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 Reprinted from ECONOMIC GEOLOGY, Vol. II, No. 7, October-November, 1907. 
 
 THE ASSOCIATION OF ALUNITE WITH GOLD IN 
 THE GOLDFIELD DISTRICT. NEVADA - 
 
 FREDERICK LESLIE RANSOME 
 
 Economic Geology Publishing; Company 
 
^U^fO 
 
 THE ASSOCIATION OF ALUNITE WITH GOLD IN 
 THE GOLDFIELD DISTRICT, NEVADA.^ 
 
 Frederick Leslie Ransome. 
 
 INTRODUCTION. 
 
 Alunite, a hydrous sulphate of potassium and aluminium with 
 the formula K2O.3AI2O3.4SO3.6H2O, is one of those minerals 
 the apparent rarity of which is probably due in no small measure 
 to inconspicuousness. The material was first noted at Tolfa, near 
 Rome, where it was used as early as the fifteenth century for 
 the manufacture of potassium alum (K20.Al203.4S03.24H20).^ 
 This salt, containing 2 parts less of AI2O3 with 18 parts more 
 of water, is obtained from the alunite by roasting and lixiviation. 
 Alunite is found also on some of islands of the Grecian Archi- 
 pelago, near Muszay in Hungary, on Mount Dore in France and 
 in a few other localities in Europe, as well as with the opal of 
 Queretaro, in Mexico. In the United States it has been described 
 in the Rosita Hills,^ on Calico Peak near Rico,^ in the National 
 Belle Mine near Silverton,^ and at Cripple Creek,^ all in Colo- 
 
 ' Published with the permission of the Director of the U. S. Geological 
 Survey. 
 
 * For the latest account of this celebrated locality, with a bibliography, see 
 L. De Launay, " La Metallogenie de ITtalie." Tenth Geological Congress, 
 Mexico, 1906, pp. 125-132. 
 
 ' Cross, Whitman, " Geology of Silver Cliff and the Rosita Hills, Colorado." 
 17th Ann. Rept. U. S. Geol. Survey, Pt. 2, 1896, pp. 52-56. Also Am. Jour. 
 Set., 3d ser., vol. 41, 1891, pp. 466-475. 
 
 * Cross and Spencer. " The Geology of the Rico Mountains, Colorado." 
 2ist Ann. Rept. U. S. Geol. Survey, Pt. 2, 1900, pp. 92-94. 
 
 'Hurlbut, E. B. "On Alunite from Red Mountain, Ouray County, Colo." 
 Am. Jour. Set., 3d ser., vol. 48, 1894, pp. 130-131. Also Ransome, F. L. "A 
 Report on the economic geology of the Silverton Quadrangle, Colorado." 
 Bull. U. S. Geol. Survey No. 182, 1901, p. 235. 
 
 * Lindgren and Ransome. " Geology and Gold Deposits of the Cripple 
 Creek District, Colorado." Profess. Paper U. S. Geol. Survey No. 54, 1906, 
 p. 125. 
 
 667 
 
668 FREDERICK LESLIE RANSOME 
 
 rado. It occurs also at Tres Cerritos, Mariposa County, Cali- 
 fornia.^ 
 
 Mr. W. Lindgren^ found alunite in the Ryerson mine near 
 Morenci, Arizona, associated with pyrite and kaolin. He re- 
 garded it (page 193) as formed by the attack of descending sul- 
 phuric acid solutions upon sericite away from free oxygen. 
 
 Until recently geologists have regarded alunite as formed 
 exclusively by the attack of sulphurous fumarolic vapors upon 
 feldspatchic rocks.^ At Cripple Creek, however, the mineral was 
 found only in the oxidized ore, and, like the kaolinite of that 
 district, is probably secondary with reference to the original tellu- 
 ride ores. De Launay,* moreover, has recently maintained with 
 cogency that the alunite of Tolfa is probably not, as formerly 
 supposed, a solfataric product, but is due to the action of perco- 
 lating surface water, charged with sulphuric acid by the oxida- 
 tion of pyrite, upon a particularly feldspathic facies of the trach- 
 yte. It is accordingly no longer justifiable to regard the occur- 
 rence of alunite as proof of former fumarolic activity. The min- 
 eral may form under very different sets of conditions and it is 
 necessary, in seeking the explanation of any occurrence of it, to 
 keep alternative hypotheses in mind. 
 
 In all four of the Colorado localities the alunite occurs in 
 regions which contain ore deposits. In the Rosita Hills and in 
 the Rico and Silverton districts it is apparently not a product 
 of superficial oxidation. No close kinship between this mineral 
 and the ores, however, has been shown, although it was doubtless 
 recognized by all the workers in those districts that there might 
 be some genetic relation between the metallic sulphides and the 
 volcanism of which the sporadic solfataric metamorphism of 
 feldspathic rocks to quartz-alunite aggregates was one manifes- 
 tation. It is worth noting in this connection that the indexes of 
 
 * Turner, H. W. " Rocks and Minerals from California." Am. Jour. Sci., 
 4th ser., vol. 5, 1898, pp. 424-426. 
 
 •"The Copper Deposits of the CI if ton- Morenci District, Arizona." Profess. 
 Paper U. S. Geol. Survey No. 43, 1905, pp. 1 19-120, 169 and 193-194. 
 
 * Iddings, J. P. "Rock Minerals." New York, 1906, p. 476. 
 
 * Of>. cit. 
 
ASSOCIATION OF ALUNITE WITH GOLD 669 
 
 two of the best and most recent works^ on ore deposits contain 
 no reference to alunite, showing that the mineral has not been 
 commonly recognized as one intimately associated with ores. 
 Neither is it mentioned, so far as I am aware, in President C. R. 
 Van Hise's work on metamorphism.^ Professor J. H. L. Vogt,^ 
 however, in discussing metasomatic vein-processes, has remarked : 
 " I would mention also the formation of alum-stone ; quartz- 
 alunite rocks ; quartz-diaspore rocks, etc. ; and also the formation 
 of bauxites, etc. But I do not know that these changes have 
 been anywhere observed in genetic relation with ore-veins." 
 
 In the Goldfield district alunite is so abundant and so gener- 
 ally associated with the intense alteration accompanying ore 
 deposition that it must be looked upon as one of the most charac- 
 teristic minerals of the gold deposits. This close association of 
 the gold with a mineral which apparently had not been recog- 
 nized in Nevada prior to the present investigation and which has 
 been reported in comparatively few localities elsewhere, seems 
 worthy of some description, especially as the relation promises 
 to throw light upon the origin of the ores. 
 
 ROCKS OF THE DISTRICT. 
 
