carkoni ferous S Carboniferous Dae ite. \ limestone w P 5 * linestone N ^. -^ \ (/t **t ^^ h S; ft \-so- i ^V 1 \ \ \ fv, 1 *~ *~ ^ ^ \ I* \^ Ddcitc. 3 Carboni fereiis Da cite. \ 1 lime, stone \ oo . .cila aa^o - - 133. ly continuous, gradually dying out to the wast, however, be- fore the large north-south fault ia intersected. In a broad way there seem to be two vertical zones of somewhat richer ore, one to the east of the shaft, the other slightly to the vreat of the shaft. In the neighborhood of the 500 feet level, very rich bodies of massive ohaloooite occur in the eastern part of the ore -body which pass with depth into ore consisting predominantly of bornite. On the same level, the western portion of the ledge contains rather lean pyritic material, which changes into rich bornite ores on the levels beneath. Lead and zinc become of greater importance on the lower levels. Along one cross-cut on the 1100 feet level, the entire width of the dike is mineralized, and the assays along its wall show a notable banding of copper and of lead and zinc values parallel to the strike of the dike. A rich copper band follows the north side of the ledge (the high values being due chiefly to bornite). This changes sharply with little intermixing of the minerals into a band lean in copper and high in lead and zinc, which is in turn succeeded by another rich copper zone, with low lead and zinc values. The silver tends to rise with the copper, but it is also present with high assays of lead and zinc. Besides these variations in distribution of the metals across the ledge, there is said to be a distinct tendency for the copper to increase at the expense of the lead and zinc toward the west; toward the east the zinc increases with decreasing .oo eio otal rttqefc riJl* ewsq rfotriw tbod-erro & . 139. ooppar values. Mineral Sequence. The mineralization la clearly later than the crystallization of the porphyry, for its earliest products break the rook in seams, or replace Its primary structures. Pyrite, aa usual, was the first sulphide to fora, and it occurs in largs Basses or scattered grains. It was accompanied by intense ailicif ioation of the ground mass of the porphyry, and by the replacement of the feldspar aad hornblende phenocryats by aoricite. Both the pyrite bodies and the altered dike-rode offer abundant evidence of subsequent fracturing, which afforded channel ways for the following copper bearing phases of the mineralization. The pyrite is greatly shattered in places (Figures 93, 94 ), and the breaks emphasized and widened by bornite or ohalco- pyrite replacement. Veinlets of quartz carrying bornite or ohalcopyrite, break the areas of earlier quartz and sericite. While it is possible that some of the copper minerals may have continued to form with the main mass of the bornite ;md chalcopyrite, the evidence of two stages, viz. one in which pyrite dominated, arid the other in which bornite and ohaloo- pyrite dominated, is very convincing. Under the microscope the ohalcopyrite and bornite nay be seen to be in intimate association. For the most part, they exhibit mutual boundaries toward each other, but here and there the ohaloopyrite is penetrated by imperfect veins of bornite, or grains of the ohalcopyrite appear as if set 190. in a oercent of bornite. The evidence is believed to indicate that here as at Engels, Evergreen, Marble Bay, a..i other carr.ps, the two sulphides formed under the same conditions and at about the same time, but that the ohaloopyrite slight- ly. preceded the bornite. Under low magnification, however, fine lines of oaalcopyrite appear in many specimens, apparent- ly outlining bornite grains. (Figure 92 ). When exam- ined tuore carefully, these lines are found to consist of irregular, somewhat elongated, disconnected blebs, and are not distinct, continuous veinlets. It suggests that, as the bornite crystallized, an excess of iron was concentrated or localized along the contacts of the grains, which resulted in the formation of the iron rich sulphide. In the diagram of mineral sequence, this idea is shown more clearly perhaps than it can be expressed here. In quantity bornite exceeds all other primary copper sulphides, and there is no reason to doubt tnat for the most part it is a direct replacement of the silicates or of the pyrite with which it is associated. The replacement of ohalcopyrite by bornite was probably only a feeble process, for the evidence that it took place is not abundant, and amount of bornite which originated in this way is without doubt relatively small. Galena and sphalerite become of great importance in certain parts of the deposit, as may be inferred from the preceding paragraphs describing the distribution of the met- als. They seem to be closely related in age to the bornite IQ a? 1 ifcfii i en 07 ^^DjtIia & ?.*Qo8 . WffA 14 SO 191. and the chaloopyrite. The galena and bornite are associated in many cases, in structures similar to the graphic structure of bornite and ohalcooite, although a greater variation in the else of the blebs ia often shown. (Figures 89, 90 ). The sphalerite and ohaloopyrite also occur in peculiarly in- timate and fine grained structures which suggest an origin by contemporaneous intergrowth for the two minerals. In a few cases, bands of sphalerite ware observed in bornite- ohaloopyrite areas, but even in these relations there seemed to be a little corrosion of the sphalerite by the bornite. The relations usually exhibited by small areas of sphalerite surrounded by bornite offer little or no evidence concern- ing sequence. Where sphalerite andigalena are abundant, the veins of the latter in the former, and the manner in which grains of sphalerite are surrounded and corroded by galena clearly indicate that the sine sulphide was the earlier mineral. Elongated blebs of galena commonly occur in the lines of ohaloopyrite blebs surrounding bornite grains, i seem of the same age as the ohaloopyrite in this form. In general the sphalerite preceded the galena in time of .. formation aad was probably a product of the same early conditions as those under which the larger part of the chalcopyrittt was deposited. The galena became of impor- tance a little later, probably associated with the abundant development of bornite, and continued to form with the later ohalcopyrite in the fine fringes of small blebs about :ia do lo 132. the bornite grains. The galena in part is the arsenical variety known as steinmannite. In physical properties it is identical with the ordinary mineral, but mioroohemical tests on the polished surface yield distinctive results. In some speci- mens, ordinary galena occurs alone or apparently intergrown with the arsenical variety. Tetrahedrite is also fairly common with the ores. It is found chiefly in the bornite, as smooth grains, or as lines of elongated blebs, associated with the bands of ohal- oopyrite previously inentioned. In a few oases, it is fair- ly abundant and there is a little evidence that it is most common as a product of the closing portion of the bornite forming period. In one case, the tetrahedrite definitely cuts sphalerite, but its relations to the galena offer little evidence of their relative sequence. On the 1200 feet level, a silver-bearing seam is picked up by a long drift to the west, which extends well beyond the limits of the ore or the porphyry. The siliceous material of the seam occurs between walls of serpentinized diabase, and contains a little pyrite, spalerite, galena (and steinmannite ) . tetra- hedrite, and native silver. The tetrahedrite is the most abundant, and probably contains the chief silver values. Native silver occurs also in the upper workings of the mine in oxidized ground, but the silver on the 1300 feet level has apparently no connection with superficial changes, and . . ,i ai . * fti , . 1* r ~ - -jffejf , . :,j8ea x -i-iev . . 35X3 . . -JW S. , ,ia 1o i-i-'Ijaw . 193. is probably primary. (Cf . Marble Bay iline, Texada Island, native silver on 1300 foot level, about 1200 feet below sea Oxidation and Enrichment Water Level. The original water level of the ttine is stated by Ransoae to have been about 400 feet below the surface, presumably at the shaft. While the ores above this level were alaost completely of an oxidized character, the lower limits of oxidation are far deeper, and seen to have little relation to the present surface of the country or to the surface of the ground ffftttr. Oxidized Ores. Below the 650 feet level, it ia noticeable that oxidized products are encountered only in the eastern portions of the Jedge. On the 650 foot level, the rr.uin drift to the west intersects a rather lean pyritio ore-body; on the saute level, passing eastward, a rich cuprite, native copper and carbonate ore-body is encountered. On the 725 fPOt level, a lean sandy pyritio body ie found to the west while to the east, oxidized and secondary sulphide ores become of importance. On the 800 foot level the oxi- dized ores occur again, but farther to the east than on the level above. The deepest oxidised material thus far encountered in the nine is found in the face of a long drift to the eastward on the 1200 foot level, which ex- tends well under the daoite flows, that cover the Carbon- t v 194. iferoue limestone. The relation of the bottom of the oxida- tion to the geologio structure is shown in the sketch below. Fig. 17. Fketoh Showing the Bottom of Oxidation and Its Relation to the ftruoture. It may be mentioned here in passing that post- mine oxidation is intense in nearly all parts of the mine. The temperature and humidity are high, and in all except the most recent drifts* the wall rooks are decomposed and softened to a depth of several inches or more. The pre-aine character of the oxidation described in the preceding paragraph, how- ever, is certain in each case, both from its lack of depend- ence on the walls of the drifts, and from descriptions given me by Mr. Ett linger, who was familiar with the conditions when the drifts in question were first opened. In the oxidized zone, cuprite is the chief copper mineral, but small amounts of malachite and azurite are commonly associated with it. Native copper and, rarely, native silver occur in small amounts both with the oxidized . d^ :. - XJfl a*i S- - - ~ 9TC I ' . ooo*s:o.c.a .,, vi" 1 tfr . .mflaf. 196. PT' grains, about .01 mm. In diameter, (Figure 97 ). The etch-otruoture of the individual grains usually possesses one set of strong parallel cracks, or less commonly two sets of different intensity and persistanoe roughly at right angles which is characteristic; of orthorhombio ohaloocite. The size of grain of the ohaloooite indicated in this way is identical with the grain of the pure bornite front the deeper levels, revealed by etching and tarnish, and was undoubtedly inherited from it* Co.vsllite, in snail amounts, is even more wide- ly distributed than the chaloocite. It occurs ae a sparse scattering of laths in a one of the most massive glance of the upper levels, and thread-like veinlets of it, almost sub-microscopic in size, break the bornite in the deepest parts of the ore. 81\i,nt laths of oovellite are irregularly scattered or distributed in indefinite bands in the steely ohalcooite from the ore -body immediately below the 500 feet level. The covellite is slightly attacked by the chaloocite, but the amount of replacement it has suffered is probably unimportant. Where the relations with the earlier sulphides may be detected, it is clearly shown that the oovellite is almost entirely a replacement of bornite. Veinlets, which are merely fine threads in ohaloopyrite or tetrahedrite, widen into broad bands composed of rosette -like sheaves of small oovellite plates when they puss into bornite. In a specimen from the 725 foot level, veinlets of bornite, large- - l iJLJ. , C3J J. ^, _,.. terfW :-ii*J 10 e$ii 'd* all xlw** exe -^8 'tioil aacsio&qs 137. ly altered to oovsliite but still containing residues of the original mineral, cut pyrite, chalcopyrite and the gangue minerals. The sharp confinement of the covallite to the limits of the veinlets clearly indicates the greater resistance of the other minerals to the attack of secondary solution. Tne alteration of bornite to co.vellite is probably the result of the first weak effects of enriching surface solutions, and many of the covellite grains in the massive ohaioocite, although probably not all, may have originally developed in the bornite, which was subsequently replaced by the ohaloocite. A second generation of covellite, formed as the first stages of the oxidation of ohalcooite, was not observed in the Magma ores, although it ia a common feature in several other deposits studied. Small spines of ohalcopyrite occur sparingly in the bornite, associated with the faint blue oovellite vein- lets, aud are probably of secondary origin. They form a very small fraction of the total amount of ohaloopyrite in the ore, however. ; . '.! a ?.. si $ at.il i ;;sev to as-o3i..3 . to 1^ DIAGRAM OF MINERAL SEQUENCE Period of ir.ineraliza 1 1 on Period, of oxiclatio; Quartz Sericite Pyrite Chaloopyrite Borriite Sphalerite Galena Tetrahedrite Coveilite Chalcooitd Cuprite, etc. Karachi te Wulfenite Native Copper Native Silver Early It Late 1 9,' S-- 199. Discussion and Summary Origin of the Primary .Ores. The close association between the porphyry and the ore suggests a common origin for the two. The dike, however, is hardly large enough to have been the source of the emmanations, and the deviation of the metals from a larger and deeper body of igneous ma- terial, from whioh ths like itself may be regarded as an off -shoot is a more probable hypothesis. The diabase and dacite are clearly ruled out of the field of possible miner- alizing agents, and the porphyry is the only rook upon which an igneous theory of origin may rest. The common association between the ores of the Southwest and porphyries of post- Carboniferous and pro-Tertiary age adds weight to this ex- planation of the origin of the Magma ore-bodies. There is, however, in this deposit no direct proof from the character of the minerals whether the thermal solutions were direct magmatic emmanations, or merely heated meteoric waters whioh had obtained their metallic burden by the leaching of ore- bearing rocks below. The ores are now at least a thousand feet below the Tertiary erosional top of the Carboniferous limestone preserved under the daoite, and this may be regarded as the minimum depth at whioh they could have been formed. In the neighboring Ray district, there is a thousand feet or so of Cretaceous volcanic material overlying the Carboniferous, . . -ft . -*-. rf-eio Xg.sK ti^'j ^o . ^w oi . wol -jo iel ^istfon , 200. but it is not known whether it extended over the country as far west as Superior or whether it is older than the porphyry, The ores are undoubtedly later than the post-Carboniferous porphyry, and earlier than the Tertiary daoite, but a closer dating does not seem possible. Probably it is safe to assume that the ores now mined were formed at depths not greater than three or four thousand feet. The geologic relations and the character of the accompanying rock alteration indicate that the deposit should be classified as an ore-body formed at moderate depths by ascending thermal solutions. The pyrite, ohaloopyrite, exoept the spines associated with covellite, bornite, galena, sphalerite and tetrahedrite are all clearly of primary or hypogeae origin* There is a distinct sequence shown by their relations, which is indicated in the diagram, but it is believed that they were all formed from ascending solutions. The pyrite is clearly broken by veins of both ohalcopyrite and bornite (Fig. 95) -ai the chalooi-yrite is for the most part some- what earlier than the bornite. From the relations exposed at the time of his visit, Ransome expresses the opinion that the bornite is of secondary or supergerie origin, although he frankly states thut the evidence is inconclusive. The ore was exposed to a depth of 800 feet at that time, an 1 there seemed evidence that the richest bornite ores occurred in . J JeJUb (e- . :1W ,Tl t j joirlj ? 201. two ehoots each related to the two zones of contact of the dike and the displaced quartz! te stratum. This re- lation Raneome believed to be most easily explained c.s a result of descending surface solutions, which had derived their copper both from the ledge above and from lenses of copper-bearing sulphides near the base of the Devonian limestone, similar to those which occur at this horizon in the neighboring Lake Superior and Arizona Kine. The ding- ling of solutions descending along the dike with solutions migrating along the quart zite which is the chief water carrier of the sedimentary series is suggested by Ransome as the probable oause for the localization of the bornite at the contact of the porphyry and quartzite. The bornite is regarded by him as a replacement of ohaloopyrite, on evidence similar to that presented on the preceding pages, and it is recognized that the chaloocite is largely a re- placement of bornite. These relations are believed by Ransoae to indicate that the bornite is the first and deepest product of secondary enrichment (supergene altera- tion), and is an intermediate step in the change of ohalco- pyrite to chalcooite. Later developments indicate that tha bornite con- tinues without notable change in relative abundance to the deepest levels (1500 feet) and does not pass into lean pyrite-ohaloopyrite . ores with de, th as the hypothesis of a secondary origin would demand. The ore-shoots parallel . .- 203. to the quartaite horizons proved to be leas definite than the first exposures had indicated, and the distribution of the bornite in general shows littls relationship to the structure of the sediments. At Magma, as elsewhere, bornite is found to be more susceptible than the ohaloopyrite to ohalcocite en- richment which is shown by the marked manner in which the ohaloocite ie confined to the bornite where both minerals are present. In a few oases, thin margins of bornite were observed arouni ohaloopyrite residues in chalcooite, and it is possible that this small amount of bornite may represent an intermediate product in the enrichment of chaloopyrite to ohalcocite. But even here, it is not certainly the case, for the bornite rias themselves may be residues, perhaps pro- tected a little longer than usual by the oonoentration of iron derived from the ohaloopyrite, or possibly by some galvanic action. The association of bornite and galena in structures whioh can be interpreted only. aa contemporaneous intergrowths or as replacements of bornite by galena is strong evidence that the bornite is not derived from descending solutions, for secondary galena is unknown in copper ores. The same argument may be made in the case of the bornite and tetra- hedrite, based on similar relationships. In summary, the persisteaoe of the bornite with depth, its relation to galena and tetrahedrite, and the lack 203. of dependence on present or past land surfaces or on the structure of the sediments, all indicate with a high degree of certainty that the rich bornite ores of the Magma Mine are of primary (hypogene ) origin. The replacement Of chal- copyrite and gangue by the bornite is regarded as a primary process, similar to the replacement -of pyrite by ohaloopyrite. The Origin of the Secondary Ores. The ohaloocite may be regarded without question as a product of descending solutions. Microscopical evidence, supporting the field re- lations, indicates that it is chiefly a replacement of bor>r nite. No graphic structures between bornite and ohalcocite were observed in the ore. In some oases the attack of the the ohaloocita developed a pattern not unlike "ice-cake structure" 1 A described by C. F. Tolman, Jr. In one specimen, a tarnish on the polished surface revealed coarse bornite grains appar- ently set in a cement of finer grains. Where ohaloooite was developed, the finer grained matrix was attacked, but the larger grains apparently remained resistant. In advanced stages of ohalcocite replacement, they appeared as islands surrounded by broad streams of ohaloocite, in which floated numerous small residues of the finer grains. The advance of the ohaloocite is probably directed merely by physical causes, such as the greater surface offered by the finer 1. C. F. Tolman, Jr, Observations on certain types of chalcooite and their characteristic atoh patterns, T.A.I.M.E., Vol. 54; pp. 402-435, 1917. 204. material, and not by any variation in the nature of the bornite. The distribution of the oaaloooite agrees with the distribution of the oxidation; i.e. it seair.s related to a surface dipping to the east parallel to the bedding, as has been described. This may be attributed to two causes, both of which undoubtedly were of importance, (l) the influence of the different strata on the penetration of oxidizing solutions, and (2) the existence of a land surface of low relief in pre-daoite times. They will be considered in order* The limestones and quartzite are both pervious rocks, and rocks in which it is commonly observed that surface changes easily penetrate to considerable depth. (Tintic, Biabee, Bingham. ) The thick diabase sill which forms the wall rock of the dike below the sediments ia rendered dense and impervious by the abundant development of serpentine uralite, a., a offers a serious check to descend- ing waters. The diabase out-crops in a band a couple of hundred feet in width north of the ledge, and dips into the ridge at an angle of about 30* parallel to the overlying ae ii cents. It does not out-crop on the southern side of the ledge, but its position and relations are indicated clearly on the mine sections. (Figure* ISA and 15B.) . The greater depth of oxidation to the east may be attributed to the increasing depth of the sediments in that direction, and to the barrier S05. offered by the diabase rising to the west. It should be kept in mind, however, that the slope of the present surface rises to the east, and that the distance of the lower limits of oxidation below the surface increase more rapidly than indicated by the quoted depths below the collar of the shaft. From the physiographic side, there are two surfaces of importance in connection with the uecondary ores, viz., the pre-dacite surface and the present surface. Front the strikingly even line shown by tha contact between the dark lava and the white underlying limestone, it is clearly seen that thd daoite flows were poured out over a country of low relief, in this neighborhood at least. The underlying lime- stone beds had been little disturbed in pre-d&cite time, and conditions of slow erosion with accompanying deep oxida- tion undoubtedly prevailed. This period was brought to a close by the floods of lava. Superficial changes of the ore were thus checked, until the faulting and subsequent erosion which broke the monotonous plains of d&oite, again exposed them. The recent surface is still one of ateep and almost precipitous slopes. Mechanical erosion is probably the dominant process by which the escarpment is retreating at present, and its rapidity is unfavorable for the production of deep oxidation. Of the two surfaces, the older seems more competent to have been the one to which the r resent deep oxidation is related. This conclusion is further supj- ported by the rough parallelism between the bottom of the 931 al 306. oxidized zone, and the liircstone-iaoite contact. There is no reason to doubt that part of the oxidized and secondary ore was formed in relation to the pre- daoite surface, and on the whole, it is very probable that the greater part of the rich chalcocite bodies were foraed at thtkt time. While it is true that enrichment can keep ahead of a rapidly retreating surface, as at Binghan canon, it is extremely doubtful if the massive and deep ohaloooite in these large ore-bodies, which represent a very complete replacement of the primary sulphides, could have been formed so rapidly. From the structure of the ridge and the high country to the east, it is probable that tl.e sill of diabase would tend to act as a dam, retaining water above its level in the mountain rather than allowing it to migrate down its upper surface. As there is abundant water in the mine at present the sediments were, without doubt, well saturated below a depth of 500 feat before the mine was opened. Any flow of the ground water under the present structural condi- tions would tend to take place from the higher country east of the mine toward the valley, which would force the water up the beds and through the diabase at the lowest outlets. The present climate is extrerr.ely arid, and^er'e' is -no reason to assume a depressed water level due to less humid conditions in the past. The height of the mine above the valley wash eliminates the possibility that the present higher level of the ground water is caused by the accumulation of detritus. io 307. From the various linee of evidence a depth of about 500 feet below the shaft u.ay be set as the lower limit to which in- tunaive oxidation can extend from the present surface, and it is believed that the deep oxidized ores and the ohalood te are cost probably products of enrichment related to the pre- daoite surface, which ie now tilted to the east at an angle of about 30 . Studios by other members of the staff of the Secondary Enrichment Investigation afford impressive evidence that the main period of oxidation and enrichment in the neighboring Globe and Miami region was earlier than the daoite and the subsequent tilting, which greatly strengthens the similar interpretation in the case of the Magma deposit* Conclusion. The results of this study indicate that the rich bornite and chaloopyrite ores of the Magma Mine are of hydrothermal origin, and formed slightly later i than the poat -Carboniferous porphyries but earlier than the Tertiary lavas. The bornite is a replacement of py- rite and chalcopyrite in part, but it is not a product of descending solutions, except to an almost negligible extent. The rich chaleocite ore -bodies were formed largely by the replacement of the primary bornite, and are a product of descending surface solutions ohiefly in pre-dacite time. OCCU. JHI) OP fiQKfilZB Donald Hamilton McLetighlin ?OL. II KSNNSCOTT, ALASKA. INTRODUCTION. The ores of the Kenneoott district are pre-iom- V inately ehalcocite. In then, bornite attains only a low A ran* on a quantitative basis, but it assumes a position of especial interest due to the critical evidence it presents on certain questions of genesis. The secondary or primary nature of the chalcocite ore-bodies offers an important problem, and its solution depends to a large extent upon the meaning of the bornite residues in the ore and upon the inter- pretation 3f various structures of the chalcocite and bornite. In addition to the great chalcocite bodies, which are confined to a limestone formation, bornite ores occur in many places in the district in a thicfc pile of altered basic lavas Known as the Nilcolai greenstone. They are similar in many respects to the general type of ores associated with basic lavas in many parts of the world, and afford an in- structive contrast to the chalcooite ores. Both classes of deposits have passed through a similar history in geologically recent times, which provides a common ground upon which their different reactions to the processes of surflcial alteration may be compared. Situation. Kennecott is situated at about 61 30' north latitude and 14-3 west longitude on the lower spurs of the southern slopes of the great Wrangell Range in southwestern Alaska. The camp is at an altitude of :ibout 2000 ft. on the VI i 01 q* 209 eastern side of the broad Kennecott glacier, which descends from the ice-fields of Mt. Blackburn, ( 161M-0 ft. ) only 23 miles to the northwest. The Kenneeott River, which springs from the end of the glacier five miles south of the camp, drains into the Nizina River which soon joins the Chitina, a broad stream flowing westward sixty miles or so through a wide structural depression to the Copper River, whose deep glaciated v-illey penetrating the Chugach Range affords an outlet to the Pacific for the waters of the entire region. The Copper River and Northwestern railroad extends from Cor- dova on Prince William Sound to Kennecott, a distance of 196 miles by way of the carfona of the Copper River and the accessible Chitina Valloy, -\iA makes the carap at all seasons of the year. A M ines. The Bonanza and the Jumbo yines, v/hich are worked by the Kennecott Copper Corporation, are in all respects the most important properties in the region. They are situated high on Bonanza Ridge, a spur extending to the south from the lofty Wrangell Range. Bonanza Peak, the highest part of the ridge, rises to an altitude of 6950 ft., which is about 1500 ft. above the Kenneoott glacier to the v/est, and the narrow valley of McCarthy Creek to the east. The mines are less than 600 ft. from the crest, and in an extremely rugged topography due to the intersecting cirques of small mountain glaciers. The mill -and other works are situated at the ter- 9tti 210 minus of the railroad at Kennecott at an elevation of ft. The ore is transported to the mill by aerinl tram-ways, which are also used for carrying supplies and members of the staff up to the mines. The Erie Mine, which lies about four Mies north of Kennecott and about 1000 ft. above the glacier, hae been developed to a slight extent by the oompiny, but as yet littlo ore has been shipped. On the eastern side of Bonanza Ridge, and about a half mile to the east of the Bonanza Mine, is the Mother Lode i.'ine, the property of a smaller company. A fair amount of development work had been done in 1915, an aerial tram- way built to McCarthy Creefc, and some ore had already been shipped. All of the deposits at the mines mentioned thus far are in the Ohitistone limestone, and the ore is to a very large extent, chalooclte. There are, however, in the viein- ity cf Kennott numerous small deposits of copper minerals in r the Nikolai greenstone, but little work has been done on any of them. To the east, on NiKolai Creek, a deposit of this sort has been slightly developed, with results somewhat promising. Many miles to the west, in the Kuskulan-a and Kotslna Districts, several similar deposits in the greenstone hvo bo?n -vorked in a small way, nd from a mine on Nugget Creak, the property of the Alaska Development Co. , a little ore hae beer, shipped. Our worx at Kennecott did not allow time to visit these distant deposits, but spsclnon* and des- criptions have been Kindly furnished by Mr. Alfred ffandtke, - 211 Literature. The region has been mpped and described in a very thorough and useful manner by the United States Geological Survey. Their most recent and valuable publica- tion for locnl information is Bulletin '44ff, The Geology and Mineral Resources of the Nizina District, Alaska, by F. H. id of fit an^ s. R. capps. (1911). The accompanying topographic and geologic maps were found to be of great benefit for all questions v/ithin the limits of accuracy of their scale, (1/62500). When the Survey work was done, however, the nines were only slightly developed, and consequently the published descriptions of the ore-deposits were of little value to us. A list of earlier papers which have contributed to the Knowledge of the region is given in Bulletin WS and is 1 appended below. A private geologic report by Professor J.D. Irving gave valuable Information and data on the large ore 1. Allen, Lieut, H. T., Report of an expedition to the Copper, Tanana, an-i Koyukuk rivers, in the territory of Alaska, in the year 1SS5, Washington oov. Printing Office, 1#S7. Hi.yes, C. Willard,. An expedition through the Yukon district: Nat. Oeog. Mag., Vol. '-, 1*92, pp. 119-162. hohn, Oscar, A reconnaissance of the Chitina River and Skolai Mountains: Twenty-first Ann. Report., U.S.O.S., pt.2, 1900, pp. 393-^0- Sohrader, F.C., and Spencer, A.C., The geology -*nd mineral resources of a portion of the Copper River district, Alaska: Special publication of the U.S.O.S., 1901. Mendenhall, W. C. , and Schrader, F. C., The nineral resources of the lit. Wrangell district, Alaska; Prof. Paper, U.S.O.S., No. 15, 1903. Mentenhall, W. C., Geology of the central Copper River region, Alaska, Prof. Paper, U.S. G-.S. , No. 4-1, 1905. Moffitt, P.H. and Maldren, A.G., The mineral resources of the Kotsina and Chitina valleys, Copper River region, Bull., U.S.O.S., No. ?iJ-5, 190^, pp. 127-175. Keller, H. A., The Copper River district, Alaska, Eng. ?rv? Win. Jour., Vol. *5, No. 26, June 190^, pp. 1273-127^. Moffit, F. H. , and Maddern, A.C., The Kotsina-Chitlna region, Alaska: Bull., U.S. a. S., No. 37*f, 1909. al is bodies mined during the first few years of the life of the Bonanza Mine. The "isometric cleavage" of the chalcocite from the Bonanza Mine was first described by Oraton and Murdoch, and its similarity to the etructiire of synthetic chalcocite, formed at a temperature of 1100, was regarded as an indication of the ^rimary origin of the mineral. The presence of bornite residues in the chalcocite and the possibility that it was derived from the bornite by replacement was, however, inen- 2 tloned later by L. C. Graton. Analyses and specific gravity determinations of chalcocite from the Bonanza Mine made in the Geophysical Laboratory at Washington have been published 3 by Posnjafc, Allen and Merlin, an-? from their results, it was conclude^ that the mineral was formed at a temperature above 91. Prom the results of a microscopical study of a suite of specimens from the Bonanza Mine, 0. ?. Tolrnan, Jr., concluded that the "isometric structure" of the Bonanza ehal- eocite vras inherited from bornite, and that the greater part of the chalcocite of the deposit was a replacement of bornite. A statement of the paragenesls of the ore-minerals is made, but, as the results are based largely on laboratory studies without field vorfc, several of his conclusions may be criticised. 1. The Sulphide Ores of Copper, T.A.I.k'.E., Vol. M-5, p. 76*, ( 1913). 2. Discussion, T.A.I.U.E., Vol. M-ff, 1915- P. W- 3. The Sulphides of Copper, ( Secondary Enrichment In- vestigation, Contribution No. 3), 2con. Ueol., Vol. 10, Jp.M-91- 535. M-. Observations on Certain Types of Chalcocite and their Characteristic Etch Patterns, T.A.I. U.S., Vol. 54- , 1916; pp. 4-02-1+41. a\ii .