STATE OF CALIFORNIA DEPARTMENT OF NATURAL RESOURCES &Y p GEOLOGY OF THE SILVER LAKE TALC DEPOSITS SAN BERNARDINO COUNTY CALIFORNIA SPECIAL REPORT 38 DIVISION OF MINES FERRY BUILDING, SAN FRANCISCO A- / ; UtAHy I SPECIAL REPORTS ISSUED BY THE DIVISION OF MINES l-A. Sierra Blanca limestone in Santa Barbara County, Cali- fornia, by George W. Walker. 1950. 5 pp., 1 pi. Price 25*. 1-B. The Calera limestone, San Mateo and Santa Clara Counties, California, by George W. Walker. 1950. 8 pp., 1 pi., 6 figs. Price 25*. 2. Geology of part of the Delta-Mendota Canal near Tracy, Cali- fornia, by Parry Reiche. 1950. 12 pp., 5 figs. Price 25*. 3. Commercial "black granite" of San Diego County, California, by Richard A. Hoppin and L. A. Norman, Jr. 1950. 19 pp., 18 figs. Price 25 *. 4. Geology of the San Dieguito pyrophyllite area, San Diego County, California, by Richard H. Jahns and John F. Lance. 1950. 32 pp., 2 pis., 21 figs. Price 50*. 5. 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Price $1.25. 36. Geology of the Palen Mountains gypsum deposit, Riversi County, California, by Richard A. Hoppin. 1954. 25 pp., pi., 32 figs., frontis. Price 75*. 37. Rosamond uranium prospect, Kern County, California, George W. Walker. 1953. 8 pp., 5 figs. Price 25*. 38. Geology of the Silver Lake talc deposits, San Bernardin County, California, by Lauren A. Wright. 1954. 30 pp., pis., 18 figs. Price $1.00. STATE OF CALIFORNIA GOODWIN J. KNIGHT. Governor DEPARTMENT OF NATURAL RESOURCES DeWITT NELSON. Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS. Chief SAN FRANCISCO SPECIAL REPORT 38 JULY 1954 GEOLOGY OF THE SILVER LAKE TALC DEPOSITS SAN BERNARDINO COUNTY CALIFORNIA By LAUREN A. WRIGHT Price $1.00 GEOLOGY OF THE SILVER LAKE TALC DEPOSITS SAN BERNARDINO COUNTY, CALIFORNIA By Lauren A. Wright • OUTLINE OF REPORT Page Abstract 3 Introduction 3 General geology 6 Metasedimentary rocks 6 Basic to intermediate rocks 9 Granitic rocks 10 Talc bodies 13 White tremolite rock 15 Green tremolite rock 15 Tremolite-forsterite-serpentine rock 15 Talc schist - 1" Summary of mineral paragenesis 17 Summary of genesis of the talc deposits 18 Age of the rock units 19 Description of deposits and workings 19 Addenda Extension area 19 Addenda area 19 Gould area Number 2 area Number 3 area 20 20 21 Number 24 area 21 Number 4 area 22 Number 5 area 23 Appendix : Notes o;i the petrography and metamorphism of the principal rock units 23 Hornfels member 23 Quartz-biotite schist member 25 Quartz-muscovite schist member 20 Forsterite marble member 27 Quartzite member 27 Basic to intermediate intrusive rocks 27 Tonalite 28 28 28 30 Mierocline-quartz-mica gneiss Emplacement of the large granitic masses. Selected references Illustrations Page Plate 1. Geologic map of western part of Silver Lake talc area In pocket 2. Geologic map of Number 4 deposit, Silver Lake talc area In pocket 3. Geologic map of Silver Lake talc mine, Gould and Addenda workings In pocket Figure 1. Location map of Silver Lake talc area 4 2. General view of most westerly part of Silver Lake mine area 5 3. Generalized block diagram of western part of Silver Lake talc-bearing area — 6 4. View westward along talc-bearing zone in hornfels member at Number 2 workings 7 5. Photo of exposure of migmatitic phase of quartz- biotite schist member 8 6. Photo of exposure of forsterite marble member con- taining diopside- and serpentine-bearing veinlets (dark) 9 7. Microsketch of marble 8. Photo of contact between tonalite and mierocline- quartz-mica gneiss 11 9. Photo of mierocline-quartz-mica gneiss 11 10. Photo of mieroeline-quartz pegmatite dikelets cross- cutting diopside-feldspar-quartz-calcite hornfels 12 11. Photo of pegmatite dike cutting talc body above adit entrance at Number 4 quarry 13 12. Generalized cross-section showing rock units and internal structure of typical Silver Lake tale body— 14 13. Microsketches of tremolite-forsterite-serpentine rock showing progressive replacement of forsterite by tremolite 10 Senior Mining Geologist, California Division of Mines. Page 14. Microsketch of white tremolite rock with grains of talcose material (center) and carbonate 17 15. Photo of typical stope in massive tremolite rock of Gould deposits 17 16. View westward along surface exposures of Gould talc bodies 21 17. View eastward of Number 2\ area 22 18. View southward of Number 4 area 23 ABSTRACT The Silver Lake talc deposits, continuously worked for nearly 35 years, have yielded about 210,000 tons of commercial talc, principally talc schist and massive talc-tremolite rock. The deposits have been extensively worked at six localities. The output now is marketed mostly as a ceramic material. The main talc-bearing zone is discontinuously exposed within a 2-mile belt. Individual bodies are as much as 800 feet long and 40 feet wide. The deposits ordinarily consist of two parallel bodies, each about 10 to 15 feet in average width. These .are part of a pre-Cambrian metasedimentary sequence and apparently represent the alteration of certain silica-poor carbonate strata. The metasedi- ments have been extensively invaded by igneous bodies of basic to acidic composition, emplaced, in general, in the order of decreasing basicity. The metamorphic history seems divisible into the following five stages: (1) the production of mineral assemblages typical of the amphibolite facies : temperatures perhaps as high as 600° C, in a dry environment; (2) a lowering of temperature, and the introduc- tion of silica, magnesia and water to produce tremolitization in certain layers of forsterite marble; (3) a further lowering of tem- perature to below 500° C, and the development of serpentine at the expense of other magnesian materials; (4) a rise in temperature and the formation of a second generation of tremolite, concurrent with the emplacement of granitic dikes; and (5) shearing, princi- pally along the margins of the tremolite bodies, yielding zones of talc schist. The additive water, silica, and magnesia appears to have been introduced mostly during the emplacement of the largest granitic bodies. A magmatic source is therefore suggested, but much or all of the magnesia may have been transferred during granitization of magnesian metasediments. INTRODUCTION The Silver Lake talc deposits have supported one of the more continuous mining operations in the Mojave Desert region of southeastern California. Through a pe- riod of about 35 years of steady though modest develop- ment, they have yielded a total of about 210,000 tons of commercial talc. This material ranges in character from schist composed almost wholly of the mineral talc to mas- sive, tremolitic rock. It is processed in grinding mills, mainly in the Los Angeles area, and has been sold prin- cipally to the ceramic, paint, and rubber industries. In recent years, almost the entire output has been consumed as a raw material in wall tile manufacture. The deposits occur in a terrane of highly metamor- phosed pre-Cambrian (?) sedimentary rocks extensively invaded by bodies of basic to granitic composition. The deposits appear to have formed by the selective replace- ment of certain silica-poor carbonate strata. This geologic setting is in marked contrast with that of the talc-bear- ing district of the nearbj^ southern Death Valley and Kingston Range region where talc deposits exist as parts of silicated zones altered from Algonkian carbonate rocks and are associated with diabase sills. The Silver Lake talc deposits more nearly resemble those of the well known Gouverneur district of northern New York. (3) Special Report 38 Figure 1. Location map of Silver Lake talc area. The workings now known collectively as the Silver Lake mine comprise one of the three oldest of the more extensively developed talc operations in California. The other two, the Talc City mine 15 miles east of Keeler, Inyo County, and the Western mine 6 miles southeast of Tecopa and lying just within San Bernardino County, were opened in the same period as the Silver Lake mine and have comparable outputs. Other more recently de- veloped deposits, such as the Ibex and Superior in San Bernardino County and the Warm Springs in Inyo County, are now being worked on about the same scale as the three noted above. (See Wright, 1950; 1951; 1950a; et al., 1953; Norman, et al., 1951; Page, 1951). Deposits similar to those at the Silver Lake mine exist at several localities in a belt extending for about 12 miles eastward to the vicinity of Yucca Grove, a settlement on the Baker to Las Vegas highway. Near Yucca Grove the Calmasil and Pomona mines (Wright, et al., 1953) have been active since the early 1940 's. Acknowledgments. This study is part of a state-wide talc investigation directed by Olaf P. Jenkins, Chief of the California Division of Mines. It was helpfully super- vised by Ian Campbell of the California Institute of Technology also. The full cooperation of the personnel of the Sierra Talc and Clay Company, the present operator of the Sil- ver Lake mine is gratefully acknowledged. Of especial assistance were Don B. Kempher, superintendent of min- ing operations, who discussed many aspects of the opera- tion and furnished the writer with maps of the under- ground workings, and Richard S. Lamar, technical director, and Alberta MacArthur, research chemist who kindly provided the chemical analyses quoted herein. Robert S. Orr voluntarily assisted in the establishment of plane table control. Helpful suggestions in the field were contributed by Drs. Jenkins and Campbell. The manuscript was read critically by these two and by Rich- ard II. Jahns and A. E. J. Engel of the California In- stitute of Technology. The writer is also indebted to Francis J. Turner, University of California, for helpful discussions and for guidance in several mineral deter- minations. Physical Features. The talc deposits of the Silver Lake area are in a group of hills about 10 miles north- east of the Silver Lake playa. The deposits are 14 miles north-northeast of Baker, a settlement on the highway of U. S. Routes 91 and 466 which connects Barstow and Las Vegas. The Silver Lake mine is 16 miles by road from Baker. It is reached by traveling 8 miles on State Highway 127 north-northwest from Baker, and thence 8 miles north- east by a graded dirt road. The two roads join at the site of the old Silver Lake siding on the Tonopah and Tidewater Railroad the trackage of which was removed in 1941. This siding which is on the north edge of the playa was formerly the shipping point for the Silver Lake mine. The talc is now trucked to Dunn siding, a point on the Union Pacific Railroad about 23 miles west- southwest of Baker. The altitude of the Silver Lake playa is slightly less than 1000 feet. The talc-bearing zone lies at an average altitude of 2500 feet, and the nearby hills are as much as 300 feet higher. Most of the talc deposits are exposed low on the southern slope of a west-trending ridge, but the two westernmost deposits underlie low rises on rela- tively level ground. West of these rises, bedrock is hid- den beneath alluvium. Talc deposits have been worked at six localities along the zone's 2-mile lenath. The five main workings shown on plate 1 are spaced at intervals of approximately 1200 feet. From west to east, these are the 1) Addenda Exten- sion, 2) Addenda, 3) Gould, 4) Number 2£, and 5) Num- ber 2 workings. At each of these, one or two shafts have ' been sunk