 For a preliminary account of the geology of Goldfield the 
 reader is referred to an earlier publication.^ A full description 
 of the geology and ore deposits is in preparation and will appear 
 as one of the professional papers of the U. S. Geological Survey. 
 It is sufficient in this place to state that the ores occur in middle 
 or early Tertiary rhyolite, latite, andesite and dacite. Nearly all 
 of these are extrusive flows, but the dacite, which contains most 
 of the important mines, is an intrusive sheet, although the intru- 
 sion probably took place beneath a cover of volcanic rocks which 
 
 * Beck, Richard. " Lehre von den Erzlagerstatten." Berlin, 1901. Stelzner- 
 Bergeat. ' Die Erzlagerstatten." Leipzig, 1904-1906. 
 
 * " A Treatise on Metamorphism." Monog. U. S. Geol. Survey No. 47, 1904. 
 
 * " Problems in the Geology of Ore Deposits." Trans Amer. Inst. Mining 
 Engineers, vol. 31, 1902, p. 150. 
 
 •Ransome, F. L. "Preliminary Account of Goldfield, Bullfrog, and Other 
 Mining Districts in Southern Nevada." Bull. U. S. Geol. Survey No. 303, 
 1907. 
 
670 FREDERICK LESLIE RANSOME 
 
 was not of great thickness. The ore-bearing rocks rest upon, 
 or cut through, a pre-Tertiary foundation of granitic rock with 
 which is involved some Cambrian shale, and are themselves, in 
 outlying parts of the district, unconformably overlain by tuffa- 
 ceous lake sediments and later Tertiary lavas. 
 
 Although comparatively unaltered representatives of most of 
 the rocks named may be found within the district, yet the meta- 
 morphism which they have undergone over much of the area is 
 of a most conspicuous kind. Dark pyroxene-andesites, dacite, 
 and rhyolite have alike been changed to nearly white aggregates 
 of secondary minerals. Such products, as a rule, retain only 
 traces of the original textures of the igneous rocks from which 
 they have been derived. To detect and interpret these traces 
 requires patient field observation of the progressive stages of 
 alteration, supplemented by microscopical study. 
 
 TYPES OF ALTERATION. 
 
 Three types or stages of alteration are recognizable. Where 
 the chemical activity has been most intense the rocks have been 
 changed to porous, fine-grained aggregates consisting essentially 
 of quartz. This is the material of which are composed the hun- 
 dreds of cragg>^ points and ledges which are one of the most 
 characteristic features of the topography of the district. Although 
 most of these ledges, so far as known, contain no ore bodies, yet, 
 until recently, no ore has been found that was not in or along- 
 side such quartz rock. Some of the discoveries made in 1906, 
 however, show that very rich bodies of ore may occur at places 
 where there is no siliceous outcrop. 
 
 The second type of alteration is one in which the rock is 
 changed to a comparatively soft, light-colored mass of quartz, 
 kaolinite, alunite and pyrite. At any given locality the boun- 
 dary between the rocks representing the two kinds of alteration 
 may be fairly sharp; but the proportions of quartz, kaolinite and 
 alunite vary widely, and consequently, over the field at large, 
 gradations in composition and hardness may be found between 
 these two typical metamorphic products. Both types are usually 
 
ASSOCIATION OF ALUNITE WITH GOLD 671 
 
 associated with the ores and are most conspicuously displayed 
 over a belt of country stretching east from Goldfield past Preble 
 Mountain. This mountain, in fact, is composed chiefly of ande- 
 site and dacite which have undergone alteration of the kinds 
 descrijDcd. 
 
 The third type of alteration has effected less conspicuous 
 results than the other two and its products are not closely asso- 
 ciated with the ores. It consists in the development of calcite, 
 quartz, chlorite, epidote and pyrite at the expense of the original 
 minerals and groundmass and is thus propylitic in character. It 
 is rather sharply marked off from the second type by the presence 
 of calcite and the absence of alunite, these minerals not having 
 yet been observed together in the same specimen.^ Rocks which 
 have undergone this form of alteration, chiefly andesite, latite 
 and dacite, still retain something of their original color and 
 texture, the principal external sign of change being a greenish 
 tint instead of the usual dark gray of the fresh rock, and a lack 
 of luster in the phenocrysts. 
 
 GENERAL OCCURRENCE OF THE ALUNITE. 
 
 Attention was first directed to the occurrence of alunite in the 
 district by the finding of small nests, or crystalline aggregates, 
 up to five millimeters in diameter, of a pale pink mineral in a 
 greatly altered nearly white rock exposed on the east shoulder 
 of Vindicator Mountain, about two miles northeast of the town 
 of Goldfield. The rock contains quartz phenocrysts and was 
 probably originally a rhyolite. The microscope shows that the 
 
 * Since this was written, very finely crystalline calcite, sericite and probably 
 some kaolinite have been found associated with much larger crystals of alunite 
 in an altered rhyolite from the east slope of Vindicator Mountain. The 
 molecular ratio of soda to potash in the alunite, as determined by Dr. Hille- 
 brand, is as 40 to 45. It is possible, however, that the result obtained for 
 potash may be high in consequence of some decomposition of the sericite by 
 the sulphuric acid used to dissolve the alunite. 
 
 The occurrence together of alunite and calcite, apparently developed simul- 
 taneously, shows that the alteration took place under conditions which did not 
 permit the free escape of carbon dioxide. It is scarcely conceivable, for ex- 
 ample, that calcite could form in the presence of sulphuric acid percolating 
 down from an overlying deposit of oxidizing pyrite. 
 
EXPLANATION OF PLATE IX. 
 
 Fig. I. Photomicrograph of thin section of altered dacite (G. 398) from 
 D level of the Combination Mine, Goldfield. Magnified 40 diameters. Nicols 
 crossed. 
 
 The section shows a characteristic pseudomorphous aggregate of alunite 
 iA), and quartz (Q) after a labradorite phcnocryst. In this case there is 
 an isotropic material (O) present which is probably opal. The groundmass 
 is a fine-grained aggregate of quartz, alunite and pyrite, the crj-stals of the 
 last mineral (black) being in some places grouped in pseudomorphous aggre- 
 gates after hornblende and biotite. 
 
 Fig. 2. Photomicrograph of thin section of altered dacite (G. 634) from 
 the 200- foot level of the Florence Mine, Goldfield Magnified 40 diameters. 
 Ordinary light. 
 
 The section shows rather more alteration than the preceding, little of the 
 original texture of the dacite remaining. Pyrite (black) is abundant and is 
 crystallized with alunite {A). The latter mineral exhibits characteristic 
 lath-shaped sections with irregular ends and felty aggregation. It has evi- 
 dently developed in some places along devious cracks. There is much quartz 
 (Q) in the rock but it is for the most part too finely crystalline to appear 
 clearly in the photograph. 
 
 672 
 
Plate IX. 
 
 Economic Geology. 
 
 FiQ 1. 
 
 Fig. 2. 
 