- 213 Excellent photographs of polished surfaces of the Bonanza ores, accompany the article, and clearly illustrate certain structures and relations of the ore-minerals. Ac-gnowledgeE'ients. Our work was greatly assisted by generous information and many courtesies from Mr. H. W. Sea- grave, the manager of the properties at Kennecott, Mr. H. D. Smith, superintendent of the Bonanza and Jumbo Mines, and Mr. W. E. Dunkle, examining engineer and geologist for the company. Excellent geologic maps of the mines, prepared by Messrs. Smith and Bunkle were of great value in facilitating on observations. Many of the ideas concerning the field relations of the ores that are presented on the following pages I owe to L. 0. Graton anl to Dr. A. M. Batenan, but the intricate way Mi in which they are woven into the entire fabric prevents spe- cif io recognition in the text, and allows me i.ieroly to ex- press at this place ny appreciation of their advice and val- uable criticism. GENERAL GEOLOGY, Nikolai greenstone. The base of the visible geologic column near Kerne cott and in t'ho entire district, is the Nikolai greenstone. The formation, as exposed on the slopes of Bonanza Ridge, consists of a thickness of a little over 3000 ft. of altered basaltic flows. The base is not recog- nized, SB the lower boundary is due to faulting for the most , - I part, and consequently its local thickness nay be consider- ably greater. The formation Is wide spread, extending in an irregulR.r belt beyond the Nigina River to the east, and with interruptions; into the basin of the Kotsina River to the west. From several good exposures, Moffit and Cappe consider it highly probable that the formation has a maximum thickness of over 4-000 ft. :r. viTvved from a peak near the nines, the formation is seen to be distinctly stratified in rather thick beds probably representing individual flows. ( Figures 19 21 ). Close measurements were not possible, but no flows were recog- nized thinner than 30 ft. A change from a dense, fine grained rock to a reddish ainygdaloidal type was observed at many horizons, and is probably the expression of the variation from the bottom to the top of an individual flow. The dense rock is a fairly i-esh cray lava with a fine-grained diabasio texture; the amygdaloid \l rocks are commonly much more altered, ~nd ire of a dull green or red color. All gradations exist oetreen the two extremes. Cliffs are frequent -along the horizons of the denser types, while the reddish rooks seem to \veather preferably to benches., ?ron a microscopic study of many specimens, the freeh rock is known to consist of laths of plagioclase, (basic ande- eine for the most part), v;ith grains of colorless augite, is accessory, Chirac teriRticilly in the ophitic texture, i'agnetite^ and a small amount of chalcopyrite an*! rarely other copper min- erals in found. The rubordlnate ground-mass way have been glassy, but none was seen in in unaltered state. The alter- . it ion of the lavas is widespread, and even in the freshest speeliaensjthere is aslight development of secondary materials. The ground mass has in all cases ^one to radial aggregates of pennine aiil deleeaite. In most specimens, the pyroxenes are partially or wholly altered. Serpentine Is also present, but its separation from chlorite In many oases is very dif- ficult. The feldspar is uore resistant, but in the most In- tensely altered types, it passes into an aggregate of chlorite and oilclte, with more or lees kaolin. Zeolites are uncommon near Kennecott, but a small amount of eplstllblte was observed in one section, probably as a replacenent of feldspar. In material from Elliott Creek in the Kotslna District, heulandlte occurs In a olnilar eo- lation, *na datollte is abundant in snail albite-bearlng veins. 1 Soffit reports thoiasonite in a specimen from Chltitu Oreek 2 and A. Knopf describes zeolites in the lavas of the White River country. The araygdule fillings are largely calclte, with some quartz, and ^hlorites. Bornlte, chalcopyrlte, and pjrrlte occur In the aiaygdules In various nlnorallzed zones, and in a few oases native copper was observed. The relations of these minerals will be dlBoussed more fully under the descriptions of the mineralization In the greenstone. In a few places, the weathering out oi the amygdules has resulted in 1. Bull. U4g, U.S.O.S., p. 61. Ph .ivgdaloids of the White Plvor teflon, Alnsxi, Econ. Oeol., Vol. 5, PP. 2^7-256. Jae; at si .'P i , 216 a vesicular root with a striding resemblance to a recent lava. The greenstone under normal conditions of erosion would tend to yield rather smooth forms, but under the severe attach of the trioutary glaciers near Kem.ecott, the upper slopes of the ridges are precipitous, although the cliffs are uHually not as bold as the overlying massive beds of limestone. The roofc gives rise to a rich, brownish-red soil, which sup- ports a luxuriant growth of grasses and shrubs, and spruce on the lower levels. The age of the greenstone i:> definitely limited in one direction by the overlying Triassio limestone, but the lower limit ia uncertain. On account or the similarity between the NiXolai greenstone, and post-Carboniferous volcanios in 1 the Skolai Pass country to the northeast, Moffit and Capps consider it probable that the formation is of early Triassio age. Chit is tone^L lines tone. The Nikolai greenstone is overlain by a sedimentary formation fcnown as the Ohitistone limestone. In a broad way the contact appears conformable, the v/ell defined bods of the limestone being parallel to the somewhat loss distinct flows of the greenstone. ( Figs.19,3 1 *- ). However, v/here mining operations have exposed the relations ma- e clearly, and in a few other cases, there is evidence of movement along or not far above the contact. The movement was probably a thrust, but in general it is not of sufficient importance to masfc the essentially conformable character of 1. Loo. cit., p. 63. fi 217 the two format ions. The surface of the old volcanic rocks on which the ChitlBtone formation was Inld down appears to have been practically flat, perhaps with slight Irregularities here and there, but with no notable relief. The first deposit consisted of about three feet of mud, now consolidated to purplish red or dull greenish shales. Near Kenneeott the shale is a persistent bed, and encountered along the contact for many miles. It tends to yield a bench, by its weathering, which forms the easiest line of travel in several regions of difficult cliffs. ( See Pig.3^o ). The shale Is succeeded by about thirty feet of an impure silicioup limestone, made up to a notable extent of flat- tened cylindrical bodies suggesting fossils. Numerous frag- no nts of crlnoid sterna were found in this horizon. Above these bods occur '1-0-50 feet of grey limestone. The rock has q dull appearance on the froshly fractured surface due to its very fine grain, and it IB oorriiaonly known to the miners is ths "dead limestone". The chief part of the formation, which overlies this type of rock, is a crystalline dolomltic limestone. It Is separated from the gray limestone by a sharp boundary parallel to the bedding for the most part but riot infrequently brewing across ii; irrsgular v/ays. Numerous inilyaes made by Mr. Dunkle havs shown the change in crystal- lino character of the limestone to be closely associated with the change in magnesia content. The f ine-gn iao-1 limestone , is fairly pure, yielding 9.30 / Ca CO^ with 3-9 Kg CO, , while the dolociitic rock averages about66.7^Ca 003 and 30. 5 The ChitlBtonellrr.estone is estimated to have a thick- ness of about 3000 ft. It passes without any noticeable break into shales, so it la difficult to place the upper lii;iit closely. The upper beds of limestone become more and more inpure, shale partings become more frequent, until finally the true line character of the foraation i lost, and the rock becomes a distinct shale. The McCarthy Shale. This upper formation, which may have a thickness in the neighborhood of 3000 ft. , has been termed the McCarthy shale. The shale is entirely re- moved by erosion from Bonanza Ridge, but it occurs to the east of fe McCarthy Creek, where it is exposed to good advantage in an imposing 2000 ft. cliff. Fossil evidence has shown the limestones anri shale to be of Upper Triaasio age. Post-TriaBsic Deformation. After the deposition of the ./earthy shale, a period of orogeuio deformation occurred. The rocks near the mines were folded into a broad anticline -ith a NW-SJE axis, and eroded to a surface of gentle relief. Tne aro.sion was sufficient to expose the underlying green- stone in many places, for the overlying sediments of the Jurassic rest on all three of the foregoing rock divisions. Kenneoott Formation. The McCarthy shale i suc- ceeded unconfonnably by the Kenneoott foraation, a series of black chalss, .^ray shales, cherts, sandstones and conglomerates., 219 which from its best exposures has been estimated to be at least 7000 ft. thick. It is known to be of Jurassic age, and rests on the even erosion surface cut across the greenstone, and the Triassic sediments. Hear Kennecott it occurs as a thin veneer of shales and cherts, resting on the greenstone, and preserved only in a few down-faulted patches. The section of Bonanza Ridge shown in Fig. IS illustrates the probable relations. In the canon of national Creek, running up from the Kennecott Mill, a thickness of several hundred feet of cherts and shales is Southwest Northeast Horizontal anJ verticali sca/es : linc.li = / mile J?lg.l#. Section through the Bonanza Eidge near the Bonanza Mine. exposed. The chert occurs in distinct beds -3 inches thick, separated by partings of shale, a fraction of an inch thick. This remarkable alternation of cherts and shales is almost identical with that found in the Franciscan series in Cal- 1 ifornia (Jurassic, radiolarian cherts) , and again in the Monterey series, (Miocene, diatomaceous cherts) (J?ig. 25 ). The significance of the rhythmical change has not been de- duced as yet. The gray shales are exposed along the lower course of Bonanza Creek, but the black shales of the formaticn 1. Hinde, S. J. t Hote on the radiolarian chert from Angel Island and from Buriburi Ridge: Calif. Univ. Dept. Geol. Bull., Vol. 1, Ho. 7, pp. 235-E40; 1894. Also A. C. Lawson, The SanPrancisco Polio, U. S. G. S. f (1915.) . ; 220 were observed in this neighborhood only as inclusions in the porphyry of Porphyry Mt. Conglomerate boulders, prob- ably of the Kenneeott formation, were observed in the bed of National Creek, but the rock in place was not found. With the exception of a small pooket of sandstone probably of Eocene age ?rest of the glacier, and the Quaternary gravels of the valleyn, the Kennecott formation completes the aedi- mentary column near the nines. PorphyrloB. The Jurassic rooks are cut by abundant intrusions of porphyritio rocks, chiefly white quartz por- phyry. The largest body is the maes which forms Porphyry yt. , ( Pig. 22 ) in 1 ' 5 to the southeast a similar body is exposed on the slopes of Sourdough Kt. The quartz porphyry la cut by several more basic dikea, but they are not abundant, and are of very subordinate volume. Under thf; microscope, the porphyry is seen to contain phenocrysts of quartz and plagioolase* The latter is com- monly acid andesine, but it is as acid as ollgoclase in some cases. Orthoolase phenocryats are rare. Biotlte aocurs in iall amounts, as short laths, usually greatly altered to Muscovite; hornblende la the only other ferromagnesian mineral, and it is very subordinate and usually oxidized to linonlta at the surface. The ground mass is micro-granitic, and is composed commonly of an interlocking raino of small quartz and orthoclase grains. Nenr the edges the rock has a glassy 221 ground raise, but even in the heart or the area, tho grain is axtreuely fine. The rook is considered a quartz dlorite porphyry by iiofrit and Capps, from a wide study of its oc- currences. Tho bodies forming Porphyry rand Sourdough itts. are referred to by iiofflt as laccoliths, but in no place was I le to se -, any evidence that they had made way for them- selves by thrusting aside the older rocks. 3oth are apparently intrusions in the Kennecott shales. The beds along the edges or th porphyry exposed in the oanon of National Creek are intimately intruded by sills, but are not noticeably disturbed. The rock or Porphyry Mt. contains numerous inclusions of black shale, many of very irregular shapes, which stand out in striking contrast to the white porphyry host. (?igs 24,26) The horizon of the black shale in this vicinity is not known, inclusions but a V 0!- iblc interpretation is that the A are blocks which have settled froir. above in the course or uagnatlc stoping. The contacts or the porphyry and the shale zenoliths are sharp, i&d there is no evi of assimilation. The Irregular ron.iK or some or the senoliths must be attributed to mechanical causes rather than chemical. AS the porphyry probably made its "'ay upward into the Jurassic terrane.by replacing the beds, arid as there is no evidence or a floor, it is prefer- able to refer to these bodies as stocks rather than lacoo- 1 lithe. 1. S'be section AA' of the nap accompanying Bulletin r'o_* the ro?i;i 01' the Sourdough i;t. "laccolith." 222. The fine grain of the roci is surprising in such large bodies. Porphyry Mt. stock is about five miles in the north-south diameter OCf 'BO 229 represents a land area of very slight relief. Toward the western end of the Wrangell group, however, Mendenhall de- scribes irregularities as great as 3,000 feet vertically in 1 2 this pre-voleanio surface, fcut for the moat part, deserip- 3 tions and photographs from neighboring districts show it to be similar to the conditions observed north of Kenneoott. The pre-volcanic surface most probably extended over the Zen- nee ott region, about a thousand feet at least above the high- est peaks east of the mines, if its present low dip may be projected. The lavas extend south on Bonanza Hidge to a point within six miles of the mines, but it is not known whether they ever continued much farther south. Areas of flat or gently rolling country at altitudes between 5,500 and 6,500 feet were observed on ridges west of the Kenneoott gla- cier (Fig. 29 ) ? south of the Chitina Valley, east of the Hiaina River and elsewhere, and they may represent remnants of this pre-volcanic early-tertiary surface, which was not buried by the lavas. It is possible, however, that these surfaces may 1 - Loc. Git., pp 56 - 57. - Shrader, if. C. and Spencer, A.C. f The Gool. and Min. Resources of a portion of the Copper Rivor dist, Alaska, U.S.G.S., Special pub.,pp 51-52, 1901. 3 - Capps, S.R., Bull. 630, U.S.G.S., Che Chiaana -\vhite River Dist, Alaska, Plate IV, p. 18, Pig.4, p. 38. . have been developed later as an early stage in the pre- glacial stream erosion, of the region. Pre-Glaoial Surface* - The main drainage channels such as the Chitina River, are along prominent structural lines, as pointed out by Moffit and Capps, and are believed by them to have been established at a fairly early date* The earliest dissection of the 7/rangell Range was probably by stream ero- sion, and a rugged topography is believed to have been in ex- istence before the intense glaciation of the Pleistocene* The remnants of flat surfaces mentioned in the preceding paragraph may be correlated with evidence from neighboring parts of t he 2 range, and their altitude and distribution make it very prob- able that ihey were developed in the early stages of the ero- sion of the region. For the most part they are too low to agree with the pre-volcanio surface. Rejuvenation of the streams, due to a broad uplift of tho region, may be con- sidered the cause of the dissection of these surfaces, which are now preserved only on the lower ridges that flank the main range. That mountain-making movements continued until very re- cent times in the Wrangell Bange is shown by the disturbance of 1 - Bull. 448, pp. 74-75. 2 - Capps, S.R., Bull. 630, Plate XVII, opp. p. 76. - . TifC r I Sejieiltfeteo aeetf 0*&. glurtA ^4qf-e" a- Jf>* . '. j .- 231 the lava beds and even of early glacial deposits in the Chisana - White River District, described by Capps. Con- sequently uplift in the Kennecott region at a fairly late time is not out of accord with the evidence elsewhere. It is difficult to estimate the degree to which the country was dissected by stream erosion before the advance of the glaciers. The Chitina Valley was probably similar in its broad features to the present topography, and Moffit and Cappa believe that the tributary stream drainage was well developed. We could find no evidence opposed to this view. It seems Tory probable that in the long period in the late Tertiary, during which the volcanics and their elevated platform were exposed to erosion, a topography at least approaching maturity had been developed. glaoirition and the Present Surface. - The present land forms have been profoundly modified by the intense glaciation of the period which is probably now drawing to a close. In the high altitudes glaoiation bus undoubtedly been long con- tinued, but as outlined above, stream erosion is considered the chief agent rtiich h^d shaped the earlier drainage lines. Upon these channels the great glaciers of the Pleistocene have im- pressed their characteristic features, eroding broad U-shaped valleys in place of the narrower canons, truncating projecting spurs and straightening the valley walls, and over-riding and 7 3: . t"X^.'o. smoothing the lower ridges. Irom the difference in elevation between the mouths of hanging tributary valleys and the main glacial channels, Moffit and Capps estimate the amount of deepen ing by glacial scour to have been between 1,000 and 1,500 feet. \ So evidence of more than one period of glaciation has been 2 observed in the Kennecott district. S. R Capps, however, has recently described indurated and deformed glacial depobits near the source of the VJhito Kiver. They are associated with lava flows, and are overlain unconformably by recent glacial deposits. The evidence is interpreted, however, as the record of an early Pleistocene glacial advance, separated by a long time interval from the latest glaoiation,- but nevertheless part of the same large period. from this information it seems fair- ly probable that there hus been similar early glaciation through out the Wrangell mountains, but in moat places, as near Konne- cott, the intense erosion of the recent glaciers has obliterated all earlier records. The glacial period in thia region has not yet passed. The broad valley to the west of the mines is still occupied by the great Kennecott Glacier, (i?igs. 28 and 29 ) which descends from the ice-fields of Mt. Blackburn (16,140 ft.) to an alti- 1 - Bull. 440, p. 44. 2 - Two glacial stages in Alaska: Jour. "eol., Vol. ii3, pp. 748-756, 1915, and Bull. 630, U. S. (J. S ., pp. 63-67, 1916. 3r c tot jciaaari ~ nfc .^roK , Rl&ai". hi ' Vj&ii J.Jt9flj[& XX9&0?0 JTJK io*Ia f Ko^al (ixl^ re- MB IXer^u > f-a/ifo-.T-.i or A!< 233 tude of only 1,400 ft. Bonanza Peak still supports five abort mountain glaciers, and even now one side of the outcrop of the Bonanza ore-body is attacked by a small glacier, which en- riched its moraine with dhaloocite until mining operations interrupted it. lilsewhere along tho ridge, especially on Porphyry Mt., the cirques are occupied by "rock glaciers", (Fig. 22 ) which may be regarded as the dying efforts of true glacial action. .in attempt is made by Moffit and Capps to outline, on their geologic map, the land areas which projected above the glaciers at the time of theiriaaxinun development. The line follows the topographic breax between the smoothed and rounded contours of the lower slopes and the rough and angular forme of the summit regions. In general it may be drawn with reasonable certainty, but near the mines above Xennecott, the sharp arretes and difficult peaks are clearly due to the intersecting cirques of the present glaciers. .01 earlier topography, smoothed by over-riding masses of ice, could easily have been destroyed by these later changes; consequently the character of the present summit topography cannot be regarded as conclusive evidence that tho entire ridge had not been ovor- ridden by the ice. There is a slight amount of post-glacial erosion along the 1 Cappa, . .., Rook glaciers in Alaska: Jour. (Jeol ..Vol. 18, pp. 359-375J Also Bull. 448, pp. 52-59. ;n .; WO Dl 333A. VII Kennecott. llq.aka. Fig. 19. The greenstone - limestone contact along the oliffa north of the Bonanza Mins. Fig. 20. Fault breaking the greenstone-limestone contact southeast of the Bonanza Mine. 333B. PLATE VIII Ker.neoott. AlaaV . Fig. 21. The spur along which the veins cf the Bonanza Mine out-crop. Note mine workings on the cliff faces. Fig. 22. View to the south from the surcmit of Bonanza Peak. Porphyry Mountain in the centre, the Nizina Riul Chitina Valllas in the back ground an3 the Chugach Mountains on the horizon; McCarthy Creek caflon to the left; a little of the moraine of the Kennecott glacier on the extreir.e ri sht. . "^' ' * ' * - J ai 333C, PLATE IX Kennecott. Alaska. Fig. 27. The Chitistone limestone in Independence Basin, southeast of Bonanza Mine. The spur on which the Bonanza Mine is situated is shown in the background on the extreme left. Fig. "'-. An inclusion of black shale of the Kennecott formation in porphyry on northern aide of Porphry Mountain. Fig. 25. Chert and shale beda of the Kenneoott formation ;ng National Creek above Kennecott. 233D. PLAT? X Kenneoott, Alaska... Fig. 26, An inclusion of ahale oi the Kenneoott formation in porphyry on the north side of Porphyry Moun- tain. Fiz. ~7. Inclusion of shale in porphyry, cut by a dike of a more bassic type. 233E. PLATE XI Kenneoott, Alaska* Fi. 2f. The Kennecott glacier an -I Mt . Blackburn from the summit of Porphyry Mountain. Note the p;reenatOTie- limestone contact on both aides of the glacier , The fold and reverse-fault above the contact ar^ shown on the cliff between the main glacier and the tributary on the right. Fig. 29. The Kennecott glacier seen frorr. the summit of Bonanza Peak. Note the greenstone-limestone contact along the cllffa on both aides of the glacier, r n' the flat surface on the moun- tains on the west side. The peaks to the right are of volcanic material. The tribu- tary glacier from Mt. Hegal joins the Kenne- cott glacier in the foreground. 233F. PLATE XII Fig. ?0. View of the Wrangell Range looking noi'th from the 9twirr.it of Bonanza Peak. Mt. Regal and the great ice-fall of the southern tributary of the Kennecott Glacier are on the right of center . Note the horizontal be is of voloanio rocka which form the higher peaks. View of the valley of McCarthy Creek from the summit of Bonanza Peak, looking to the north- east. Chltiatone lirreatone beia form the cliffs in the foreground, wi '.h softer beds of McCarthy shale above, *nd the sediments (chiefly shales) of the Kennecott formation on the smoother dis- tant slopea. The volcanic bedsi of the higher peaks are prominsnt. This view is pBnoramic with Fig. 70. 233G. PLATE XIII Kenneoott, Alaaka, Fig. 3". The Jumbo Mine ani Glacier, sn3 the northwestern cliffs of Bonanza Pet- . Fig. "5^. The Jumbo Mine ->n ; dlscier, beneath the southern cliff of Jurcbo Caatle. Fig. Ik. The limestone-greenstone contact along the cliffs north of the Jumbo Mine. The outcrop of the Jurrbc Vein is njarke.1 by the airall adit on the nearest spur on the right side of the picture, Just above the oontaot. The Tram House of the Jxurbo Mine is shown in the lower ria;ht fore- ground. The Kannecott Glacier ir>ay be seen to the left. The view ia panoramic with Fi. 33. 33H. PUTS XIII A Korneoott Alaska... FIT. 3i(.a . Berp;sohrund of the Jumbo Glaoier, Northwest aide of Bonanza Peak. . 3 J * . The contact of the Chltistone limestone and Nikolai greenstone (beneath) a-c a prospect near the Erie Mine. The thin shale bed at the bottom of the limestone is well shown. Fitt. "^-'1.0. The bench alone; the liraestone-^rsenstons con- tact nep.r ths f^rie about 1000 ft. a- bove the Kennecott Glacier. lower courses of the streams from Bonanza Ridge, Tout as the glacial period is still in existence near the mines, water is an unimportant agent. MINERALIZATION IN THE OREENSTONE. The mineralization in the greenstone is a formation vide feature. Traces of Copper are coupon in nearly all parta of the region where the ronxo are exposed, but few prospects give ouch promise of eventually supporting mine'-. ?oria of Deposits. The mineral deposit:? in the greonston'B oo<->ur in tho following I'oras (1) veins brea&ing morose the bedding ox' the flows, (2) disseminated deposits of replacement origin, and ( 3 ) anygdule fillings. The first group is the most important from a commercial standpoint. The veins in the Kennecott district ire rather short, and few were observed of sufficient perils tones to carry them through tho thicJcnoss of more than two or three of the flows. For the niopt part thoy . are small, and are to be measured in inches or fractions of an inch, but in a few exceptions they attain a thicfcnosa of several feet, with their other diuensione correspondingly greater. The disseminated ninerilizatlon is either related to definite veins, or to shear zones along which circulation 01 solutions could tajce place. No ores of this sort are Known large to bs of value although ar-;iK o:' the .-rr-jsnstons contain suail ^ A 235 iiuountn of copper. The distribution of the aniygdaloldal deposits is of course limited to horizons of once vesicular lava, and llite the disseminated mineralization, there is a dependence on neighboring veins or sheared zones. Near Konueoott, deposits of this sort are not common, although native copper and bornite v/ere observed in amygdulss in a few cases. Gangue Minerals. Calcite and quartz are the com- monest r>;angue minerals in the veins, although in some cases epidote becomes of equal importance. The veins are of re- vlicenent origin for the most part, as ia clearly shown by the relations of their minerals to the wall work, but in seven! localities lar.^e quartz crystals lining cavities v-ore observed, and there could be little doubt that open spaces had existed, and probably played an important role. Snail veinlots of albite, usually associated 'vith epldote, ire found fairly coononly, and in a few cases, datolite ^as observed associated with the other ganguo ninerals. The araygdule fillings are very probably of the eaLie origin as the vein fillings. Pormiivs, delessite,( and possibly other chlorltes), serpentine, and calcite are the usual minerals. ?ine green borders of fibrous serpentine were apparently the first to fora; a broader layer or plaooroio "hlor: uine probably) corcaonly succeeds the serpentine, passing toward the center, and the interior of the cavities in many cases i: formed -oader flafces of delessite. SJJ Calcite is the iaet to form, and where it occurs, it is nearly always in the center. Quartz and epidote are lese common In the araygdulea. A mineral resembling prehnite in most or its properties, but not positively identified, oooure with native copper in a few oases, but the araygdules composed of zeolites, and native copper, which are plentiful in the 1 White River district in similar rocks, were not observed in the Kenneoott region. ROCK. Alteration. The rook alteration accompanying tho miner a 112 at ion ia similar to the general propyilitization of the greenstone, but more intensive. Chlorite is more abundantly developed near the veins, and little of the original Character of the work, generally remains. Heulaniito and a few other zeolites less positively identified ( probably harmotone and eplstilbite in two oases) replace the feldspar near certain veins. Caloite is wide-spread as the final product, replacing nearly all pro-existing minerals. The order of Liinorai succession can not be stated in detail, but for the veins and the replacements of the wall rock, albite, epidote, quartz find datolite may be regarded as representatives of the earlier phases of the mineraliza- tion. Similarly the zeolites and calcite belong to the closing phases. .>,. Pl r ' Primary Ore-Miner a Is. Bornlte and chaloo**te are the most important of the ore-minerals. In some cases, pyrlte 1. Knopf, A., Loc. it. ir.- common, but not in the vicinity of Konnaoott. The bornite ind chalcopyrlte are clearly primary. Their contacta show the usual mutual outlines which are believed to be charac- teristic of primary relationships. The opinion has been ex- 1 pressed, that in . certain cases the bornite is secondary, and should be expectej. to decrease with respect to the chalcopyrite with depth. none of the depoeits aro more than prospects thore is little positive evidence of changes In a vertical direction, but the microscopic relations, which are similar to those observed in ores of known primary character, show that this view is unwarranted. The ore-minerals corrode the rock minerals or the gingue, and are clearly of later forma- tion, calcite, however, is an exception, being in part later than the sulphides, as it distinctly breaka thair grains in veins in uany cases. The time relation between the jre- mlnerals and the zeolites is not known. Both bornite and ohilcopyrito were observed in amygduxes, usually with oalcite, with which the sulphides were apparently contemporaneous. Secondary Alteration. The auount of surficial alteration i small. The sulphides are themselves exposed to the air on the out-crops, and in no case is there a zone of complete oxidation, ilalachite uvi liiaonite in varying amounts ^ro always present, however. Native copper and cuprite oo>ut in the few oases* whera thoro ie post-ore jnovonont it admits only to a few feet* In:? ore tends to rru.ke out along the earlier oross-breaks to a slight degree, but rarely extends far from the main fracture zone although frequently far enough to cause a local swelling of the vein* In parts of both mines, more then one vertical fissure is mineralized, and in these localities, good ore-bodies are common along cross-breaks between the two veins. Bedding planes have played a similar role in directing the ore-solutions ~ut are less prominent than the cross fractures* Zonae of fine ohalcocite ve inlets are common in certain portions of the ore, but disseminated sulphide bodies do not oocur. In peneral, the ore chows a marked dependence upon the main i!j only slightly modified vertical or nearly vertical fracture syctetn, and by the other /A structural features with the exception of the flat fault, vvhioh usually forms the bottom of the ore* The relations are '- generalized In tho idoalisoo sections shown "below. / on a i tuJind! section Cross fractures - N Cf " s fracture Trans fers e sec tton \Ho sca/e) 35. Idealized longitudinal and transverse sections of the ore -"bodies of the Xonnooott -..Lines. The largest ore-bodies aro in the lower portions of tho vertical ; cure zone near the flat fault. Passing upward, strut! graphic; ally, tho mineralization becomes v?eaker, and the line representing the upper limit of finable ore is, in a broad ,vay, strikingly parallel with the dip of the flat fault and trie bedding of the limestones. (Fig. 35 ) - At tho Erio :,:ine. tho ores are also formed along vertical fissures, "but they differ from the greater deposits In that the fractures cross the greenstone- limestone contact t and ninortlisatlon occurs ttlong the "breaks in both rooks. The veins in the groenutone are thin and of no commercial vc.lue, ..'hilo those in the limestone aro wider and promising, Tho ores in the limestone are in the lower siliceous beds, or In the grey limestone, at least as far as they had "been exposed at the ti^e o'" our work. The Mother Lode mine depends on ores formed along a system of eteep or vertical fractures in the dolomltio limertone, far above the horizon of the other oro-bodies of the district. The fiseuring ie distinct, "nit is:? less intense than at the Bonanza Mine. The lower limit of their ore is not known at present Gun,-;ue Minerals and rock-alteration* One of the most remarkable features of the ores in the Chitistone limestone is the great simplicity of the gungue minerals, and the lac'k of siliclflcatlon of the wall rook. The wall rook ie commonly reoryi tallized along the voins, forming ooarse, grained maesos of calclte and dolonlte, but in many places, stringers of ohaloooite ooour directly in limestone of normal prein, which even under the microscope, shows no alteration of any sort. Only a few umall Drains of quartz were observed in a study of a large number of thin sections. lio silicates .vere detected either in tho field or in the course of tho laboratory study of the material collected. The reorystallizatlon of the limestone alon? the veins, and the deposition of new ealcite, or dolomite tie vein-filling, is probably a phase of the mineralization, "but evidently pre- ceded the development of the sulphides in some plaoee for In many oases tho coarse crystals of the carbonates were observed to have been Attacked along thalr cleavages or othor lines of structural weakness by voinlets of ohalaooito. Tho formation of the carbonates, however, extended considerably beyond the limits of the oro, and consequently the continuation of tho fraoturee is marked by the veins of carbonates. Barren veins of these minerals are common in various parts of the Chltlstone formation, however, as would be expected In a limestone country. The gangue minerals in the veins at the Eria Mine show an interesting dependence on the wall rook* In the greenstone, quartz predominates and oalclte is subordinate. Tho same fracture, followed Into the limestone contains a gangue com- posed only of caloite and dolomite r.ith no quartz. )ro-!ninerals Relative abundance. The ore-minerals, with the exception of obvious products of oxidation, are as follows, lifted in the order of their quantitative importance :- (1) chaloocito, (~) covelllte, (3) bornite, (4) enargite, (T) chaloopyrite, (6) lusonltef? ) , (7) tennantlta, (8) pyrlte, (9) sphalerite, and (10) galBna. Of theso, chalcocite is by far the most important. In tho flold, we estimated that It formed 90 - 95$ of tho sulphide ore, and no modification of this figure seems necessary from the reiralts of laboi'atoi'y work. Co/ellite is fairly abundant, compared v/ith the ro- taaitfing minerals , and when ite wide -spread distribution in fine threads and lathe throughout the ohalcocite is tukan into account, it is not unlikely that it forms from 3 to 5/S of the ore -minerals. The remaining sulphides probably con- stitute lesci than of tho ore. *f these, bo.