on talc bodies. In common practice, the talc has been removed by drifting and overhand stoping. The sixth locality is at the eastern end of the zone. The development here, known as the Number 4 work- ings, is mainly a group of irregular drifts, gently in- clined stopes, and rooms connected to the surface by a vertical shaft, Still other deposits, the Number 3, about- 300 feet south of the Number 2\ deposit, and the Num-| ber 5, about one mile north of the main talc-bearing belt, have been only prospected or worked on a very small scale. Because precipitation in the Silver Lake area averages less than 3 inches annually, the stream channels drain- ing into the playa are dry except during heavy rains. Shallow water stands in the playa itself for not morel than a few days each year. The mine workings are alsoi dry even to their lowest levels. The water consumed at the mine is trucked from a well about 5 miles to the south. The region contains a very sparse vegetation ; most of I the hillslopes are virtually barren bedrock exposures.) Some of the slopes in the mine area, however, are over- lain by relatively thick talus and remnants of an earlier alluvial cover. Historical Sketch* The original claims on the Silver Lake talc-bearing belt were located in 1911 and 1912, some by G. E. Gould, others by M. E. Stearns and asso- ciates. The latter organized under the name " Western i White Talc Company." In 1918 the Pacific Coast Talc * Most of the data in this section were kindly supplied by Walter I\. Skeoch, now president of Southern California Minerals Company, formerly vice-president of Pacific Coast Talc Company which operated the Silver Lake deposits from 1018 to 1941. Silver Lake Talc Deposits - iniifM'ii MP -. Figure 2. General view, looking northwestward, of most westerly part of Silver Lake mine area. Addenda deposit and shaft in right foreground. Addenda Extension deposit and shaft in left mid-distanee. Higher areas around shafts underlain mainly by metasedimentary rocks; lower areas mainly by granitic rocks. Beyond lies Silurian Dry Lake occupying southeastern extension of Death Valley trough, bordered by Avawatz Mountains to left and Silurian Hills to right. Company was formed by Robert W. Glendenning with Glendenning remained as resident manager for the capital contributed by a group in Washington, Pennsyl- Pacific Coast Talc Company until early in 1920 when vania. The company soon acquired two of the claims George Ames succeeded him. Ames retained the position from Gould, and opened a mill and office at 2149 Bay until 1932 ' and was in turn succeeded by P. E. Thomas. Street, Los Angeles, and operated the property until Smce 1923 T W / K " %^J*d been f 1 ™ ma " a f £ f the -.r..-, , „ f ,,. » f, -r, .« ~ . m , ^ company. Late in 1935 Thomas and Skeoch left to or- 1941, when all holdings of the Pacific Coast Talc Com- • :, r, ,, « ,.. . M . , n tj , ° , _,. _ . _ . gamze the Southern California Minerals Company. By pany were purchased by the Sierra Talc Company, the that time the mine had yielded 85;000 tons of talc w s present owner, now named the Sierra Talc and Clay Lockhart, who for several years had served as president, Company. The estate of M. E. Stearns still owns 16 then came west from Washington, Pennsylvania, to per- claims. These are leased by the Sierra Talc and Clay sonally manage the operation. In 1936 a 21-hole diamond Company which, in addition, now owns four claims and drilling program, under the direction of F. T. Roberts, a mill site. was undertaken and completed. The holes, totaling about Table 1. Output of Silver Lake mine 1915-52 indu- 2 > 700 fe ^ in length, were estimated by F. T. Roberts * sive. Data from records of California Division of to have developed about 25,000 tons of talc, mainly in Mines and Sierra Talc and Clay Company. the Number 4 area. Period Short tons The operation has been under the supervision of Don 1915-19 900 B. Kempher since 1942. About 90,000 tons of the talc iwr i>q ^^rfn have been mined by the present owners, mainly from 1930-34 20407 *^ e Number 4, Gould, and Addenda Extension bodies. 1935-39 ~_I 32^516 The original or "Gould" shaft, sunk at a point high 1940-44 38,919 on the most extensive talc exposure, was begun in 1919. 1945-47 (three years) 32,098 I n 1925 the shaft was intersected by the Gould tunnel -o 34 ' 0;)8 driven eastward along the talc-bearing zone. In 1934 and Total 210,618 • Unpublished report, 1936. Special Report 38 Ejce>z~a 7XA TION Granitic rocks Hi!il/i. Limestone Qurartzite and qirartz-muscozsite scnist Qtxartz-biotite leftist Diqpside- feldspar rocfr ivitH talc-tremolite hodzj Figure 3. Generalized block diagram of western part of Silver Lake talc-bearing area. 1935 other deposits were opened by the Addenda, Ad- denda Extension, and Number 4 workings. In the period 1941-48, the Number 2\ workings were active. The early operators sought only the relatively pure talc schist for use in the manufacture of cosmetics. Later, tremolite rock also was mined. In 1925, talc from the Silver Lake mine was first sold to manufacturers of rubber goods ; in 1927 it was first marketed as a ceramic material. Previous Investigations. Past references to the Silver Lake talc deposits have dealt largely with the mining operations and include only brief discussions of the geo- logical features. A short description of talc deposits "near Riggs" and mined by the Silver Lake Talc Com- pany was provided by Diller in 1914 and cited by Cloud- man, et al. in 1919. Diller, however, may have referred to deposits in the Silurian Hills about 6 miles north- west of the present Silver Lake mine. Brief descriptions have been provided periodically by members of the Cali- fornia Division of Mines; by Tucker in 1921; Tucker and Sampson in 1930, 1931, and 1943 ; Wright in 1950, and Wright, Stewart, Gay and Hazenbush in 1953. Others who have commented on the deposits and opera- tions are Ladoo (1923), Wicks (1931) and Engel (1949). Previous studies of the geology of the Silver Lake re- gion have been of a reconnaissance nature. The deposits lie about 2 miles west of the western margin of the Ivanpah quadrangle the geology of which has been mapped by Hewett (1954). In this quadrangle the area lying closest to the Silver Lake mine is shown by Hewett to be underlain by an extensive body of Mesozoic quartz monzonite. Miller (1946) has described metasedimentary and granitic rocks exposed in the hills between the mine and the Silver Lake siding. These and similar rocks ex- posed near Halloran Spring (about 5 miles west-south- west of Yucca Grove) were believed by Miller to be earlier pre-Cambrian in age and to belong to the Halloran Complex as defined by him. Although the rocks at the Silver Lake mine were but briefly mentioned by Miller (1946), he noted both granitic and metasedimentary rocks, and tentatively correlated them with the Halloran Complex. GENERAL GEOLOGY Metasedimentary Rocks General Features. The metasedimentary units shown on the accompanying maps are part of a considerably thicker section exposed for hundreds of feet both north and south of the talc-bearing zone. The continuity of the section has been broken by the emplacement of granitic rocks which predominate in the western part of the mine area where they enclose numerous metasedimen- tary islands (pi. 1). In mapping, the writer recognized five metasedimen- tary rock members as persisting through most of the western 5,000 feet of the talc-bearing zone. In upward succession these are here named the hornfels, quartz- Silver Lake Talc Deposits Quartzite member 25 Marble member Quartz-muscovite schist member Quartz-biotite schist member Hornfels member 155 biotite schist, quartz-muscovite schist, forsterite marble, iand quartzite members, and are shown with distinguish- ing symbols on the map. Other occurrences of metasedi- imentary rocks are indicated only by lithologic symbols I with no stratigraphic correlation implied. The petrologic | features of the five members are outlined in table 2. Table 2. General features of metasedimentary rock sequence in western part of Silver Lake mine area. Maximum thickness Name in feet Description Upper units 400+ Complex of metasedimentary bodies, principally quartz-biotite schist ; di- opside-feldspar hornfels and quartz- ite less abundant ; interlayered with tonalite and granitic gneiss. Quartzite, light gray, medium-grained, massive, compact, vitreous ; con- tains thin layers of amphibolite. 60 Marble, dolomitic, olive gray, medium- grained. Generally shows crude planar structure. Contains abund- ant disseminated silicate grains (forsterite, clinohumite, chrysotile, antigorite, talc) and small silicate veinlets (diopside, chrysotile, anti- gorite). 125 Schist and quartzite composed mostly of quartz and muscovite ; orange, fine- to medium-grained. Contains layers of marble and amphibolite. 185 Schist and quartzite composed mostly of quartz and biotite ; gray, medium- grained ; commonly contains migma- tite. Hornfels composed mostly of feldspar, diopside, quartz and calcite ; green, fine- to coarse-grained ; thin lamina- tions characteristic ; also contains layers of mica schist, tremolite schist, massive tremolite rock, talc schist, quartzite, and marble. A common host for pegmatite and lamprophre. Lower units 400+ Complex of various metasedimentary rocks (schists, hornfels, quartzite) interlayered with tonalite, granite gneiss, and silexite. The most characteristic structural features shown by the outcrop pattern of the metasedimentary units are (1) a general lack of deformation apart from the uni- form tilting, broad folding, and cross-faulting, and (2) a parallelism of the metasedimentary planar tex- tures with contacts of sedimentary origin. Many bodies that clearly were once sedimentary strata, such as marble layers in quartzite, quartzite layers in schist and hornfels, and amphibolite layers in schist and quartzite, are commonly only a few feet thick and several hundred feet long. Such layers have nearly straight or broadly curved traces. They are ordinarily parallel with each other, with comparably undeformed contacts that separate the five principal metasedimentary units, and with the schistosity and foliation of the units. The following are general descriptions of the physical ] and distributional features of the five units. Descriptions I of their petrographic features and notes on meta- morphism are contained in the appendix at the end of this report. Hornfels Member. The lowermost and most northerly of the five metasedimentary members is also the most Figure 4. View westward along talc-bearing zone in hornfels member at Number 2 workings. Stoped material is composed mostly of tremolite and talc. Wall rock is largely a diopside-feldspar- quartz-calcite hornfels. heterogeneous. Because a green diopsidic rock with a hornfelsic texture predominates, the name "hornfels member" is applied. This member, however, contains all of the large concentrations of talc and tremolite, as well as subordinate proportions of other non-hornfelsic rock types, principally mica schist, and quartzite. The principal rock types of the hornfels member exist mainly as distinct units in sharp contact with one another, but the textural and mineralogic variants of the diopsidic hornfels commonly have gradational relationships. The northern boundary of the member is mostly in contact with granitic rocks; but large