ASSOCIATION OF ALUNITE WITH GOLD 673 
 
 feldspar phen.ocrysts, as well as the groundmass, have been 
 altered to an aggregate of quartz and alunite, the optical deter- 
 mination of the latter mineral being checked by independent 
 chemical tests in the Survey laboratory by Dr. W. F. Hillebrand 
 and Dr. W. T. Schaller. With the alunite is associated a subor- 
 dinate quantity of an anhedral/ colorless mineral of high refrac- 
 tive index, strong birefringence and possessing a single well- 
 marked cleavage. The mineral is biaxial and optically negative, 
 the plane of the optic axes coinciding with the cleavage. It 
 resembles diaspore but differs from that mineral in being optically 
 negative. It has not yet been identified. Undoubted diaspore 
 has been found intergrown with alunite, in the altered dacite of 
 the Florence mine. 
 
 Microscopic study of the bleached, altered rocks of the district 
 has revealed alunite in nearly all of them. The rhyolite forming 
 the summit of Columbia Mountain, just north of the towns of 
 Goldfield and Columbia, has been changed to a quartz-alunite 
 rock, the alunite, with some quartz, forming pseudomorphs after 
 the feldspar phenocrysts which probably ^vere originally ortho- 
 clase. Alunite is very abundant in the altered andesite and dacite 
 of Preble Mountain and vicinity and is present in practically all 
 of the altered dacite which forms the country rock of the Com- 
 bination, Florence, January, Red Top, Jumbo and other mines 
 near Goldfield. It also occurs in abundance directly associated 
 with the sulphide ores in these mines, in some places, particularly 
 in the January mine, as a soft, easily pulverized material, of snow 
 white or faint pink color, which is not always distinguishable on 
 mere inspection from certain pure forms of kaolinite which occur 
 in the mines of this and other districts. At Goldfield, the deli- 
 cate pink tint of the alunite is rather characteristic and serves 
 often as a rough-and-ready means of distinguishing it from the 
 kaolinite with which it is often associated. Close inspection of 
 the altered rocks in which alunite is abundant usually reveals 
 
 *AnhedraI = without crystal faces, xenomorphic, allotriomorphic. See 
 Cross, Iddings, Pirsson and Washington, "The Texture of Igneous Rocks," 
 Journal of Geology, vol. 14, 1906, p. 698. 
 
674 FREDERICK LESLIE RANSOME 
 
 this faint pink color in the aggregates which have replaced the 
 original feldspar. 
 
 As a constituent of altered rocks in the Goldfield district, the 
 alunite does not have complete crystal form. It is tabular in 
 habit, the basal pinacoid being well developed and the rhombo- 
 hedral faces being absent. In thin sections it shows distinct 
 cleavage parallel to the basal planes and parallel extinction with 
 reference to the cleavage. The birefringence is apparently 
 rather stronger than the difference y — a = 0.018 given in the 
 table of Levy and Lacroix.^ the interference colors in good thin 
 sections being mostly yellow but rising in places to red or blue 
 of the first order. The mineral gives a positive uni-axial inter- 
 ference figure, in sections showing no cleavage. The index of 
 refraction is distinctly higher than the balsam of the slide (1.54), 
 that given for the alunite of Tolfa being ••=1.572 and e= 1.592.* 
 The most decisive optical means of distinguishing the mineral 
 from the optically negative sericite which it resembles in habit and 
 refraction, is by means of the quartz wedge or gypsum plate. 
 Sericite shows the higher interference colors when the section 
 is turned with the cleavage at right angles to the direction of 
 greatest elasticity (X) of the plate or wedge, while alunite shows 
 the brighter colors when the basal cleavage is parallel to this 
 direction. 
 
 The soft massive alunite, such as occurs in ilie January mine, 
 when gently crushed on a microscopical slide and stirred in a 
 drop of water, shows under a high power of the microscope thin, 
 colorless scales of hexagonal outline. The average diameter of 
 those examined is less than 0.05 millimeter, and such is their 
 tenuity that no crystal faces can be recognized on the edges of 
 the basal-pinacoidal scales. 
 
 COMPARISON OF FRESH AND ALTERED DACITE. 
 
 Inasmuch as the largest and richest ore bodies are in the dacite 
 and as the rock is uniform in character, it was chosen for a close 
 
 1 (I 
 
 Les Mineraux des Roches." Paris, 1888. Also Iddings, J. P. " RocV 
 Minerals." New York and London, 1906. 
 "Iddings, J. P. Op. cit., p. 476. 
 
ASSOCIATION OF ALUNITE WITH GOLD 6ys 
 
 study of the alteration effected by the ore-depositing agencies. 
 Such comparison is especially favored by the fact that the most 
 intense alteration is often local and consequently it is possible to 
 obtain specimens of fresh and altered rock within moderate dis- 
 tances of each other. The two specimens of dacite analyzed and 
 compared wxre taken about one and one fourth miles apart. 
 
 The unaltered and typical dacite is a dark gray rock with 
 abundant phenocrysts of plagioclase, up to a centimeter in length, 
 lying in a dark aphanitic groundmass. There are present, also, 
 less numerous phenocrysts of augite and biotite with occasional 
 anhedral grains of quartz. The volumetric ratio of phenocrysts 
 to groundmass is estimated at rather less than i to 3. 
 
 Under the microscope the feldspar phenocrysts prove to be 
 labradorite of the approximate composition AbiAnj. They are 
 perfectly fresh, with slightly rounded euhedral^ outlines. A 
 noticeable feature is the presence in many crystals of closely 
 crowded minute inclusions of glass which in some cases consti- 
 tute cloudy cores or kernels, in others sharply bounded shells, the 
 center of the crystal being clear. The augite crystals are roughly 
 euhedral or subhedral, with a maximum measured extinction 
 angle, Z t\C, of about 45°. The biotite occurs in the usual 
 short, rather irregular pseudo-hexagonal prisms. Both biotite 
 and augite are entirely fresh. The quartz phenocrysts are anhe- 
 dral, with rounded, embayed or sharply angular outlines. 
 
 Hornblende is very abundant but is confined almost exclu- 
 sively to the groundmass, most of the crystals being comparable 
 in size rather with the feldspar microlites than with the pheno- 
 crysts just described. The largest hornblende crystal in the thin 
 section studied is about half a millimeter in length. It should 
 be said, however, that in dacite from other parts of the district 
 the hornblende occurs as phenocrysts. 
 
 The feldspar microlites in the groundmass are at least in part 
 labradorite of as calcic composition as the phenocrysts. They, 
 the hornblende, and a little magnetite and apatite, are imbedded 
 in an abundant glass, which in transmitted light is gray, with 
 
 * Euhedral = bounded by crystal faces ; idiomorphic, automorphic. 
 
676 FREDERICK LESLIE RANSOME 
 
 incipient crystals, or crystallites. One crystal of titanite, 0.3 
 millimeter in length, is in the thin section studied. 
 