-nite is the most generally distributed. Grains over an inch in diameter are vory rare .however, but e;aall ,- ec :d the size of a millemeter or less in diameter are oorrunon. Locally, enarglte is 1 .poi'tant. In the Bonanza -.Una It is probably more abundant than the bornite, nut for tho district as a whole, bornite undoubtedly forces it into fourth plaoe. Chaloopyrite occurs megascopioally only in e few eraall bunches, and is so rare, that it has been mistaken for 0ld by the miners* Lusonite (?) and tennantite are not uncommon under the microscope, "hut were not observed in the field. Pyrite IB noticably absent in most parts of the deposits, and its total amount is exoeeollngly small. Sphalerite and galena are microscopic rarities. Chalcoolte. Xwo distinct types of chalcocite occur in the deposits in the Chitistone 1" :>ne, which r e tormed for convenience in the field, atooly^ j.:halcocite and crystalline phaloooite, roepeotively* Tha Tormer is a compact massive mineral, '. th no , conchoids! fracture, and a metallic steady lur.tor. The crystalline chslaocite appears to have & .jrarmlar structure on a fracture -surface , as If the material /ore composed of an aggregate of individual crystals of about the coarseness of a medium-drained marblo. Uo crystal faces were observed on any of the grains, however, "but the crystalline appearance 1$ largely duo to an imperfect cleavage. The "stoel;/ ohaleoclto" is most abundant in the Bonanza Mine, the "crystalline" in the Jumbo Mine. The tv.'o types ooumonly grade into each other -Tlthou!; sharp Boundaries, but in & few rare instances, "bands o~ the coarse nu-terial were found ir. the fino an, 1 visa versa. The distinction between the t^o types IB lost oompl^t on the freshly polished surface, for both assume the same even bluish tint in reflected llrht. In the course of a few hours, however, a slight tarnish usually forms, revealing the grain of the material, end often the cryBtallorraphlc structure. In the coarser grains, a pattern of triangles or rectangles is formed by lighter b&nde or Strips cutting areas v;hioh tarnish to f. deeper blue. (Fig. 119 ) Tho structure is identical v/ith the lattice structure, previously described (page I? 1 *- > figures lM-6,152). -^hin the polif<:hed eurfacee with dilute nitric acid and ,ith dilute potaoaiuni cyanide solution usually reveals the In of the mineral. The crystalline chalcocite appears com- Jt el: sei, 251 posed of large grains oe.-nentod by smaller. (?!#. 10? ) Both large and small grains develope lattice patterns when tchod, the smaller ones imperfectly as vould "bo azpeote . c-o^el orally the pmallT rains develops only one set of strone lines, similrr to tha orthoryor-tbio etoh-cleave.gf ;?. - -.ineral is emphasized by '.;o no en trie clacks in the individual grains, or by scalloped YQ inlet s "nates. Small open spaces rarely exceeding 6 in. in their -roatest dimensions hsve been ob- served in the ore, and in a few, the chalcocits of the walls possesses a -.veil developed mesmlllary surface. Tho chalcocite, :.ir the mineral v/hi ch it replaces, may most reasonably be inter- preted as en open space flllinr-. The various structures of the chalcoclte are well brought out by voinlet oarlonatss or other alteration products which are abundant In all pt-rte of the ore-bodies. The lattice structure ie particularly wejl shown in its various degrees of fineness (figures 103 and 116} As has been -nentioned, tha concentric structure in likewise revealed by the carbonate voinlets, and much more clearly than it oan be shown by etching. ' th the esoeptior of a certain fraction of the oovellite, whicsh is plainly associatet; ,vith tho carbonates, the ohalcocite is the latoet sulphide to form. In the ease of every other miner- 1 ,'n tho lint on page 303 , there is oloar evidence that it has ru'fored partial replacement by ohaloocite, In some parts of the deposit at least. The relative ease of repl at of the earlier minerals by ah.ilcooite varies notably as is commonly found ta ])0 the case ii; other c<;jnps. Bornite, us usual, nroas ' . 01 most reedily v'ith ohalcopyrite a poor second. : nargrite and tennejitite ero less easily replaced, tut not ae resistant as the luzonitp' ": ; /hieh often remains as fine voinlets or rings, partir j.ly corroded, in the chaloooite after all neighboring material had "been consumed* The relations between tho ohalcooite and other sulphides will "ho de so ribs d in great or detail in the paragraphs dealing v7j.th the other rninerGls. Oureful chonioal studios of chaloocita from tha Bonanza and Jumbo 'linos have been made in the Geophysical Laboratory in Washington. Tv.-o Analyses ana specific gravity determinations 1 have been published , and are as follows: Specific rrr.vity Per oent Per oont Per oont Per cent Total at 5 Gu 3 ye Si Og 5.610 77.99 21.48 0.26 0.13 99,86 5. GOG 77.56 1.55 0.55 0.18 99.84 The analyses of ohaloooite collected during our work in 1915 are in striking aocord with the earlier results. Material 2 from tho Jumbo liines yielded the following values: - Specimen Steely glance Crystalline glance Cu 77.90 77,38 Ag .07 .09 s .06 .04 Pe .17 .17 S 21.33 21.48 Insoluble G.in?ruo .10 Of Ca .11 D .02 U ) (?) .11 1. i: . Poenjak, L. T.Allen, and H.ii. Merwin, Loo.oit.,p. 508. 2. K.T. Allen, private communication Deo., 1915. Coarse grained crystalline glance from the Bonanza ..:ine was also found to have almost the same composition that the other chalcooites from the region have. Dr. Allen writes:"! find the copper to "he 78.007., the silver O.)5,w, the insoluble gangue 0.03-/i. It alBO seems to contain a trace of arsenic. It seems to me that the remarkable uniformity in composition of such a great body of ore is very significant as to the uniformity in the chemical cunditiona of formation." The chalcooitee Bent to the Geophysical Laboratory were selected after iuinsraloArar.nio examination au examples of ae pure material as could bo obtained. Vary minute grains of bornite were observed under tho highsst magnification, and their presence probably HO counts for the iron in the analyoas. In sorae specimens, especially t,ho cteely glance, .fine grains of luzonlte (?) occur, and possibly a little onargite . The arsenic is due to the presence of these minerals .vithout doubt. A little oovellite was seen as very fine threads in tho chalcocite, but almost negligible in amount. The surfaces of the chalcocite exhibited the faint bluish mottling common in the Kennecott ore. Concerning specific gravity determinations on the new ~1 material, Dr. Allen -/rites: "I ho.vo determines the gravity of both Kn 1166 (crystalline chalcocito ihe Bonanza Mines) and KJl ISOOz ( crystalline chalcocite from the Jumbo ...ine) by both the Archimedes and the piiknoaoter methods, anu find, con- trary to our expectations, a similar porosity in both. The 1. Private communication, (Jan. 1916). 255 specimen Xn 1200s however io still nearly li htor than a solution of Cu S in Ou:; o should "bo. We can at present account "or this? only by supposing the specimen to possess that muo'h pore space in capillaries too fine for water to penetrate under our conditions. The results are aa follows :- Kn. 1166 a. Archimedes T.othod at 25 .'"91 b. likno-oter " " " 5.G28 Kn laOOs a. .-rohi'no: !:hod at 25 3.473 b. Pllcnoraotor w " " 5.531 Covollita. Covolllt - >f t-,vo distinct ageo may "be recog- nised with oertclnty in the 7nnneoott orac?. Part of the jovelllte ic oarlier- than the ohalcooite, ancl seams aaaooiated Va the primary rainer&lizatlon. An even ^r--.- -tor amount, however. Is conclusively shown, ^oth "by field and microscopical ovidenoe, to "be the first product of tho o 7.1 Act ion of the ohalcooite this part of th-3 oovellite ia olo--oly assocjiatec- in its davol^r iiont and distribution with malachite, azurite and litr.ani^e, it .ill "be r:.ncicloro " on the pages dealing with oxidation. The earlier oovellito occasionally oocurs as broad crystalline "oar.de through, ^he ore, "but it is ooraraonect hort lathe ecatterad in the ahalcocite. Locally, the "bloct crystdlr- o abundt-nt encup-h tj resemble tho distribution if he in a filifbesi (rJ-j- 10 9 ) but ordinarily they are very small and fora a subordinate pt.rt of the 256 total sulphide. Pine threads of covellite are almost always present, even in the purest ahalcocite. Bands, an inch or so wide, of ooarsely crystalline covellite penetrate the massive ehalcocite in a vein-like manner in several plaoes but when studied under the microscope, the chalcooite is olearly shown to be later, as it forms fine veinlets along' the boundaries of the oovellite crystals and even breaks across them. (Pig. Ill) The oovellite bands undoubtedly formed as veinlets in an earlier sulphide* In several oases, the chaloooite near the oovellite contains residues of bornite, and it is most probable that in many oases at least, the oovellite was developed in that mineral. The oovellite crystals have thoir longer axes perpendicular to the course of the vein, and on the polished surface, they offer a striking exhibition of the ploocroio properties of the mineral, varying from a deep blue parallel to the plane of polarization to pale bluish .vhite -vhan a* right angles. In a f e v specimens, believed, when collected, to be en- tirely composed of oovellite the microscope revealed a complex a.~p-regate of blunt covellite laths, with chaloopyrite and bornite filling the small angular spaces between them (Pig. 110 ) a relation very similar to the feldspar laths and the ferromag- nesian minerals in a meoium grained diabase. The covollite laths are sh&rply bounded, and broak across bornite- chaloopyrite contacts without the slightost ohange. The ordinary criteria of sequence among minerals of igneous rocks, if applied here 257 would put the oovollito earlier than the bornlte and chalcopyrite for the latter apparently moulded themselves about the oovellite crystals* On the other hand the usual ae< uenoe elsewhere in the deposit argues that the oovellite is later than the iron sulphides. The disregard of the oovellite for bornite or chalcopyrite grains is however difficult to explain on this basis, for under ordinary conditions of replacement, the greater ease of the alteration of bornite to oovellite than of ohaloopyrite to covellite is very definitely shown. It is possible however that the ohaloopyrite in these associations is a replacement of bornite, and developer with the covellite, but the blooky grains in which it occurs are unlike the forms commonly assumed by chaloopyrite of such origin, Bornite. In nearly all parts of the deposit, small amounts of bornite are found, but it is less abundant in the crystalline type of chalcocite, and most abundant in steely glance in the Erie Mine and certain parts of the Bonanza Mine. It is most common in small grains and patches v/ith rather smooth outlines in chalcoolte. They rarely exceed an inch diameter, and are most abundant as tiny speoks of microscopic size. The larger rraina are often broken by veinlets of ohalcooite which afford clear evidence of replacement. (Fig. 11# ) The veinlets usually follow cracks in tha bornite, which may contain malachite or limonite in some cases. The oxidized material, however, rarely extends beyond the limits of the bornite, and only in the case of the larger voins does the crack continue on into the chalcooite field. (Pig. 117 ). The malachite voinlets in regular lattice patterns in the chalooaite either do not extend into the bornite or else follow curving oraoks in it v/ithout yield- ing definite patterns. (Fig. 120). The bornite etches .vith its usual fine-grained lines "but their orientation rarely agrees with the structural directions of the surrounding ohaloooite. Definite lattice patterns between bornite and ohaloocite, similar to those observed in Bisbee ores (page >7rf ) do not occur at Kenneoott, but in some pieces, the distribution of faint bluish residues and the pattern of blue and white chalcooite strongly suggest the last stages of replacements of the lattice type. This impression gains support from the occurrence of residual spines or more correctly plates of bornite in ohaloooite, in symmetrical orientation to the pattern in the ohaloooite, which is strikingly similar to the association common in bornite-chaloocite ores from Butte and Bisbee, where the lattice structure is important. In places where the inter- secting spines are abundant, the relations resemble forms assumed by Certain Intergrovrbhs of minerals (magnetite and ilmenite for example) and of artificial products (ohalcopyrite and bornite; certain iron carbon compounds in steel.) Careful study, however, of the contacts invariably yieidi evidence of ohalcocito replacement of "hornite, and establishes the residual nature of the spines. teli The lattice structure IB shown also by spines of ohalcopyrite in bornite, similar to the relations described previously at Engels, LaFleur i;t.. Copper Lit., and Seven Devils. The ohaloopyrite in many oases eho./s the same dependence on chalooclte voinlets in the bornite as has been noted in the other deposits, but the intense development of ohalcocite in the Kennecott ores usually masks the evidence of incipient alteration. However, it is clear here, as in the other camps that the ohaloopyrite is a product of the reactions by which the ohaloooite was pro- duced from bornito. In places the change becomes of quantitative importance, and the intersecting spines in the fine grill become so numerous as to yield an almost solid mass of chalcopyrite. (figures 1*4 ) Certain narrow strips of bornite often with sharp offsets seem lees subject to alteration to ohaloopyrite, and usually remain untouched. They greatly resemble voinlets of bornite in chaloopyrite, and in the extreme oases of ohalcopyrite formation it is difficult to escape from the interpretation that chalcopyrit was the earlier mineral. However, by tracing the relations from the incipient stages, where a faint band of chaloopyrite spines is forming along the margins of these strips of resistant bornite to the final stages, where the main field of bornite has been completely altered and only the strips remain, the sequence of bornite to chalcopyrite may be firmly established. By these changes, intimate mixtures of hornite and ohalcopyrite aro produced. Prom them, the bornite is extracted 260 in many cases by alteration to eovellite or less commonly to chalcooite. Extremely complex patterns of covellite, in fine lines, specks or vo inlets in chalcopyrite result, BO intricately end finely spaced that under low magnificat lone, the surface appears almost as if it were a single mineral with a peoular shade of yellow. No typical graphic ctruotures between bornite and ohal- oooite were observed in the Konnecott ores, but in some cases, in chalcooite clearly of replacement origin, residues of bornite assume forms very similar to the shapes of the blebs in graphic areas. Ihese sub-graphic structures, as they may be termed, are usually on the edge of bornite areas, and are part of broad borders or margins >f chalcocite which are ..ithout question a replacement of tho bornite. In the preceding paragraphs, the results of various alterations of the bornite have been considered. The relations of the bornite to contemporaneous or earlier sulphides remain to be considered, and will be treated under the headings of the following minerals. Bornite continued to form under the conditions which pro- duced the earliest covel.lite, if the evidences presentee; on pages 256-257 is to be accepted. This weak continuation or renewal of bornite formation is shown by fine voinlets, which break later structures, and later linerala as luzonite or^ rarely, the covollite. ' f - r -vrv. ' ' . . g*lc | . . - 261 A puzzling occurrence of bornite In nodules about the size and shape of pigeon's eggs wae noted near the Erie Mine. The nodules are in the lo.ver siliceous limoctone, and apparently have no connection with the veins or ore -bodies either in the limestone or in the underlying greenstone. Ho structure could be observed in the bornite v/hich would surest a replacement of a fossil or of some impurity in the rook. In one nodule studied microscopically the bornite is cut by numerous vo inlets. The centers of the ve inlets are usually malachite and oalcite with llmonite on the sides. They are bordered by thin margins of oovellite which in turn is succeeded inward by narrow bands of chaloocite. Many grains of the bornite contain fine lattices of ohaloopyrite spines which in plaoes range down to almost submiorosoopic size. Certain bornite grains, with unusual yellowish tints, are most probably filled v. ith chaloopyrite of this character, too fino to be detected by our highest magnifications. The nodule presents a v/onde fully complete record of the alteration of bornite. The slight stains on the surrounding limestone indicate that little except water and oxygen and calcium carbonate have been added to the nodule and that little has been removed. Consequently, the chief work of the oxidizing process hae been to rework the mineral combinations, with little addition or subtraction of material. As the alteration advances into the nodule along: cracks and seams, the copper of the bornite is transposed into ^halcocite. The iron liberated by 'II 262 this reaction is In part foroed ahead of the main alteration and concentrated in the protected interior producing the lattice of ohaloopyrite, end in part finds its way into the principal channels, where, vith the acidity of the solutions re. ucod by calcium carbonate from the limestone, it is oxidized and hydrolyzed to lirnonite, tnd therefore little iron escapee beyond the limits of the nodule. The continuing attack of oxidation yields covellite from the chaloocite with malachite as the final product. The nodule affords convincing evidence of the details of the alteration of bornite to secondary pulphldeo. It is not u case of enrichment if the entire module is considered for no copper has beon add eel, but its processes are identical h ;.hose v/hiah take plaoe in the enrichment of larger ore-bodies. The sequence of alteration products is not simple but it may be illustrated fairly completely by the diagram below. The alteration of one mineral to the other is in the direction o? tho arrows. Bornite. Litnonite and Malachite : i :.36.iagram of mineral sequence in bornito nodule from limestone near Erie Mine. inargite. Enargite ooours abundantly in coarse crystalline masses in a few places in the Bonanza Mine, and in small amounts throughout the ore, espeoially in the steely ohaloooite. None was observed megascopioally in the Jumbo ore. In the crystalline ehaloooito, which is the most important type in this ore-body, there is little even under the microscope. On the whole, it is far less abundant than bornite, although a considerable mass of it was exposed on the 100 *L of the Bonanza LJine, where the mineral was first observed by Dr. Bateman. The enargite is usually associated with ehalcocite, which replaces it, yielding a peculiar brecoiated structure, v/ith angular fragments of the enargite set in r cement of blue ehalcocite. In ^orae cases etching develops a well-defined triangular pattern in the chalcocite. The enargite apparently alters lose easily than bornite, and the replacement to chulc5oclte generally does not seem to be far advanced. The relations of the enargite to the other sulphides (except the chaloocite) are not definitely shown* In most oases, it io oleerly a replacement of the carbonate of the wall-rook* Small definite crystals have been observed, in the unaltered limestone, with no traces of earlier or later sulphides. (Fig. 121) Certain enargite crystals In the 11 iestone show signs of mechanical changes, beln^ bent and fractured. The fractures were cemented by calcite or by oovellite, of the early typ . Bornite and ohaloopyrito were observed in close associations th the oovellite in relations similar to those described on page 256 which suggest that the enargito is an earlier product than the iron-bearing sulphides. . Unde.r the microscope, the polished surfaces of certain sections of the enargite crystals were found to be distinctly pleocrolo, when the specimen was rotated in polarized light. When tho longer axis is parallel to the plane of polarization (the plane of tho reflecting- mirror), the mineral assumes a pinkish tint; at right angles it becomes white or slightly bluish when contrasted with the normal enargite white* tii-ny sections however show no trace of pleocroisra. Chaloopyrite. Besides the ohaloopyrite previously de- scribed whloh is a replacement of bornite in the lattice structures, the mineral also occurs in small grains which are clearly contemporaneous cr earlier than th3 bornite with which they ere associated. In tho typical cases, the two ages of chalcopyrite may be ro&dily separated, but in lesa definite forme, in whioh more complete replacement may have obscured the original relations, it is difficult or impossible to distinguish the . Tho earlier chalcopyrite forms clean grains, asroolat with the bornite ir. mutual structures for the most part, but occasionally corroded by the bornito or broken by its veinlets in ways which convincingly show its earlier origin, (figure 126 The chaloopyrite, however, is an uncommon feature in the deposits, and probably never was of importance. Luaonite. Voinlets or grains of a pinkish mineral, which has not been positively identified, have been observed In a number of specimens iu the ohalcocite or the lose common minerals. / It agrees in tnineralo graphic properties with the 265 Iu2on1te in Dr. L T urdooh's standard collection. It resembles onargite In general properties, "but possesses a deeper pink color in reflected liftht and is never pleocroio. The mineral does not e?iet in euf-'ioient ouantiti^s in the Kanneoott ores to permit analytical determination, "but qualita- tive tests on small fragments indicate I/he presence of arsenic and trie absence of antimony and bisoiuth. The luzonite, or at least the "pink enargite", is a rather late product, for its. veins break all other sulphides except chalcooite. It roplaoe; iiornite in the complex ohaloopyrite- bornite mixtures, producing an apparent lattice of ohaloopyrite in a field of luaonite. Elsav/here it breaks the ohaloopyrite in veinlets, and even cute boldly across the broad plates of '.ovellite. In several cases, it I'orrae borders around the cryetnls of oovellite, a- i " doporited at a later time. It plays e prominent part in tho peculiar aonoentrio structures found in various parts of the ore, and preserves the curves of these J -r tures even after the less resistant minerals hnve boon oomplotely replace^ by chaloooite. Tennantite. Tennantite is rarely abundant enough to "be visible in the hand-specimen, but it is common in certain complex mixtures o " bornito, ahaloopyrite and covellite. It is in pert a replacement of ohaloopyrite, as corroded coren of th :"neral are common in the tennantite grains. The grains, however, usually possess oryott-lline outlines, and their arrangements in raany cases, sugffeet that they were formca along the ./alls of open spaces. The crystals are often rimmed \?ith thin borders of ohal- oopyrite .vifeh indicates conclusively a recurrence of the for- mation of this mineral. Fino voinlets of the le-tor bornite also out the tennantite crystals. The tennantlte is also partially replaced by luzonite which develops in one or two instances a lattice structure identical in appearance with that between bornite and later minerule. The tennantite is relatively resistant to rep] a :nt by CDvellite and chaleooite. Pyrita. ryrite is very unoommon in the ore-bodies, but It is fairly wide-spread as a sparse scattering of email cubes in the lower siliceous bo the Chitistona formation, where it apparently ni-s no connection with the mineralization whioh produced the copper deposits. Under tha flat fault in the Jumbo ...ino , a fo 11 bunches ware found associated with partially oxidize copper ores and Much limonite. It is possible that the^e oiay be of the sort previously mentioned, and were in the limestone before the ores were formed. Small grains have been observed in tha raidst of chaloocite and covallite, but th&y are of rare occurrence. Sphalerite. A few large grains of sphalerite have been observed in severe! specimens but they yield no information con- cerning the relation oi' the mineral to tho other sulphides. The sphalerite i. iier than tha chalcocite, but apparently not readily repl&.oed by it. Galena. inly a few small epeolce of galena, doubtfully identified by microohemioal tests,. /ere observed. Concentric and "bunded structures. Information concerning the mineral sequence presented on the preceding pages h&8 "been gained chiefly from certain peculiar knots in tho mass of the monotonously uniform ohalcocite, in which the rarer minerals of the deposit are concentrated. In the majority of these oases, the various sulphides posse SB remark able concentric or "banded structures, ./hi oh, on account of their genetic signifi- cance are worthy of separate discussion. Chaloopyrite and bornite apparently form the ground mass, upon which most of the other sulphides have been built. The formation of the chaloopy- rlte and bornite was succeeded "by the deposition of tennantlte, both as a partial replacement of the earlier sulphides and as crystals a parently forming drusy coatings on the walls of small vugs. A later phase of the mineralization again pro- duced ohalcopyrite, closely followed by covellite in abundance with a little lusonite(?) and bornite. The ohaloopyrite of the second generation forms rims about the tennantite crystals or cuts them in veins* It is most abundant, however, as a replace- ment of Tiornite, in complex aggregates of spines as previously described. The covellite is always abundant. It is prominent in large radial flakes in the centers of the oonoentic areas. ?rom the relations to the crystalline tennantite bands, it is very probable that it constitutes he final filling of open spaces remaining in the earlier minerals, or else a replacement of other material ,;ith similar radial structure which had filled euoh cavities. There is, iuwever, little evidence of any earlier nineril associated with the oovelllte in this form. 'OOaiq SAJ '. 9S i1l ' , tiO< Inni ^ Elsewhere the oovellite replaoes bornlte, and In places so thoroughly that little of the original sulphide remains. Here and there, smell angular grains of chaloopyrlte or of bornite ooour between the covelllte crystals. Ho olue Is given as to their relativ^ a^es. They are probably contemporaneous, but it oannot "be stated positively. A little bornite, In feeble veinlets is apparently formed at about the same time as the crystalline oovellite. The volnlets out ohaloopyrite of both agee, and also the tennantite. The luzonite is in part oon- tamporeneous and in part later than the crystalline oovellite. It forms veins in the tennantite and oovellite, but also occurs as sharp crystals, euhedral toward the oovellite fields. In a few Instances, fine ^ims of the luzonite were observed follow- ing the serrate outlines of the oovellite blades. Chaloocite replaoes all of the preceding minerals and structures. In soat speoi iens, it ic present only as veins or bands parallel to or breaking across the complex aggregate; In others, more extreme replacement hao left only coarse sheaves of oovellite ( figures 12#-129 ) or rings or bands of luzonlte or of tennantite. Some trace u of these minerals are al'-vays present in the pebbly ohalcoolte, or in the mammillary ohaleocite, and it is very probable that these structures v--er-) inherited from such complex masses of onrlier sulphides. Enarglte is rare in the knots of concentric sulphides. It is chiefly a direct replacement of the limestone, rather than an open space deposit or o replacement of other sulphides. atata i .>?&! K.' ! 1W ' A*r Art ? :o { ?5 ^eiw>. 269 Oxidation. Distribution. The products of oxida- tion are prominent in all parts of the deposits. In no place, ho\?9Ver, is the alteration of the original min- erals complete, but, on the other hand, there are few Places where no traces of oxidation exist. The degree of oxidation varies locally v>ith changes in the ore minerals or wall, roofcs but, in general, It is surprisingly constant throughout the deposit. certain structural features, hovever, off.r especially favorable conditions. Where cross-breaks of post-mineral age intersect the veins, the alteration is found to be most Intense. Bedding planes and the fractures along which the ore has been localized -ilMO afford channel ways for descending solutions; in the Jumbo mine, for example, the upper surface of the -one of the flat fault is a particularly favored locus for oxidation, but it is less marked in the Bonanza Mine. In general, the cross-breaks, which allow un- altered surface solutions to descend to great depths through barren and chemically stable limestone, are more important agents In promoting oxidation than the breaks parallel to the ore, for in these, later solutions descending from the surface are quickly robbed of their powers of oxidation by the e r tnily alt r I smlphides in the upper parts of the deposit. Through- out the msnive sulphide-ore-, the alteration IB con- fined to the surfaces of joints or snail cracks, or to the walls of vugs. The alteration, however, is feeble; it is rarely pervasive, and only where an isolated chaloocite stringer happens to be attacked, does it ap- proach completion. Nevertheless, the oxidized copper u.inerale in the aggregate fora a sufficient fraction of the ore to have warranted the building of a special plant for their treatment by an ammonia leaching process. The sulphides are exposed on the present surface, and show no greater alteration there than in the deepest workings. Broad faces of chalcoclte, broken by the mechanical disintegration of the cliffs, have not developed more than a tarnish fron their exposure to the air. At the Bonanza Kine, the disintegration of the vein has enriched the t r ilus and the araill ^l^nior ' it to such an extent that the ice and broken rock Constitute a f'ood ore. This persistence of sulphides ir. looae ,-ial, exposed as favorably \ as possible to\ tr^fe influence of oxidation, illustrates forcibly the nefli^ible extent of aurficial deooiapo- i f ; ition undor present, climatic conditions. X oi Underground Temperatures.- The mine temperatures do not rise above the free sing point of water at any time of year, except in a few placea on the Tipper levels where the ventilation is good, or in confined drifts where many men are working. The mines are therefore absolutely dry, even on the lowest levels. Ice is a common mineral, occuring as splendid frost cryatals on the walls of drifts near the surface, and as partial or complete fillings of fissures or other cavities at all depths. There is no circulation of water at present, consequently no oxidation is possible ex- cept by cold dry air, which is negligible. Water Level.- There is no indication of a frosen groundwater level. The continuation of abundant oxidation to the bottoms of the nines indicates that under earlier conditions, the water level was below the deepest workings developed at present, which are more than 1000 ft. below the crest of the ridge above the Bonanza and Jumbo Mines. This deep oxidation indicates almost certainly that the topog- raphy proceeding the present glacial period was one of marked relief, and therefore of active erosion. The alteration of the sulphides was checker] by the increasing aridity due to the glacial climate, and the various phases of the oxidation have been preserved v;ith unusual completeness* Minerals due to Oxidation. - The minerals, obviously formed by oxidising processes, are as follows; (1) malachite, (2) limonite, (3) covellite, (4) antlerite, (5) azurite, (6) 272 arsenates of copper, (7) clialcanthite and (8) cuprite . They are listed In the probable order of their abundance. Some quartz in the antlerite may be due to oxidation* Malachite is the commonest product of oxidation. It is abundant as a replacement of chalcocite, or aa cavity fillings in vugs or other open spaces. As has been de- scribed, it penetrates the chalcocite in veinlets, whose orientation is controlled by the structure of the sulphide, producing lattice patterns, or scalloped bands in the materi- al with the concentric structure. It is also a replacement of limestone to a slight extent. Coarse crystals of calcite with malachite developed along their cleavages were observed. Malachite produced in this way is far less abundant than that which has replaced chalcocite or the other sulphides directly. Azurite is closely related to the- malachite in origin. It is unusually abundant in proportion to malachite in the Kennecott ores. Vugs in the chalcocite lined with azurite crystals are very common, and in nearly all places where malachite is developed, there ia some azurite formed with it. Ho definite age sequence between the two carbonates could be established however. In some places one would be the older, in others the reverse would be true. There is a slight tendency for the ratio of azurite to malachite to increase in the heart of massive ore-bodies, and to decrease in the zones of more intense oxidation. Limonite is widely distributed throughout the deposits. It is moat intensely developed in close association with the sulphides. Stains penetrate the limestone or are dif- fused through gouge for notable distances, but the percentage of iron is much lower than in material adjacent to the ore. Pure massive limonite is rare however. It ia nearly always mixed with malachite or azurite, or else with residual limey material. The abundance of the limonite is surprising when the low iron content of the ore is considered. In the largest chalcocite masses, as in the Jumbo Mine .where there is al- most no iron, limonite is least abundant; in the few places where born ite -ind chalcopyrite were found, limonitic altera- tion is very prominent. This relation suggests that the limonite is derived directly from residual iron-bearing sul- phides in the chalcooite, but the distribution of abundant limonite is by no means limited to the places where the iron-rich sulphides are present. As has been stated, no gossan remains at the present surface. Limonite is as plentiful in the deepest levels as in the shallow parts of the ore. Covellite is the first product of the oxidation of chalcocite at Kennecott. Its relation to the chalcocite is conclusively shown by the field associations and con- firmed by its distribution along malachite veinlets ob- QG a ener ^oolaxfo erit at aeJ5i er iw eeoxjlg e emjwa on Trf i served tinder the microscope. Zonea of chalcoclte partially altered to oovellite exist between unaltered chalcocite on one side and oxidized minerals on the other. Under the mi- croscope, rosettes and sheaves of covellite commonly accom- pany malachite veinlets in chalcocite (Fig. 103), The spots of covellite, spreading out from the channol-ways, resemble the products of decay advancing into wood. It is usually closely followed by the carbonates, and there can be little doubt that they are successive products of the same general processes. Antlerite and Chalcanthite are associated with secon- dary covellite in a remarkable ore-body in the Junbo Mine. A large, roughly pipe-shaped mass of oxidized ore occurs parallel to the dip of the beidlng at base of one of the large sulphide stopes. The overlying chalcocite is the pure, crystalline type common in the Jumbo Lline. It passes without sharp boundary into a purplish friable material, resembling covellite, but porous and with a low specific gravity. In the intermediate zone between the two extremes, chalcanthite occurs in an intermittant band of 1/2 to 1 inch in thickness. From the results of chemical studies on this material by Dr. 2. 