elongate masses of biotite and muscovite schists are included in the granitic rocks and appear to be remnants of immedi- ately underlying metasedimentary units. The hornfels member ranges from 15 to about 150 feet in exposed thickness. The thinning is caused principally by the encroachment of granitic rocks and is not, in general, a stratigraphic or deformational feature. The hornfels member appears to have resisted such encroach- ment more effectively than the bordering mica schists. Consequently, in the western part of the mine area, where the granitic rocks are the most extensive, masses of the hornfels member exist as elongate islands. The 8 Special Report 38 Addenda and Addenda Extension talc deposits are in two such islands. The hornfels itself is essentially a diopside-feldspar- quartz-calcite rock; but garnet phlogopite, serpentine and talc are locally prominent. Most of the talc, serpentine, and phlogopite in the diopsidic rock ap- parently occurs within a few feet of the borders of the talc-tremolite bodies. The diopsidic rock ranges in color from pale green to grayish olive green, is generally fine-grained, although some is medium- to coarse-grained. In most exposures, it is dense and tough, but in some it is relatively friable. From place to place within the member, the rock ranges from thinly and evenly layered to massive. Schist, composed of various proportions of tremolite, actinolite, phlogopite and alkali feldspar, and locally containing biotite, forms conspicuous layers. It ranges in color from dark through light green to yellowish gray. In sunlight, the phlogopite-rich rock has a lustrous, golden sheen. Layers of tremolite-phlogopite-albite schist, from a fraction of an inch to several feet thick, ordinar- ily separate the commercial talc bodies from diopsidic wall rock and also occur locally within the wall rock. • Other layers within the diopsidic rock contain only tre- molite or actinolite. Thin beds of quartzite commonly can be traced for several hundred feet. Lenses of marble occur locally. In a very thin part of the member, east of the Gould work- ings, marble is the principal rock type, but has been extensively invaded by pegmatite. Pegmatite and lampro- phyre, in dikes or irregular bodies, are particularly abundant in the hornfels member. The talc-tremolite bodies, discussed in a succeeding section, consistently oc- cur near the center of the member. Quartz-Biotite Schist Member. A quartzose, metasedi- mentary unit, distinguished by the presence of biotite in various proportions, and commonly showing a schistose texture, persists for the full length of the talc-bearing zone. This, the quartz-biotite schist member, everywhere overlies the hornfels member ; but the two are ordinarily separated by bodies of granitic rock or lamprophyre. The full thickness of the quartz-biotite schist member is about 150 feet, but invasion by granitic material has caused a marked thinning for much of its exposed length. Other biotite-rich metasedimentary units exist below and above the five members considered here ; but, of the five, only the quartz-biotite schist contains biotite as a characteristic mineral. Its texture ranges from massive granular to schistose, and from fine- to medium-grained. The more massive varieties, represented by exposures south of the Gould workings are largely quartzites with subordinate proportions of biotite, muscovite, and feld- spar. These quartzites show a crude planar structure caused principally by layers of contrasting mica content. The quartzitie varieties, which are typically light gray, grade laterally and westward into dark gray, distinctly schistose rocks containing as much as 30 percent biotite and appreciable proportions of muscovite and feldspar. Biotitic layers characteristically alternate with felsic lenticles or bands one-eighth inch or less thick. The rocks of this phase commonly grade into migmatite (fig. 5) which, in turn pass gradationally into poorly foliated granitic rocks. Both the migmatite and granitic rocks contain pegmatite and aplite dikelets that cut across the planar structures. In the area south of the two Addenda workings the biotite schist grades into the granitic rock through a belt of migmatite from 50 to 100 feet wide. Virtually all of the truly migmatitic rock in the mine area is, in this manner, associated with the more mica- ceous phases of quartz-biotite schist. The quartzitie phases of this member, as well as the various non-schistose rocks of the other members, are, in general, free of migmatite, but contain granitic material in relatively large sill-like masses. Quartz-Muscovite Schist Member. The quartz-musco- vite schist member, of a pale orange to dark yellowish orange color, is in sharp contrast with the gray under- lying quartz-biotite schist. Through most of the area mapped, the quartz-muscovite schist member ranges from 35 to 125 feet in thickness. It pinches out eastward at a point south of the Number 2\ workings, but farther east, in the vicinity of the Number 4 workings, this unit or one very similar is present and is much thicker than in its western occurrences. The member is composed mostly of fine-grained, very even layers that are alternately schistose and quartzitie, and that range in thickness from a fraction of an inch to several feet. The schistose rock commonly contains scattered iron oxide grains giving a pepper-sprinkled appearance. Locally interbedded with the schist and quartzite, are thin layers of quartzose amphibolite and elongate lenses of marble similar to the forsterite marble of the overlying member. Forsterite Marble Member. The forsterite marble member, a crystalline carbonate unit containing abund- ant magnesian silicate grains, persists from the area south of the Gould talc bodies westward for about 4,000 feet to the alluvial overlap. In this area, the member ranges from 45 to 65 feet thick. South of the eastern part Figure 5. Exposure of migmatitic phase of quartz-biotite schist member. Silver Lake Talc Deposits flGURE 6. Exposure of forsterite marble member containing diopside- and serpentine-bearing veinlets (dark). >f the Gould workings it terminates against a granitic nass. On fresh surfaces the marble is generally light-olive ,'ray, but a dark gray or dark, greenish gray color is not incommon. It is a medium-grained, dense rock, charac- teristically massive, but showing a crude planar struc- ure. It weathers to a hackly surface which is mostly pale rellowish brown, but upon which dark brown-weathering reinlets stand in prominent relief (fig. 6). The carbonate Taction is principally calcite. The veinlets, composed >f diopside, serpentine, antigorite, calcite, and opaque grains, are mostly one-eighth to one-half inch thick. They ire much more abundant in the member itself than in he marble of the underlying lenses. Disseminated grains consisting predominantly of ser- jentine, forsterite, clinohumite, and antigorite, also pro- rude from the weathered surface. Many of the veinlets parallel the general attitude of the member, but others •risscross it at seemingly random positions (fig.- 7). The lisseminated grains are commonly clustered in planes ;hat also parallel the general attitude of the member. Quartzite Member. The uppermost of the five per- sistent metasedimentary units is a vitreous quartzite that n all of its exposures overlies the forsterite marble raem- aer and separates it from granitic rock. It is a medium light gray, medium- to coarse-grained, dense, vitreous rock. The member also contains a small proportion of thinly layered quartzose amphibolite similar to the am- phibolite occurrences in the quartz-muscovite schist member. The quartzite was not traced east of the area south of the eastern part of the Gould workings where it abuts against a granitic mass. The exposed thickness of the member ranges from 2 to 25 feet. Because granitic rocks border it on the south, its original thickness may not be indicated in these exposures. Basic to Intermediate Intrusive Rocks Basic to intermediate intrusive rocks are widespread in the Silver Lake mine area, but are much less exten- sively exposed than the granitic rocks. Most of these bodies are dikes or irregular pods less than 50 feet in maximum dimension ; but a few are relatively thick, tabular bodies several hundred feet long. Most are composed of about two-thirds andesine and one-third mafic minerals. The mafic fraction of the smaller bodies ordinarily consists of biotite and horn- blende in nearly equal proportions, to form a hornblende kersantite lamprophyre. In the larger bodies there are all gradations from this rock through biotite-poor diorite to tonalite. The smaller bodies are persistently fine-grained. The larger bodies are generally fine- to medium-grained, but some of the diorite has hornblende blades as much as an inch long. Both the lamprophyre and the more felsic phases are characteristically gray in color. The dioritic phase or- dinarily has a diabasic texture, but the biotitic phases commonly show parallelism of mineral grains, and much of the rock is markedly schistose. In general, both the abundance and size of the basic bodies increase from west to east within the area of plate 1. The bodies are par- ticularly numerous within or adjacent to the hornfels member; but, unlike the granitic bodies in the hornfels O 5 mm. . A Figure 7. Mierosketch of marble containing forsterite partly to wholly replaced by chrysotile, clinohumite (unaltered), and antigo- rite (shred-like). From forsterite marble member. 10 Special Report 38 member, they have not visibly altered the bordering meta- sediments. The largest of these more basic bodies in the mine area separates the hornfels and quartz-biotite schist members south of the Number 2 and Number 2£ workings. It is approximately 1000 feet in length and 300 feet in maxi- mum width. The lamprophyre of this mass locally grades into diorite and is also cut by dikelike bodies of tonalite, aplite, and pegmatite, each of which has sharp contacts. A smaller body, formed along the hornfels and quartz- biotite schist contact between the Gould and Number 24 workings, shows a transition from lamprophyre to diorite to tonalite. This body is approximately 300 feet long and 50 feet wide. It is mafic near the hornfels contact, but becomes successively more felsic from north to south toward the quartz-biotite schist contact. The transition is mostly gradational, but it is also marked by a north to south mafic to felsic change in the composition of dike- lets that intimately penetrate the rock. The tonalite, aplite, and pegmatite dikes and dikelets that cross-cut these two larger, more basic bodies show that much of the more basic rock of the mine area is older than most or all of the granitic rocks. The local gradation of diorite into tonalite suggests that the time of formation of the tonalite followed closely. Some of the lamprophyre, however, appears to have intruded tonalite and silexite and suggests that some of the basic material is late. A dacite porphyry also post-dates even the granitic sequence described below and appears to be unrelated to it. The dacite porphyry is a fine-grained, medium gray rock composed of about one-half plagioclase, one-fourth biotite, and the remainder, quartz, hornblende, and opaque grains. It occurs in elongate bodies, from a few inches to about 10 feet in width, that transect both the