 A chemical analysis of this rock, made by Mr. George Steiger 
 in the Survey laboratory, is given under I. in the table on page 680. 
 
 The arbitrary norm of this rock, calculated from analysis in 
 accordance with the plan of the " quantitative classification,"^ is 
 as follows: 
 
 Norm of DAaxE. 
 
 Quartz 17.34 
 
 Orthoclase 15.01 
 
 Albite 25.68 
 
 Anorthite 21.96 
 
 Zircon 37 
 
 Diopside 5.21 
 
 Enstatitc 4-63 
 
 Magnetite 4-87 
 
 Ilmenite 1.52 
 
 Apatite ... .34 
 
 (Water 2.95) 
 
 99.88 
 
 As the rock contains much undifferentiated glass no close cal- 
 culation of its real mineralogical composition, or mode, is pos- 
 sible. The norm and chemical analysis show that the rock is a 
 tonalose in the terminology of the quantitative classification. 
 
 The altered dacite, of which a chemical analysis by Mr. George 
 Steiger is given under II., was taken from the D level of the 
 Combination Mine, about 235 feet below the surface and in a 
 part of the mine where no apparent oxidation had taken place. 
 The original water level in this mine appears to have been at 
 about 130 feet from the surface, near which depth the oxidized 
 ore gives place to sulphides. The rock analyzed is the typical 
 wall-rock of this and adjacent mines. 
 
 * Cross, Iddings, Pirsson and Washington. " The Quantitative Classification 
 of Igneous Rocks." Chicago and London, 1903. 
 
 For the benefit of the reader who may not have had occasion to familiarize 
 himself with modern petrology it may be explained that the " norm '* is a 
 classificatory expedient and is usually very different from the actual mineral 
 composition, or "mode." The dacite for instance really contains no ortho- 
 clase, the potassium being combined in biotite and in the glass of the ground- 
 
ASSOCIATION OF ALUNITE WITH GOLD 677 
 
 The altered rock is light-gray, flecked with numerous dull 
 white spots which represent the original feldspar phenocrysts. 
 The few quartz phenocrysts are unchanged, but the ferromag- 
 nesian, or, more briefly, femic, minerals have wholly disappeared. 
 Pyrite, in small, disseminated crystals, is abundant. 
 
 Under the microscope, it is seen that the only original con- 
 stituents are the quartz phenocrysts, even these showing some 
 development of pyrite along irregular microscopic cracks, and an 
 occasional small crystal of zircon. The feldspars are changed 
 to aggregates of minutely crystalline kaolinite and tabular crys- 
 tals of alunite, the latter being sometimes arranged in sheaf-like 
 bunches. (See Plate IX.) The alunite has the form character- 
 istic of its occurrence in this district, sections cut at large angles 
 with the basal pinacoid having the shape of narrow laths with 
 irregular ends. The length of these lath-shaped sections rarely 
 exceeds half a millimeter. The alunite, when cursorily exam- 
 ined under a low-power lens, may easily be mistaken for sericite 
 (muscovite), although of course this error is quickly dispelled 
 on closer study, sericite being optically negative and alunite 
 positive. The interference colors, moreover, although bright, 
 particularly in the larger crystals, are of a lower order than those 
 of sericite. Quartz also accompanies the alunite and kaolinite in 
 some of the pseudomorphs after labradorite. (See Plate IX., 
 Fig. I.) 
 
 The former femic constituents can be recognized from the fact 
 that they contain little or no kaolinite, the quartz and alunite 
 being here associated with abundant pyrite. In some cases the 
 pseudomorphous aggregate still preserves a suggestion of the 
 former cleavage of the biotite or of the outlines of hornblende. 
 
 The groundmass is changed to a fine-grained aggregate of 
 kaolinite, quartz, alunite and pyrite. The last-named mineral 
 occurs also in irregular microscopic veinlets. 
 
 As the rock is wholly crystalline and is made up of minerals of 
 approximately constant composition, it is possible to calculate 
 with some precision the actual mineralogical constitution from 
 the chemical analysis. This, in percentages of mass, is as follows : 
 
678 FREDERICK LESLIE RANSOME 
 
 MiNERALOCICAL G)MPOSITION OP ALTERED DaCITE. 
 
 Quartz ( SiOj) 49-38 per cent, of mass. 
 
 Kaolinite (AU0a.2Si0r2H,0) 23.99 
 
 Alunite' (IG0.3Al,Os4S0..6H,0) 157.^ 
 
 Pyrite (FeSi) 7.^ 
 
 Water (H,0) .. 25.^ 
 
 Other constituent;* ... 1.17 
 
 loaoo 
 
 Compared as they stand, the two analyses show close agree- 
 ment in silica, alumina, titanic oxide and phosphorous pentoxide. 
 When all the iron in both analyses is calculated as ferrous oxide, 
 that of the altered rock shows a loss of only 0.7 per cent., so that 
 in this respect also the two analyses are very close together. On 
 the other hand, the analysis of the altered rock shows a loss of 
 practically all the lime and magnesia, most of the soda and one 
 half of the potash. It has gained a large quantity of combined 
 water, nearly 6 per cent, of sulphuric anhydride and nearly 4 per 
 cent, of sulphur. 
 
 Such a direct comparison of analyses, however, while indi- 
 cating in this case the general character of the change that has 
 taken place in the dacite, is really a comparison of equal masses 
 of the two rocks, or, more strictly speaking, of the two powders 
 as prepared and weighed for analysis, and as Mr. Lindgren* 
 pointed out some years ago, would accurately portray the char- 
 acter of the alteration only under two conditions, namely, (i) 
 that the specific gravity or density of the rock is unchanged by 
 the metamorphism, and (2) that the alteration has not produced 
 any change of volume in the rock mass as a whole, such a change, 
 for example, as takes place when peridotite alters to serpentine. 
 It is essential in an investigation of this kind to determine what 
 materials have been added to or subtracted from a unit volume 
 
 * The molecular proportions of potash and soda as calculated from analysis 
 II., page 680, are respectively 11 and 14. All of the potash and 8 of the soda 
 molecules are needed to satisfy the 75 molecules of S0». Consequently the 
 Goldfield alunite is a soda-bearing variety. 
 
 ' " Metasomatic Processes in Fissure-veins." Trans. Am. Inst. Mining 
 Eng„ vol. 30, 1901, pp. 591-595. 
 
ASSOCIATION OF ALUNITE WITH GOLD 6yg 
 
 (or unit mass) of fresh rock in order to produce a given altera- 
 tion product. 
 