2. Allon and Dr. H. E. Merwin of >. the Geophysical Labratory, the following information was sent us:- Che composition of the covollitic material is:- 275 "Cu S 50.44 per cent Cu S 4 . 5E K 45.20 ' 4*56 100. per cent The quartz crystals are all v;ell formed and each is doubly terminated. The 45$6 copper sulphate was a surprise as it was not detected under the microscope. Each grain of covellite is very minute and is probably surrounded by a zone of sul- phate". The presence of the quartz was not suspected. The over- lying chaloocite contains practically none, consequently it may have been introduced with the oxidizing sol\itions. In the field the covellite can be detected cutting the chaleocite in a net-work of fine veinlets. The material is very difficult to polish, but even on imperfect surfaces, the relation to chaleocite was clear. The covellite is without question a replacement of chalcocite. That the reacting solutions contained copper sulphate is indicated by the presence of the chalcanthite. Chalcan- thite, however, is very uncommon under normally humid condi- tions, and its formation here must be attributed to the in- creasing aridity at the beginning of the glacial period. 1. S. T. Allen, Private communication to L. C. Graton, Jan. 1916. ,no aw Q&$ Chalcolite Transverse section Scale. : - 10 feet Ch&lcoc it e LonqituJinal section 37 Chalcocite-covellite-antlerite ore-body, Jumbo i.iine. The aone of covellitic material which is a foot or so in width, forms a rim around an underlying larger mass of light greenish material, which 'in the field we believed was malachite contaminated with sulphates. The boundary betv/een the covellite and this oxidized product is fairly sharp, but fine stringers of the sulphide extend in lacy masses several x'eet from the . s>U. I .IB. line of contact. Specimens of the purest material which we collected were studied by Dr. B. E. Allen, who wrote as fol- lows concerning the mineral : - "The particular basic sul- phate of copper which you find in Alaska turns out to be 3 Cu 0. 30 3 2H 2 0, and is in all probability identical with antlerite which is in turn identical with stelznerite. It is very difficultly soluble. I am inclined to connect its formation with the presence of limestone. Of course, basic copper sulphate is formed by the action of water on a dilute copper sulphate solution which contains no free acid." In later work in the Geophysical Laboratory, it was found that the product of the reaction of a 10> cuprl!c sulphate solution on marble in the lump at room temperature yielded a product much like the material from the Jumbo .Mine. tfrom ohis evidence it seems very probable that the basic sulphate is the product of solutionscarrying copper sulphate reacting with the underlying limestone, xhe field relations are in accord with this interpretation. Ehe lace work of covellite veinlets may be resi'dues of chalcocite veinlets in original limestone. Very similar conditions have been ob- served on the edges of unaltered chalcocite ore-bodies, Cuprite is very rare in the deposits in the limestone. One little nest of octahedra coated with malachite was found 1. Private communicate to Jj. C. Graton. . he: et in the Bonanza :.!ine, bxit the mineral was not observed else- where except in small amounts under the microscope. The common oocurrenoe of cuprite associated with native copper, described in connection with the greenstone deposits, offers an interesting contrast, and indicates the control of wall rook on the character of the oxidation. Native copper has not been observed in the deposits in the limestone. Arsenatea of copper are formed in snail quantities by the oxidation of the enargite-bearing ore. They are in most cases mixed with malachite, and have not been identified min- eralogically. Mixtures of onarite and chalcooite have been observed to oxidize more readily than the pure enargite. The chalcocite is apparently the more resistant. The influence of oxidation on the wall-rock is manifested chiefly by the replacement of calcite or of dolomite by copper carbonates, basic sulphates, or limonito respectively. The limestone being inert to simple oxidation shows no traces of descending solutions away from the ore except veins of calcita or dolomite in coarser crystals than the grain of the rock. However, near the ore, especially where the development of li- monite is most intense, the limestone is altered to a sandy, incoherent material, apparently by the partial leaching of the rock, which is && readily attributed to the attack of the acid, produced by oxidation of the sulphide or the hydro- lization of the ferric sulphate which produced the limonite. M '. Xw . I Off* t &J.P.&- . leetsoo aJt , eio erf* ier. . .>we 279 This is especially notioable in the Jumbo Mine near where pyrite was found and on the under side of sulphide masses. THE ORIGIH OP THE COPPER DEPOSITS IU THE GREEHSTOHE. Similarity t Other Regions* - The mineralization in the greenstone is similar in many important features to that described in numerous widely separated regions where copper minerals occur in basic lavas. The forms assumed (veins of rather slight persistence, disseminations, and amygdule fillings) the metallic minerals (bornite, chalco- pyrite, native copper and cuprite) and the gangue minerals (calcite, quartz, epidote, chlorite and - although not abundant - zeolites, datolite and prehenite) are all charac- teristic of this world-wide type of mineralization, Source of the Metal . - The distribution of the copper through all parts of the Copper River region where the green- stones occur, supported by the common association of copper with basic extrusives elsewhere, points definitely to the lavas themselves as the source of the metal. Lindgren g writes: "Basic igneous rocks such as gabbro, diabase, basalt, 1. Lake Superior region; White River District, Alaska; eastern Oregon (20 miles west of Baker City); Triassic Traps of Hew Jersey and Conn.; near Lurray, Va. ; South lit., Pa.; the Bay of Pundy, Hova Scotia, the Paeroer, north of Scotland; Sterling in Scotland; Oborstein a.d. Nabp, Germany; Sao Paulo, Brazil; the Kristiania Region, Horway; Hew Guinea; the Trans- balkalian provinces on the Dochida River; Monte Catini, near Livorno, Italy. (Por a general discussion and references to literature of the above districts see Lindgren 's Mineral De- posits, p. 392.) 2. Mineral Deposits (Ma Graw-Hill, Hew York, 1913) p. 393, ., ij . i: .Becfl oo aJLfncertiffi leqqoo . '.?A ie vitj biqe , . at ,TJ - , o.l*& . . : 2SO some andesites and basaltic flows designated melaphyres or amygdaloids probably always contain copper, in some cases as much as 0*1 or 0.2 per cent., but commonly about 0,02 per cent, of the metal. According to Volney Lewis and F. P. Grout, the copper is present as a silicate pos- sibly in part as a sulphide such as bornite or chalcooite-" In the Kenneoott district, ohaloopyrite has been observed as an accessory mineral in the lavas apparently remote from notable mineralization, and in all probability the sulphide is a primary constituent of the rook. Ho analy- ses of the greenstone have been made however, but the gen- eral distribution of copper is shown by the commonness of faint stains of malachite in fractures along which waters have percolated, Theories of Cone ent rat i on of the Copper in Basic Lavas. Although most writers are in general agreement in regard- ing the basaltic lavas as the source of the copper, there is considerable difference of opinion concerning the mechan- ism by which the concentration of the metallic mineral was effected, Whitney, Pumpelly and Wadsworth have attributed similar ores in the Lake Superior region to concentration and deposition by descending waters, Van Hise regards the 1, Moffit and Cappa, Loo, cit. ')h 8* .io -i-fco SQ -o 1*0 6u A .f 9il^ lo d'.noo tst' .')C . ^rl'c , aoor- .cr a.v. og , tsncoo ^xt^Jtiq e 3ieifv, .d .^s^txrife ni a0ic . , 281 zeolites, which are usually closely associated with the copper minerals, to be products of descending sur- face waters or similar percolating waters in the zone of cementation, the material for the zeolitization hav- ing been extracted from the rock itself , although he somewhat later attributes the Lake Superior copper de- 2 posits to ascending solutions* Weed believes ores as- sociated with basic lavas in the Appalachian region in Virginia and Pennsylvania to have been formed by infil- trations from the surface. According to a theory ad- at vanoed by Lane for the I*ake Superior deposits, the lavaa are believed to be submarine effusions, and the concen- tration of the copper is attributed to the action of heated sea water on the beds. Zeolitic deposits in the Watchung basalts, in which a little copper occurs, are 4 believed by C. H. Fenner to be the result of lava flows encountering water in shallow lakes or playaa. The chan- ges effected are attributed to readjustments of the miner- al combinations along fractures and in the rook itself 1. C. H. Van Hise, Monograph 47, U.S. G.S., 1904, pp 333 and 633. 2 W. H. Weed, Types of copper deposits in the southern United States - T.A.I.H.S., Vol. 30, 1900, pp. 449 - 504. 3. A. C. Lane, Salt ?ater in the Lake mines, Proc., Lake Superior Min. Inst, vol. 12, 1906. 4* C* U. Fenner, The Watohung basalt and the parageno- 8i3 of its zeolites, Annals., H. Y. AC id. Sci. t vol. 20, part 2, pp. 97 - 187, 1910. noo la under the influence of steam and hot waters* Lewi! as- sumes hot solutions of cuprous sulphate released from in- trusive trap magma to account for native copper in certain other How Jersey deposits. In the region about the head- waters of the White Kiver, Alaska, chalcocite, native cop- per and a black hydrocarbon occur with oaloite and various zeolites in amygdaloids, possibly of the same age as the Nikolai greenstone* The deposits are described by Adolph g Knopf , and are attributed to hot solutions circulating through the lavas soon after their extrusion* The flows are believed to be submarine from fossil evidence in in- terbedded tuffs and breccias* She development of albite, epidote, chlorite and da- tolite in certain of the veins in the Hikolai greenstone indicates that they are not the product of cold meteoric waters, for these minerals are not formed under such con- ditions* The later minerals, calcite and the zeolites, are undoubtedly the products of milder conditions, but probably part of the same sequence* Veins similar to those at the Brie Mine, which break across the greenstone- limestone contact can not be related to cooling effects of the lavas, and were without question formed under later and very different conditions. The shorter veins, throughout the formation, which rarely break through more than three 1* J. Y. Lewis, Gaol. Sur* Hew Jersey, 1906, p. 131, A. Knopf; Loo* cit. . zabass o L&fygoR& ai J01> 9i{'I ,eco*aneei, , 08 61 Vf C Affi _ ixx .^ V n ^ w ^.*.., ^ or four flows, may be the products of the final stages of the vulcanism. but they are quite similar to the later veins. ' The information concerning the submarine or sub- aerial extrusion of the lava is too indefinite to base any theories upon. Ho evidence favoring a submarine ori- gin was observed near Kennecott; the liraonitio horizons suggest land surfaces, but do not prove it* The intrusions of the porphyries in post- Jurassic time -,vere wide-spread phenomena, and the unequal heating to which the country must have been subjected, undoubted- ly accelerated the circulation of the ground-water. Heated solutions, traversing the copper-bearing rocks, are agents capable of causing noteworthy changes and may be the cause for some of the greenstone veins, such as those breaking the limestone-greenstone contact. The importance of this circulation and possible structural relations will be dis- cussed more fully on the pages dealing with the origin of the ores in the limestone. The long-continued volcanic activity in the Tertiary may have had a similar effect, but it was probably neither as localized nor as intense as the heating caused by the porphyry. The ores on Uugget Creek in the XUskulana District are stated to be later than important structural changes in the greenstone. if so, they are not products of the cooling lavas, but must be attributed to later causes. 1. A. Wandtke, Oral communication. at? u .0 i Porphyry intrusions are common In this re- gion also, and may be the indirect agents, as dis- cussed above* Certain ores in this district are close- ly related to the porphyry, and are probably contact effects, but their mineral character (pyrite, magne- tite and ohalcopyrite with garnet) is in striking contrast to the ores in the greenstone in general and argues strongly against the porphyry as the general source of the copper. The porphyry intrusions may have had an important part in the formation of the deposits, but it is not believed that they supplied the metal* The chaloooite in the greenstone deposits is clearly secondary in part, and a replacement of bornite. Hear Kennecott, chalcocite is only slight- ly developed in the greenstone* In the Xuskulana district, the ohaloocite is of two ages* The older, which is more abundant, is either a primary inter- growth or a replacement of bornite. If it is of the latter origin, it may possibly be secondary and related to an older topography. The ohaloocite of later age is due to feeble enrichment from a sur- d :.5ni 9; mn J&ti OT^ /i^ aJt e^ioc. i A baa , ,-n4 face of rapid degradation, probably of inter-glacial Re'surae'. The source of the copper in these deposits is believed to be the lavas them- selves; the concentration of the metal may have tafcen place in part by heated solution" as the lavas cooled, but it ia probable that the stronger veins are of 1-ator origin, and were deposited by the circulation of solutions heated and set in motion by the in- trusions of the post-Jurassic porphyry. The chal- cooite is secondary in part, and a replacement of bornite. Chalcooite in graphic and siiailar struct- ures in bornite from the Kuskulana District is poss- ibly primary; the origin of chalcocite in these structures will be considered separately in a later section of the paper. 2X6 THE ORIGIN OF THE COPPER DEPOSITS IN THE LIMESTONE. Summary of Significant Features From our observation in the field and in the laboratory, and from the results of v r ork, in the OeophyRical Laboratory, many features concerning the ores lu th< limestone have been definitely established. The moat important points which must be taken into consider- ation in any theory of origin for the deposits, are sum- marized in a following paragraph. ^ho first ten observations bear chiefly on the problems concerned with the primary origin of the deposits - the source of the copper, the manner of concentration, and its ^resent loci. The question whether the chalcocite was 'formed directly as a replacement of limestone or a replacement of older sulphides depends largely on the observations listed f rom ( 11 ) through (17), while the problem of the secondary or primary origin of the chalcocite requires the consideration of the remaining points of the list as well. The list of a ignifioant features is as follows: (1) The dependence of the ore on the greenstone-limestone contact ( the Mother Lode deposit an apparent exception ); (^) the widespread distr lout ion of copper in the under- lying greenstone; (3) the dependence of the ore on definite fractures, and the iacfc of disseminated sulphides; (4-) the definite lower limit ( stratigraphically ) of the ore in the larger mines; ; - . i *. : aeTj-tffiel j; _e '10 Jell (5) the gradual decrease In ore (ohalooclte) passing upward, strati^raphioally; ( 6 ) the sulphides a replacement of limestone In nearly all places where the relation Is shown; ( 7 ) the occurrence of small Knots In the ore character- ized by the complex concentration of the less abundant min- erals, and usually by concentric structures ; () the laefc of alteration in the wall rocl: (except par- tial recrystallization); ( 9 ) the occurrence of enargite in notable amounts in the Bonanza L'ine; (10) the lacfc of definite relations between the ore and any intrusive rocfc; (11) the great size of the ore-bodies; ( 12) the variation In texture of the ohalcocite ( steely and crystalline types); ( 13 ) the dominance of chalcoclte over other ore-minerals and its unusual purity; (1U-) the results of analytic and synthetic vorfc on Xen- neoott ohalcooite by the Geophysical Laboratory; constantly low copper content and specific gravity of the oh loocite; chaloocite-oovellite solid solutions; the high and low tem- perature forms of chalcocite; ( 15 ) the replacement relations which the chalcocite exhibits toward all other sulphides (except a part of the covellite ); sac ilOUSTSJlF '1C . o s cacti '^^^^^^^^^^^^^B^^^H ' ' . ;( 8{H;>f : ^io *o c { ^s. } ' iscieof 2X8 (16) the common distribution of traces of bornite throughout the chaleocite ( most abundantly in the steely Bonanza glance, less abundantly in the crystalline Jumbo glance ); (17) the definite structural relations occasionally shown between the chalcocite and bornite; (Iff) complete lacfc of surfioial alteration under pres- ent climatic conditions; (19) wide-spread but partial oxidation throughout the mines; laci of dependence or. present surfaoe, and lack of -Ilialnution with depth within the limltw of the mines in 1916; (20) the existence of physiographic conditions which afford land surfaces of possible relation to the secondary pro- cesses,- pre-glacial topography of notable relief; older Tertiary topography of gentle relief; pre-voloanlo peneplain in early Tertiary, and pre-Kennecott post-Chltlstone peneplain; (21) the existence of structural features - low dip and flattening of limb of anticline to the southwest; the in- fluence of the flat fault as a barrier for descending solutions; ( d2 ) the slightly greater abundance of bornite in the chaleocite from the Erie Mine, at a lower elevation on the limb of the anticline than the Bonanza and Jumbo deposits; (23) the occurrence of bornite, in slightly more abundant ai.iounts, in the open-out at the Bonanza iiine under the zone of the "flat- fault." .fittl ill ' ifa eJin. ^ rs ^r ^. ?- jfiwo^ XO 1O A_L oe er; o ser. . ' ;nlfllvi. 2S9 The Origin of the Primary Ore. The only possible sources of the copper as far as our knowledge of the district extends ar<3 the greenstones and the post-Jurassic porphyries. The remoteness of the Tertiary volcanics, and *he lack, of a;.y known mineralization associated with them raaKes it ex- trer.ely Improbable that the copper was derived fron any phase of their activity. The close association between porphyry intrusions and ores in many of the copper camps in the Southwest suggests at ths outset that similar relations may exist in this dis- trict. The porphyry, however, near Kennecott shows no traces of mineralization, either in the heart of the Porphyry Mt. stocfc or near the edges, or in the smaller bodies. At the Erie Mine, a three-foot dike cuts ths vein, but it is uniainer- alized and aost probably later than the ore, although this cannot be established with certainty. On the southwestern side of Porphyry lit., low-grade gold ores have been found. Little information was available and the region was riot visited. The gold is associated with pyrite, anrt the miner- alization is apparently very different from that near Kenneoott. In the Kuskulana region, a few deposits of little importance seem related to porphyry contacts; as has been mentioned the .:ineralizatlon Is in distinct contrast to that in the green- stone or in the limestone, and indicates a different origin from the larger deposits of the region. air -1- 290 There is the possibility that the ores were deposited from solutions which had emanated from the por- phyry magraa, but hich had travelled ;?orae distance before depositing their load. The presence of enargite in the ore suggests an igneous Bouree, but the confinement of the ores to definite fractures and the lacfc of Bilicificatlon both ii.'iieate that the conditions of ore-format ion were not in- tense. Nevertheless it is difficult to believe that emanations* evenfrona remote magma, would be so poor in silica and so relatively rich in the metala to have produced ore-bodies of the Kennecott ty. At 3isbse, however, rich ores do occur in almost unaltered limestone in parts of the mineralized zone remote from the prophyry, yet there is little doubt that they wer"e derived from the igneous rocfc. Consequently the poss- ibility that the Kennecott ores were formed in this way can not be completely eliminated, although the probability that it is the correct view is not great. The deposits in the limestone are believed by iloffit 1 2 and Capps and by J. ].'. Irving to have been derived from the . lely disseminated copper in the greenstone. It ie suggested that the leaching of the limestone may have been facilitated by the heating due to the intrusions. "It is believed that the copper ta&en into solution by circulati. xtsr was carried ii.to trunk channels and depot? ited there when the conditions favorable ..... Most frequently deposition tooK place 1. Loo. cit., (Bull. 445), pp. 2. Loc. nit. noil in the greenstone formation, but at times the copper-bearing waters passed outside the greenstone and into the overlying . 1 limestone before giving up their mineral load." The greenstone affords a competent source for the copper frou a quantitative standpoint, for it is copper- bearing almost wherever exposed. Concentration of copper in the greenstone is known to have taken place in pont-Chitistone time, for ve inlets of the typical greenstone typ^s break across the greenstone-limestone contact at the Erie Mine and else- where. The relation of the ores to the contact is striking in all cases except in the Mother Lode Mine. Even there it is v not impossible that the ore may increase or at least continue until the contact is encountered. Small stringers of the chaisocite in the limestone are not infrequently en- countered as one follows the contact along the cliffs between the Jumbo and Erie Mines. Definite fissuring is lacking in all these cases, however, and none of the seams persist many feet, but usually die out along bedding planes or short cross- fractures. The lower siliceous limestone beds are apparently unfavorable for mineralization. At the Jumbo and the Bonanza Mines the existence of strong and persistent fissures, both of the vertical zone and the flat fault, gavs the ascending solution^ acoen, to the purer limestone, and also formed im- portant channel ways in which a concentration could take place, 1. Mofflt and Capps, LOG. oit., p. S2. . ' ne$wJ> i :IJt 292 It remains a disputed fact among geologists whether a metallic concentration of this sort can be effected by col^l meteoric 'waters. However, ordinary meteoric waters undoubtedly become more active agsnte if accelerated both in 'ir'culation and chemical action by dli'ferentlal heating due to igneous intrusions. In the Kennecott district, the wide- spread intrusions of post-Jurassic porphyry probably had this effect. Where definite circulation along channel ways was established, copper leached from the greenstone would be pre- cipitated, if conditions were favorable. Where these channel- ways crossed the contact, ind the fissures per- isted in the limestone, or encountered other dominant fractures, these con- ditions were supplied;v/here the limestone is unbrfcfcen, the mineralization rarsxy extends far above the greenstone. The reactions involved in the solution and precip- itation of the copper according to this theory of origin are not known. It is possible that the copper was transported 1 in a colloidal state. Tolman and ClarJc have shown thai col- loidal dispersions of copper sulphides may be induced by hydrogen sulphide, and flocculated by calcium carbonate. The existence of hydrogen sulphide in sufficient quantities in the greenstone, however, is a mere speculation. It or other agents could conceivably hive been provided by emanations from the porphyry. The precipitation of the colloidil copper by calcium carbonate solutions offers an attractive explanation for the localization of the ores in the lower beds of the 1. The dispernion and precipitation of copper sulphides from colloidal suspension, Econ. Geol.,Vwl. ^, p. 559-592, limestone. The hypothesis finds possible support in the Concentric and mannilary structures observed here and there in the ore, which resemble certain colloidal products, but rom our present knowledge of colloids it is doubtful whether the great bodies of sulphides, clearly replacements of lime- stone could have been formed in this way. The solutions deposited little except the metals, sulphur and arsenic. The quartz ind. silicates in the veins in the greenstone probably were derived from readjustments of mater ill in the adjacent wall rocfc, with little trans- portation or addition involved, for these minerals cease ab- ruptly at the limestone contact. It IR possible, however, that the two rocfcs exert sufficiently different precipitation ef- fects on the solutions to account for the .changes. in resume., the theory of derivation of the copper by leaching of the greenstone offers a competent source for the metal and a satisfactory mechanism for its concentration, an explanation of the relation of the ores to the greenstone contact, and of the lac* of sillcification In the limestone. Its laiin points are supported by positive evidence. Opposed to it may be mentioned the lack of strong veins in the green- atone beneath the larger ore-bodies, and the occurrence of enargite in the limestone. The first objection is not vital; the material v/as not necessarily derived from the greenstone immediately belo v : the Contact, but may have migrated to the -'I; favorable ohannelways In the limestone from notable distances. The occurrence of enargite is puzzling. At Butte it is re- garded as a product of intense mineralization; hero the condi- tions ware undoubtedly rcl; tively mild. So enargite was ob- served in the greenstone, and although the presence of small quantities of arsenic throughout the lavas is not impossible, its presence is a serious difficulty in the way of complete ac- ceptance of the leaching theory. THE NATU3K OF THE CHALCOCITB. Three possible explanations for the Kennecott chalcooite may be offered: (1) diroct deposition from heated ascending solutions, as a direct replacement of the limestone, (2) a replacement of bornite and to a lesser extent of other sulphides by asceruling solutions, or (3) a replacement of bornite and to a lesser extent of other sulphides by descending meteoric solu- tions. It must be admitted at the outset that the evidonoe is conflicting. In the final sumrnary the weight given the various arguments will probably throw tha scales one way or the other, but due to the different values given the evidence by several investigators interested in the problem, a close agreement is hardly to be expected. Evidence from studies in the Geophysical Laboratory. In the course of work at the Geophysical Laboratory knowledge of oortain properties of ohalcocite was obtained which has a dis- tinct bearing on the Kenneoott problem. Synthetic copper sul- 1 _ E. Posnjak, K.T.Allen, unu H.E.Merin, Loo.oit. : ' . I ;TT Aaf 3el*Tarc' 295 phidas formed under certain conditions were found to contain more sulphur then IB demanded by the ratio 2 Cu : 3. It was shown that these products were solid solutions of cuprous and ouprio sulphide. Pure cuprous sulphide - chaloooite - was found to "be dimorphous, the inversion temperature being at about 91C.. Increasing the amount of cupric sulphide dissolved in the cuprous sulphide raises the inversion temperature. This takes place until cuprio sulphide reaches a concentration of about 8 per cent, after which an inversion is no longer observed. The crystals of chaloooite formed at higher temperatures in various ways are isometric. tfhen ooolecl below 91 C, inversion takes place, and if etched the characteristic etch-cleavage of or- thorhombio chaloocite develops. Cuprous sulphide, however, containing more than & per cent of ouprio sulphide if formed above 91 C does not develop the characteristic etch-cleavage of orthorhombio ohalcocite at lower temperatures, but retains an octahedral parting which etches out more or less clearly. Tlu octahedral parting of the artificial material yields patterns on tine polished surface which are similar to the etch- structures of the Kenneoott chalcooite (Pig.l39) "It is fur- ther significant that analyzed samples of the Bonanza ore show about nine per cent of dissolved cuprio sulphide, a quantity sufficient to prevent isometric crystals which may have baen formed above 91 from going over into the orthorhornbio form as the cooling progressed. Etched surfaces of some of the ana- lyzed samples of this ore contain grains in irregular veins ;r eifrcw georitf which etoli like ohalcooite (ortho rhombic) and are thus shown to be the low temperature for.. They may have formed above 91 and contain too little cuprio sulphide to prevent their in- version, or they may have formed below the temperature of in- version*''^ In examining material sent to the Geophysical Labor- atory from our collections, H. K. Ller.via found a number of twinned crystals in the chalcooiie, which he believes from oaftas- uremente of the angles to be octahedral twine of ohaloocite. If his interpretation is correct, it is the first recognition of isometric ohalcocite in nature* The possibility of ohalcocite, with over 8 per cent ouprio sulphide, when formed ut ordinary temperatures, assuming the oc- tahedral form, hay not been positively eliminated. It is stated, however, in their paper thet if solid solutions containing over per cent of covellite oouid be crystallized at temperatures just below 91 , they would probably etch like ordinary ohalcooita. Front th work of the (Jeo physical Laboratory a fairly strong case is presented for the thesis that the Kenaecott chaloooite is of primary origin and formed at a temperature above 91 G, but as it will be shown later, the octahedral structure of the chalcocito may be explained in another way that does not demand a high temperature for Its formation, which consequently weakens the foroe of thoir conclusions. 1 - Kugene Poenjak, B.T.Allen, and H.il - F. ,TAllen, Private communication. 297 videnoe that ths chaloooite IB a replacement of bornite. Structures identical with the isometric patterns described above have been observed, however, in ohaloooite whioh is clearly a replacement of bornite. They were first described by Graton and :.iurdoeh^in 1913. The etch-cleavage in some of the ohaloooite was believed by them to be inherited from born- ite. It was definitely pointed out that such cleavage in sec- ondary ohaloooite could not in all cases be distinguished from the regular (isometrio) cleavage vto ioh they believed to be characteristic of primary ohalcooite. In a more recent article on the etoh -patterns of ohalco- oite, C. ?. Tolrnan, Jr., comes to the same conclusion regard- ing the replacement origin of the chaloooite with the triangu- lar or rectangular pattern, and in addition states positively that this pattern proves the ohaloooite to be a replacement of an earlier mineral, generally of bornite. Material from the Bonanza iiine at Kenneoott, and from the Apache Mines, Hma Co., Arizona, is described by him in whioh the derivation of ohaloocite from bornite is clearly shown. The blue and whita patterns on slightly tarnished polished surfaces are re- garded by Tolman as certain proof of replacement origin. It is believed "that the color is due to the unmixing of the sol- id solution rof ohaloocite and oovellitel whioh develops sub- 1 - L.C.Graton and J. Murdoch, Loo.olt. 2 - Loo.oit. &% ox* . . . 