metasedimentary and granitic masses. At one place, south of the Gould workings, discontinuous bodies of dacite porphyry have been emplaced along a northwest- trending fault. The fault has displaced both metasedi- mentary and granitic rocks an apparent horizontal dis- tance of about 15 feet. Other dacite porphyry bodies to the east have a similar trend. Granitic Rocks The granitic rocks of the Silver Lake mine area form a group that ranges in composition from tonalitic to silexitic, and in texture from granular to schistose and gneissic. Three rock types comprise most of the granitic material: (1) a granular to poorly-foliated rock, desig- nated as tonalite, but locally grading into true granite ; (2) a microcline-quartz-mica gneiss; and (3) a micro- cline silexite. These three persist as relatively well-de- fined units throughout the area, but as the tonalite and microcline-quartz-mica gneiss are commonly too intri- cately associated to be shown on the accompanying map (pi. 1), the two are shown with a single pattern. Dikes and dikelets of pegmatite, aplite, and granite, though subordinate in volume, are widespread. Each is com- posed principally of microcline and quartz. Age relationships between the three principal units are not well shown. As indicated below, the three may well be essentially contemporaneous, although some of the tonalite is probably earliest. The following descrip- tions are largely of the distributional and compositional features of these rocks. Petrographic data and a brief discussion of mode of emplacement are included in the appendix at the end of this report. Tonalite. The tonalite unit comprises more than one- half of the granitic material in the mine area. Tonalite, together with a subordinate proportion of microcline- quartz gneiss, comprises most of the granitic mass that surrounds several large metasedimentary "islands" in the vicinity of the two Addenda workings. Nearly all of the granitic rock south of the Gould, Number 2^, and Number 2 workings is likewise tonalite. In this area the rock commonly occurs in sill-like bodies, as much as 50 feet thick and 1,100 feet long. The bodies are particularly large and numerous in the quartz-biotite schist member ; but several also exist in the quartz-muscovite schist member. In the area south and east of the eastern part of the Gould workings, tonalite occurs in large, irregular masses that contain very elongate metasedimentary in- clusions, of which most are quartz-biotite schist. Some of the inclusions are several hundred feet in length. The inclusions are ordinarily arranged in parallel patterns and have attitudes similar to those of larger nearby metasedimentary masses. In general the tonalite unit is a light to medium gray, medium-grained rock that contains about one-half pla- gioclase feldspar (oligoclase to andesine) and one-fourth potash feldspar^ muscovite and biotite. In some places it also contains scattered euhedral microcline pheno- crysts as much as 2 inches in length. In many of its oc- currences, it appears to be structurally homogeneous ; in others, it has a crude planar structure that approxi- mately parallels the attitude of the nearest metasedimen- tary masses. A pronounced schistosity is shown in two elongate masses of a granite phase of the tonalite unit. Both are more than 300 feet long and are indicated separately on the accompanying map. One is north of the Eastern Ad- denda workings; the other is north of the Gould work- ings. As noted above the tonalite unit postdates much of the basic rock. Its relation to the microcline-quartz-mica gneiss and to the microcline silexite are less clear. In many places a narrow zone of tonalite lies between the gneiss or the silexite and metasediments. In the western part of the area, such a zone separates metasediments of the quartzite, forsterite marble, and quartz-muscovite schist members, from microcline-quartz-mica gneiss to the south. This zone is 8 to 50 feet thick and is continu- ous for more than 1,500 feet. It is in sharp contact both with gneiss (fig. 8) and metasediments; within a few feet of the gneiss it contains numerous large microcline crystals. A zone of the tonalite unit discontinuously sep- arates microcline silexite from the lower border of the hornfels member throughout the eastern part of the area, Here, too, contacts are sharp, but the large microcline crystals are missing. The pattern of these zones at first suggests an intru- sive origin ; but the distribution of the microcline crys- tals near the contact with gneiss, and the fact that tona- lite bodies were not observed to cut the gneiss or the silexite do not support the intrusive concept. Indeed, the zones appear to be a less siliceous and less potassic con- Silver Lake Talc Deposits 11 :gure 8. Contact between tonalite (below) and microeline- quartz-mica gneiss (above). ct phase of the mierocline-rich units. At a locality just ,st of the Gould workings, a dikelike septum of silexite :tends into tonalite and appears to postdate it. Thus, e field evidence indicates that the tonalite is in part ,rlier than, in part contemporaneous with, the more )tassic granitic rocks. Microclin.e - Quartz - Mica Gneiss. Microcline-quartz- ica gneiss is a distinctive and prominent rock type in ie western part of the area mapped. It commonly oc- irs in small masses that are intimately associated with nalite; but the gneiss composes nearly all of a large •anitic mass that is exposed south of the quartzite mem- ;r in the area of the two Addenda workings. It is sepa- ited from the metasedimentary rocks that border it on ie north by the thin zone of tonalite noted above. In the granitic rock exposed north of the quartz-bio- te schist member in the Addenda area, the microcline- lartz-mica gneiss is subordinate to tonalite with a mod- ■ately well-defined planar structure. In this area, rela- vely small bodies of the gneiss are interlayered with tnalite (fig. 8). The two units are generally separated 7 sharp contacts, but neither rock appears to penetrate ie other. In mapping east of the Addenda area, virtu- ly no microcline-quartz-mica gneiss was observed. The gneiss is a light gray, medium-grained rock com- 3sed of about one-half potash feldspar, one-third lartz, and subordinate plagioclase, biotite and musco- ite. The foliation is caused principally by layers of iigned mica flakes spaced as much as 4 cm but averag- ig 1 cm apart. The material between the biotite layers a feldspar-quartz mosaic that is characteristically ren-grained, but locally the gneiss contains microcline "ystals as much as 1| inches long. Many of the gneiss iposures show a pronounced lineation produced by nail crenulations in individual folia and by broad cren- lations in folia groups. Microcline Silexite. Microcline silexite is extensive and widespread in the area of plate 1 east of the Ad- denda workings. The rock is white to light gray, medium-grained, and composed of about seven-tenths quartz, one-fourth microcline, and small fractions of plagioclase feldspar, muscovite, and biotite. A local feldspathic phase consists of normal granite. The silex- ite also contains wispy schistose masses, but these are uncommon. In most of its occurrences, the silexite is re- markably uniform, both in texture and composition. The rock is vitreous, compact, and tough. In sunlight, weath- ered surfaces show quartz cleavage flashes that give the appearance of a rock composed largely of feldspar. The rock generally has a faint planar structure marked by an alignment of mica grains. The microcline silexite is the most resistant of the rock units in the mine area. The largest body supports the hills along whose lower southern slopes most of the talc deposits are exposed. This mass was mapped for a dis- tance of 3,000 feet and it may well continue much farther to the east. No silexite, however, was observed in the area of the Addenda workings. Many, much smaller masses of microcline silexite are exposed south of the hornfels member in the central part of the mapped area. Some irregular silexite bodies, less than 100 feet in longest dimension, are surrounded by tonalite. Other bodies are thinly elongate with lengths of as much as 450 feet, and separate tonalite from quartz-biotite schist. A few small, irregular masses of the rock occur within quartz-biotite schist. None appears to exist simply as a fracture filling as do so many of the granitic dikes described below. The relatively sharp contacts of the silexite and its occurrence in irregularly shaped bodies in various parts of the section, suggests an intrusive origin. North of the Number 2\ workings, however, the silexite grades later- ally and almost imperceptibly into the mica schist septa that appear to be part of the metasedimentary section. The schist shows no evidence of distortion in the grada- tional zone, and has attitudes parallel to those of meta- sediments nearby. Upon casual observation, the silexite has a quartzitic appearance. However, the irregular out- FiGi'RE 9. Detail of mierodine-quartz-mica gneiss. Note cross-cutting aplite dikelet at top. 12 Special Report 38 J?5v^^ ;^ ' '^W'^- ■VJQ .:>•% --rr %ts Figure 10. Microcline-quartz pegmatite dikelets cross-cutting diopside-feldspar-quartz-calcite hornfels. Note dark contact zone rid in ferruginous amphibole. Tonalite at upper right. lines of many of the bodies, contacts that transgress bedding features of metasedimentary units, the lack of bedding features in the silexite bodies, and the very uniform quartz-microcline ratio, all point against the quartzite origin. Microcline-Quartz Dike Rocks. Dike rocks, composed mostly of quartz and microcline, but with textures rang- ing from aplitic to pegmatitic, are widespread through- out the area mapped. From dike to dike within this group, there are apparently all gradations in grain size. One type or another was noted in crosscutting relation- ship with all of the previously described rock units ex- cept the microcline silexite. The most irregular in outline and generally the larg- est of the granitic dikes are the pegmatites (fig. 10). These have formed as elongate lenses or irregular pods in most of the rocks of the area. The pegmatites are simple in mineralogy and internal structure. Most are merely bodies of coarse-grained granitic rock in which the grains do not exceed 5 inches in diameter, and which contain no prominent minerals other than quartz and microcline. Such dikes range from a few inches in width and a foot or two in length to as much as 30 feet wide and 200 feet long. The pegmatite dikes within the talc bodies rarely exceed 3 feet in width. They commonly consist entirely of graphic granite. The granite and aplite dikes generally are narrowe and of more uniform thickness than the pegmatites These finer-grained bodies also are as long as 200 feel but most of them are much shorter. The microcline-quartz dikes within the hornfels mem ber are commonly bordered by concentrations phlogopite, chlorite, or ferruginous amphibole. Border of phlogopite or chlorite, which were noted only aloni dikes in talcose or tremolitic rocks, occur as well definet schistose layers that lie close against the dike wall (fig. 12). Ferruginous amphibole accompanies chlorite and tremolite accompanies phlogopite in many of thes layers. Such schistose borders are as much as an inc thick. The chloritic borders are black to dark green an present a striking