 The changes undergone by a given volume of rock can be 
 determined most simply if the alteration is unaccompanied by 
 swelling or contraction of the whole rock mass. The altered 
 dacite shows no indication of increase of bulk. The field evi- 
 dence of such swelling would comprise irregular fracturing and 
 squeezing of the rock, the presence of curved slickensided surfaces, 
 and pressure effects in the microscopical texture of the altered 
 dacite. Contraction would probably be more difficult to detect. 
 Any notable degree of shrinkage, however, would be likely to 
 record itself by the development of many irregular open cracks 
 or by some modification of the original texture of the rock, such 
 as the distortion of phenocrysts. Evidence of either kind being 
 absent, it may be concluded with some confidence that whatever 
 volumetric change has taken place is to be sought in a compari- 
 son of the porosity of the two rocks and not in change of total 
 bulk. 
 
 The porosity of the fresh dacite is exceedingly slight and is 
 not apparent under the microscope. An accurate specific gravity 
 determination of the rock powder used for analysis, made by Mr. 
 George Steiger, gave the figures 2.654.^ He also determined the 
 specific gravity of the whole hand specimen as 2.63, the differ- 
 ence between the two results being an approximate measure of 
 the porosity. The specific gravity of the altered dacite, which 
 is noticeably porous, as calculated from the mineralogical com- 
 position on page 6yy, assigning to the 1.17 per cent, not accounted 
 for the average specific gravity of the other constituents is 
 2.769.^ Determinations by Mr. Steiger gave 2.766 for the pow- 
 dered rock and 2.49 for the hand specimen. These figures indi- 
 
 * The specific gravities of the powders were determined by placing weighed 
 quantities in water in a tared picnometer and boiling them at low temperature 
 under reduced pressure. They were then kept for some hours in a thermostat 
 and quickly weighed at 25** C. The specific gravities of the rock masses were 
 obtained by weighing them in air and quickly in water. The figure 2.49, ob- 
 tained for the altered dacite, is probably slightly too high in consequence of 
 some absorption by pores open to the surface. 
 
 'The figures used for the specific gravities of the minerals were: Quartz 
 2.66, kaolinite 2.61, alunite 2.60, and pyrite 5.0. 
 
68o 
 
 FREDERICK LESLIE RANSOME 
 
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ASSOCIATION OF ALUNITE WITH GOLD 68 1 
 
 cate that the substance of the altered rock has gained 4.2 per cent, 
 in weight and that, notwithstanding this gain, a given volume 
 of rock, under the supposition of constant volume, has suffered 
 a net loss of 5.3 per cent, in weight consequent upon an increase 
 of porosity. 
 
 Since the volume of a given mass of rock, in cubic centimeters, 
 is, for all practical purposes, equal to its weight in grams divided 
 by its specific gravity, the percentage of pore space in the fresh 
 dacite is obtained from the equation 
 
 287.4 _ 287.4 ^^874 ^ 
 2.63 2.654 2.63 ' 
 
 where 287.4 is the weight of a specimen of dacite in air, 2.63 
 the specific gravity of the rock mass, 2.654 the specific gravity 
 of the poreless mineral constituents, and x the proportion of pore 
 space which in this case is equal to 0.9 per cent. In the altered 
 rock, the proportion of pore space is 9.9 per cent., showing an 
 increase in porosity of 9 per cent, of the initial volume. 
 
 The chemical changes undergone by the dacite are shown in 
 the accompanying table. Columns P. and IP. are not essentia] 
 as regards the process illustrated although they represent a step 
 in the reasoning. They are of value also as showing that specific 
 gravity determinations of rock powders, while useful in calcu- 
 lations of the mineral compositions of rocks, are of minor impor- 
 tance in a comparison of this kind, in which it is necessary to 
 know the specific gravity of the mass as a whole, including its 
 pores. Incidentally, it may be remarked that when the specific 
 gravity of a rock is given in connection with its chemical analysis, 
 the method by which the density was determined should be 
 indicated. 
 
 In columns P. and IP. are given the number of grams of each 
 constituent in 100 cubic centimeters of each rock, the figures 
 being obtained by multiplying the percentage figures in columns 
 I*, and IP. by the respective specific gravities of the fresh and 
 altered dacite. The gains and losses of each constituent are 
 given in grams in column III., and in percentages of total initial 
 
682 FREDERICK LESLIE RAN SO ME 
 
 mass in column IV. The last column, V., g^ves the percentage 
 of loss of each constituent and perhaps most clearly displays the 
 nature of the change. 
 
 It appears that the rock has lost a little of its silica, more of 
 its alumina, nearly all of its magnesia and lime, three fourths of 
 its soda and two thirds of its potash. On the other hand, the 
 water has greatly increased, a large amount of sulphuric anhy- 
 dride has been added and the iron has been converted to pyrite. 
 The iron oxides in column P. correspond to 11.08 grams of 
 metallic iron, while the pyrite in column IV. corresponds to 8.4 
 grams of iron and 9.6 grams of sulphur. The quantity of iron 
 originally in the dacite was therefore not only sufficient to form 
 all the pyrite in the altered rock but has suffered a loss of over 
 27 per cent. 
 
 Comparisons of fresh and altered rocks are sometimes made 
 by assuming the constancy of one constituent and recalculating 
 the analysis of the altered rock accordingly. Such a method, in 
 the present case, would give erroneous results. 
 
 The metamorphic agent was evidently a strongly acid solution 
 carrying sulphydric and sulphuric acids, and possibly sulphurous 
 acid. Whether the sulphuric acid which was doubtless the direct 
 agent in the formation of the alunite was present in the original 
 solution from the first, or whether it was derived from the hydro- 
 gen sulphide, by oxidation at some stage in the movement of 
 the solution toward its point of attack upon the rocks now visible, 
 is an undecided question. The absence of sulphates from contact 
 ore-deposits indicates, however, that oxygenated sulphur com- 
 pounds are not given off directly from solidifying magmas. The 
 solution was capable of decomposing the silicates in the rocks, 
 carr>'ing part of their constituents away, reacting with the iron 
 of the magnetite and silicates to form pyrite, with the potas- 
 sium and aluminium to form alunite and with the aluminium to 
 form kaolinite. The large percentage of loss in calcium, potas- 
 sium and sodium shows that the active solution was far below 
 its saturation point for these elements and was, therefore, entirely 
 different in character from the supposedly alkaline solutions 
 which deposited the large class of sericitic and calcitic gold- 
 
ASSOCIATION OF ALUNITE WITH GOLD 683 
 
 quartz veins^ exemplified by those of California. Before pro- 
 ceeding farther with genetic considerations I shall briefly describe 
 the mineralogical character of the Goldfield ores. 
 
 THE GOLD ORES. 
 
 The typical unoxidized ore as it occurs in the dacite near Gold- 
 field consists of pyrite, bismuthinite and a reddish-gray cuprifer- 
 ous mineral having the general composition of tetrahedrite. 
 Native gold is usually associated with these minerals, particu- 
 larly with the bismuthinite and tetrahedrite, and in the rich ore 
 may be easily visible. These minerals are often arranged in 
 successive layers or crusts around silicified fragments of dacite, 
 the inner crust being usually the richer. 
 