8 malachite azurite. The group is formed at about 0C., as much of the ore is frozen throughout the year. It is the i*esult of the present v^dose circuit; tion, and tho copper is migrating chiefly as malachite . " The mineral sequence given by Tolman is confirmed, in a gen- eral way, by our work; but from field observations and larger collections our mineral list is considerably greater. Ho klap- rotholite was observed, however, and the rarity of galena hardly '. . entitles it to "be mentioned with bornite. Tolman'e statement of the temperature of formation of the primary ore is "based on little real evidence. In his third -rroup of minerals, we oan not confirm tho occurrenoe of tenorite. Limonito is not men- tioned, although it is an extremely abundant mineral. It is doubtful if asurite can in gener&l be regarded as a replacement of malachite . Copper migrating as malachite by means of a frozen vadose circulation is rather surprising. To return more definitely to the topic under discussion, thero is good evidence that an important part of the chaloooite at Kenneoott is a replacement of bornite. The fine grains of bornite, common in many parts of the ore, are most simply in- terpreted as residues. When they are of sufficient size, re- placement relations are usually shown. The triangular pattern in tho ohaloooite is similar to the structure remaining after a lattice replaoeiaont of bornite. The distribution of bornite specks and spines in a few specimens lends force to this inter- pretation for the origin of part of the Kennecott ohaloooite. If the ohaloooito is a replacement of bornite two origins are possible; (1) it is the final product of the ascending pri- mary solutions, or (2) it is a product of descending meteoric solutions. Secondary versus Primary orirln for the Chaloooite . It is obvious thf.t secondary enrichment is virtually Impossible under the present climatic conditions at Xennecott. It is also un- likely thr.t enrichment of any importance was associated with ' frag 300 the oxidation now prominent throughout the deposit. Tho ox- idation is not intensive and is limited to fractures for the most part upon which the ohaloocite shows no dependence. The topography and climate immediately preceding the present gla- oial epoch probably supplied the necessary conditions for its development. From the evidence presented by Gapps, two gla- oiul periods seem probable; the oxidation is possibly the pro- duct of the inter-glacial period. The various surf GOO s to whioh secondary ores could be re- latoJ have boon described in detail on pages 22.J-23 1 *-. The most recent is the surface of low relief developed during the period of pre-glaoial erosion, which to Judge from remnants could not have been far above the mines. During this time the ores may well have boon exposed to long continued oxidizing influences in so permeable a medium as the Chitistone limestone. The long period of erosion which produced the even surface beneath the Tertiary lavas could also havo afforded favorable conditions for enrichment. If projected, this surface ?;ould pass not far above the top of Bonanza Peak. The peneplain out across the Chitistone limestone and the greenstone, whioh serves as a floor for the Jurassic sediments, may likewise have offered a surface from whioh enrichment could take place if the ores were then in ex- istence. At a point less than two miles from the mines, the erosion at this time was sufficient to strip the limestones ooia- 1 - S>. 3. Cupps, oo.cit. 'iMD Jt ! - -. orffl ifi V;Q 06 , ., ne* pletely from the greenstone. Although it is not possible to determine the actual relation of these surfaces with greater precision, it is evident that there were at various periods physiographic conditions whioh may have permitted enrichment on an important scale. Structural conditions at the Jumbo, Bonanza and Erie :.iines are such that a email amount of erosion would make a large amount of copper available for enriching solutions. The plunge of The ore -bodies is approximately parallel to the dip of the limestone. There is good evidence that the dip flat- West East Poss/i/ e position O f early Tertiary surf ace 3# Sketch illustrating relation of ore to successive erosion surfaces. tens to the southwest as the crest of the anticline is ap- proached. Consequently the ores which have been eroded prob- ably had a lower dip than those remaining. If a 30 dip is assumed, which is the least favorable case, it is evident that . .01308 tux*' T O al err 302 if the surface is lowered 100 feet, 200 feet of the ore meas- ured along the dip would be removed. With a decreasing dip this ratio would increase. The concentration of the descending solutions laterally upon the underlying ores would be effi- ciently effected by the barrier offered by the gouge layer of the flat-fault. These factors, in addition to the well-known ease with which oxidation penetrates to great depth in limestone, offer unusually favorable conditions for the ooncentration of copper by descending solutions. The occurrence of a slightly greater amount of bornite In some of the chalcocite from the Erie Mine than la comraonly found in the Xennecott ores, is a slight argument favoring a secondary origin, for tho Erie Mine is farther down the limb of the anti- cline and its ores would have been farther below the older sur- faces to which the enrichment must have been related than is the case in the other deposits of the district. In the summer of 1916, some chalcooite with unusually abun- dant bornite was encountered in the open-out of the Bonansa Mine, at a horizon below the zone of the flat-fault. The bornite IB in material which is undoubtedly more resistant to percolating solutions than the overlying rook, and its preservation in such places is quite in agreement with the relations expected under the conditions of the hypothesis of secondary origin for the chaloooite. Ihe ease of bornite enrichment is another factor worthy of ~i el ' I v " >on j'v. eJimod trr-a'- . 303 no to in favor of a secondary origin for the chaloocite. The presence of chaloooite of unquestioned secondary origin in the bomite nodule from the limestone near the Brie iline shows that the enrichment oan take place in the presence of calcium carbon- ate, although the alteration in this oase is on a small scale. Opposed to the secondary prigin of the chaloooite are the following arguments: The chalcooite occurs in limestone which for the most part shows no traces of aoid attack. Although the enrichment of bornite produces comparatively little aoid, the great quantities involved if all the ohalcooite were formed in this way would surely seem to have been sufficient to have had a pronounced ef- fect on the wall rock. It should be pointed out in this connec- tion, however, that according to the equation established by 1 to Sies ;, Allen and Merwin, bornite may alter a mixture of ohaloo- A cite and oovellite, as follows: CU5 Pe S 4 + Cu S0 4 = 2 CU 3 + 2Cu 3 + F 304 with the production of no sulphuric aoid. The analyses of ap- parently homogeneous chalcooite from the Kennecott deposits are uniformly low in copper, and are interpreted by Dr. alien to in- dicate about 9 per oent of dissolved cupric sulphide. In addi- tion there is a small amount of free covellite distributed rather generally through the ore'. This total however, does not yield the ratio demanded by the equation, but it is sufficient to reduce the amount of aoid produced by approximately a quarter. 1 - Loo. cit.,p. 461. "TO Tjriaf ^xi>nc9fa >j 9bn&ae! 304- The only evidence of acid attack Is the sandy limestone, which was observed in a few places near the ore, some of which was associated with abundant limonite, and certain peculiar irregu- lar masses of gouge-like material of uncertain origin* The former may be attributed as well to the acid released by the hydrolyzing of the ferric sulphate. To what extent the greater insolubility of the dolomite would influence its behavior toward acid attack is unknown. If the chalcocite is attributed to the action of descending waters, it implies the movement of larger quantities of copper sulphate solution through tho ore, and necessarily through the adjacent limestone. It has been established from both chemical and geological data that carbonates or basic sulphates of copper tend to form under such conditions, which would remove copper from solution and hinder the formation of secondary sulphides. It is evident that this process would probably be of little in-* fluenoe on solutions descending through the rich sulphide ores, but it would be expected to check the development of chalcooite in fine stringers in unaltered limestone. The occurrence of chalcocite in thin veinlets in oalcite or dolomite, a few of which show no traces of malachite, azurite or sulphates, is a strong argument against the formation of the ohaloocite from de- scending copper sulphate solutions. The replacement of such large masses of bornite would in- volve the removal of a great quantity of iron, probably as fer- . ajp 3fil-e7 > ofr Jtatoeqxe r o wi 305 rous sulphate. In neutral solutions, suoh as those in a car- bonate rook, limonite would form readily if mild oxidizing con- ditions were encountered; if the iron remained in the ferrous condition reaction with the wall-rock might be expected to yield siderite. Important masses of siderite believed to be of simi- lar origin occur at Bisbee. The limonite in the deposits is not as abundant aa would be expected if the ohaloocite were en- * tirely a replacement of bornite under secondary conditions. The distribution of limonite in part suggests that it is a product of later oxidizing conditions and not of those which produced the chaloocite. Siderite has not been observed, and it is cer- tainly not present in any noteworthy amount on the levels now exposed. These observations offer definite objections to any genetic process which would involve the transportation of large quantities of iron through the limestone in the form of soluble sulphates. Passing upward perpendicularly to the stratification, the chaloocite in the veins gradually pinches out* The fissures continue and are usually marked by reorystallized limestone along their walls. The chaloocite in the upper beds cannot be said to be more intensely oxidized than that in the lower, and there is no overlying zone of oxidized ore. It is improbable* therefore, that the ohaleocit^ coul... have been formed by ver- tically descending waters, for there are no known higher sources from which the copper could have been derived. If seoondary, the copper along the upper edge of the ore-bodies must have been de- ! -X 08 lislJffSjK Hi -i ^11 001 e$: -i ar tl erfcfr 2:i .Jbeietnuoone eiew s of J&. oi-XIgw eifct rf^Jtw noito^ci col bla v io eet r atirtodiJ aetfei ;cl 9 etf ^n.f.il;fiiru' , . a^ ton ' *o-< lnoffiU to col tm j^c tsXJDiro i[e *o. :.ticw ui JnaBSiq; *o ^ lello aitoii 1 ; v^ieecfo eeerf'.? e8o rtl eJjtoc iXXste^s J&e3itiixn ^jJIawau diJ5 Boo i aw ijterf* :{* flsdJ 5sJtJbco -^leaxte^ni e-xoai ed of MSB . S^Jt^lTSVO OK &1 S' a *rf* tfiil* krft ->Tdri ;> SifiJ' I I s 5 H 5 * I 5 ! Vt O * 4* 2 I S 1 VI V 5 8 I I O -J 3 r-4 O O ft O 4 I * A *< I t 4 rl 8. o o Vi o I* v4 8 lJ si 3 JH O S 'H O - d 4* -d i T I K I 4 u g 4 H 'tf 3 III * O, ss s "g ss ||i >H vi O 4* r v ?8 ! 3 o ,a O rt >' IMJ^ 3 5 . Vi * o .s 4 4 4 ^ l 4* l*Tfll v< 4 i:i2H Hi Us vi & 5 v. *j| 9 i o A A .11 4* O O V 4* 8 5 1 g * 4 ^ fl , S,p 53^ N 9 w I o _ S o E d 4 TI J< H * M 4 4* in -a V '. ** * o - i - 1 " ?* - 9 +* ? B u i, \-.. K. H B M **^* . ! O *t f t ; * r * "*" E tf ' "1 S-fl ~ c, O t 9 t* v -i- * r g g 'X O. 1 ' .* fy > *r H |*| 6 5 ?S *S: 1 ^^ 9 k- ' - ' . At A " ff4 * 0^> H * . A A 4^ %T* ** ti ft^ *Ti III 307 rived by almost lateral movement. With a 30 dip it is con- ceivable that descending solutions following the horizon of the flat-fault could have spread far enough to have accomplished this change (Figure 35 ) , but it does not aeem probable that it could 1 .., ,j en done with such searching completeness. On the whole, the continuation of pure oh aloo cite in small stringers into the upper beds is a strong argument against the hypothesis of secondary origin. The arguments supporting the various hypotheses of the ori- gin of the ohaloooite are summarized in tabular form on page 306 . It is apparent that it is difficult to settle the question defi- nitely, but the evidence on the whole presents a stronger case for the view that the chalcocite is primary than that it is sec- ondary. The arguments supporting the latter merely show that conditions favorable for deep enrichment exist; they do not meet the detailed objections raised against it. It is certain that part of the chalcocite at Kenneoott is a replacement of bornite; the poasibility that some of the chalcocite may be a direct re- placement of limestone at temperatures above 91 c, however, cannot be eliminated. dUUQIAEY ATC> CONCLUSIOflS. The mineral deposits at Kennecott consist of two distinct types: (1) small but numerous bornite and chalcopyrito deposits of little commercial importance in the Nikolai greenstone, and larrre ore-bodies of ohaloocite in the Chitistone limestone. Jft :o *, r tne f atcio t>9 L 30S The basic lavas, now greenstones, aro believed to have been the source of the copper in both types. In the deposits in the greenstones themselves, the copper may have been concentrated in part by volcanic after-effects, but the larger veins were pro- duced by later circulation of heated solutions. It is suggested that the solutions may have been derived from meteoric waters, accelerated in circulation and chemical action by the heat of the extensive post -Jurassic intrusions. The ores in the limestone are also believed to have been formed by similar solutions, which had derived their copper con- tent from the greenstone. Where fractures extended into the lime- stone, and other structural conditions were favorable, the solu- tions broke across the contact from the greenstone, and deposited the primary sulphides of the great ore-bodiea. The primary sulphides in the limestone wore formed chiefly as replacements along the fractures, and never occur disseminated through the rock. Locally, deposits were made in small open spaces. The conditions of temperature and pressure were probably mild* The only rock- alt oration is a partial recrystallization of the car- bonates of the limestone. No silicates and a very subordinate amount of quartz occur in the ores. The list of ore-minerals and a diagram of sequence are given on page So ? Bornite, enargite and part of the chaloopyrite were the earliest ore-minerals to form. The bornite probably continued to. be deposited somewhat longer than the chalcopyrite, but the two minerals are believed to have been essentially contemporaneous. . . }MC $Q f srad - /oe 9 A 5 *ijl uX. 310 lo Ifo u/ O& ae. 3 /O O& ae. copper occur, but are not common. A small amount of chalcocite, for example that in the bornite nodule near the Erie Mine, may be attributed to the period in which the malachite a.nd limonite were developed, but it is almost negligible in quantity. The oxidation is not related to the present surface or to the pres- ent climate; it took place under pre-glacial, or possibly inter*. glacial, conditions, when the climate was milder, and the relief somewhat like that at present. If the mineralization continues with depth, there is little possibility that there will be notable decrease in the richness of the copper minerals. If the chaloocite is primary, little change would be expected; if secondary, it can be a replacement only of bornite, hence the change to primary ore would be a grad- ual one and not marked by serious diminution in copper values. Diagram of Mineral Sequence. Mineral Period of Primary Mineralization Period of oxidation Tertiary Interglaoial mite Chalcopvrlte Enargite Pyrlte Tennantlte Covelllte ( primary) Luzonite Sphalerite - aa ^^^^ - (if cfafcocitf ? ' Chalcocite Primary theory Secondary " Covellite ( Secondary ) Linonite Malachite Chalcanthite Antlerite Areen^tes of Copper Cuprite 310 Bornlte was probably by far the moat abundant. The ohalcopyrite is replaced to a slight extent by tennantite, which in addition to this relation, also oocura as fine crystals on the walls of open-spaces. The tennantite was followed by the development of oovellite in coarse crystals in veins or in radial masses prob- ably filling cavities, riflth the covellite, & small amount of luzonite (?) was formed, as thin veinlets, or crusts on earlier crystals* ,.n almost negligible amount of sphalerite and galena accompanied the primary mineralization. The earlier minerals and their relations are almost oblit- erated by intense replacement by chalcooite. The bornite was most completely replaced, but all of the earlier sulphides suf- fered. The chaloooite may be also a direct replacement of the wall-rook in part, although it cannot be settled with absolute certainty, it is believed that the chalcooite is the last prod- uct of the primary mineralization, and not the result of proc- esses of secondary enrichment. Chaloopyrite and a little bornite reoccur, probably as by- products of the changes which produced first the primary covel- lite, and later, the chaloooite, from the earlier iron-bearing sulphides. If the ohaloocite is secondary, part of the ohalco- pyrite is also secondary. The first product of the oxidation of chalcocite at Konne- oott is covellite. This is usually followed by limonite, mala- chite and aaurite, but in one important case, by chalcarthite ani antlerite (a basic sulphate of copper). Cuprite and arsenatos of I a- o aesee PART IV GENE RAL DISCUSSION 313. PART IV GENERAL DISCUSSION GENERAL FEATURES OF BORHITE DEPOSITION Bornite in Deposits of MaKiratio or Pneumatolytio Origin 1 Some Properties of Magtnatio Sulphide Ores. Mag- matio sulphide ores are commonly regarded aa those in whioh the sulphide minerals were forced directly from a magma in a manner analogous to the crystallization of the rook min- erals. The ores should possess the structure of the igneous rook with whioh they are associated, and form an integral part of its body. The gangue -minerals of such deposits should be the characteristic minerals of igneous rocks. Alteration products such as garnet, wollastonite or tremolite, and especially those of hydrothermal origin suoh as epidote, serioite, chlorite, carbonates or zeolites should be either absent or of later origin than the sulphides. Sulphides in many pegmatite dikes usually possess the properties mentioned above as essential characteristics of Biagmatio deposits. In most of these oases, however, there is reason to believe that the concentration of volitale con- stituents was greater than in a normal magma arid that the deposition of the sulphides was related to these agents. Consequently for the sake of completeness in classification, 1. Compare Lindgren, Mineral Deposits, p. 735. 1 diJieaoi'I etaoS 8ii>nlEB ai '-0 -n l 313. it seer; a proper to regard the origin of such deposits as pneumatolytio rather than directly magmatio. The two groups, however, are closely associated, and no sharp line can be drawn between their,, for it is probable that pneumatolytio action plays a more or less important part even in those bornite ores which are moat closely related to magmatio conditions. With changes toward milder pressures and tem- peratures, a similar gradation may exist between pneumato- lytic a-.l hyirotherrcal ores, but the point at which minerals of generally recognized hydrothermal origin appear may be established as the point at which true pneumatolytio condi- tions commence to give way to hydrotheriral conditions. gyrrhotite-Chalcopyrite -Bornite Ores. The ores near Ookiep, Little Namaqualand, are the only rrag&atio depos- its of the pyrrhotite-ohalcopyrite type, in which bornite is an important mineral. The distribution of the ore-bodies in broad lenses or sohliers in unuaual baeio rooks, which exhibit a remarkable degree of differentiation, suggests that the sulphides were derived directly from the magma. The sequence of the sulphides and their relations to the rook silicates indicate, however, that metasooatic replace- ment also played an important role in their formation. 1 From the experinents by Vogt, it is very prob- 1. L. J. H. L. Vogt, Die Silikatsohnrelzlosungen, pt. 1, Videnskabe-Selskabets. Skrifter, Math.-Naturv. Klasse, Kristiania, 1903, No. 8. W. Lindgren, Mineral Deposits, p. 761. 314. able that sulphides dissolved in a dry silicate malt would be the earliest constituent to separate out from & cooling solution. The late position of the sulphides in the se- quence of gangue arid ore -minerals is clearly out of harmony with these results, and indicates with certainty that differ- ent influences controlled the deposition of the sulphides in nature from those provided by the conditions of the experiments. Vogt suggests that the retention of mineral - izers in the magma due to the great pressures, was prob- ably the chief factor which determined the late position of the sulphides in nagrcatio orea. Concentration of metallic constituents is effected in residual portions of crystalliz- ing ir.agaa, as is shown convincingly by ores in pegmatites and in high-temperafture veins. In abnormal magmas in which a high metallic concentration had been produced by differ- entiation, this final segregation would probably produce a still richer and more fluid extract capable of migration and localization in definite ore-shoots. Such late magmatio solutions would be expected to corrode and replace the ear- lier silicates, and, as the character of the solutions changed, the earlier sulphides would become unstable and would be forced to yield to later products. Such an explanation, which is offered for the Ookiep ores, may also harmonize the apparently conflicting field and microscopic evidence at Sudbury. The high metal- lic concentration of the ir&gma in the lower portions of the .3as; 315. Sudbury basin may be attributed to direct differentiation in the liquid phase controlled by gravity, while the con- centration of the sulphiies into definite ore-shoots, with the accompanying replacement of the silicates and the suc- cessive or e -minerals, may be attributed to the agency of mineralizers in the final rr.agcr.atio extracts. The importance of niineralizers in the magma is suggested by the presence of abundant znioropegrnatite throughout the overlying norite 1 and granite. Tolman and Rogers believe that the Sudbury ores are late aagmatic products and claim to have recon- ciled the diametrically opposed viewe of origin. They state, however, that the ores were not formed by the sinking of the sulphide constituents, but offer no explanation for the con- centration of the ores near the bottom of the ir.agwa chamber. 2 Lindgren states that the temperature of forma- tion of magmatio deposits proper ranges from 700- 1500 C. He places the temperature of the niagmatio ores associated with pegmatite* at about 575C. Tolman and Rogers, however, emphasizing the replacement relations between the high temperature silicates and the sulphides, state that the formation of sulphides in all types of high -temperature de- posits takes place probably not higher than 300- 400C. While the value of the work on temperature variations is 1. Loo. oit. 8. Mineral Deposits, p. 188. 3. Loc. cit. . 316. appreciated, it nay be undesirable to try to exprees with too great exactness the temperature at which the sulphides in deposits of different types font. It seems reasonable to assume, however, tnat in deposits intimately related to igneous activity, there is a general decline in temperature from the initial conditions of the melt to the close of the mineralization. Though local or temporary reversals in this progressive cooling may be expeotad, the general tend- ency is probably sufficiently dominant to justify the as- sumption that the time relations of any mineral to the other products of the mineralization may be regarded as an approxi- mate indication of the relative intensity of the temperature under which it was formed. In the deposits at Ookiep, where bornite is involved, the sulphides formed later than the rock-silicates (hypersthene, plagioola.se, biotite), and con- sequently their temperature of formation may be assumed to be lower than that at which the silicates crystallized. The upper limit of the temperature of formation depends on the temperature of the crystallization of the rook minerals and is probably within the range set by Lindgren. There is no evidence which limits the temperatures in this case to the low values given by Tolman and Rogers, although the lateness of bornite in the sequence of silicates and ore-minerals suggests that it was for rued at a considerably lower temper- ature thai; the latest rock-minerals. Pueug-.atolvtic Ores. In the remaining deposits 'Tezgc 1 TOfl /i3Isi td^ & . 317. described in this group, the orea are of a somewhat different type, and clearly more dependent on pneumatolytio conditions. The sulphides occur in pegmatites or in other dike rooks in which numerous miarolitic cavities and the character of the associated minerals clearly indicate the importance of gaseous agents. In the deposits at Copper lit., La Fleur lit., and at Engels, sulphides occur apparently as primary constituents of pegmatite dikes. In the first two deposits, bornite is the chief mineral in this association; at Engels chaloopyrite is more abundant in the pegmatites. At Evergreen the bornite is confined in a large measure to the dike in which the parent rock occurs, and this is also the case at La Fleur Mt. except where the syenite and syenite- pegmatite dikes out gabbro. At Copper Mt. and at Engels, however, the disseminated sulphides in the plutonio rook constitute the chief ore-bodies. In the pegmatites and in the parent dikes at Ever- green and La Fleur Mt., the sulphides are later than the silicates but there is almost no rook alteration connected with the mineralization, except changes attributable to pneumatolytic agencies, as the development of aegirite-augite and fluorite at Evergreen, or the tourmaline at Engels or Copper Mt. The ores in the plutonic rook at Engels, Copper Mt., and La Fleur Mt., on the other hand are usually accom- panied by important changes, and are believed to have been deposited under a range of conditions, decreasing from the - -,;$qefc . ioar ; xl- I 2lL '. 10000 t&fc., 9C .; aeaUJb . ' , oic . 8Y ffl - fe ^5. . i.jrfj :. ; 38 X ' . . , a f. eg 318. Intensity of the pne meat oly tic stages through successively milder temperatures to the feebleness of final hydrothermal emanations. The formation of the sulphides in the wall rook was preceded by the development of hornblende over a wide zone at Engels, converting the original norite to the pecu- liar diorite or metanorite, but at Copper Mt. and La Fleur Mt. this change was more closely confined to seama. The sulphides soon followed, but were accompanied by an increas- ing amount of epidote, serioite and chlorite, indicating that the original pneumatolytio conditions had given way to hydrothermal. In the closing stages at Engels, the final effects are shown by the development of carbonates and zeo- lites. The association of the hydrothermal minerals with the sulphides is so marked in the disseminated ores at Engels and at Copper Mt., that these deposits are properly classed as ores of hydrothermal origin rather than pneumatolytio. In all these deposits, however, it is fairly certain that the earliest formation of the bornite took place under pneumatolytic conditions. At Evergreen and La Fleur Mt., the chief mineralization was confined to this period but at Engels and Copper Mt., the formation of the sulphides con- tinued and became of even greater importance under succeeding hydrothermal conditions. The absence of pyrite in deposits of the magmatic- pneumatolytio type (including Ookiep) is noteworthy. Magnetite is usually abundant (except at Evergreen j. In all cases it is the earliest ore-mineral, and indicates that the deposition of sulphur was unimportant in the early phases of the miner- alization. The association of sulphur with the increasing copper suggests that it may have played an important part as a mineralizer. 1. Tolman and Rogers (loc. oit., p. 15, footnote 25) con- sider sulphur as a aineralizer of importance in the magmatio stage in this type of deposit. . i 3lio nation-. <{l&*ol& *ao(B j*w & 238 OdC olo e erf? 1 js ca si aafcldql e: - 1 jfl^ift ' - 1 jfl^ift ' a flWXSlWl *A ,8Ol*iJ>nOO Qt.$TtlQ8& f t^ j ;w nol^sllassfilw %1 s'i i: l x Jrffi - . tofiJ liOi/ SJg* i? > 319. The praotioal absence of pyrite and the dominance of magnetite in the early phases of the mineralization in these deposits rcay be interpreted as an indication either that sulphur was not present in notable concentrations at the start but was introduced later with the copper, or that the temperature preceding the deposition of the copper was above the dissociation point of pyrite. The dissociation- pressure curve of pyrite, rsoe.-tly established by Allen and 1 Lombard indicates that although pyrite is practically stable up to about 600C., beyond that point the sulphur vapor pressure necessary to prevent the dissociation of pyrite increase* very rapidly with the temperature. It sug- gests that if the dissociation interpretation is accepted, and if the conditions of the dissociation experiments may be applied directly to the conditions in nature, then the temperatures at which the magnetite formed were not below 700C. Somite in Deposits of Con tao t- Met amor phio Origin Bornite ores produced by oontaot-metamorphio ac- tion may be divided into two classes: 1. the ores associated with intense localized alteration of invaded sediments and intrusive along the contact; and 2. the ores associated with a widespread develop- ment of hydrothernal minerals such as epidote, chlorite, and sericite, in both the county rook and the igneous rock. 1. A method for the Determination of Dissociation Pressures of sulphides and its application to oovellite (Cue) and py- rite (FeS 9 ), A. J. S.,Vol. 43. p. 193. (1S17). 330. The ore-bodies of the first group, auoh as the Warble Bay, Whitehorse and Seven Devils deposits, are usually smaller, and nay be considered of direct contact -metamorphio origin in the strictest usage of the word. The ores of the second group, which are of greater commercial importance, and are well illustrated by the extensive deposits at Bisbee, seen to be the product of milder conditions that way extend far- ther from the igneous mass and in which contact- me tamorphic conditions may grade without break into hydrothermal. The source of the netals is clearly the intrusive rook in both oases, but in the latter the distribution of the ores is likely to be less closely related to the actual contact than in the former. In the three deposits of the first type which have been studied, bornite is the chief ore-mineral, but in each it is associated with almost equal amounts of ohaloo- pyrite. In part it is contemporaneous with the ohaloopyrite in period of formation, but probably continued to form some- what longer. Both sulphides are later than magnetite, hema- tite or pyrite. In these deposits pyrite is an unimportant mineral and the early deposition of iron usually tales the form of magnetite and speoularite. The similarity of contact to the group previously described _ rr.etarr.orphic deposits, suggests that the initial conditions under which deposition took place in the former were not far rarroved from the early phases of deposition of the ore- minerals in magmatio-pneumatolytio deposits. The occurrence -Ti.- - \*[k '3 sae . BC J 4 published by A. F. Rogers. In material from Sierra Oaoura, New Mexico, it is clearly shown that the bornite was derived in part at least by the replacement of pyrite, and the ohal- oooite by replacement of the bornite. A little chaloopyrite and oevellite are also found as replacements of bornite. The replacement of hematite by pyrite, the two ages of hema- tite, and the climatic changes deduced by Rogers, are, how- ever, difficult to follow. The question of the origin of ores of the "Red 2 Beds" type cannot be regarded as finally settled, but most writers are in agreement that in general the sulphides were deposited from cold meteoric solutions. Bornite noiules i: t the lower beds of the Chitis%one 3 lirr.estone near Kenneoott, and in shales in Uashonaland, 4 southern Rhodesia, described by F. P. Mennell, are most 1. Origin of Copper Ores of the "Red Beds" type, Eoon. Geol., Vol. II, pp. 366-380, (1816). 2. The Sierra Osoura ores and others of the "Red Beds" type have been attributed to heated ascending solutions by L. C. Graton, Prof. Paper, No. 68, U.S.G.S., (1910). 3. p. 261. 4. Minaralogical Magazine, Vol. 17, p. Ill, (1914). -3* 1 . .-.iv 93 s.n'ax. . )*f' > d lii^JtO 339. case of similar relations in pyritic ores, but in none of the bornite ores which have been studied, does the continu- ance of the chaloocite with depth alone establish the primary nature of the replacement. The possibility cannot be denied that the enrichment of bornite by surface agencies may take place under favorable circumstances as deep as any of the ohalcocite-bornite ores observed in the course of the work of the Secondary Enrichment Investigation. Selective Enrichment* The relative ease with which the change from bornite to chaloocite takes place is an important factor in explaining the features of enriched bornite ores* The common- est ore-minerals associated with bornite may be placed in the following sequence with respect to their resistance to altera- tion to chalcocite: pyrite, enargite, sphalerite, ohalcopyrite, bornite. Between the pyrite and enargite may be grouped tennantite and tetrahedrite, and between the chalcopyrite and bornite, galena should be placed, but the evidence fixing the positions of these members of the series is not' entirely satisfactory, and their exact places are not definitely es- tablished. The information by which the relative ease of replacement by chalcocite is established is gained in small part from field observations, but chiefly from the relations observed under the reflecting microscope which alone are really reliable. -cKi-sTfroe taj .orci/ys iq erf* -fai r . '10 JO* '3 Oi 1 ? rtei r(e :! .:xe i- : 340. Evidence such as that afforded by the disseminated ores in Sacramento Hill at Blsbee fox example oloarly shows the greater resistance of pyrite to alteration than of bornite. There, it will be recalled, the ratio of pyrite to chaloocite in the enriched portions ol the deposit is approximately the same as the ratio of pyrite to bornite in the deeper ores, which indicates clearly that the pyrite was little changed by conditions which had completely altered the bornite* Under the microscope, the comparisons are based on the vol- umes of the different primary minerals which have been re- placed by chalcocite or its associated products under the same conditions, or on the persistence of grains of one mineral which remain in fields in which others have been partially or completely replaced* Examples are numerous of veins of ch&lcooite or oovellite in bornite pinching or dis- appearing when the crack along which they have developed passes into chalcopyrite. (Figure 155*) Enarglte frequently remains unenriched in material in which the chalcopyrite has been attacked. Pyrite itself is attacked to a certain extent, but the amount of chalcooite which ie now attributed to pyrite replacement, ie much less than it was a few years ago when the relations and distribution of the primary minerals were less thoroughly understood* The later minerals, chalcopyrite and bornite particularly, are often in replacement relation- ships to the pyrite, either breaking it in veins or, where 7 341. more abundantly developed, occurring as & cementing material about the grains. (Figures 93 ,9 h ) Where the bornite or ohaloopyrite are completely replaced by chalcooite, the same forme are inherited, and in the resulting product the ohalcocite appears to have been derived by the replacement of pyrite, although ae a matter of fact this mineral may have remained completely resistant to the ohalcocite attack. There is little evidence of exact comparative value with respect to tennantlte and tetrahedrite. Under oonte conditions, they are corroded by the ohalcocite, but usually they remain resistant in fields in which surround- ing sulphides are partially or completely altered. The evidence with respect to sphalerite and galena Is somewhat conflicting. According to Lindgren 1 , the altera- tion of sphalerite to covollite Is the deepest change which takes place at Moreno i, but this statement was made before the reflecting microscope was in common use among geologists and hae not been confirmed by the evidence it has yielded concerning these ores. R. U. Over beck 2 states that sphalerite is more easily replaced by chalcocite than is bornite but his photographic evidence is not convincing. For the most 1. The copper deposits of the Clilton-Morenoi District* Prof. Paper, 3, U.S.G.S., (1905). 