color contrast to the granitic an tremolitic rocks that they separate. Chlorite-borderei graphic granite dikes locally contain elongate blade of chlorite, as much as 3 inches in length, that exten into the dikes from the dike walls. Some dikes i tremolitic rock do not have schistose borders, but in thes the bordering tremolite ordinarily is altered to dee green actinolite. Many of the dikes in tremolitic roc contain inclusions of tremolite or ferruginous amphiboh Microcline-quartz dikes in diopsidic hornfels common! have clusters of ferruginous amphibole (actinolite? blades along their margins (fig. 10). Most of these blade are much larger than the grain size of the hornfels, an> Silver Lake Talc Deposits 13 re as much as half an inch in length. The ferruginous mphibole has formed at the expense of diopside, but lso contains inclusions of diopside, feldspar and quartz lat apparently represent unreplaced grains of the ornfels. TALC BODIES General Features. The rocks mined as talc in the ilver Lake area are composed principally of magnesian ilicate minerals. Listed in order of decreasing abun- ance, these are tremolite, talc, chlorite ( 1 ) , serpentine, rid forsterite. Tremolite forms an estimated three- >urths of the volume of the bodies. In the remaining jurth, talc and probably chlorite (?) are markedly in scess of forsterite and serpentine. Calcite ordinarily is resent, but in amounts of 2 to 3 percent. Within the odies, however, these minerals are not evenly distrib- ted, but form several rock types with contrasting tex- lres and compositions. All of the material shipped from the area has been btained from bodies within the hornfels member. One datively small body, which is not being worked, occurs outside the member 300 feet south of the Number 1\ workings. This body, the Number 3 deposit, is enclosed in green diopsidic hornfels which occurs as a lens apart from the member. Although in many places granitic rocks or lampro- phyre have invaded the hornfels member and border or cut the talc bodies, where the member exists in its normal thickness of about 150 feet, the talc bodies ordi- narily lie in a zone very close to the center of the member. The mineable bodies range from a few tens of feet to about 800 feet long and from 5 to 15 feet wide (figs. 15 and 16). Two such bodies generally parallel each other and are separated by a layer of diopsidic rock 10 to 20 feet thick. As the operators refer to the higher of the two bodies as the "hanging-wall vein" and to the lower one as the "footwall vein," they will be so designated in this report. Individual bodies terminate in several ways. Com- monly they narrow gradually and lens into diopsidic rock. Others are brought against diopsidic rock by Figure 11. Pegmatite dike cutting talc body above adit entrance at Number 4 quarry. 14 Special Report 38 I & <0 § ^ & ^ is ^ & Si <0 \\ / > - I \ I § I W *s -k> ft 1 S> -SB I W Pi I 3, O XI a OB A" / ' \ •' - >\ '\\\ - ' - W\, ' s^ ^ 3 >0« O 3 a xs c c 3 cj o h M C o .a c o £ to T3 ■K 01 c o iH Ed 35 & o M fa Silver Lake Talc Deposits 15 cross-faults. Several of the bodies in the Addenda work- ings end abruptly against irregular masses of pegmatite. The principal cause of the discontinuity in the Silver Lake deposits, however, has been the emplacement of the larger masses of granitic rocks. One or more of the talc body terminations at each of the five workings is against rock of the tonalite unit. Microcline silexite transgresses the surface exposures of one body at the eastern end of the Gould workings. The deposits contain three principal varieties of rock in which tremolite predominates : ( 1 ) a snowy white rock with subordinate fractions of the mineral talc, (2) a pale bluish green, virtually monomineralic tremolite rock, and (3) a pale yellowish green to pale brownish gray rock that also contains appreciable proportions of disseminated forsterite and numerous serpentine veinlets. The first-mentioned variety is by far the most common, but the other two were noted in abundance at several places. Snowy white talc schist is a fourth abundant and widespread type. A pronounced and very even planar structure characterizes all of the 3ommercial talc bodies and each of the contained rock types. In general, it parallels the structural features of the associated metasediments and large granitic rock masses. The planar structure of most of the commercial talc rock, however, is caused mainly by textural and mineralogic variations, and not by dimensional align- ment of mineral grains. Alignment parallel to the over- all plannar structure is shown by most concentrations of the mineral talc and locally by tremolite blades. White Tremolite Rock The snowy white variety of tremolite rock is ordi- tiarily composed of more than three-fourths tremolite ; but all gradations exist between almost monomineralic rock and talc schist containing virtually no tremolite. Most of the tremolite needles are between 2 mm. and 2 cm in length (fig. 14). The more tremolitic phases of the rock generally have a decussate texture ; but locally they are schistose and in other places tremolite needles have grown normal to the rocks planar structure. In mining operations these high-tremolite rocks character- istically break from the face in tough, irregular blocks. The talc in the white tremolite rock occurs in two principal habits ; one is equant, the other platy. The observed equant grains, as much as 3 mm in diameter, are disseminated through the tremolite, and represent replacements of it. They commonly contain aligned tremolite residua, but pseudomorphs of talc after tremolite are rare. The platy grains of talc occur in schistose layers which lie parallel to the planar structure and over-all attitude of the deposit. Layers of this type range in thickness from a small fraction of an inch to 4 or 5 feet, to form mineable bodies of talc schist. In the thinnest layers, aligned talc shreds lie across the decussate tremolite needles in a markedly contrasting texture. Tremolite residua askew the schistosity are common in the schist. The calcite, which forms 2 to 3 percent of the snowy, white tremolite rock occurs in seams and in grains interstitial to the tremolite needles. It forms distinct, but cloudy crystals. No replacement textures involving this calcite were observed. Green Tremolite Rock The pale bluish-green tremolite rock, composed of needles mostly less than 1 mm long, is much finer grained than the snowy white variety. Thin sections of typical specimens contain only tremolite whose decus- sate arrangement causes this rock to be very compact and tough. The green tremolite rock ordinarily occurs in layers, less than 3 feet in width, which parallel the other layered features of the talc bodies. This rock was particularly well developed near the hanging wall of the Number 1\ deposit, now largely worked out. Where the white and green varieties are in contact, the white tremolite vein- lets commonly extend into the green rock ; in other places the two types grade into each other. Tremolite -Forsterite -Serpentine Rock A serpentine- and forsterite-bearing tremolite rock is abundant in the Addenda and Gould deposits and exists locally in the Number 4 deposit. In the Gould workings it occurs mostly as irregular masses, several feet in long dimension, enclosed by white tremolite rock; but locally Table 8. Analyses of representative samples of Silver Lake commercial talcs and wall rock, computed, on percentage by weight. Alberta J. McArthur, Sierra Talc and Clay Company, analyst. 1 2 3 4 5 6 7 8 3iO» 51.17 0.64 0.36 29.80 11.25 0.20 0.10 3.79 0.67 2.37 0.03 57.40 1.29 0.86 23.91 13.55 0.33 0.11 2.12 0.08 None Tr. 58.90 0.57 0.30 25.14 12.66 0.26 0.11 1.60 0.11 None Tr. 59.25 0.53 0.30 25.80 11.82 0.13 0.06 2.24 0.13 0.02 None 58.12 0.60 0.28 28.65 8.11 0.27 0.11 2.89 0.09 0.06 Tr. 59.05 0.78 0.42 28.67 5.81 0.41 \ 0.18 / 3.71 0.19 0.42 Tr. 56.29 1.07 0.43 28.31 9.26 1.06 4.02 56.70 M,0. 9.16 FetOi \ 2.04 FeO / MgO .... 8.55 CaO 14.49 Na 2 1.26 KjO - 5.50 ELO+ 0.90 HsO— 0.31 CO. 2.30 MnO 0.03 Total 100.38 99.65 99.65 100.28 99.18 99.64 104.44 101.74 1. Forsterite-tremolite-serpentine-calcite rock from Gould workings. 2. Green tremolitic rock from Number 2J workings. 3. White tremolitic rock from Gould workings. 4. White talc-tremolite rock from Gould workings. 5. Green talc-tremolite rock from Number 2J workings. 6. Talc-tremolite schist from Gould workings. 7. Commercial talc blend. 8. Dlopside-feldspar hornfels wall rock from Number 2J workings. 1G Special Report 38 O 5 mm. Figure 13. Micro-sketches of tremolite-forsterite-serpentine rock showing progressive replacement of for- sterite by tremolite. Tremolite partly replaced by talc (upper left) and by carbonate (lower right). Chrys- otile veinlets cut all other minerals. in these deposits and extensively in the more northerly Addenda body this rock occupies most or all of the entire width of an individual talc body. The tremolite-forsterite-serpentine rock is pale yel- lowish green to light brownish gray. It is composed prin- cipally of tremolite, but forsterite and serpentine, in various proportions, form as much as one-third of the rock. The rock also persistently contains 2 to 5 percent of carbonate material. Talc, though irregularly dis- tributed, commonly comprises several percent of the rock. As in the white tremolite rock, most of the tremolite needles are 2 mm to 2 cm long, and have a decussate tex- ture giving the rock an unusual toughness. In hand specimen, however, this tremolite is generally darker than the tremolite of the white rock. Forsterite or talcose ghosts of forsterite are dissemi- nated throughout the rock, but are most abundant along planes that parallel the other planar features of the deposits. Most of the forsterite is in relatively large, equant grains as much as 5 mm in diameter. Some of the grains, however, are elongate, and in cross-section have length to width ratios of as much as three to one. Microscopic studies show that individual needles and needle aggregates of tremolite project into forsterite grains and commonly extend completely through them (fig. 13). The former presence of large forsterite in- dividuals is shown by the alignment of residual grains that now exist in a tremolite mesh. In most thin sections, however, the outlines of the original forsterite grains can be determined with reasonable surety, and indicate that the mineral formed between one-eighth and one-fourth of the volume of the original rock. Chrysotile serpentine has formed as alteration rims about forsterite grains, as a pseudomorphic replacement of tremolite, and in a network of both megascopic and microscopic veinlets that extend across forsterite, tremo- lite, talc and carbonate grains and grain aggregates in- discriminately (fig. 13). These veinlets are colored green and are the most distinctive megascopic feature of the rock. The larger veinlets rarely exceed one-fourth inch in width. Most lie parallel to the other planar features of the deposits. The microscopic veinlets and an appreciable proportion of the larger veinlets, however, lie athwart the planar structure with seemingly random orientation. The chrysotile serpentine veinlets are abundant and widespread in the forsteritic tremolite rock, but both chrysotile and forsterite are lacking altogether in the other rock types of the talc bodies. Where the