 The brownish copper-bearing mineral, here provisionally called 
 tetrahedrite, contains antimony and sulphur, as chemically deter- 
 mined by Dr. W. T. Schaller, and is therefore not bornite, 
 although it goes by that name in the district. It appears so 
 commonly as a constituent of the best ore that there is a strong 
 suggestion that it is auriferous, particularly as the microscope 
 fails in some specimens of reputed rich ore to reveal any free 
 gold. Further investigation of the mineral is in progress. It 
 may possibly prove to be the orthorhombic species famatinite 
 which contains the same constituents as tetrahedrite, but in dif- 
 ferent proportions, and is characterized by a reddish tint. Bis- 
 muthinite, the sulphide of bismuth, is also regarded by the miners 
 as a sign of rich ore. 
 
 Concentric shells of ore minerals about greatly altered rock- 
 fragments are rather characteristic of the best ore. Specimens 
 from the Combination mine show an inner zone of free gold and 
 quartz up to an eighth of an inch in thickness. This is covered 
 by a shell of tetrahedrite and this by an outer crust of pyrite. 
 In the Florence mine similar fragments show, first, a shell of 
 pyrite, then one of tetrahedrite, and, finally, a thick crust of 
 quartz speckled with native gold, tetrahedrite and pyrite, and 
 transfixed by needles of bismuthinite. Some of the particles of 
 
 *See Lindgren, W. " Metasomatic Processes in Fissure Veins." Trans. 
 Am. Inst. Mining Eng., vol. 30, 1901, p. 668. 
 
684 FREDERICK LESLIE RANSOME 
 
 gold are embedded in the compact quartz, others are inclosed in 
 the bismuthinite. The different crusts are not in every case 
 sharply defined nor are they necessarily continuous. 
 
 The native gold of these ores is often in particles so fine and 
 so closely crowded in the gangue that the precious metal resem- 
 bles a streak of yellow ocher. The proportion of silver is rather 
 small, the average of thirty-four assays of rich ore from the 
 Mohawk mine, made for Mr. J. W. Finch, giving 330 fine ounces 
 of gold and 46.5 ounces of silver to the ton. 
 
 Tests and chemical analyses of the ores made for scientific and 
 technical purposes nearly all show a little tellurium, although no 
 tellurium mineral has been recognized in the mines close to Gold- 
 field. Tellurides of gold, not sufficiently well crystallized for a 
 determination of their species, occur in the Jumbo Extension and 
 Goldfield-Belmont mines, near Diamondfield, about four miles 
 northeast of Goldfield, and both tellurite and emmonsite (or dur- 
 denite) are found in the oxidized ore of neighboring properties.' 
 
 A small speck of sphalerite was found by Mr. W. H. Black- 
 burn in the Goldfield-Belmont, but galena has not been noted in 
 any of the mines so far as known. Chalcopyrite, a common min- 
 eral in most mining districts, was not found at Goldfield in the 
 course of the present investigation, but it is said to occur sparingly 
 in the Sandstorm and Florence mines. 
 
 The common gangue of the unoxidized ore is quartz. This is 
 usually compact, almost flinty, in texture, although porous in 
 mass, and in most cases bears unmistakable evidence of having 
 resulted from the silicification of dacite, rhyolite, or andesite. 
 Large vugs and conspicuously crystalline quartz, such as are 
 found in typical veins the world over, are practically absent from 
 the Goldfield district, where the free development of quartz crys- 
 tals in open spaces is represented only by drusy films, lining pores 
 left by the solution of phenocrysts or incrusting small interstitial 
 cavities in brecciated material. 
 
 Associated with the quartz in much of the ore are soft white 
 substances, such as the miners commonly call " talc." Chemical 
 
 * Ransome, F. L. " Preliminary Account of Goldfield, etc." Bull. U. S. 
 Geol. Survey No. 303, 1907, p. 36. 
 
ASSOCIATION OF ALUNITE WITH GOLD 685 
 
 and microscopical tests of the white material show that in most 
 cases it is either alunite or kaolinite or a mixture of the two.^ 
 Sericite, supposed when the preliminary report^ on the district 
 was written to be also abundant, proves on further study to be 
 very rare and is certainly nowhere an abundant or characteristic 
 mineral in the ore deposits. Gypsum, while not known with 
 actual sulphide ore, is very abundant in the altered rocks within 
 a hundred feet or so from the surface and does occur crystallized 
 with quartz and pyrite on the Goldfield-Belmont mine, below the 
 belt of oxidation. 
 
 As a rule, the gold and auriferous sulphides are embedded in 
 quartz. Alunite, however, is often so close to the gold as to 
 appear in the same microscopic field and the telluride of gold in 
 the Jumbo Extension mine is partly embedded in alunite. 
 
 The ores, it should be noted, do not occur in typical veins but 
 in masses of shattered, altered rock of very irregular shape. In 
 small part only do they appear to have filled visibly open cavities 
 or fissures. 
 
 For descriptions of the oxidized ores, which have been of so 
 much importance at Goldfield, the reader is referred to the 
 advance report cited and to the forthcoming memoir on the 
 district. In some places they contain alum derived from the 
 alunite. 
 
 THE ORE-DEPOSITING SOLUTIONS. 
 
 The mineralogical character of the Goldfield ores and the 
 alteration of the wall rock show that the solutions or vapors 
 which deposited them carried gold, copper, bismuth, antimony, 
 a little arsenic and tellurium, hydrogen sulphide, and probably 
 sulphurous and sulphuric acids. The solvent action of the solu- 
 tions upon quartz was comparatively slight, as shown by the fact 
 that the original phenocrysts of the dacite still retain their char- 
 acteristic embayed outlines when the rest of the rock is altered 
 
 ^ Mr. E. A. Collins, in his recent good description of the Combination mine 
 (Mining and Scientiiic Press, vol. 95, 1907, P- 398) refers to the ore as "a 
 mixture of soft kaolinized material and hard dacite." The " kaolinized ma- 
 terial " is chiefly alunite. 
 
 •Bull. U. S. Geol. Survey No. 303. 1907, p. 35- 
 
686 FREDERICK LESLIE RANSOME 
 
 to quartz, alunite and kaolinite. The quartz set free by the 
 decomposition of the silicates has for the most part recrystallized 
 as fine-grained aggregates. A small proportion, as we have 
 seen, has been carried away in solution, perhaps in part to be 
 recrystallized in small neighboring fissures, and in part permeat- 
 ing the surrounding less altered rocks. Large masses of coarsely 
 crystalline vein quartz are absent from this district and it seems 
 necessary to conclude that the ore-bearing solutions were not 
 only poor in silica but that, in spite of their energetic attack upon 
 the rocks, they were not good solvents for quartz. The appar- 
 ently intense silicification of the country rock in proximity to 
 fissures appears at first glance to point to a diflferent conclusion. 
 The conversion of a rock containing 60 per cent, of silica to a fine- 
 grained, pyritic, quartz aggregate does not, however, necessarily 
 imply the direct addition of 40 per cent, of silica. The result 
 may be attained by the removal of most of the bases, the con- 
 version of the iron to pyrite and the development of a porous 
 texture such as is favorable for ore deposition and is character- 
 istic of the Goldfield ledges. Moreover, it is probable that some 
 of the silica shown to have been abstracted from the altered dacite 
 may have been concentrated in the immediate vicinity of the 
 fissures. Certainly, whatever silica was originally in the solu- 
 tions did not travel far from the fissures through which they rose. 
 