3 * A metallographio Bt. ud y o.|. ....thecop. of Maryland^ EC on. Geol., Vol. II, pp. 151-1787^1916) , See figure 9. -z 42. part sphalerite is slightly more resistant to tha secondary sulphides than ohalcopyrite. Galena ia not very commonly teen in relations which throw definite li-;ht on its ease of replacement, but on the whole it is fairly certain that it is more resistant than bornite. There is apparently no tendency for blebs of galena in partially enriched bornite to be aought out by the chaloooite, while on the other hand ohaloocite has been observed associated with blebs of bornite in areas of galena. In the Marble Bay deposit at Texada Island, there is evidence that galena it attacked by ohalco- oite more readily than chalcopyrite, and it is probable that this relation is the general one. Intermediate reactions between those described for pure galena and those described for steinmannite 1 (the arsenlc-actijaony variety) have been observed. It is reasonable to assume that the rate of re- action with copper sulphate solutions would also vary, which might account for its present indefinite position in the se- quence. From the results of experiments in the Geophysical Laboratory 2 , it was found that the sequence of minerals, bcbed on the weight of material altered in two months in 1.25 per cent solution of oupric sulphate at 40C, is as as follows; galena (greatest) (5.5), bornite (3.5), ephaler- 1. J. Murdoch, Loo. oit., Mineral tables, p. 133. 3. E. G. Zieeo, E. T. Allen, and H. E. Herwin, Loc. cit. 'Od o * i / '.: .: 343. ite (l.l),ohaloopyrite (!) 'Based on volume, the series ie : galena (3.0), bornite (3.1), sphalerite (l.l), ohalco- pyrite (1.0). In precipitating power, bornite follows ohaloopyrita, but in volume of mineral altered it follows galena. As the microscopical determination of the ease of replacement is based on volume relations, the second series is more significant for comparison with natural relations* The ease of replacement of bornite is well shown, but the strong reaction in the oaae of galena is surprising. How- ever, as is mentioned by the authors of the paper, the a- mount of material dissolved varies notably with the surface exposed, and it is very probable that in minerals which po- ssess a good cleavage the actual surface available for at- tack ie greatly increased by incipient cracks along the cleavage directions, developed during the grinding. This factor would be especially important in both galena and sphalerite, and may explain their higher position in the series established by chemical work than observations of associations of these minerals in o^g-deposits would accord them. QrosQQpio Relations. Knowledge oi the microscopic relations between the earlier minerals and the group of later sulphides is always necessary to interpret the nature of changes in any ore- deposit, but it ie of especial value in the case of bornite 3,4. ores,, where the greater depth to which enrichment can ex- tend mokes the relations observed in the field leae definite ae criteria, and throws a higher relative value on the inter- pretation of intimate structures observed under the micro- scope. Simple Structures* Under conditions of enrichment of normal intensity, bornite, aa do the other sulphides, alters to ohalcooite along velnlete whose course is clearly determined by preexisting oracke, or along grain or gangue boundaries, where definite channel ways are afforded for the enriching solutions* (see figures 41, and 7^). The distribution of the chalcocite is dependent largely on these easily recognised causes, and usually Dhows little tendency to be controlled by the more delicate influences of crystallographic nature or other internal properties of the bornite. In certain deposits in ores near the bottom of the enriched zone, the chalcocite works its way around the edges of the individual bornite grains within the massive material which results in a peculiar pattern on the polished surface of large and snail bornite areas apparently floating in ohalcoclte. Where the bornite grains are small, replace- ment advances more rapidly, consequently a matrix of finer grains is often eaten out from around the larger, producing a pattern on the polished surface to which C. F. Tolman, 1. Loc. cit., Fig. 13. /I 345. has given the descriptive naae, "ice-cake structur- The Lattice Structure^ The replacement of bornite, however, under certain conditions may develop other structuree core complicated than the simple ve inlets or rime described above. In one prominent type, which ie commonly termed the "lattice struct lire", the replacement advances in a differen- tial manner, apparently controlled by crystallographio factors. Plates of ohaleocite, covellite or chalcopyrite develop in tha bornita, parallel to two or three structural directions, and when sectioned by the polished surface yield a reticulate grill ol intersecting strips ox bands in a field of bornlte. (:-"iCures M , 7^ , 77 ,13^,137 ,1^6 , and 152.).. Chalcocite is the most abundant mineral in this relation with bornite, but oovellite and chaloopyrite are by no means uncommon. The structures are very similar in the three cases, and indicate with little doubt that the orientation of the plates is due . to the common hoot mineral. En&rgite and luzonite (?) have alno br:en observed in lattices in bornite, but they are un- common and appear to be of primary origin. On the polished surface, the lines of the replacing mineri,l in the lattice structure are usually oriented para- llel to three directions but a fourth direction ie shown oocssionslly, which suggests that the bornite yields to the attacking solutions along octahedral planes. The octahedral cleavage cracks, developed by pressure on the polished sur- face, form a pattern very similar to the lattice structure, I /*- 346. an agreement which strong 3y euggests that this peculiar re- placement structure was determined by the bomite cleavage* The octahedral form of both the lattice structure and the pressure cleavage in bornite la farther suggested by their eimllarity to the etch-structure of synthetic isometric ohalcocite which ia known to follow the octahedral parting.^ (Figures 135 and l4o.) The replacement origin of the minerals associated with the bornit in the lattice structures is convincingly shown in most of the deposits st\>died, but since other explanationa have been advanced by two writers for similar water ial in the Butte ores, it will be neoeesary to consider the evidence in detail. Chalcopyrite, when developed in bornite in asso- ciation -?ith ohalcooite or covellite of recognized secondary origin, nearly always assumes the lattice fora (fi-rureaW , 7? , .13^), and its distribution near veinlete or in the neighborhood of the secondary sulphides clearly indicates its dependence on the same agencies as those which produced the copper sulphides. There can be little doubt that the mirsral is & common product of the alteration of bornite. Chalcopyrite platee in bornite similar to those of the lat- tice structure have been produced artificially in the Oeo- 'hysioal Laboratory 2 by treating bornite with ouprio sulphate 2. T. Allen and H. E. Merwin, Loc. dit., p. 52- 2. E. G. Zies, E. T. Allen, and H. E. Merwin, Loc. oit., . 479. - . 347. solutions. Chaloocite is the chief product of the reaction but within the bornite grains away from the direct attack by the reagents, chalcopyrite was formed* AB chaloopyrite Is alao produced by the action of sulphuric acid on bornite , its formation in thie experiment and in nature is attributed by the members of the Geophysical Laboratory staff to the action of sulphuric acid produced by the alteration of bornite o to chalcooite or covellite* In oases where the development of chaloopyrite is accompanied by shrinkage, it is quite probable that the alteration was accomplished partly or wholly in this way (figures^ and lj-5), but where no volume change can be observed, which is the commoner case, there must have been a distinct addition of iron to the space originally occupied by the bornite* The reaction is probably more com- plicated than simple acid attack. From the evidence in the ores themselves and from the synthetic work in the Geophysical Laboratory it may be regarded as definitely established that chalcopyrite associated with bornite in the lattice structures is of replacement ori- gin, and that it is closely associated with the alteration of bornite to chalcooite and oovellite, a change which or- dinarily tak-58 place under secondary condition* 1. Ibid., p. 476 2. Ibid., p. 480 aid* svz 6 348. The possibility of "reticulate inter growth*" of bornite and chaloopyrite under primary conditions, however, cannot be entirely eliminated for somewhat similar structures were formed artificially at the Geophysical Laboratory in a sulphide melt from which products close in composition to bornite and ohaloopyrite respectively crystallized. But, while there is a general resemblance in a broad way between the structures, the details are distinctly different, and easily distinguished. Forms exactly similar to these syn- thetic structures have never been observed in natural prod- ucts. All the available evidence, however, indicates that the chalcoclte, oovellite and ohalcopyrite in lattice struc- tures with bornite is of replacement origin* The common distribution of oovellite spines around the margins or grains or along the border of veinlets in bornite, often closely associated with chalcocite or prod- ucts of oxidation, makes the replacement and the secondary origin of oovellite in this relation so apparent that other interpretations have not been suggested. The chalcocite in the lattice relation with bornite is believed to be of replacement origin by Graton and Mur- doch 1 who first described the structure. In a later paper by J. C. Ray 2 , dealing with the Butte ores, the opinion is 1. LOG. cit. 3. The paragenesia of the ore minerals in the Butte dis- trict, Montana, Boon. Geol., Vol. 9, p. 479, (1S14). 349. expressed that the bornite in lattice structures in the "covellite zone" was formed by the elimination of iron from the chaloocite along crystallographic planes. He states that the precipitation of ohalcocite in a colloidal state is strongly suggested and that the formation of chalcopyrite and bornite is believed to be due to a reaction of iron locally taken into solution. In a discussion 1 published more recently, however, he apparently regards the chaloocite in the lattice patterns to be an incomplete replacement of bornite, and his excellent photographs strongly support this view. Julius Segall** from a study of similar material, interprets the relations to indicate a replacement of ohaloo- cite by bornite, along crystallographic directions of the former mineral. W. L. Whitehead 3 , however, from a study of material from many localities, supports the earlier inter- pretation, advanced by Graton and Murdoch, and regards the evidence presented by Ray and Segall to be unconvincing. In all the ores studies in the course of this work, there has been distinct evidence that the chalcocite in the lattice structures is closely associated with chalcocite, which from its occurrence in veins or rims, is plainly of replacement origin. The beginnings of the lattice patterns 1. J. C. Ray, Econ. Geol., Vol 11, pp. 179-185, (1916). 3. The origin and occurrence of certain crystallographic intergrowths, Econ. Geol., Vol. 10, p. 467, (1915;. 3. The paragenesis of csrtain sulphide iatergrowthe, Econ. Geol., Vol.11, p. 10, (1916). 350. have often been observed as fine lines penetrating the bornite from the edges of ordinary veinlets or rims. In these early stages, the stripe of chalcooite are most readily interpreted as veinlets in the bornite* All degrees of ohaloooite abund- ance may be explained aa the results of the widening and ex- tension of these veinlete, with the accompanying gradual re- duction and elimination of the ieolat-ed rhombs and triangles of bornite* When examined under high magnification, these residual shapes are often found to have blurred bluish borders, due to a fringe of tiny bornite specks severed but not yet destroyed by the advance of the ohalooclte. The blue color in these transition zones may be due in part to exceedingly minute bits of covellite, which is known to form as an inter- mediate product. Hazy boundaries of this sort are common accompaniments of replacement by secondary ohalcooite> and these relations in the lattice structure prove beyond doubt that the ohalcocite is of metasomatio origin. While the views advanced by Hay and Segall may seem to explain the relations observed in certain special oases, it is believed that the abundant evidence of replacement exhibited in many districts offers a more generally applic- able interpretation. None of the relations described by Ray in his earlier paper presents any objection to the re- placement origin of the ohalcocite. Segall 'e arguments are based largely on the resemblance of the lattice patterns to the triangular etch-structure of ohalcooite, and on the 351. occurrence of bornlte in vein-like bands along cracks in ohaloocite or bornite-chalcocite areas. Chalcocite v^ith the triangular type of etching, however, is uncommon except where associated with bornite. The usual orthorhombio etch-pattern of chalcocite ie not similar to the bornite-ohalcooite lattice (Structure. Moreover, it is difficult to conceive of the me- chanics of a replacement process which would develop angular grains of the new mineral between regular vein-like strips of other. The interpretation would demand two ages of bor- nite, for the evidence of the later age of some of the chalco- cite is undeniable in moat deposits. Borders of bornite, such as Segall illustrates, alone irregular cracks in areas of ohalcocite or of chaloo- cite and bornite (figuresl't6andl 50), often in the lattice relationships, certainly resemble veinlets of bcrnite cutting the surrounding; material. As a matter of fact, in this kind of occurrence as well as in related structures in which mar- ginal rims of bornite surround fields of mixed ohalcocite and bcrnite, detailed examination under high magnification often shows that the bcrnlte is not later than its surround- ings, but is instead residual and is yielding) though with apparent resistance, to the same kind of replacement by chalcocite ae that which day be shown by the bornite in the general field of replacement. 1 1. For an excellent example eee J. Murdoch, Op. cit., Frontispiece! Fig. 2. 353 Irregular stripe of bornlte in fields partially altered to chalcocite, or rime of bornite about areas con- taining chalcooite and bornlte have ben observed, which on first inspection apparently support Segall's interpretation, via., that the bornite is a replacement of the chalcocite. However, detailed examination under greater magnification rarely fails to reveal fine veinlets of chalcooite or other relations indicating partial alteration of the bornite to the ohalcocita, which make it highly probable that the elongated forme of the bonnlte are inclusions and not veins. The pos- sible explanation may be suggested that this peculiar pres- ervation of the bornite from chalcocite attack was due to the concentration of outgoing iron solutions produced by the re- placement reactions. The differential attack of the ohaloooite in the lattice structure whereby certain strips are altered and others left suggests the explanation that the alteration was produced slowly under mild conditions, probably by dilute solutions act- ing for long periods of time. Where enrichment is intense, the lattice structure is rare; it is commoner* In the deeper parts of the enrichment zone where bornite is the only mineral I altered, or in protected, impermeable cores. This distribution is clearly shown at Bisbee. The structure is well known in ohalcocite ores of accepted secondary origin, and it seems probable that the milder and more slowly acting processes of seoondary enrichment would be more favorable for its production 8i ft' 353. than the more intense action of heated ascending solutions of the period of primary mineralization. The occurrence of a structure of auch delicacy under two sets of contrasted con- ditions, while possible, would be rather unusual, and the presence of the lattice or oloeely related structures in the ores at Kenneoott and at Butte may be regarded as an argument in favor of ji secondary origin for the ohaloooite in these disputed occurrences. Residual Structures Related to the Latti ce Structure . Associated with the lattice structure are various modifications, in part closely related in form, and in part transitions to other structures. Of the former, the most prominent is the occurrence of definite, sharply bounded plates of bornite in the chalcooite, or ohalcocite-bornite lattice, which apparently remain resistant, to the replacing reactions. (Figures 1^3, 1^7 andlW). From the appearance of these plates on the polished surface, the name 'spine structure" has been commonly applied to it, but as this expression is not accurately descrip- tive for the rather blunt sections of the bornite plates, as revealed on a polished surface, the term spline structure 1 is suggested. Where associated with the chalcocite-bornlte lat- tice, the plates or splinee are usually oriented symmetrically but not parallel to the lattice directions. The edges against the chalcocite commonly yield evidence of replacement, and 1. Spline, a thin plate in a slot, a term used in con- nection -with machinery and carpentry. - . . c . 354. /" *t i where the enrichment is intense, they may be broken by numer- ous irregular cross veinlets of ohalcooite. (Figure 143 . ) They rarely yield a lattice pattern by their decomposition, but the alteration along their edges is often controlled by the directions of the adjoinia^ lattice. (Flguresl^Tandl 1 ^.* ) The strips of bornite isolated between long bands of ohalco- oite in the lattice structure are rarely as long as the splines described above, and are usually broken by regular cross-bands of ohaloocite. Their contacts with the chalcocite are of the hazy indefinite type for the most part. Consequently there is little indication that splines aro normal residues of this sort. There is a distinct suggestion that the bornite in these forms is of different character from that which filled the surrounding areas, for the difference in resistance to ohaloocite replacement is very marked and la difficult to ex- plain on other grounds. This possibility will be discussed more fully on later pages. Numerous, sharply defined grains of bornite are often left scattered through the chalcocite field, after the replacement has almost obliterated the angular bornite resi- dues of the lattice structure. The grains are usually very small, mere dote on the polished surface, but by their true color they are in notable contrast to the indefinite residues of bluish bornite which mark the last stages of the lattice replacement. The specks are usually irregularly scattered but their distribution preserves the lattice pattern by their . -It 'a eo. 355. avoidance of the white etripa of chaloocite which mark the lines along which the first replacement was accomplished. In eorae of the Dutte ores in which this spot structure is particularly prominent, (Fi-rurelV?), the arrangement of the grains sometimes shows curving banded patterns imposed upon a field in which the lattice structure ia still strongly evidence. The spot structures are most prominently developed in the Butte and Bisbee bornite-chalcoclte ores* In some oases, where the grains were somewhat larger than usual, they assume smoothly irregular, forma, very similar to blebs of bornite in the graphic stiucture. 1 While the most typical graphic structures have not been observed in this association, where the surrounding ch&lcocite had clearly been formed by lattice replacement of bornite, the approach to true graphic forms is so close in many oases, as to afford a strong argu- ment that the graphic and lattice structures are of related origins. As in the case of the splines, the existence of a more resistant type of bornite is suggested by the presence of these sharply defined grains, remaining in fields in which all the rest of the bornite has been nearly or completely altered. Chalcooite Derived from Bornite. Chalcocite, which has been derived from bornite 1. See photograph by J. C. Ray: Econ. Geol.. Vol. 11, p. 184, Fig. 6, 1916. arf * fa iei 'v '-/ >- *.*w-* ' 358. An inheritance of cryetallographic orientation of one mineral by another ie not an uncommon phenomenon, but the inheritance of internal crystal structure ie undoubtedly rare. It is perhaps difficult to comprehend the inheritance of that particular molecular arrangement which determines cleavage, but it is conceivable that certain planes of weak- ness, parallel to the cleavage directions, which directed the advance of the first stripe of lattice chalcocite, could be preserved as apparent cleavages or pseudo-partings in the chalcocite in the same way that a visible crack which has localized replacement on a coarser scale, often remains as a line of weakness within the final product. This property, which is neither cleavage nor parting merits a separate designation, and for it the term herifrage (from the Latin heres, heir as in inheritance, and fragilis, from frango, break) is proposed by L. C. Qraton and the writer. It may be defined as the property of breaking or etching along particu- lar planes of systematic arrangement acquired by replacement of an earlier mineral which possessed cleavage in correspond- ing directions. The triangular etch-structure is known in secondary ores although it IB not a very common feature, but it cannot be limited to chalcocite of secondary origin. It is most strikingly developed in the great ohaloooite ore-bodies at Kenneoott, for which a primary origin is favored. There is the possibility that the development of bornite lattice struc- / *M. ture and of the related triangular pattern in chaloooite is facilitated at temperatures above 91 C, at which the chalco- cite possesses an isometric form similar to that of bornite, but the relationship is certainly not confined to such con- ditions. Although the distinction between the isometric structure of hi^h temperature chaloocite and the structure inherited from bornite cannot always be made, it is certain that material possessing this structure is either of primary nature or, if secondary, is a replacement of bornite. Con- sequently, chaloocite of this sort would either be expected to continue with depth, or to be succeeded by primary bor- nite, which would not mean a very serious decrease in copper values . \ 360 THE GRAPHIC STRUCTURE. Description The tarn "graphic structure" has been applied to the relation between bornite and various other, sulphides in which the minerals are associated in a manner resembling my- merkitic or micropegmatitic intergrowths of quartz and feld- spar. The structure is characterized by the association of the two minerals (rarely more) in minutely sinuous strips of varying widths or elongated blebs and lobes of sub-equal size. The strips are simple or irregularly branched and in some oases complexly interdigitated. A rough parallelism in one or more directions is not uncommon, and in some instances it approaches a regularity sufficient to suggest a relation to crystallography directions* The contacts between the two minerals are nearly always sharp, and rarely offer any evi- dence indicating relative sequence. As many tongues of bor- nite extend into the associated mineral, and as many projec- tions of the latter are apparently severed by the bornite as the reverse* In other words the boundaries of the two miner- als are mutual. These associations have commonly been called "graphic intergrowths", but since the word "intergrowth" is often used in the sense of simultaneous formation of the two components, and since such an origin cannot be univer- sally established, a term that is merely descriptive, like 361> graphic structure is to be preferred, for it implies no theory of origin. The following minerals have been observed associated with bornite in the graphic structure:-- ohalcopyrite, galena, enargite, tetrahedrite, tennantite, klaprotholite, and ohalco- cite. Of these, the bornite-ohalcooite association is far more abundant and of much greater inter eet than the others, but as it will be treated at greater length, the occurrence and significance of the less common minerals will be dis- cussed first* Primary Minerals Associated with Bornltfi in the Graphic Structure* The ordinary primary structure exhibited between bornite and ohalcopyrite is not similar to the graphic form 7 (Figures ^7 and 67 ), but in a few oases, as at the Evergreen Mine the relations between the two minerals are practically those ordinarily observed in the well-known bornite-ohaloocite '& graphic pattern. (Figures ^5 and ^6.) The bornite and chalco- pyrite are generally believed to be essentially contempora- neous, and in this particular case there is every reason to believe that the graphic structure, which is exhibited be- tween them, is of direct primary origin, and the result of the simultaneous deposition of the two minerals. In the neighborhood of the graphic structures, however, the rather common feature of slight primary corrosion of ohalcopyrite by bornite can usually be seen, which is regarded as an indi- bev "0 ., Ixe ai 382. cation that the initial conditions of contemporaneous de- position were succeeded by those under which ohalcopyrite yielded a little to bornite replacement t Bornite and galena in graphic relations are more common* The distribution of the two minerals indicates that both are primary in all examples in which the structure has been observed, and the absence of all evidence of replacement nature, indicates that the two minerals are of simultaneous deposition and that the structure between these minerals like- wise is that of a contemporaneous intergrowth. The bornite usually predominates over galena and is the host mineral* The bornite-galena intergrowths differ from the usual form of the graphic structure by the marked variation in size of the galena blebs (figures 9 and 90), which range in some cases from fairly coarse grains down to the finest specks that can be distinguished under the oil- immersion lens* An intergrowth of bornite and galena, coarse enough to be visible with the 1 unaided eye, has been described by H. P. Collins. Tennantite and tetrahedrite are uncommon in the graphic structure, but occur occasionally accompanying other minerals in this relation. In certain fields of graphic structures between chalcocite and bornite a few of the light colored blebs are found to be tetrahedrite or tennantite, but in the same shapes and relations as the more abundant chalco- 1. Mineral epical Magazine } Vol. 13, p. 336, (1903). jcevo af?.; -.: 3 la'x . ? ( .; 50 OC vT f 363. cite. A similex association of tetr&hedrite and tennantite with galena-bornite graphic intergrowthe has been noted* Graphic blefcs of enargite in bornite have been seen in a few specimens from Butte. An unknown pinkish mineral from Butte also shows graphic relations to the bornite* A few grains of klaprotholite, a rare eulphobismuthite of copper, which has been doubtfully identified in bornite from the Marble Bay Mine. Texada Island in connection with this work occurs in forme closely simulating the bleb* Of the graphic structure. A. F. Rogers, however, has described klaprotho- tite in graphic relation with bornite in ore* from Butte and other camps, but in general the mineral occurs in very un- important quantities* Rogers states that it is later than the bornite. Chaloooite in the. Graphic Structure. The minerals in the graphic relation which have been described in the last few paragraphs are of primary origin, and with the possible exception Qf klaprot hoi ite, are probable intergrown simultaneously with the bornite. The origin of ohalcocite in the graphic structure with bor- nite is less easily settled, however, and offers a problem of definite commercial as well as scientific interest. The relations upon first inspection suggest a contemporaneous 1. Econ. Geol., Vol. 17, p. 584 (see Fi. 4), 1916. . ixow sJMtf rffrr. e^ awiiuc iff . MHO i .. r 364. origin for the ohaloooite and bornite, and henoe that the chalcooite was formed under primary conditions, The abund- ant distribution of chalcocite as a secondary mineral in most camps, however, make 8 any relation which may indicate a primary origin worth especially careful atudy, and demands a critical examination of the evidence before any other inter- pretation than secondary can be accepted* Bornite and chalcocite are occasionally associated In a structure identical with that often observed between chaloopyrite and bornite. (FigureslOlaJid 102*) The structure ie similar to the graphic pattern in that the two minerals exhibit mutual relations for the most part, but differs in the larger size of the blebs and in their often pointed and scalloped forms* The two structures are probably of about the same origin* Literature. The graphic structure between bornite and ohalcooite was first described by F* B. Laney in ores from the Virgilina District, Virginia. It had been previously o suggested that the deep ohalcooite ores in the district were of primary nature, and this view was supported by Dr, Laney who regarded the graphic structure as the result of a primary intergrowth of ohalcocite with bornite . Graton 1. F. B. Laney t Proc. of 0. S. Nat. Museum, Vol. 40, p. 520j and The relation of bornite and chalcooite in the cop- per ores of the Virgilina District of N. C. and Va., Econ. Geol., Vol. 6, pp. 399-411, (1911). 3. L. C. Graton, Mineral Resources, U.S.G.S., 1907, Part I, p. 630. SAX 8 li t ei/oivs: 365. .nd Murdoch in their paper on the sulphides of copper accepted the graphic relation as an indication of contempo- raneity, and hence a criterion for the primary nature of the chalcooite. The same interpretation was given the M structure by C. C. Gilbert and G. E. Pogue in ores from 3 the North lit. Lyell Mine in Tasmania* R. M. Overbeck described chaloocite and bornite in graphic structure in ores from Maryland, and believed that the two minerals are contemporaneous. It is suggested in the description of a microphotograph that the bornite may in part be later than the chaloocite. Bornite and chalcocite in irregular associations somewhat akin to graphic relations were described by A* F. A Rogers in 191*-, and believed by him to be of replacement character. The similarity to the structures described by Laney was pointed out, and it was suggested that the same origin may be applied to both. No proofs were advanced* In an earlier article on the Butte ores, the same writer came to the conclusion that the deep chalcooite at Butte 1. L. C. Graton and Joseph Murdoch, loc. cit., p* 768* 2. C. C. Gilbert and G. E. Pogue* The lit. Lyell Copper dis- trict of Tasmania* Proc. U. S. Hat. Museum, Vol. 45, pp* 609-625. 3. R. M. Overbeck, A met all graphic study of the copper ores of Maryland, Econ. Geol., Vol. 11, pp. 151-178, (1916); see also, B. S. Butler and H. D. kcCaekey, Copper ores of the Hew London Mine, T.A. liU.E., . Tol. 49, p. 287, (1915). 4. A. F. Rogers, Secondary enrichment of copper ores with special reference to microscopic study, Mining and Scientific Press, Oct. 31, 1914, p. 686. 5. Upward Secondary Enrichment and ohalcocite formation a * Butte, Montana, Vol. 8, pp. 781-794, (1913). 366. Is in part a replacement of bcrnlte, but specific reference was not made to the graphic structure* Chaloocite in graphic structures in bornite from En^ele, California was described by A. F. Rogers, and believed to be du* to a peculiar break-down of bornite into ohalcoeite. He ex- preset i th doubt whether graphic relations due to simul- taneous intergrowth ever exist between these minerals* 2 In similar material, Ray, ae has been mentioned, apparently regarded both the bornite and chalcocite as replacements of covellite, but considered them to be of simultaneous origin* Later he stated that the graphic structure may originate by the replacement of bornite by ohalcooite. The view has been advanced by L. C. Oraton 4 for graphic structures in general that some may be simultaneous intergrowths and others may originate by replacement. The replacement origin of the chalcooite in the graphic structures is favored by W. L* Whitehead, whose views are well supported by hie discussion and excellent mlcrophotographs . A. F. Rogers, 6 in a later article, comes 1. A* F* Rogers, Engele article, Econ. Geol., Vol. 9, p. 381. (1914;. 