tremolite- forsterite-serpentine rock is in contact with the white tremolitic rock the chrysotile serpentine veinlets extend to the contact but not beyond. The mineral talc occurs in the serpentine- and for- sterite-bearing tremolite rock in several habits. The talcose material to which the forsterite has commonly altered is dark-brown, felty, and very fine-grained. All stages in this alteration are shown. The completely tal- cose ghosts of forsterite appear to be most abundant near contacts with the white tremolite rock. Not only do such ghosts preserve the outlines of the partly tremolitized forsterite grains but the intricate network of serpentine veinlets, characteristic of the forsterite, also commonly remains. These veinlets are much less altered than the forsterite but their disposition suggests that they formed when the forsterite still existed as discrete grains, and not after the alteration of forsterite to the feltv talcose Silver Lake Talc Deposits 17 material. Talc also occurs in colorless, fine-grained aggre- gates that have formed at the expense of tremolite, and less abundantly, as a replacement of the chrysotile ser- pentine. The carbonate material is ordinarily very fine-grained and occurs in veinlets and irregular aggregate masses. In both occurrences it is intimately associated with chryso- tile. The veinlets of both minerals occur with mutually crosscutting relationships that probably indicate virtual contemporaneity. The carbonate aggregates formed largely, if not wholly, at the expense of tremolite. That the occurrences of tremolite-forsterite-serpentine rock are older than the white tremolite rock with which they are associated is shown in several ways. In many places the tremolite-forsterite-serpentine rock is cut by bhlogopite-filled fractures along which white tremolite needles have grown normal to the fracture walls. Granitic ^likes and dikelets that cross this rock are also bordered by zones of white tremolite. Such zones range from a fraction of an inch to several feet, and are commensurate In size with the granitic bodies they border. An older age for the tremolite-forsterite-serpentine rock is suggested by forsterite ghosts surrounded by serpentine-free, white tremolite, and by the transgression of serpentine veinlets by white tremolite rock. The smaller bodies of tremolite- forsterite-serpentine rock, in displaying these features, exist as residual islands in a sea of white tremolite rock. The tremolite-forsterite-serpentine rock that occupies most of the thickness of one of the Addenda bodies is (bordered on both the hanging- and foot-walls by 1- to p-foot zones of white tremolite rock that appear to have formed along shear zones localized by the contacts of the leposit with hornfels. O 5 mm. flOUKE 14. Microsketch of white tremolite rock with grains of alcose material (center) and carbonate. Talcose material probably ghost of forsterite grain. Figure 15. Typical stope in massive tremolite rock of Gould deposits. Talc Schist As noted above, the talc-rich, schistose layers range in thickness from a small fraction of an inch to several feet. The larger layers commonly contain a very small pro- portion of tremolite and form a distinct rock unit. The thickest of these talc schist occurrences persistently lie along the foot walls of the deposits (fig. 12). Similar, though generally smaller, schistose concentrations of the mineral talc lie along the hanging walls and within the central parts of the commercial talc bodies. The talc in all of the schistose layers is snowy white, micaceous, and very friable. Individual flakes are com- monly a centimeter or more in diameter. In aggregate, the flakes produce a wavy schistosity, and have a lustrous sheen. Thin sections of material from the larger concen- trations show that talc, though the predominant mineral, is mixed with lamellae of colorless chlorite ( ?) (not talc grains viewed along optic axis), and contains tremolite in moderate to very minor proportions. Summary of Mineral Paragenesis The tremolite-forsterite-serpentine rock contains the earliest of the observed mineral assemblages in the de- posits. Of the minerals now composing this rock, the forsterite was the first to form and was subsequently partly to wholly replaced by tremolite of an early gen- eration. Chrysotile serpentine and carbonate material, 18 Special Report 38 both in veinlets and irregular masses, formed next ; chryso- tile mainly at the expense of forsterite and tremolite. A very fine-grained felty, dark-brown, taleose material has partly to wholly replaced the forsterite. Such altera- tion is particularly pronounced near contacts with white tremolite rock, and is indicated in taleose ghosts of pre- existing forsterite grains now enclosed in the white tremolite. The- talc of this origin is also veined by chrys- otile serpentine, a minor part of which also has been altered to talc. Because this serpentine characteristically occurs in intricate vein networks that elsewhere are du- plicated only in unaltered forsterite grains, the networks are believed to be relic and older than the brown taleose material. The white tremolite against which the serpentine vein- lets terminate, and which has grown into the tremolite- forsterite-serpentine rock from fractures, and from the borders of granitic dikes and dikelets, is tremolite of a second generation. The two generations are clearly dis- tinguished by the serpentine veinlets that cut the early tremolite, but not the later tremolite. It cannot be assumed necessarily that the white tremo- lite rock has everywhere replaced pre-existing forsteritic rock, but their spatial relations and distinctive composi- tions do indicate that the rocks represent different pe- riods in the metamorphic history of the talc bodies. The time of formation of the green tremolite rock is not clear ; but the veins of white tremolite that it commonly contains indicate that at least part of the green rock is earlier than part of the white. The talc in the equant grains and schistose layers in the white tremolite rock is later than the tremolite. Though replacement textures are commonly missing in the large bodies of talc schist, these clearly are large scale developments of the smaller schistose layers, and also postdate the tremolite. SUMMARY OF GENESIS OF THE TALC DEPOSITS The metamorphic history recorded in the rocks of the Silver Lake mine area appears divisible into five dis- tinct stages. These stages are summarized below and are considered in more detail in the appendix of this report. First, a rise in temperature, to a maximum perhaps in the range of 500° C. to 700° C. produced mineral assemblages typical of the amphibolite facies as defined by Eskola (Barth, 1949, p. 344). Forsterite-bearing marble developed from silica-poor dolomite ; diopside- plagioclase-microcline-calcite-quartz hornfels from im- pure dolomite containing abundant alumina, silica and alkalies. Relatively pure sandstone was changed to quartzite ; impure sandstone to quartz-biotite schist and quartz-muscovite schist. That rather high pressures and a stressed environment pertained is indicated by the pronounced foliation in each of these units, and by the dimensional alignment of most mineral grains. The widespread and abundant oc- currence of diopside, which Bowen (1940, p. 245) has shown develops in the fourth of thirteen steps during the progressive metamorphism of siliceous dolomite, suggests a moderately high temperature. That the tem- perature probably did not exceed 800° C. is indicated by the persistent quartz-calcite association and the ab- sence of wollastonite, which at this temperature ordina- rily would have formed by the reaction of quartz and calcite. The emplacement of the larger basic bodies, pre- dating the more acidic bodies, may well have been contemporaneous with this early stage of metamorphism. The second and third stages were marked by wide- spread hydration and magnesia and silica metasomatism and probably by a lowering of temperature. These are evidenced principally by the abundant formation of tremolite in certain marble layers that were largely or wholly forsterite-bearing, and by the still later forma- tion of serpentine veinlets cutting grains of carbonate and all of the pre-existing magnesian silicate minerals. That the temperature of the third stage was below 500° C. is indicated by the presence of serpentine which Bowen and Tuttle (1949) have shown cannot exist at higher temperatures regardless of pressure. The decus- sate arrangement of the tremolite indicates a non- stressed environment. The fourth stage is recorded mainly by the develop- ment of the second generation of tremolite, the later age of which is shown most clearly by the occurrence of nearly white tremolite in replacement veins cutting the darker tremolite-forsterite-serpentine rock, and by the white tremolite contact zones which border the granitic dikes that cut this rock. That much or all of the white tremolite rock was formed during this later period is shown by the widespread occurrence of residual j masses of tremolite-forsterite-serpentine rock in the white tremolite rock. Here too, a non-stressed environ- ment is indicated by the decussate texture of the newly formed tremolite. The abundant development of the mineral talc, par- ticularly at the expense of white tremolite, records the fifth stage, after the emplacement of the late granitic dikes, and probably during a period of decreasing tem- perature. The common occurrence of zones of talc schist along the margins of bodies of white tremolite rock, and locally within such bodies indicates that much of the talc formed as a stress mineral. Why virtually all of the tremolitization was confined to certain layers seemingly near the same stratigraphic position is not clear, particularly with the existence of untremolitized marble layers nearby. The possibility that the parent rock had a markedly different chemical com- position should not be overlooked. Part of the magnesia, for example, could have been supplied by magnesite in the original rock. The abundance of serpentine-diopsidei veinlets in the other marble layers, however, indicates! magnesia metasomatism did occur and probably was also effective in the tremolitization. The localization of tremolite appears to have been largely an effect of physical nature of the parent rock and the degree of access permitted the tremolitizing solutions. The source of the additive MgO or H 2 could have been in the magmas that yielded the basic or acidic rocks or in the sedimentary rocks themselves. Unlike the talc de posits of the southern Death Valley-Kingston Range re- gion to the north, those at the Silver Lake mine are not persistently associated in space with basic bodies, nor do the basic bodies in the mine area show any metasomatic contact effects. As noted above, most of the hydration and magnesia metasomatism appears to have been con current with the emplacement of the large granitic bodies and possibly an effect of solutions originating in Silver Lake Talc Deposits 19 the granitic magma. Magnesia, however, could also have b«.en derived during the granitization or assimilation of high-magnesian sediments (see appendix section on em- placement of large granitic bodies). AGE OF THE ROCK UNITS Because the rock units of the Silver Lake mine area are not in contact with and have not been lithologically correlated