 Whether the solutions were notably ferruginous is doubtful. 
 As compared with other districts, the pyrite at Goldfield is not 
 abundant and all of that in the country rock can, as has been 
 shown, be accounted for without any addition of iron. There 
 has, in fact, been an abstraction of iron which may have been 
 sufficient to form the pyrite in the actual ore, upon the supposi- 
 tion that the dissolved iron, like the silica, migrated toward the 
 fissures. 
 
 In general the heavy metals brought in by the solutions 
 remained within or very close to the multitude of small, irregular 
 fissures that afforded opportunity for the depositional process. 
 The sulphur acids, on the other hand, penetrated the wall-rock 
 for considerable distances. 
 
ASSOCIATION OF ALUNITE WITH GOLD 687 
 
 It is very probable that the ore-bearing solutions contained 
 carbon dioxide, although the ores and the rock adjacent to them 
 are free from carbonates. Field evidence in this district, as well 
 as in the Cceur d'Alene district^ in Idaho, indicates that this con- 
 stituent, when of volcanic origin, becomes active in the formation 
 of carbonates only at comparatively low temperature and that it 
 tends to penetrate to greater distances from the source of supply 
 than some of 'the other constituents. The third, or propylitic 
 type of alteration described on page 671, is thought to be largely 
 due to the extensive permeation of the rocks by solutions con- 
 taining carbon dioxide. It thus constitutes an outer aureole to the 
 more intense metasomatism associated directly with the ores. I 
 hope at a future time to study in detail the relation of the meta- 
 morphism particularly described in this paper to that of this outer 
 zone. 
 
 It may be recalled, in this connection, that Sainte-Claire Deville 
 and Leblanc,2 in their studies of Italian volcanic gases, found 
 that sulphurous and carbonic acids never occurred together, 
 although the association of sulphydric and carbonic acids was 
 frequent. They concluded that, as a rule, the emanation of 
 carbon dioxide marks a later and cooler stage of solfataric activ- 
 ity. The transition from one type to the other, however, can 
 hardly be abrupt. 
 
 Just what became of the lime and magnesia removed from the 
 altered dacite has not been determined. A part, at least, of the 
 calcium was probably taken into solution as hydrous sulphate 
 and deposited in fissures as gypsum, this mineral being known 
 to occur in the district in places where its formation can scarcely 
 have resulted from the action of surface waters. Still another 
 part and some of the magnesium may have been carried as car- 
 bonates into the rocks affected by metamorphism of the third or 
 propylitic type and there deposited metasomatically as calcite or 
 dolomite. It has not yet been ascertained, however, whether 
 
 * Ransome and Calkins. " The Geology and Ore Deposits of the Coeur 
 d'Alene District." Profess. Paper U. S. Geol. Survey. In press. 
 
 '"Memoire sur la composition chimique des gaz rejetes par les events 
 volcaniques de I'ltalie meridionale." Ann. de Chimie et de Phys., vol. 52, 
 1858, p. 45. 
 
688 FREDERICK LESLIE RANSOME 
 
 there has actually been any addition of calcium or magnesium to 
 these carbonatized rocks. 
 
 Two general hypotheses are entertainable with reference to 
 the source of these solutions. They either came from above, the 
 sulphurous or sulphuric acid having been derived from overlying 
 deposits of oxidizing sulphides, or they came from below. In 
 the former case the solutions were presumably cold ; in the latter 
 case they were in all probability hot. The difficulties in the way 
 of the descensional hypothesis, however, appear to be insur- 
 mountable. 
 
 All available geological evidence indicates that the ore deposits 
 of Goldfield were formed comparatively near the surface and that 
 consequently they do not represent the final stage of a long period 
 of erosion and progressive downward enrichment. The irregu- 
 larity and sporadic occurrence of the ore bodies would seem to 
 be alone sufficient to render any such long-continued enrichment 
 impossible. We are driven then to suppose that if descending 
 solutions effected the intense metasomatic action described, these 
 must have derived their chemical energy from the oxidation of 
 moderate quantities of pyrite comparatively near at hand. Under 
 such circumstances, the effect produced appears to be greatly 
 disproportioned to the available agency. Such a process might 
 account for the development of kaolin and perhaps some alunite 
 within the oxidized ore, but hardly for the extensive metasoma- 
 tism of the country rock which is not limited to the vicinity of 
 known ore bodies and continues far below all traces of oxidation 
 as measured by the presence of iron oxides. For it is to be borne 
 in mind that the alteration described in this paper is character- 
 istically associated with unoxidized ores and that gold, pyrite, 
 tetrahedrite, bismuthinite and the other minerals found in such 
 ores were deposited at the same time that the neighboring rock 
 was changed to quartz, alunite and kaolinite. If the alunitiza- 
 tion is the work of cold descending solutions then the rich ores 
 of Goldfield are entirely the product of the same agency. In 
 the light of our present knowledge of ore deposition, this seems 
 improbable. 
 
ASSOCIATION OF ALUNITE WITH GOLD 689 
 
 The intensity of the alteration and the character of the solu- 
 tions, as shown by the composition of the ores and by the meta- 
 somatism of the country rock, point to hot ascending waters as 
 the effective agent. Even the alunite, in spite of the occurrence 
 of the mineral in the oxidized ore of some of the Cripple Creek 
 mines and the view held by De Launay with regard to the Tolfa 
 occurrence, is suggestive of more intense chemical activity than 
 is usually manifested by cold oxidizing solutions. The Gold- 
 field region, moreover, is known to have been the scene of vol- 
 canic activity throughout most of Tertiary time. That the solu- 
 tions were essentially emanations from a solidifying body of 
 dacitic magma — the reservoir whence came the intrusive dacite 
 and the extrusive dacite vitrophyre of the district, is regarded as 
 a probable hypothesis. It is significant in this connection that a 
 chemical analysis of the glassy, effusive dacite vitrophyre shows 
 3.35 per cent, of water driven off below 110° C. and 4.06 per 
 cent, lost above that temperature, a total of 7.41 per cent. The 
 intrusive dacite on the other hand contains 2.95 per cent, of 
 water, 2 per cent, being driven off above 110° C. The dacitic 
 magma thus originally contained a large quantity of water and 
 that much of this water has been expelled during crystallization 
 is shown by the difference of over 4 per cent, between the water 
 of the two analyses. 
 