3. J. C. Ray. Econ. Geol., Vol. 9, pp. 463-487, (1914). 3. Econ. Geol., Vol. 11, pp. 179-185, (1916). 4. L. C. Graton, Trans. A. I. LI. E. , Vol. 58, p. 597, (discussion). 5. W. L. Whltehead, The Paragenesis of Certain Sulphide Intergrowthe. Econ. Geol., Vol. 11, pp. 1-13, 6. A. F. Rogers, The so-called graphic intergrowth of bornite and chaloocite, Vol. 11, pp. 587-5S-3, Econ. Geol., (1816, ) 367. to very similar conclusions and advances some additional arguments. It ia apparent from this brief survey of the literature, that there is notable divergence of opinion concerning the origin of the ohalcocite in the bornite- chaloocite graphic structures* The evidence in most oases admits of more than on* interpretation, and it is difficult to advance arguments of sufficiently compelling nature to settle the question in a satisfactory and final manner. Our own observations confirm the relations described by the other workers in nearly all oases, but our interpretation of their value as arguments differs somewhat. The evidence favoring the two opposing views of origin will be summarised in order in the following paragraphs, and on the later pages, the significance of the various relations will be discussed. Summary of Evidence. Evidence favoring a contempo- raneous origin lor the bornite and chaloocite in the graphic structure may be summarized as follows: (1) the similarity of the graphic structure to the structures of metallic eutectics, (2) to the structure of quarts and orthoclase in micropegmatite; and (3) to the structures between bornite and primary ore-minerals; (4) the sharp contacts and mutual boundaries be- tween the bornite and oh-.lcocite; (5) the occurrence of the otmlcooite in the heart of bornite grains with no visible channel ways for the entrance of altering solutions; 368, (6) the lack of relation between the ohaloo- cite and grain boundaries and gangue contacts; (7) the contrast to the structures of bornite and chaloocite in specimens in which the ohaloocite is definitely known tc be of secondary origin; (3) the parallel orientation of chaloooite in the blebs over considerable areas* The arguments supporting the view that the chalco- cite is a partial replacement of the bornite may be enumer- ated as follows: (1) the occurrence of occasional microscopic veinlets of chalcoclte connecting the ohalco- cite lobes; (2) the lack of discernible boundaries between the chalcocite of the veinlets and the chalcocits of the lobes; (3) the common cr yet allograph ic orientation of the ohaloocite in each; (4) the gradation and merging of chalcocite of replacement origin with the chaloocite of the bornite-chalcocite graphic structure; (5) the presence of small spines of chalco- pyrite in the bornite blebs; (6) the replacement by chaloocite of blebs of other minerals (ae klaprotholite or galena) associated in graphic relations with the bornite; and (7) the presence of bornite "residues" arranged with a faint suggestion of the lattice struc- ture in ohalcocita blebs of the graphic struc- ture. The points in both lists will be discussed in the order of their enumeration, beginning with those which favor contemporaneous origin of the two minerals. j*>tjf $,.Ci * ^fflHBOfllq^l ; Si rfci'v&Jbr'x/. ad^ lo 363. Arguments Favoring a Contemporaneous Origin. (l) and (2). Although the similarity between the graphic structure and metallic? eutectics is striking, the evidence is not sufficient either from our knowledge of conditions of ore-deposition or from experimental work to state that the graphic structure in sulphides is of eutectio or eutectoid 1 origin. Whitehead draws attention to the discordance be- tween the structure of metallic eutectice and the graphic structure, shown by the presence of an excess of only one constituent in the former, whil--> in the latter, both con- stituents, bornite and chalcocite.are commonly present in addition to the association of the two in the graphic areas. Relations strictly in accord with artificial products formed in a closed system and in a limited time, however, should not be expected under the changing conditions which must have obtained during the formation of the natural sulphides. Both quartz and feldspar in excess are common accompaniments of micropegmatite yet this structure is believed by nearly all observers to be a. simultaneous intergpowth ^f the two 2 minerals. E. S. Bast in has presented evidence which indi- cates that the coarser graphic structures in pegmatites are not of eutectic nature, but he believes that the components are of contemporaneous origin. The sulphides may likewise 1. LOG. cit. p. 5. 2 E. S. Baetin, Origin of the pegmatites of Maine, Jour. Geol, Vol. 18, 1910. pp. 397-320. Bull. No. 445, U.S.G.P., 1911. I0a at i-: l a to. ^ 7 1; tXBC' sqoi -i Af\ is AH fc-V *J ~jJ . 570. be simultaneous Intergrowtha even though not euteotios, al- though the argument baaed on eimilarity of pattern is slightly weakened if it oannot be admitted that an origin similar to the metallic euteotice is possible. Further relations of the graphic structure to euteotice are discuseed on later pages. (3) The graphic relations between chalcocite and bornite are very similar to th* associations between bornite and certain primary minerals which have been Described on pre- ceding pages* For the most part, the minerals of this group are unknown or uncommon as replacements of bornite, and in general there is good evidence to believe that they are of contemporaneous origin with the bornite* (4) T 7hile sharp contacts are common features of sulphide* in primary relations to each other, their existence in the graphic structures does not constitute definite evidence of contempo- raneous origin* Examples are numerous of chalcocite, clearly of secondary origin, with sharply defined boundaries against bornite, and indications are accumulating that many primary aeeooiations of minerals with clean-out outlines may likewise be a result of replacement. (5) The occurrence of chalcocite in the graphic patterns in the heart of bornite graina has now been observed in enough cases to greatly reduce the possibility that ohalcooite is related to definite visible channel ways which are unexpoeed by the section revealed on the polished surface. It is con- i & . - >: to e: }V 371. oeivable and probable that material can be transferred by channel-ways of sub-microscopic size, or by diffusion through the massive sulphide, for movement of this sort must take place in the ordinary process of replacement to supply new material and to remove waste products through the compact layer deposited in the earlier stages of the reaction. The replacement of chaloopyrite or bornite by chalcooite due to descending copper solutions affords a definite example of this action for although the replacing sulphide adheres firmly to the primary mineral, and forms an apparently impermeable coating, the alteration often goes on to completion in spite of it. Even as Burning this mechanism, no explanation is offered for the frequent lack of alteration of the bornite in the outer portions of the graine, which are normally most easily affected by the replacement processes. Under the usual conditions, there is no known reason why the attacking solutions should have withheld action until the centers of the grains were reached. While graphic chalcocite is by no means confined to the heart of grains in general, the occurr- ence of many definite cases in this relation and the diffi- culty of explaining the association by the ordinary mechan- ics of replacement, offers a strong argument againet the view that the structure could originate by the metasomatic alteration of a homogeneous mass of bornite. (6) and (?) The increasing number of observations of chalco- watt sriT .ctiM t }tee>ir rf 1 *t adf a Silt 375. not necessarily parallel to the strong direction of the chaloocite cleavage. With these mod ifioat ions it is apparent that the parallel orientation of the chaloocite in the graphic struc- tures possesses little value as an argument favoring con- temporaneous origin for the two minerals. This exhausts the arguments which have been advanced in favor of the contemporaneous origin of the ohalcocite and bornite, and on a whole it ie apparent that they present a strong case against the view that the graphic structures could originate by the replacement of massive, homogeneous bornite* They make it clear that many properties of the graphic struc- ture are closely related to those of contemporaneous inter- growthe, but nevertheless the evidence does not show with certainty that the chalcooite in the graphic patterns is of contemporaneous origin with the bornite. Additional considera- tions are presented on later pages. Diaouseion of Arguments Favoring a Rep^ace.ment Origin. The arguments of the second group, viz., those favoring a replacement origin for the chslcocite, are discussed in order below. (1) Whitehead states that the graphic structure originates by the replacement of bornite by cbalcocite along minute inter- lacing veinl te. The characteristic pattern la attributed 1. Loo. cit., pp. 6-9, Figures 6, 7 and 8. ll 376. to a stage of the metasomatism in which these veinlats are broadened in places and the bornlte masses rounded where the ve inlets intersect. In many graphic areas, how- ever, even the fine veinlete described by Whitehead cannot be observed under the oil-imnersion lens, and from our ex- perience, it would seem that although they are not an un- common accompaniment of the graphic structure, they are not invariably present. If the graphic structure originated in the manner suggested by Whitehead, it is difficult to under- stand why the degree of replacement in these structures, now known from many widely separated districts, should be so similar. Intersecting lace-works of veinlete have been observed in bornite in which no tendency toward the graphic structure could be detected, but between this stage and the typical graphic structure, no relations have been noted which could be intermediate steps in the change. No explanation Is offered by this plan of origin for the differential charac- ter of the replacement. Chaloocite developed in this way would not be expected to differ so distinctly in distribution from replacement chalcocite in general, consequently the ob- jections raised by the points discussed under preceding bead- ings remain in force. Replacement under theee conditions would be expected, as is stated, to yield rounded bornite blebs or lobes, but not rounied chalcocite masses. The mutual relations between th$ two minerals offer serious objections to this theory of origin. j c (2) Etching shows that chulcooite of veinlete breaking bornite blebs does not continue across Intervening chaloocite fields* In some oases, where a crack was observed to con- tinue, it seems very probable that the chalcooite along such channels is later than the chalcooite of the graphic areas, yet there is usually no definite boundary between the two chalcocites at the main ohalcocite-bornite margin. The ve inlets are usually very narrow, consequently the line of contact between the two chalcocites on the polished surface would be very short, and a break in continuity might not be visible* A physical boundary, emphasized perhaps by a variation in color, might be expected, if the chalcocite were of two ages, but its absence does not afford a convincing argument in favor of the hypothesis that the ohaloocite in the graphic structure is of the same age as that in the ve inlets. (3) The directions of the etch-pattern of the ohalcocite in the small ve inlets are difficult to determine, and from our Orfn work, there is no evidence to indicate that the two chalcooites are in parallel or yst allograph! c orientation. From ^hitehead*a statement, however, that the chaloocite of the veinlete is continuous with that of the blebs it is to be inferred that h? believed tha chaloooite of the veinlets to be in orystallographic orientation with that of the blebs. But, 1. Superior Mine, Engela, California for example, p, "jrl ai atyixa 378. even if this Is established, It ie not a conclusive argument against the later age of the velnlets for the orientation of the ohaloocite in them coull conceivably be determined by that of the pre-existing chaloooite, just as the orientation of the later quartz in a quartzlte is determined by that of the quartz in the original sand-grains* (4) In addition to the fine veinlets discussed above, it is very common to find chaloooite of the graphic structures passing without detectable break into ohalcocite which is olearly of replacement origin. (Figures 70, 75 *nd S2,) 1 Bands of chalcoolte forming alteration rims about bornite areas, widen into patches of chalcooite interwoven with bornite in th* graphic pattern. Furthermore, fine veins or tongues of ch&lcocite in some oases penetrate the sharp walle of the graphic bornite blebs or the surrounding mas- sive bornite. (Figure. #2.) Blue hazy boundaries, of the kind already described, are sometimes found between bornite and ohaloooite in the graphic association, and show beyond question that the replacement conditions had been active to some extent at least, though in parts of the same graphic areas, however, these hazy boundaries may be entirely want- ing. These various relations oan of course be explained as the effect of later secondary ohaloocite imposed on earli- 1. A photograph by A. F. Rogers in Econ. Oeol., Vol. 11, (1916), p. 588, Fig. 6, illustrate^ this point well. . 379. er material deposited contemporaneously with the bornite. It is possible that lat*r ohalcocite would ao weld itself to the earlier that the boundary would show no break in color, cryetallographic orientation or other properties, but it is more difficult to accept this explanation in these cases than in those previously discussed where only fine veinlete are concerned* Except for the graphic relation itself, there is often no reason to assign to the ohalcocite of these struc- tures a different origin from that of adjoining material which without question has been formed by replacement of bornite. On the whole , the relations between the graphic chalcoclte and material of replacement origin are in many oases so intimate and so general, that they constitute a very serious argument against the universal application of the hypothesis of strictly contemporaneous deposition for the two minerals in the graphic structure. The strength of this objection, however, ia very greatly reduced by a modification of the hypothesis which will be presented later. (5) The formation of chalcopyrita as a product of the altera- tion of bornite to chalcocite has been commonly observed in material from many districts, as has been mentioned previously, * and consequently the occurrence of chaloopyrite spines in the bornita of the graphic areas suggests a replacement origin for the associated chalcocite. The argument is weakened, however, by the fact that chaloopyrite ie not a common ac- companiment of the graphic structure, and that there is the 380. possibility that it may be the product of neighboring re- placement reactions, unasaooiated with the chalcocite of the graphic areas, for the formation of secondary chalcopyrite at any point commonly precedes the formation of chalcocite, and may take place some distance ahead of the first traoea of enrichment. On account of these reasons the occurrence of occasional opines of chalcopyrite in the graphic struc- tures does not offer positive support for the hypothesis of replacement origin for the graphic structure. (6) It Is conceivable and easily granted that chalcocite associated with bornite in the graphic structure could have originated by the replacement of an earlier mineral similar- ly associated with the bornite* The other minerals which have been observed in this association with the bornite, . however, are in general lees susceptible to replacement by ohalcooite than is the bornite itself and as a matter of fact, with the exceptions noted below, no evidence whatever to support such a view has been observed* Galena may be an ex- ception but there are certain peculiar features commonly ex- hibited by the galena-bornite graphic structures which are not found in the chalcocite patterns, and which make it probable that the chalcooite was not derived from this mineral* A. F. Rogers 1 has described specimens from several districts in which klaprotholite and chalcocite are associated in the same 1. Econ. Geol., Vol. 11, pp. 583-593, (1916). 381. graphic area with bornite, and he believes that the chalco- oite was formed by the replacement of the klaprothollte blebs, which in turn had previouely replaced bornite. While this interpretation ie a possible one, and may be sufficient in certain special cases, the rareness of the mineral and the absence of partial stages of its replacement, makes it ex- tremely unlikely that it of fere an explanation which ie generally applicable. It should be noted that even if this hypothesis is accepted, the explanation is only postponed one step, for it is still difficult to understand how klaprothollte replaces bornite in these peculiar forms. (7) In material from Engels and from Bisbee, fine stripe of bornite were observed in chaloooite between bornite blebs similar to those of the graphic structure. The arrangement and association of the bornite strips suggests that they are residues from a replacement of the lattice type. If this is the case, the replacement terminated against the lobns of graphic bornite with a sharp even boundary as if against a mineral of great resistance to alteration. Lattice struc- tures sharply limited to certain areas have been described on earlier pages. Usually the evidence suggests that under certain conditions, chalcoolte in the lattice pattern can develop uniformly in all parts of certain grains without extending in the slightest to neighboring grains, but only in the few cases in the two districts mentioned have re- lations been observed which definitely suggest that the grains ;s~ * subject to this peculiar type of metasomatism may be of graphic outline. The apote and specks of apparently more resistant bornite which remain scattered through the field in certain Butte and Bisbee ores, after the main mass of the bornite has been awept away by lattice replacement, sometimes occur in twisted, curved blebs suggesting graphic forms. Since they are clearly residues in chalcooite of replacement origin, their resemblance brings support for the view that the chalcocite in graphic areas originated in this way. If this were the case, however, it would seem that partial stages should 'be commonly found, and the practical lack of these is a strong objection to this genetic mechanism* From a review of the arguments on both sides of the controversy, it is apparent that neither view can be established without serious modification. The hypothesis of contemporaneous origin fails to explain important relations in many examples, and the hypothesis of simple replacement origin ia also inadequate to give a complete solution. The evidence ie so conflicting that neither extreme view can be established to the exclusion of the other, and it indicates the difficulty of explaining all features associated with the graphic structures by a single kind of mechanism. Hypothesis of Contemporaneous Deposition Modified by Replacement. The most serious objection to the hypothec is of strictly contemporaneous origin is the close association often observed between ohalcocite in the graphic areas and . iei .^a . . - ' a vrf < 383. chalcocite of replacement origin. A modified form of the hypothesis, however, reconciles the evidence if it ia assumed that although the chalcocite was initially deposited con- temporaneously with the bornite, it continued to form after the bornite had become unstable, which resulted in the de- velopment oi replacement relations. The hypothesis thus modified is more in agreement with the usual relations observed between primary minerals than is the view of strictly contemporaneous deposition. The relations between the bor- nite and chalcocite would be very similar to those believed to exist between chalcopyrite and bornite in many deposits. The sequence of th- minerals implied by thie hypothesis is expressed graphically in Fig. 40 . Increasinq amount of minerals ,te or Fig. 40 fta lenotfi of time, since initial deposition 40^ t Di-^ran:. of Sequence of Bornite and Chalcocite According to the Pria-ry Theory for th- Chalcocite in Graphic Structures t _ -.'js si it \i .... - e. ' yX wvtf ... A :>\t 384. Tha hypothesis in this form assumes that with the changing conditions of the period of primary mineralization, a point was reached at which ohaloooite commenced to deposit with the bornite, but with progressive changes in concentra- tion of the solutions (probably a gradual reduction in the iron-copper ratio), bornite ceased to form, became unstable, and tended to alter to ohalcooite. It is conceivable that the contemporaneous deposition of the two minerals was of temporary euteotic character* of which the graphic form is the expression, but that these conditions were generally interrupted more or leas promptly by the addition of compo- nents favoring the production of chalcooite, which thus brought about the super-imposition of replacement phenomena upon the euteotio pattern. By this hypothesis, graphic structures which are unassooiated with any evidence of re- placement are explained equally as well as the types in which replacement is apparent. The hypothesis harmonizes the evidence which favors a contemporaneous origin for the two minerals, with the strong evidence which indicates that part of the sane chaloooite is often of replacement origin. As already pointed out, the occurrence of graphic structure among certain pairs of sulphide minerals which rarely if ever exhibit replacement relations toward each other makes it reasonable to conclude that they are contempo- raneous intergrowths, especially as we know that simultaneous graphic structures can be produced as final crystallizations xa 01. . .' itiq ,lrfW 91 % CXfcWOw . ,.'J-i '.{?'. ," TjB YE 385. in alloys. Among tnese natural intergrowths the mineral ooirjnon to moat of them is boruite, from which, because of ita sequence position, the inference nay be drawn that graphic structures are most favored in the late stages of mineralization. This deduction may render more acceptable tha idea that the graphic structures between bornite and chaloooite are actual intergrowths, since primary ohaloooite if it exists at all, is in part later and probably just later than bornite. No certain weight, however, can be attached to this speculation. If the interpretation is correct, it of course means that part of the ohalcooite in replacement relations with bornite may be of primary origin, a fact which makes chaloooite the last member of the sequence of primary min- erals* The sequence, in its simplest terms, would be mag- netite or pyrite chalcopyrite bornite chaloocite. The first three members whose positions are definitely es- tablished, are in agreement with the sequence if arranged according to decreasing iron suvl increasing copper; hence on chemical grounds chaloooite is the logical end member. Although chalcooite is definitely known to be of secondary origin in most of the important copper deposits, and hence formed at ordinary temperatures, synthetic results indicate that it is stable at higher temperatures, and may be formed under the usual conditions which are believed to have prevailed during the deposition of primary sul- t JD0* *. . . atUtAOi phides. In the Geophyeioal Laboratory, ohalooolte crystals have been formed in the dry way at a temperature above 350C, and in the wet way at temperatures as high as 350C. In- crease of temperature is also known to aooelerate the rates r^ a of the reactions by which secondary ohaloooite is formed. In general chuloooito in the graphic patterns occurs most prominently In the deeper iarts of bornite- 3 bearing deposits. Nearer the surface the structure is usually obliterated by the more complete replacement of the bornite by chalcocite, and evidence of its former existence is largely removed. For the most part, the field relations indicate that the ohaloooite in the graphic structures at any definite place in the ore was formed earlier than the period of intense development of secondary ohaloooite. The evidence of primary origin offered by the deep ohalcocite, while very suggestive* cannot be regarded as fi- nal, however, for the reason that in none of the cases now known are the workings deep enough to be beyond the influence of descending solutions, and beyond the point at which see- on l&ry ohaloooite has developed from bornite, with a few ex- ceptions mentioned later 4&*4 whose character is not fully determined^, fchft eg--" -Jr^y *.1t.*r.-it*"' -f hi-ir vi fc f.vf A<. 1. Eugen Posnjak, E. T. Allen, and H. E. Kerwin, loo. oit ., pp. 5*0-521. 2. E. G. Zeis, E. T. Allen, and H. E. lierwin, loo. oit 3. Butte, Eiigeis, Virgilina, for examples. e. ,0 02o wrotfja tyj -al '4 1. YW i . i 'i aao. ' ?oe n -^cf sto dflittO . Jbi -ysi4i ai -II * Le?. : ess i -Xft Jon el ic ^B M. , Virgil ina District, tiie dines are not ever 450 ft. deep, and although thers is very definite evidence that the rooks are extremely tight, the long continued exposure to erosion cakes it surprising that the bornite ores are not core com- pletely enriched. . If the hypothesis of primary ohaloooite is accepted, it becomes necessary to attempt to distinguish between pri- mary ohaloooite of repl¢ent origin and secondary ohaloo- oite in the same relatiouu. At the Superior k'ine near Engels, and in the Kuskulana District, Alaska, the secondary and climatic ohalcocite formed under recent physiographic conditions is A so feeble that it is in definite contrast to the chaloooite in the earlier graphic or related structures. But in cost oases where the secondary sulphides are well developed under present conditions, ohalcooite definitely recognized as sec- ondary in origin, may be traced without change into chaloo- oite of replacement origin intimately associated with the graphic areas. No break in structure or other properties can be detected, and any limit to the extent of the sec- ondary ohaloooite is quite arbitrary. While this is obviously a difficulty in the way of the acceptance of the hypothesis, the possibility of chaloooite assuming similar replacement forms and similar phyeioal properties under primary conditions as under 1. F. B. Laney, loc. oit. p. --- -' f fl> *- 3 Jber . 386. secondary cannot be denied. Although chaloocite fortred at temperatures above 31C. is isometric, it alters to the usual orthorhombio form when cooled, unless it contains over 8$ 1 dissolved oupric sulphide. With this one exception, the synthetic material formed at higher ten^eraturef is identi- cal with natural chaloooite. Consequently it seems that there is little hope of distinguishing primary and secondary prod- ucts by microscopic means, where both are in ordinary re- placement relations to the bornite, and the lack of break between definitely recognized secondary chalcocite and the ohaloooite associated with the graphic structures, as at Engels, cannot be regarded as a convincing argument against the primary origin of the latter. Hypothesis of Selective. Replacement. In the dis- cussion of the properties of graphic structures on preceding pages, the features were emphasized, which oppose the hypoth- esis of simple replacement origin, for the ohaloooite. In resun.e', the occurrence of chaloooite in the graphic structure often in the heart of bornite grains, the lack of dependence of the chalcoolte on gangue boundaries, the abrupt termina- tion of graphic areas against masses of bornite, and the contrast to the forms of ordinary replacement are difficult to explain as results of rnetaaomatio processes and offer very eerioua objections to the hypothesis. 1. Eugen , Posnjak, E. T. Allen, H. E. Iterwin, loo. ait. tft* 9*lC .Ub '; ^ r erf *o * So e: i sfe$. i ' ' - 389. On the other hand it will be recalled that there is fairly definite evidence that the development of ohaloo- oite in many graphic structures was actually accompanied by replacement reactions. The microscopic evidence has al- ready been summarized. There are a few cases in which the 1 field evidence is significant. At Bisbee, Arizona, and 2 at Rising Star Mine, Shasta Co., California, the ohaloo- oite ores, in which the graphic structures occur, appear not 'to oontinue into the deepest workings. Although the deposits have not been examined with sufficient thoroughness to es- tablish these observations as definitely as might be desired, the evidence is very suggestive that the ohaloooite in the graphic structures is related to the distribution of secon- dary enrichment. At Butte, when the deepest known ores con- taining ohaloooite and bornite in the graphic relation occur, definite observations, which will not be detailed here, make it apparent that from our present information, the possibility of the secondary alteration of bornite even at the great depths attained there cannot be eliminated. If, however, the replacement hypothesis is to be ade acceptable, it must be modified in such a way that it offers a competent explanation for the peculiar but particu- larly characteristic features of graphic structures, which are so suggestive of contemporaneous nature of the mineral 1. See pages 173-17^. 3. L. C. Graton, personal communication. rff .8 .fct erf* 9qac 390, components. An explanation must be offered for the selective manner in which the bornite is attacked, in which the obvious lines of weakness are neglected aud the advance of the ohal- oocite is directed into irregular yet fairly constant forma. To do this, the hypothesis nay be suggested that bornite ex- ists in two forma, one of which is slightly more resistant to the influence of enriching prooeaaea than the other. The difference between them is assumed to be sufficient to allow one form to remain unaltered under certain conditions of mild, probably long-continued attack of altering solutions, while the other is completely replaced. According to this hypothesis, tne graphic atruoture would be explained as the result of selective replacement of the more easily altered type of bornite from a contemporaneous intergrowth of the two. The replacement relations usually associated with the structure could be readily considered to be slight modifications of the rr.ore resistant type, or leas commonly incomplete replacement of the feebler type. As replacement tendencies beo&me more and more intense, the resistant type would be expected to yield, ai>d the earlier structure would be gradually obliterated. This explanation harmonizes the conflicting evi- dence in a very complete way. The evidence o contemporane- ous intergrowth structures may be accepted without modifica- tion, for they are attributed to the primary relations of the two bornites, and the evidence of replacement relations -At? &J JB a fa - . '- '*; IHl *- o*j 391. between the bornite and ohalcocite oar, also be taken without discount for a aetu.aon.atio origin ia assigned to the ohaloo- oite . The conception of two bornitee, of course, cannot be confined to the graphic structure alone, but must find support in features observed in other forms of bornite re- placement. In the diacuasion of the lattice structure, the oocurenoe of plates or splines, apparently more resistant than the surrounding material, was described. (Figures lJj-3, 1 117, 111?, .and 1^9 . ) , In these associations it is usual- ly evident that the surrounding material had been altered to a ohalcooite -bornite lattioe with remarkable uniformity throughout the field, while the splines of bornite had been attacked only slightly or not at all. This relation when considered in the light of the new hypothesis, at o:.ce suggests that the splines of bornite are of the more resistant sort, while the tain field was of the easily altered type. The relations in the lattioe structure would indicate that widespread replacement of bornite by chaloo- oite in this pattern is irobably confined to the leas resist- ant bornite. The symmetrical relation of the splines to the surrounding lattice shows that the two bornites are either of the same or closely related crystal forms. In a few instances a slight development of chalcooite in the lattice form has been observed along the rrargins of the splines, and it is probable that the same structure exists . . 393* in them as in the surrounding field, but that it is less easily developed by the chaloooite. Two forms of bornite associated in the graphic pattern would suggest contemporaneity in formation, but splines of the more resistant type surrounded by the easily altered sort would suggest an age difference which, however, might be only slight. Two interpretations tray be offered for the second relation: (l) that the splines were the earliest to font, probably as skeletal crystals, a;.d that the angular spaces between then were filled later by the less resistant bornite, or (&) that the easily altered type was the first to fora, and was then altered to the resistant bornite by agencies which advanced along crys- tallographio planes, similar to the invading chalcooite at a later time. In our present lack of knowledge concerning the possible nature of ' the differences between the two bornites, if suoh exist, further speculation is useless, but it may be suggested that since the earliest product to crystallize from a melt or solution is often the purest, as has been suggested' by Dr. E. T. Allen, and since pure substances are usually more resistant to alteration than impure, the first interpretation is probably the better one. The sharply defined spots and irregular blebs of bornite which remain as residues in ohaloooite derived by lattice replacement, also indicate the existence of a more resistant type of bornite than that which had surrounded . 