with rocks of known geologic age, their age remains in doubt. It will be recalled that Miller (1946), largely on the basis of their metamorphism, tentatively assigned an earlier pre-Cambrian age, and correlated them with similar rocks near Halloran Spring which were named by him the Halloran complex. It is true that throughout the southern Death Valley and Kingston Range region to the north only rocks of earlier pre-Cambrian (Archean) age display a degree of metamorphism as high as that of the metasediments of the Silver Lake mine area. However, in a geological study of the Silurian Hills about 6 miles northwest of the mine area, Kupher (1951, p. 1456) has reported that rocks of the Pahrump series (later pre-Cambrian) "can be traced from unmetamorphosed sedimentary rocks into feldspathized, intruded, and metamorphosed rocks which previous workers called 'Archean'." The meta- morphosed Pahrump rocks, however, cannot be traced laterally into the metasedimentary rocks of the Silver Lake mine area and several factors seem to make such a correlation very tenuous on the basis of present knowl- edge. The Pahrump rocks observed by the writer in the southeastern part of the Silurian Hills appear consider- ably less metamorphosed than those of the Silver Lake mine area. Moreover the lithologic sequence of the mine area is characteristic of none of the occurrences of the Pahrump series known to the writer. If the mine area metasedimentary rocks are the same age as the Pahrump, the granitic rocks may be correlative with the various plutonic rocks of late Mesozoic or early Tertiary age so common in the Mojave Desert. The only other granitic rocks recognized in the region to the north occur in bodies older than the Pahrump within earlier pre- Cambrian units. The planar structures, well displayed in the granitic rocks of the mine area, are not character- istic of the younger rocks,* but do characterize the earlier pre-Cambrian granitic rocks. Neither can the presence of talc deposits in both the Pahrump and Silver Lake mine rocks be considered a basis for correlation. The Silver Lake deposits have a much more complex metamorphic history and differ from the Pahrump de- posits in numerous textural and mineralogical features. They have formed by selective replacement and are seemingly unrelated in space to basic dikes such as those near or in contact with the Pahrump deposits. Pending further geological investigations in the region surround- ing the Silver Lake mine, the writer prefers to retain the earlier pre-Cambrian, Halloran Spring complex des- ignation as originally suggested by Miller. DESCRIPTIONS OF DEPOSITS AND WORKINGS Addenda Extension Area The commercial talc in the Addenda Extension area (pi. 3) is confined largely or wholly to two parallel bod- * Hewett, D. F., personal communication. ies 8 to 15 feet apart. As exposed at the surface, each body is about 10 feet in average width and 300 feet in length. Each strikes west-northwest and dips southward at an average angle of 55° to 60°. To the east they ter- minate against tonalite and pegmatite. To the west they are flanked by alluvium and may extend beneath it well beyond the line of overlap. These bodies apparently con- tain the largest undeveloped reserves of talc in the mine area. The Addenda Extension deposits differ from most of the others in the area in that the mineral talc appears to be the principal constituent. It occurs abundantly in white tremolite rock as well as in talc schist. Hard, blocky tremolite rock is subordinate ; forsterite-bearing material was not noted. Much of the hard tremolitic rock exposed underground lies next to the foot-wall of the more southerly body. By mid-1953 the Addenda Extension bodies had been developed by a 60-foot vertical shaft and about 250 feet of level workings. The shaft was sunk in 1938 and a small tonnage was removed from a tunnel and stopes that extend eastward at the 16-foot level. In 1951 and 1952 the two bodies were encountered in a 40-foot cross- cut driven S. 30° W. from the 65-foot level. In February 1953 the more northerly body was being followed to the west-northwest by a drift, then 80 feet long. The more southerly body had been followed in the same direction for 70 feet to a point where it narrows to a thickness of 4 feet, and east-southeastward for 50 feet to a cross-cut- ting pegmatite dike. Actual terminations of neither body appeared to have been reached. The talc had been stoped. only above the west-northwest drift on the more south- erly body. Diamond drill hole 19, driven north-northeastward at a 45° angle from a point 112 feet S. 30° W. of the shaft collar, encountered the more southerly body at about 25 feet down-dip from the 65-foot level. "Quartz" was re- corded as encountered where the other body would have extended. This may have been a permatite dike and does not necessarily indicate the downward termination of the body. Addenda Area Surface exposures of the talc-bearing zone in the Addenda area (pi. 3) extend laterally for about 520 feet, but the individual bodies are less continuous. As noted above, in plan the tale bodies exist as part of a metasedimentary island in a sea of granitic rock. The zone terminates against tonalite to both the east and west. In general the bodies strike westward and dip southward at an average angle of about 50°. At the surface the Addenda zone appears to be broadly divisible into three parts. A 120-foot segment at the west end and a 260-foot segment at the east end contain commercial talc in the twin layers so common at other localities in the mine area. These layers average about 10 feet thick and are separated by 5-foot to 15-foot thicknesses of hornfels and mica schist. The central segment, about 140 feet long, is separated from the others by cross-faults and granitic bodies. Here the talc exists in less regular bodies. It is most extensively exposed in the segment 's eastern half where three bodies seemingly lie one above the other. 20 Special Report 38 The talc in the Addenda area has been mined down- dip to a maximum of about 85 feet from the highest surface exposures. The lojr of diamond drill hole 18 indicates that the two bodies in the eastern segment extend at least 30 to 60 feet farther down-dip. This hole was drilled northward at a 45° angle from a point about 150 feet south of the southerly body. As explored to date the northerly body at the east end has consisted mostly of tremolite-forsterite-serpentine rock. White tremolite rock has been confined largely to layers 1 to 4 feet thick near the margins of the bodies. Talc schist occurs in layers, from a few inches to 2 feet thick, along the footwalls of the more northerly bodies. In February 1953 the principal workings consisted of a 70-foot shaft, inclined at an average angle of about and joined at the bottom to about 660 feet of drifts and cross-cuts. The shaft was sunk on the middle body of the central segment and the level workings driven mainly in the two bodies of the eastern segment of the Addenda zone (pi. 3). The Addenda deposits were first seriously worked in the mid-1930 's, when talc was removed from several shallow cuts and stopes and the shaft was begun. In 1946 the shaft was driven to its present depth. In the subse- quent level work each of the bodies in the eastern seg- ment of the zone has been found to terminate against tonalite at a distance of about 330 feet east of the center- line of the shaft. By February 1953, much of the talc between the tunnel level and the surface had been stoped. Gould Area The Gould talc bodies in the west-central part of the Silver Lake mine area (pi. 3), are the largest developed to date, were the first to be opened, and have been the most extensively mined. They strike westward and dip 50° to 65° southward. Although these bodies are dis- continuously exposed for a lateral distance of nearly 1600 feet, virtually all of the Gould output has been obtained from an 800-foot segment. Within this segment the mineable talc occurs largely in two parallel bodies each ordinarily 10 to 15 feet thick, and separated by a 10- to 20-foot thickness of diopsidic hornfels and tremolite-mica schist. Faults, some parallel with the talcose zone, others that cross it cause the bodies to pinch and swell abruptly and to be off-set 10 to 20 feet. The Gould bodies consist mostly of white tremolite rock. Tremolite-forsterite-serpentine rock, existing as irregular, residual masses in the central parts of the bodies, is common. Talc schist occurs as layers, from a fraction of an inch to two or more feet thick, which are best developed along the footwalls of the bodies, but which also exist along the hanging walls and within the bodies. Layers of green tremolite rock are present, but inconspicuous. The Gould bodies were first developed by means of the Gould shaft, begun in 1919 and sunk on the footwall vein at an angle of about 50°. The early output of the mine was obtained from drifts and overhand stopes, mainly in the footwall vein, and joined to the shaft. In 1925 the operation was facilitated when the shaft was joined at about the 130-foot level by the Gould tunnel (fig. 16), driven eastward from the small canyon north of the camp (pi. 3). The tunnel, which now ends at a point about 1100 feet east of the portal, for much of its length, consists of parallel drifts following both the footwall and hanging wall veins. By 1940 the footwall vein was thoroughly stoped and was believed to have been nearly depleted from the surface to its downward termination against granitic rock * at a reported maxi- mum of 255 feet down-dip. Following an 8-year idleness, the Gould workings were reopened in 1948 and continued active in 1953. For about two years, mining was confined to segments of the hanging wall vein that lay above the main tunnel and, were by-passed in the earlier work. Because the part of the hanging-wall vein that lay below the tunnel level also proved to be largely unworked, in 1951 an inclined winze was begun from a point about 200 feet east of the western ends of the principal Gould talc bodies. By early 1953 the winze had been sunk on the vein for a distance of 85 feet, and drifts driven east and west at this level for a total of about 250 feet, also in the hanging-wall vein. The operators planned to extend the drifts and deepen the winze to the commercial limits of the vein. Number 2 Area In its surface exposures, the main part of the Num- ber 2 talc-bearing zone is about 400 feet long. It too consists of two parallel veins separated by a schist and hornfels layer from 15 to 25 feet thick. These strike westward and dip 60° to 80° southward. Of the two, the footwall vein is the longest and is 15 to 25 feet thick for most of its length. It has yielded most of the talc mined in the Number 2 area. The hanging-wall vein thins markedly down-dip and westward. The veins are composed of white tremolite rock and subordinate talc schist. Although extensively exposed at the surface, the talc bodies of the Number 2 area have been mined no deeper than 55 feet down-dip. At about this level, for most of its length, the zone appears to pinch, ordinarily an effect of the encroachment of granitic and basic bodies along its southern border. The footwall vein has been followed by two inclined shafts about 200 feet apart. The more westerly shaft, now filled to the 50-foot level, was sunk in talc at least to this