 The limits of a brief paper do not permit of a full discussion 
 of the question of the thickness of the overlying rocks at the 
 time the ores were deposited. General geological evidence which 
 will be set forth in a later publication indicates that the ores were 
 deposited at comparatively shallow depth. 
 
 ORIGIN OF KAOLINITE. 
 
 Some discussion has been waged in the literature of ore 
 deposits regarding the origin of kaolin and Mr. Lindgren^ has 
 recently intimated that, although he himself formerly referred 
 to it as characteristic of certain classes of veins, kaolinite should 
 
 '"The Relation of Ore-deposition to Physical Conditions." This journal, 
 vol. II., 1907, p. 120. 
 
690 FREDERICK LESLIE RANSOME 
 
 not be considered as a gangue mineral of any class of ore de- 
 posits except those formed under the influence of oxidation. In 
 a subsequent paragraph he somewhat modifies this statement by 
 the expression of belief that " kaolin is rarely formed by alkaline 
 hot water at any considerable depth below the surface." 
 
 Much kaolin originally described by various authors as formed 
 by the solutions which first brought up the ores associated with 
 it, has later been proved to have resulted from oxidizing proc- 
 esses, it being one of the most characteristic products of the 
 action of percolating acid solutions upon aluminous rocks. The 
 kaolin at Cripple Creek is a good example. Much so-called kaolin, 
 moreover, has been shown to be sericite, and some is perhaps 
 alunite. These facts, however, seem scarcely to warrant the total 
 elimination of kaolinite from the list of gangue minerals found in 
 unoxidized ore deposits. The mineral undoubtedly occurs as an 
 original constituent of some of the ores of the San Juan Moun- 
 tains, Colorado.* Mr. W. H. Weed^ has described it as a product 
 of the metasomatic action of the waters of Boulder Hot Springs, 
 Montana, upon granite, and according to Mr. S. F. Emmons' 
 the mineral is an abundant original constituent of the ore of the 
 Bassick mine. Kaolinization has also been noted by Bela von 
 Inkey* in the dacitic country rock of the Nagyag veins, the mate- 
 rial being afterwards investigated by F. Kollbeck,** whose analysis 
 indicates a mixture of kaolinite and sericite. 
 
 At Goldfield, the intimate association of the kaolinite with the 
 alunite, gold and sulphides shows that all were formed at the 
 same time and by one general process which was anterior to and 
 entirely independent of oxidation or weathering. These latter 
 
 ' Ransome, F. L. " Economic geology of the Silverton Quadrangle, Colo." 
 Bull. U. S. Geol. Survey No. 182, 1901, p. y^. 
 
 '"Mineral Vein Formation at Boulder Hot Springs, Montana." 21st Ann. 
 Report U. S. Geol. Survey, 1900, part 2, p. 253. 
 
 • " Geology of Silver Cliflf and the Rosita Hills, Colorado." By Whitman 
 Cross. Accompanied by a paper on the mines of Custer County, Colorado. 
 By S. F. Emmons, 17th Ann. Rept. U. S. Geol. Survey. 1896, part 2, p. 432. 
 
 * " Nagy4g und seine Erzlagerstatten." Budapest, 1885, p. 143. 
 
 • " Untersuchungen iiber die Zersetzung des Quarztrachyts neben Gold- 
 erzgangen von Nagyag." Oesterr. Zeits. f. Berg- und Huttenwesen, vol. z'^* 
 1888, pp. 25-27. 
 
ASSOCIATION OF ALUNITE WITH GOLD 691 
 
 processes have probably also resulted in the formation of some 
 kaolin, but with this secondary development the present paper is 
 not concerned beyond the pointing out of the distinction between 
 the two modes of genesis. 
 
 CONCLUSION. 
 
 The recognition of alunite as a characteristic constituent of the 
 Goldfield ores and the demonstration of its genetic relation to 
 them establishes a new type — that of alunitic and kaolinitic gold- 
 quartz veins, jn the classification of epigenetic deposits based 
 upon the kind of metasomatism effected in the wall-rock by the 
 ore-depositing solutions. While the Goldfield deposits are prob- 
 ably too irregular in form to come under the usual definition of 
 vein, yet in all that relates to genesis of the ores there is no essen- 
 tial difference between them and what is usually termed a meta- 
 somatic fissure-vein. 
 
 It is not believed that the Goldfield district is unique in the 
 possession of this type. Other examples are likely to be found 
 among the great number of ore deposits associated with Tertiary 
 volcanism, particularly when more investigators of mining dis- 
 tricts realize the importance of close studies of rock alteration, 
 such as have been so admirably carried out in this country by 
 Mr. Lindgren and in Europe by the late Professor Stelzner, and 
 when they accept neither kaolinite nor sericite on faith and ex- 
 ternal appearance. 
 
 In spite of the mass of evidence which indicates the deposition 
 of a large class of gold veins by highly siliceous alkaline solu- 
 tions, it is clear that some deposits, and those of remarkable 
 richness, may be formed by acid solutions. To explain this 
 essential difference in the vein solutions is one of the most ab- 
 sorbing problems connected with ore genesis. It is possible that 
 emanations from a crystallizing magma may be normally and 
 initially acid but become modified by passage upward through 
 the rocks. If there is any truth in this suggestion, then ores 
 such as those of Goldfield are deposited comparatively near to 
 the source of the solutions. Of course, the extent to which 
 
692 FREDERICK LESLIE RANSOME 
 
 initially acid emanations would be neutralized and modified in 
 their ascent through fissured rock would depend not only upon 
 distance but very largely upon the kind of rock traversed, the 
 quantity and character of admixed surface-derived waters, and 
 the pressure and temperature gradients. 
 
 It may also be pointed out that the constituents removed from 
 the Goldfield rocks are those which are deposited extensively in 
 the production of gold-quartz veins of the sericitic and calcitic 
 type. Consequently, conditions are easily conceivable under 
 which the acid solutions of Goldfield might have ascended through 
 a much thicker series of rocks and g^ven rise to sericitic and 
 calcitic veins. 
 
 Finally, the relation of the alunite and kaolinite at Goldfield 
 suggests the attractive possibility of the future discovery of 
 metasomatic veins of the pure alunitic type, without kaolinite. 
 
 While the available facts in the Goldfield district appear to 
 justify the conclusions drawn in this paper, the cautious reader 
 may well share with the writer a certain reserve in judgment, 
 in view of the moderate depth, about 300 feet, to which study of 
 these deposits has necessarily been limited. The possibility that 
 more than one process has contributed to the concentration of 
 the sulphide ores and to the alteration of the rocks is one that 
 cannot be entirely eliminated until the mines have been opened 
 to greater depths.