4 393. their. Their clean boundaries, irregular forma and distribu- tion pl-oe then, la contrast to the usual angular residues produced by a lattice replacement . Their resemblance in some cases to forms of graphic structures suggests that the ohal- cocite of the latter cay have been derived by lattice replace- ment . Chaloooite occurring in the graphic relation in the heart of bornite grains has been mentioned. Similar cases in which the outer portions of grains are apparently more resistant than the cores have been observed in which the bornite in the centre is partially replaced by chaloo- 1 cite in the lattice pattern (Fig.lW ), which may be ac- cepted as proof that the chaloooite is of metasomatio origin. The resistance of the bornite in the outer parts of the grains is probably to be explained in the same way as the survival of the splines, which has just been discussed. It is worthy of note in this connection that the centers of bornite grains on the polished surface frequently are strong- ly attacked by artificial reagents, whereas the marginal portions are more resistant. The lattice structure as has been mentioned has been observed well developed throughout definite grains of bornite, yet sharply terminated against others which are 1. J. Murdoch, Op. cit, Frontispiece, Fig. 2. It should be noted that the specimen illustrated is a crystal determined to be of proper bornite form. This removes the possibility that the outer rim of bornite is of later deposi- tion. 10 *> i apparently untouohecl by the ohalcooite. The strips of ohal- oooite are out off as regularly and as consistently as if against another mineral possessing complete resistance to their attack. This selective replacement is usually more apparent in the early stages of enrichment, but it has been seen in specimens in which ohaioocite is the predominant member of the lattice. Such differential attack cannot be attributed to chance, and it seens to imply that there is a variation in the resistance of bornite which can direct and control the advance of chaloooite alteration in the early stages. In ores from the Koteina District, which have been described previously (page 241), the development of the secondary copper sulphides was found to leave certain ir- regular strips and bands of bornite untouched. The ohalco- oite and oovellite seem confined to definite grains, but usually permeate the bornite thoroughly within these limit*. They rarely assume the lattice pattern. In the zones re- sistant to ohalcooite, however, ohaloopyrite ia developed sparingly, but in excellent lattice forms. If the theory of two bornites is applied, the relation indicates that both sorts are capable of yielding lattice replacements, an observation which finds confirmation elsewhere. The forma- tion of ohalcopyrite may be attributed to the accumulation of iron released by the replacement reactions which pro- duced the ohaloooite and oovellite in the more easily al- rff .til 10 jbadi 395* tared boraite. On polished surfaces of ores from the Bonanza and Jumbo Mines, at Kenneoott, angular, broken strips of bornite have b@an observed in chaloopyrite. When first examined, they at once give the impression that the bornite is in veins in the ohalcopyrite and should therefore be considered to be of later age* If a strip, however, is traced into a bornite field, its continuation is usually marked by a delicate bor- der of fine ohaloopyrite spines, which, gradually disappear and allow the "vain" to lose its identity in the massive /^bornite. (Fig. 123 , 12fc , \K% , and 1^.) The summation of the evidence (page 259) presents such a strong case favoring the replacement of bornite by ohalcopyrite, that it forces the belief that the bornite strips are not veins in the chaloopyrite, but residues, which for some reason remain resistant to chaloopyrite attack, as do the splines in the lattice structure to the chaloocite attack. Direct evidence indicating two types of bornite, however, is not abundant or especially significant. There is a slight variation in the hardness of different grains or different parts of the same grain in some oases, which is shown most definitely by the relief on the polished surface. Variations of this sort may possibly be attributed to changes in properties due to crystallographic orientation but not very probably for the mineral is isometric. Harder rims about softer cores have been observed in material from the . > 396, 1 Kuskulana District, Alaska, but in no cases have the hard and soft types been found associated in forms similar to graphic patterns. An array of arguments against the view of two types of bornite may be raised, but none of very positive or final character. The oolor of bornite on the 2 freshly polished surface is very constant. Slightly yellowish tints have been observed, but it is most probable that they are due to submiorosoopio spines of ohaloopyrite, and not to any variation in the character of the bornite. The etch-patterns of bornite offer no evidence of two types possessing different properties. The cracks are either of the regular sort within the limit of the grain, or evenly irregular in a way which carries no significance. Strips with a slight variation in pattern have been observed in a very few oases, and may possibly be related to the splines but the orientation of the cracks suggests that the structure is probably due to twinning. Bornite front different dis- tricts often shows a distinct variation in vigor of reaction with the ordinary reagents, but the character of the re- action seems to be constant within a limited field, as far as our experience goes. The regional variation may be due to purely physical characters, such as porosity, or the presence of submiorosoopic impurities. The constant ohercioal character of natural borr.ite 1. A. Wandk/e, personal communication. 2. J. Murdoch, Op. Cit., p. 35. 397. may be regarded as established with oertuinty and there is little likelihood that true variations in composition ooaur. 1 In work done in the .Geophysical Laboratory, which will aoon be published, bornite was found to possess no inversion points, suoh as the one between isometric and orthorhombio forms of ohaloocite at 91. As far as the properties of bornite are known at present, there is no definite physical or chemical evidence to indicate that it possesses more than one form* A rather serious objection which may be raised a- gainst the hypotheses of selective replacement is that no partial stages of the metasomatio changes have been observed with certainty. Bornite residues, in the lattice pattern in some oases, should be found in the ohalcooite blebs and lobes the of the graphic structure according to mechanism suggested, but in general they are completely absent. Indications of residual bornite strips have been found in two or three cases, but the evidence is hardly definite enough to settle the matter, and although it lessens the weight of the objection, it does not remove it* Chalcocite which has been formed by the lattice replacement of bornite usually possesses a tri- angular etoh-pattern; the ohalcooite in the graphic structures commonly yields the orthorhombio etch-cleavage. (Fig. 136.) Consequently it is improbable that the ohalcooite in the 1. Private communication. v O V 398. graphic relation with bornite was derived by a replacement of the lattice type, which greatly weakens the force of one attractive explanation acoording to the hypothesis of selec- tive replacement. Summary and Conclusion The relations of the ohaloooite and the bornite in all graphic structures cannot be satisfactorily explained either by the hypothesis that the two minerals are of strict- ly contemporaneous origin or by the view that the ohaloooite is produced by simple replacement of bornite of ordinary homogeneous character. Normal replacement origin will appar- ently not account for any of the graphic structures. Strict- ly contemporaneous origin will account for many. If the hypothesis of contemporaneous origin is modified to the extent that it allows replacement conditions of bornite by oh&loocite to follow upon the heels of contem- poraneous deposition, it becomes nore acceptable, and affords a satisfactory explanation for nearly all the observed fea- tures. The ohaloocite fits into the later phases of the se- quence of primary minerals in a logical manner and chemical knowledge of its properties and range of stability offers no objections to its being placed there. Opposed to the view is the difficulty of finding any break between second- ary ohalcooite and that closely and intimately associated with the graphic structure. In two districts, the apparent T MN>JtflA ' JO 8 IdDtaftdc .*r.i";,.--; -ello x 399. dependence of chaloocite in the graphic structure on the distribution of the secondary sulphides is also of opposing nature, but more careful field and laboratory studies are required before the strength of the last argument oan be fully established. If it is upheld by more detailed work, it offers a serious objection to the hypothesis of primary chaloooite. The explanation baaed on selective replacement depends on the strong evidence of the importance of metas- ometiam in close association with the graphic structure, and the apparently greater resistance of boraita in certain forms to alteration. It finds support in the satisfying way that it harmonizes conflicting evidence of contemporaneous and replacement structures, and in the many features in other structures which suggest two types of bornite, such as splines, rime, resistant specks and abrupt terainations. It is seriously weakened by the lack of known chemical or pronounced physical variations in the properties of bornite, ani by the rareness of any forms which may be interpreted as latex-Mediate stages in the replacement. Of the four possibilities which have been suggested, viz. striotly contemporaneous deposition, modified contempo- raneous depostiion, selective replacement and normal replace- ment, it is noteworthy that the first three agree in assum- ing that the graphic pattern Is controlled by a primary structure in the ore, and each of these iieas has something ao fl,eo ,^ao i* {*'oa,& a ei le di : 401, is merely one of the many problems which seek to oonoeal themselves behind the intricate structures and varied behavior of bornite, toward the solution of which this contribution has been directed* MICE OPHQgQGHAPHS BOOKS 402. PLATF TIV Photographs of Polished Surfaces of Ores from the Evergreen fr'ine, Colorado.' Fig. kl. (x fO diameters) Bornite (gray), partially replaced by chalcocite (light), working in from gangue (black) contacts. The feathery material in fine lattice orientations in the chalcooite consists of very fine-grained ohalcopyrite, ani a little bornite, with some oxidized material. For discussion see page 75. A grain of ohalcopyrite (light) in the lower part of the field remains re- sistant to the ohfllcocite. Fig. &2. (x diameters) Chaloopyrite (light) and bornite (dark). In part associate.! with mutual boundaries and in part in relations indicating replacement of chelcopyrite by bornite. Some spines of secondary ohalco- pyrite penetrating the bornite from gangue (black) contacts. Fig. k-3. (x diameters) Primary chalcopyrite (light; opj) an-! bornite (dark) with mutual boundaries. Bornite partially replaced by secondary chaioc- pyrite, in imperfect lattice structures, sc- oompanied by shrink 9 ge cracks. Fig. Wt.. (x ^0 diameters) Bern! te (gray) with a little primary chalcopyrite (light; cpj) in smooth grains and more abundant secondary chplcopyrite (op?) developing at the expense of the bornite as rirr:3 along gangue veinlets, or as imperfect lattices. 45-- 40 3 v PLATS XV Photograph a of Polished Surfaces. Fi<". ^5. (x "0 diameters) l^vergreen Mine, Colorado. Bor- nite (lark) and chalcopyrite (lisht) in graphic association. . Secondary chalcopyrite developed in bornlte bleb in up jar portion of the fiel^. Note shrinkage crooks . FJP. '-6. (x 50 diameter 3) ^^rgreen Mine, Golor n ,''o. Bor- nlte (dark) be "h in graphic structures -.vith chalcopyrite alonp; ganifue veinlets (blp.ck). Fig. k-7. (x meter a) Evergreen Mine, Colorado. Bor- nite (dark) both in mutual relations vrtth chalco- pyrite (lir^ht) -n1 as a replocement of chalco- pyrite ir. definite veinleta. Fig. Itf?. (x ?0 diametera) Evergreen Mine, Colorado. Bor- nite (dark) in mutufl relations toward primary chalcopyrite (light) --ni t lly replaced by secondary chplcopyrlte, in the lattice structure. Fig. k9. (x !!0 diameters) Marble Bay Mine, Texade lalani, B. C. Bornite (gr->y) with an intemiittant border of klaprotholite (?) (white), and ohr.lcoci te, and chalcocite (li^ht gray). Qpngue , t Fi?. 50. (x kg diameters) Merble Bay Mine, Texnda Inland, B. C. Chalcopyrite (li^ht) anl bornite (dark) in primary relations which nli^htly surest the earlier sp,9 of the chnlcopyrite. 404, PL/TS 7 VI PhQtosraph-3 of Thin ^eoti one of Hooka and Ores froii; ^n.^;-:;, Ce.ll fornjp. .. Fig. 51. (x 20 diameters) Bornite (bL?ok) raplaoing rock-roinarala, with only slight development of chlorite end epldote. Fig. 5?. (x 90 diameters) Creased niools. Broken apatite prism in strained feldspar. Fig. 53. (x n. diameters) Creased niools. Feldspar phenooryst in granodiorite porphyry, showing recrystallization. Fig. 5^. (x 20 diameters) Same aa in figure "?, but without crossed niools. Fig. 55. (x 16 diameters) PoikJlitJc hornblende, with feldspar qni magnetite chiefly. f^, 56. (x "*>? diameters) ^uh^dral epidote crystals included in bornite. I %>,.... .- 405. PUTS XVII Photographs of Thin Seotiona of Hooka ond Ores from ffngels t Call for ni P .' Fig. 57. (* 67 diameters) by magnetite ani Chlorite also in Hornblende corroded by oryatal surrounded chlorite end bornite, thin foils between magnetl te . (x ~? diameters) M?p;netite r?n^ bornite with chlorite along boundaries of magnetite grnina. (x " dimeters) Hornblende cryetsla containing amell inclu^ion^ of magnetite end spin 1 (spn.), \ corroded by surrounding magnetite. Note abundant inclusions of apatite in the and the occurrence cf bornite in ragged with chlorite. Fig. 60. (x 6" diameters) Bornite en1 chlorite replacing aaphibole. Magnetite in smoother grains. * ** 406. PLATE XVIII Photographs of Ores from Engels. California. Fig. 6l. (x 30 diameters) Thin section. Hornblende par- tially replaced by calcite, developed along cleavages of the former. Fig. 6?. (x 5^ diameters) Thin section. Crossed nicols. Plazioolase partially replaced by heulandite (heii.). and both partially altered to natrolite (white). Fig. 63. (x Ui diameters) Thin section. F.piiote vein- lets cutting albite and chlorite. Also patches of epidote. Fig. 6^. (x *iO diameters) Polished section. Bornite (white) with inclusions of chlorite (dark laths); magnetite (gray), earlier than the borni te. Fig. 6~. (x 65 diameters) Thin section. Ore-bearing di orite cut by siderite (?) veinlets. Fig. 66. (x 650 diameters) Polished section. Chlorite (dark) corroded by borni te. 407 PLAT* TIX Photogrpphg of Polished Surfaces of Ores ""from ffngels, Cali " Fig. 67. (x 7^ Hamsters) Chalcopyrite (dark) and bornite (light) with mutual boundaries. (Jangue, black. Fig. 6g. (x 50 diameters) Bornite (gray) with chalco- oite (light) developed along chlorite (dark) inclusions in the bornite, *n^ slong gangue veinlets. Fig. 69. (x 60 diameters) Bornite (dark) with chaloo- oite (liht) both in graphic associations (upper part of ths field^ and in veinleta, in part alon? gangue (black) contacts. Fig. 70. (x 70 diameters) Bornite (d?rk) with chalco- olte (lir,ht) moatly in graphic structures. Note the extension of bornite through the field alonp, the intem-ittant line of gangue (blaok), with ohalcooite along the centre of the bornite sone. Fig. 71. (x '"^ diametera) A oortion of the field of Fitr. 70. Fig. 72. (x 650 diameters) A portion of the field of Fig. 70. Bornite, dark; chplcocite, light. 408. PLATE T* Photographs of Poliahad Surfaces of Ores "ffngela, Ca fornia." Fig. 7^>. (x SO diameters) Veinlets and patches of chaloo- cite (light) in bornite (dark) in part along gangue veinlets (black). Note subgraphio form of some of the larger chalcocite grains. Fig. 7 1 *. (x 70 diameters) Bornite grains (lark) with riirs of chalcocite (light) in altered diorite (darker gray). Note the imperfect beginnings of B lattice structure between the bornite and chalcooite in many places. Fig. 71. (x 70 diameters) Bornite (dark) with chalco- oite (light) both in graphic structures and in veinlets in the bornite. Note cleavage cracks in the ohalcocite about holes in the polished surface. Fi?;. 76. (x 2?0 diameters) Field of ch^lcocite (white) cut by a gangue veinlt (carbonate; black), with bornite in intermittant strips alonp: the margin in the chaloooite. 409. PLAT?! 7X1 Photographs of Polished Surfaces of Ores rrom Angels, California . (Photographs on this page by W. L. Whitehead.) Fig. 77. (x "0 diameters 1) Bornite (dark) cut by lattice of chalcopyrlte (li^ht). Gangue, black. Fig. 7 . (x 900 diameters) Lattice structure developed in bornite (bn) by chalcopyrite apines (op) with chalcocite rims (cc). Fig, 79. (x 300 diameters ?) Bornite (dark) and cha loo- cite (light) in graphic structure. Fig. 0. (x 1000 diameters) Photograph at higher magni- fication of portion of the same bornite-chalco- oite graphic structure as shown in Fig. 79* Note that the chalcooite is dark and the bornite is li?ht, due to a different color screen used in this case. The similarity in appearance of the two photographs in spite of this change is a striking example of the mutual relations of the two minerals. 410, PLATS Photographs of Polished Surface 3 of" Ores from' "FngeTg, California,. Fig, l. (x 250 diamatara) Bornite (dark) and chaloo oita (light) in a fine-grained graphic atruo tura. Apportion of the field shown in Fig. Fig. 2. (x 250 diameters) Chalcooite (light) in graphic structure, rear edge of bornite grain (dark), end also rasooiated vr j th bornite in vainleta and along gan^ue oontact. Fig. f! T . (x 2^0 di&metero) Intsrgrowth of magnetite (liht) ?n1 ilroenite, in whicjh the letter is largely altered to leucoxer.3 (gray). A little hematite (white). Fig. ?k. (x 70 -!laweter8) Superior Mine, Angels, Cali- fornia. Magnetite grains (shadowed, due to M?h relief) set in a field of bornite (dark) with chalcocite (light) in graphic associations 411 PLATE XXI I I Phptogr? pha of Pol i 3h n d 3ur f r- peg of Or eft frog the 3ur:^r JOT MJn? , near Tnsrels, California Fig. 5. (x 70 diameters) Magnetite crystals, shown by black bordara due tc hip;h relief, surrounded by bornite (dark) containing blebs of chaloo- oi te (light) suggesting imperfect graphic structures (subgraph! c otrrotures). t Fig. ?6. (x 70 diametera) Magnetite grains, shown by black borders, with small inclusions of bornite, and surrounded by bornite containing chalcccite in graphic form. Inclusion;-? of ohlorlte in the bornite. Note lr ok c >ndence of the chaloo- oite on marp;in< of the bornite areas or on chlo- rite inolusiono. Fig. #7. (x 70 diameters) 3irailrr to Fi^. Fig. , (x 70 diameters) Bornite (d'-rk) with chalco cite light in sra.'hic structure. Magnetite grains, shown by hifth relief. 412. PLATE XXIV Photographs of Poll ehed Surf ace of Orea from the Marine. Mine, Super for 7 Arfzona. Fig. 9. (x ?50 diameters) Eornlte (dark) in graphic structure with galena (white). Tetrahedrite (td), light grey. Fig. 90. (x 250 diameters) Similar to Fi?. 9. Note great range in oize of galena grains. Fla;. 91, (x h~ 3iametera) Ohalcopyrite (white) p.nd bornlta (a very li^ht p;r*y) broken by later minerals (quarte in p-rt). Fisr. . . (x '-~ .Uametero) Bornite (dark) snl ohsloo- pyrf te (light) in vein-like strings of elon- gate i bleba, often around bornite grains. (The VRriatlone in shade of the bornite are due to ataina on the plate). 93 ( J i-5 diameter a) Pyrite (rough) broken by veinlets of bornite (smooth). Fig. 9ii. (x fc" diameters) Pyrite (rouh) broken by veinlets of bornite (darker sroooth). A primary replacement. 413 PLATE XXV Photoe.ra.oha cf Polished 3ur-faoes of Ores "line. Superior, Arizona. Fig. 95. (x lJ-5 diameters) Pyrite grains broken by bornite veinlets. Fig. 96. (x ^5 diameters) Pyrite trains broken by bornite veinlets. Replacement of pyrite more advanoed than in figures and jt. Fi". 97. (x "?50 diameters) Chaloooite etohed with nitrio pci-l, revealing grained structure and orthorhombio nature, ahown by the one set of strong craoks in each grain. Fig. 9^. (x diameters) Chaloocite (light) with covellite (dark). / y v ' -. i -' v 414. PUTS XXVI :'. _ __!__ ' __ __. J2 ____ __ . fro? the KuaVu). ::.;- -I :'?: -^ in f- "^ : ""Tfcnrieoott 'Districts, _ Alaska Fig. 99. (x 50 diameters) From flugget Creek, Kuskulena District. Bornite (dark) and chaloopyrite (light) in mutual relations, but with a suggestion that the bornite is somewhat Ipter than the chalco- pyrite. Fig. 100. (x 30 diameters) From Elliot Creek, Kotsina District. Chalcopyrite (light) conodedby bornite (dark). Fig. 101. (x 50 diameters) Nugget Creek. Bornite, (gray) ' ch.^lcocite (white;, in aubgraphic relations. be lack of dependence of chr>lcocite distribu- tion upon calcite (dark gray) contacts. Fie. 10?. (x 50 diameters) Rugget Creek. Bornite (dark) with chslcocite (light) in structures similar to those often shovm bdtween chalcopyrite and similar to those often ohorn between chaloc- pyrite and bornite; associated in on? small erea in the graphic structure. Fi?. 10^. (x ^0 diameters) Bonanza Mine. Kennecott. Chalcoclte (light) elterin?, to malachite (dark) along structural planes. / little oovellite (intermediat? f^rsy) between malachite and chalco cite. Fig. 10'-. (x 50 diameters) Chalcoclte from vein in th* greenstone near the Bonanza Mine, Kennecott. Partially altered to malachite, along parallel lines rsv93lln the orthorhombic structure. Com- re with figure 103 in whioh the chalcocite de- ae triangular pattern, beliaved to be isometric. 415. PLAT 1 !! XXVII Photographs of Polished Surfaces of Ores from Kennecott, Alaska* 105. (x diameters) Bonanza Mine, Steely ohalco- cite, etched with nitric acii. There is a alight suggestion of the concentric structure of "pebbly ohaloocite" in it, but the material la massive in the handspeciman. Fl*. 106. (x ?0 Uameters) Chalcocite, etched with pot- assium oyni1e oolution, showing variation in grain, and the triangular etch-pattern In nearly all oases. Fig. 107. (x "0 diameters) Chalcooite, etched with pot- assiufr cyanide, revealinrr triangular etch- pattern. Fig. lOf. (x ^0 dlanseters) Chalcocite, etched with potasaiuc cyanide solution. For discussion of these etch-patter re, see pages 25 and 356. ;... "7- 416. PLATE XXVIII. Photographs of Polished Surfaces of Ores from Ke nne oo 1 1 , A laska . Fig. 109. (x ?0 diameters) Bonanza Mine. "Diabasio" coTellite. Covellite (dark, mottled due to pleocroism), with chaloopyrlte (white) and bornite (bn, light gray) between the covellite lathe. Fig. 110. (x 2gQ diameters) Bonanza Mine. "Diabasic" covellite. Covellite (dark), with ohaloo- pyrite (white) and bornite (light gray) be- tween the laths. Fig. 111. (x 40 diameters) Bonanza Mine. Covellite plates in radial and scalloped patterns, partially replaced by ohalooolte (white). Fig. 112. (x 40 diameter) Bonanza Mine. From the same as Fig. 111. Fig. 113. (x 65 diameters) Bonanza Mine. Covellite (dark) in chalcocite (light). Fig. 114. (x 50 diameters; Jumbo Mine. Complex ag- gregate of chalcopyrite ani bornite in con centric structures. Partially replaced by -ovellite an i luzonite (lightest), Coarser plates of covelaite filling space cetween the curved masses of ohu-lcopyrite anl bor- nite. 417. ' PLAT? XXIX Photographs of ?o 11 ?/r ;H. ; : ' . :' Ore s ' from Ker.i-eoott , .Alaak" . Fig. 11^. (x 2#0 diameters) Bon^n^a Mina. Bornite, etched \*ith potaaaiur cyanide aolution, yielding brick- like pattern. Black squares and rectangles due to small blocks springing out between oraoke of the etch structure. Fig. 116. (x 50 dj -:rr.etflra) 'FYie Minft. Bornite (bn) and chalcocite (cc, almost the aan-iS color in this photograph), ^rtiplly altered to malachite vrith intermediate covellite in the ohalcocite. Ths triangular structure in the chalcocite and its absence in the bornite, >e ^hown by the malachite veinleta, ia notevyorthy. Fig. 117. (x ?0 liametera) Bonanza Mine. Chplcocite altering to malachite alon^ structural lines. Fig. 1121. (x diameters) Erie Mine. Bornite (gray) partially replace' by chslcocite (lip/nt). Triangular structure shown by ;hite vein- leta in the chaloooite, but not in the bornite. Lack of chaloooite lattice in the bornite is noteworthy. Fig. 119. (x 50 diameters) Erie Mine. Bornite (dark) partially altered to chaloocite (light) along irregular veinletr?. The strip^ of white chalco- cite in the blue type reveal the structure of the chalcocite. This structure is emphasized by > ^laohite veinleta (black). Fig, 120. (x 50 diameters) Erie Mine. Bornite (tn) chplcocite (oc, somewhat lighter) in a large area -3 f? veinlete in bornite. Note contrast in structure developed by the malachite veinlets in the chalcocite and in ths bornite. 418. PLATE XXX Photographs of Polished Surfaces of Ores from Kennecott, Alaska^ Fig. 121. (x ^6 diameters) Bonanza Mine. Enargite oryatala in limestone. Fig. 122. (x to diameters) Jumbo Mine. Ch?lcopyrite (light of the earliest generation partially replaced bornite (darker). Fij. 1T5. (x 2fO diameters) -Jumbo Miiw. later age developing in bornite in feathery lattice structure, Chalcopyrite of (dark) in part Fid, (x 2?!0 diameters) Jumbo Mine. Chalcopyrite (light) developing in feathery lattice struc- tures in bornite (dark). Note the apparent reaiatar.ee of the bornite in certain zonsa, often in vein-like strips with characteristic angular offsets. Fig. 125. (x abaters) Jumbo Mine. Bornite (dark) re- placing chalcopyrite of the earlier generation (light). 419. PLATE XXXI Photographs of Polished Surfaces of Orea f r o re Ke rine oo 1 1 , At a ska. Fig. 126. (x ?0 diameters) Jumbo Mine. Chalcopyrite (light) of the earlier generation, partially replaced by bornite (dark). Black lines are cracks in the polished surface. Fig. 127. (x fO diameters) Jumbo Mine. Similar to Fig. 126. Fig. 12g. x fO diameters) Bonanza Mine. Cove Hi te dark) partially replaced by ohalcoolte, light). Two bands of bornite residues slightly lighter) in the ohaloocite. The specimen possesses a scalloped structure but it is not well shown in this part of the field. Veinlets of malachite in the ohaloo- oite in one zone. Fig. 1?9. (x ?0 diameters) Bonanza Mine. Covellite (dark) in radial plates, partially replaced by ohalooolte . - ^- - -'- 420. PLATE XXXII. Photographs of Polished Surfaces of the Kenneoott, Alaska. Fig. 130. (x ?0 diameters) Bonanza Mine, open out. Bornite (dark) and chalcopyrite (light) in scalloped structures. Broken by azurite veinlets. Fig. 131. (x 20 diameters) Jumbo Mine. Complex associa- tion of chalcopyrite (light), bornite (dark), and covellite (dark, mottled) in concentric structures. The covellite is largely a replace- ment of bornite. Fig, 132. (x 50 diameters) Bonanza Mine, open cut. Chalcopyrite (light) and bornite (dark) with chaloocite (a little lighter than the bornite) in scalloped structures. Fig. 133. (x 50 diameters) Jumbo Mine. Complex association of chalcopyrite and bornite in exceedingly intricate mass (mottled, light gray), with later luzonite (white) and covellite (mottled gray), in concentric and scalloped structures characteristic of certain "knots" in the ore. For discussion see page* zw to 2<>8 . Crystals of bornite or chaloopyrite with rims of luzonite (?) occur on the right side of field. Covellite replaces bornite, and forms fringes of radial plates about margins of curved masses of the earlier minerals. The luzonite is in part later than the covellite. 421. PLAT? XXXIII Photographs of Polished 3ur faces of Ores . Jr. lattice Fig. 13^. (x 270 diameters) Ajo, Arizona. Chaloopyrite A structure in bornite (light gray). Chaloo- oite and malachite (dark gray) around the mar- glna of the grain. Gangue, black. Photographed by Murdoch. Fig. 135. (x diameters (?) ) Virgllina, Virginia. Bornite (dprk) with ohaloocite (light) in graph! o structures and asoociated with mutu- al boundaries. Photographed by Murdoch. Fig. 136. (x 50 diameters (?) ) Messina, South Africa. Bornite (light) and ch?>lcocite (dark) in graphic structure. Etched -#ith nitric acid showing the orthorhoabic structure of the ohalcocite, an-! its parallel orientation throughout the graphic pattern. Photographed by Alfred Wandtke. Fig. 137. (x 750 liametera) Ajo, Arizona. Bornite (d.irk) with ohalcopyrite (light), devslop-fl in lattic* structure. Photographed by l.-lur^och. 422, PUTS XXXIV Photographs of. Poll shad Surfaces. Fig. 13. (x 67 diameters) Messina, South Africa, Chalcocite treated with nitric acid showing orthorhorablc etch-pattern. Photographed by Alfred Fig. 139. (* 6~ diameters ) Synthetic chalcocite from the Geophysical laboratory formed above 91C with an excess of sulphur, Etched with ni- tric acid, reveallnrr the octahedral parting of the isometric hich temperature chalcocite. This and the following miorophotographs are by Murdoch, Fig, UK), (x 65 diameters) Synthetic chaloocite aimil-r to that in figure 139. Fig. 1M. (x 50 diameters) Similar to figures 139 and Fig, 1*|2, (x 70 diameters) Butte. Bornite (dark) vith chalcocite (lip;ht), possessing a blue and white lattice, suggesting replacement of bornite through the lattice stage. Fig. 1J-3. (x ^" diameters) Butte. lines of bornite (dark) in chaloocite (light) and broken by irregular veinleta of ch?,lcooite. PU 433. PLATT? XXXV Photographs of Poliahad Surfaces of Ores. ("urdoch) I Fig. l^Jj-, (x 63 diameters) Butte, Montana, Chaloooite (white) '*ith bornite (dark) along margins of grains, and "spot structures" in the adjoining ohalcocite grains. Fig. 1^5. (x 3^0 diameters) Butte, Montana. Spot struc- tures, and subgraph! o structures, between bor- nite (dark) and chalcocite (light). Note curved, banded distribution of the bornite specks Fig. lJj-6. (x 65 diameters) Butte, Montana. Bornite (dark) with chalcocite (light) in the lattice pattern Note the resistant rim of bornite along the left of the grain. Fig, lfc-7. (x 65 diameters) Splines of bornite (dark) re- maining in a field in which the bornite is largely replaced by chalcocite (light) in the lattice structure. The splines, however, are slightly replaced by ch-plcooite in veinleta, developed parallel to the lattice directions. Pig. 148. (at diameters) Bonanza Line, Zennecott, Alaska. Bornite (dark) partially replaced by ahalcopyrite flight), developed in the lattice pattern. Bor- nite Bplinee remain little altered. 14-9. (x diameters) Butte. liontana. Bornite spline in a field of bornite-oh: loocite luttioe, show- ing incipient alteration to Chaloooite parallel to the lattice directions. 424 iOOl PUTE XXX7I Photographs cf Polisher! Surfaces of Ores. (Murdoch). Fig. 150. (x 61 diameters) Redruth, Cornwall. Bornite (dark) along cracks in chalcocite (light). Fig. 151. (x 1?5 diameters) Hedruth, Cornwall, Similar to Fig. 150. The relations elsewhere and the structure of the chrdcocite indicates that the chaloocite is R replacement of bornite. The preservation of these veinlika strips may possibly be due to outgoing iron. Chalcopyrite frequently occurs along their centre lines. Bornite Fist. 152. (x 150 diameters} C OFP er Queen. Bisbee, Ariz.- (dark), partially altered to ohalcocite (lisht), which develops the lattice struc- ture in the bornite. Fig. 153. (x 50 diameters) Calument *nd Arizona Mine, Biabee, Arizona. Chaloopyrite grains with halos of bornite in n field of chalcocite. Cores of pyrite in a few cases. Fig. 151J-. (x 115 diameters) Bonanza Mine, Kannecott, Alaska. Bornite (dark) partially replaced by intricate chaloopyrite lattice (light). Vein-like strips of bornite remain, which at this magnification make it seem certain that the sequence j j the reverse of that stated. For discussion, see page 359. Fie. 1~5. (x itO diameters) Butte, Montana. Bornite (light gray) partially altered to covellite (dark) alon?; veinlets, with adjacent chalco- rite (white) ^nd pyrite (rough, with high relief) untouched. 435. PLATE XXXVII ' ^Mqihed Surfaces of Ores f ror. 3haata Co .", Co lif orni g ._ (Photographed by J. Murdoch) 1^6. (x diameters) From Sutro Mine. Bornite (dark) and chalcooite (lip;ht). The chalco- cita possesses s grained structure and a mottled appearance shown by the variation in color on the polished surface. Not minute sub0;raphic forma of the bornite in the chaloo- cite. Fig. 157- (* diameters) From Sutro Mine. The aame field as in figure 156, but etched with ni- tric Bold, revealing the orthorhombic nature of the chnlcooits, and showing the different orientations of ths individual chalcooite grains. The fine linaa of the etch-pattern of the bornite are shown faintly, especially on the right side of the ch^lcocite field, and the structure is emphasized by the char- acteristic blocks which have sprung out be- tween the cracks. Fig. 156". (x diameters) From Sutro Mine. Bornite (dark) and mottled ohalcocite (light) in re- lations similar to those shown in figures 157. 1*^9. (x diameters) From Afterthought Mine. Cal- oite (black), partially replaced along ita cleavages by bornite (gr w y) nd ohalcocite (light). n . chplcocite i a believed to bs a replacement of bornite. 436 Plate XXXVIII. Photographs of Polished Surfaces of Ore. (Photographed by Murdoch). Fig. 160. (x diarr.eters) Butte, Montana. Chalcopyrite (white) developed in bornite (darkj in the lat- tice structure. Fig. 161. (x diameters) Butte, Montana. Chalcopyrite (white) developed in bornite (dark) in part in the lattice structure. Fig. 162. (x 45 diameters) Artificial products close to ch^lcopyrite an 1 bornite in composition, crystallized from a melt in the Geophysical Laboratory. (chalcopyrite, light; bornite, dark). Fig. 163. (x 85 diameters) Redruth, Cornwall. Bor- nite crystal containing symmetrically dis- tributed chalcopyrite in the lattice struc- ture* Fig. 164. (x 770) Seven Devils, Idaho. Exceedingly fine lattice of chalcopyrite in bornite. Fig. 165. (x 770) Seven Devils, Idaho. Exceedingly fine lattice of chaloopyrite in bornite. 1940 GENERAL LIBRARY - U.C. BERKELEY U.C.BERKELEY ENGINEERING LIBRARY