depth. The latest dump material indicates that the shaft bottomed in granitic rock at a point about 20 feet deeper. A drift-adit, joining this shaft at the 50-foot level, extends for the full length of the foot-wall vein. To the west it forms the bottom of a large stope ; to the east it connects three open cuts (pi. 3). The more easterly shaft, sunk at a 50° angle from the floor of the easternmost cut, is 40 feet long and is joined to a 20-foot eastward drift at the 12-foot level and to about 115 feet of level workings at the 40-foot level. Here the footwall vein has been stoped eastward for about 75 feet. On the same level a 40-foot cross-cut intersects the hanging-wall vein which here appears to contain only a 2- to 3-foot thickness of commercial material. Between the Number 2 and Number 2\ workings are several exposures of talc-tremolite rock, apparently on relatively thin or discontinuous bodies (pi. 3), but which indicate a likely area for future prospecting. Diamond * Skeoch, W. K., personal communication, 1953. Silver Lake Talc Deposits 21 Figure 16. View westward along surface exposures of Gould talc bodies. Two parallel bodies are bordered by diopside-feldspar-quartz- calcite hornfels. Quartz-biotite, schist member and tonalite exposed on hill at upper right. drill holes 1 to 6 were sunk in search of down-dip exten- sions of the western part of the main Number 2 zone and of these other, more westerly bodies. When logged at the time of drilling, the holes were believed to have been entirely in roek of little or no commercial interest. Cal- careous talc units which were noted in holes 3 and 4, however, suggest that the western part of the hanging- wall vein may extend considerably deeper than its proved limits. Number 3 Area As noted above, the talc in the Number 3 area, though associated with a lens of diopsidic rock (pi. 3), is not actually part of the hornfels member as defined in a pre- vious section. Instead the lens lies about 300 feet south of the trace of the member. Here the talc is confined to a single, irregular body with a surface exposure about 280 feet long and 45 feet in maximum width. It trends west- ward, dips southward at an average angle of about 50°, and is composed mostly of talc schist. A small tonnage of this material has been shipped. The body has been followed eastward by a shallow drift-adit whose floor is about 40 feet lower than the highest outcrop. The talc exposed in this adit appears to be 8 to 10 feet in average thickness and persists in the face. West of the adit entrance, the body is exposed for about 70 feet, but appears to have nearly pinched out at the adit level. That the central part of the body is also quite shallow, is indicated by the log of diamond drill hole No. 18 sunk north-northwestward at a 45° angle from a point about 90 feet south of the talc body (pi. 3). The log shows no talc, but that the drill passed mostly through so-called granite and schist. N umber 2i Area The tale bodies of the Number 2\ area (pi. 1) are the smallest to have been seriously mined. The surface ex- posures of the body most extensively worked are now largely hidden beneath dump material, but underground workings show a maximum horizontal dimension of about 200 feet. It strikes westward and dips about 60° to the 22 Special Report 38 Figure 17. View eastward of Number 2-} area. Hill at upper left underlain mostly by silexite. Lower area underlain mostly by meta- sedimentary rocks and basic intrusive rocks. Number 3 body in right distance. south. The body, now largely removed, averaged about 10 feet in width and was followed down-dip for 200 feet. At this level, it appeared to have nearly pinched out. The material mined consisted mainly of white tremo- lite rock associated with prominent, though subordinate layers of green tremolite rock. Talc schist, in a layer one to two feet thick, ordinarily lay along the hanging wall. The mining was hampered by an abundance of a granitic and basic dikes. The Number 2\ shaft was begun in 1941 and continued in use until operations in this area were suspended in 1947. The talc body was followed for 140 feet west and 45 feet east on the 75-foot level, for 130 feet east and 130 feet west on the 130-foot level, and for 100 feet west on the 200-foot level. At these points the body terminated against granitic rock. From the eastern limit on the 75- foot level, about 280 feet of exploratory tunnel was driven unsuccessfully in a search for a downward exten- sion of another body exposed at the surface about 200 feet to the northeast. One branch, extending for about 130 feet eastward, is mainly in granitic rock ; the other, extending about 130 feet east-northeastward, is mainly in hornfels (pi. 1). Number 4 Area In the Number 4 area metasedimentary units and later bodies of basic to acidic composition exist in an easterly extension of the terrane that contains the deposits noted above. The exposures of Number 4 deposits are confined to a triangular area, about 700 feet on a side. The talc bodies lie near the southern end of a wedge-like, south- pointing block which is bordered on the west by a zone of highly brecciated and faulted rocks barren of talc and on the east by a belt composed mostly of granitic rocks. Within the area of the accompanying map (pi. 2) the block is broadly folded into a syncline on the west and an anticline on the east. Most of the talc has been re- moved from shallow, gently dipping bodies on the crest and flanks of the anticline. As in the western part of the mine area, the talc occurs mostly in two parallel bodies separated by a 5- to 15-foot thickness of diopsidic hornfels. As elsewhere in the Silver Lake area, the talc bodies are bordered mainly by hornfelsic and schistose rocks characteristic of the hornfels member described above. Irregular bodies of granitic rocks and lamprophyre, as well as pegmatite dikes, are abundant and widespread. Because the lamprophyre ordinarily shows a pronounced schistosity, it closely resembles biotite schist of metasedi- mentary origin. Much of the biotite schist pattern shown on plate 2 may actually be lamprophyre. Exploration to date indicates that the principal body is elongate and tabular, and has a maximum dimension of about 700 feet in a northeasterly direction and a maxi- mum thickness of about 40 feet. By 1950 it had been mined to points where it terminates by lensing or fault- ing, and had been largely removed. Other talc bodies in the area appear to be much smaller. Most of the com- mercial talc mined to date has been of the talc schist variety. Hard, blocky tremolite rock is very subordinate. Previous to 1936 the Number 4 area had been in- extensively worked and developed. The inclined shaft shown on plate 2 had been started and a small tonnage had been recovered from a lens in the west part of the area. In that year the principal body, whose surface ex- posures were very small, was largely outlined by dia- mond drilling. Eight holes (nos. 7 to 14) ranging from 20 to 223 feet long, were drilled. Each encountered talc of commercial quality and thickness to indicate a reserve estimated by the operators at about 25,000 tons. Sub- sequently, near the southwestern edge of the talc body, a vertical shaft was sunk 75 feet. The body, encountered Silver Lake Talc Deposits 23 ,t a depth of 57 feet, was mined by low-angle raises en- arged to form modified room and pillar workings. In 1945 the collapse of the roof of the workings near the 2rest of the anticline permitted the removal of several |thousand tons of talc by open-pit methods. This opera- tion was suspended in 1948 and the workings have since been idle. Number 5 Area The Number 5 area, which lies about half a mile north of the Number 2 workings, has yielded a small tonnage of talc but has lain idle for many years. Here, as in the principal talc-bearing belt to the south, the talc occurs fn a green, hornfelsic member and mostly in two parallel bodies. The bodies of commercial interest appear to be confined to a zone about 180 feet in exposed length and 30 feet in average width. It strikes N. 45° W., and dips (about 75° to the southwest. The zone terminates against granitic rock to the southeast and is flanked by alluvium ito the northwest. Beneath the alluvium the bodies may iextend well beyond the line of overlap. They are com- posed principally of white tremolite rock, but commonly Contain a footwall layer of talc schist one to two feet (thick. The workings consist of a shaft about 30 feet deep ind several trenches. APPENDIX Notes on the Petrography and Metamorphism of the Principal Rock Units Hornfels Member Petrographic Features. Microscopic studies of about 25 thin sections of the truly hornfelsic facies of the horn- fels member show that, despite a wide range in the relative proportions of minerals, its mineralogic and tex- tural features are persistent. Diopside and feldspar are he most abundant and widespread constituents. Quartz and calcite are common. Garnet and phlogopite, though much less abundant, are locally prominent in certain layers. Tremolite and dark green amphibole are irregur larly and sparsely distributed. In a few thin sections the diopside has partly altered to talc or serpentine, but such alteration is rare. The diopside, feldspar, and quartz grains are generally of uniform size and average less than 0.3 mm in diam- eter. From place to place within the member, these min- erals are present in widely different proportions, and locally any one of the three may predominate. The pro- nounced, thin banding typical of much of the diopsidic facies is caused chiefly by alternating diopsidic and felsic layers. Most of the diopside is in subhedral grains ; but locally it occurs in very irregular grains, several millimeters in diameter, and deeply embayed and transected by grain aggregates of quartz and microcline. In the specimens observed in thin section, the diopside content ranges from about 10 to more than 95 percent. Both plagioclase and potash feldspars are abundant ; locally, either comprises half to two-thirds of the rock. A few hornfelsic layers contain quartz and alkali feld- spar to the exclusion of diopside. Most of the plagioclase is albite or sodic oligoclase, but some was observed as calcic as bytownite. Locally bytownite grains, as much as 6 mm in diameter, enclose much smaller equant diop- side grains. The garnet, straw yellow in transmitted light, occurs in small, irregular grains as part of the diopside-feld- spar-quartz mosaic. Garnet grain clusters are character- istically elongate parallel with the banding of the rock. Calcite grains are persistently the coarsest. Although some are as much as 1 cm in diameter, most are less Figure 18. View southward of Number 4 area. In the vicinity of open cuts principal talc body dips toward viewer. South of hill, body dips southward toward vertical shaft at upper left. 24 Special Report 38 than 1 mm. The calcite occurs in veinlets and in irregu- lar grains that enclose smaller grains of the silicate min- erals. In thin section, specimens typical of the schistose phases of the hornfels member are shown to be uniformly- fine-grained, but to have a wide range in mineral compo- sition. Most are composed of grains from 0.5 to 1.5 mm in length. Thin section studies indicate that some layers contain only tremolite or actinolite ; others contain trem- olite with subordinate mica, phlogopite or biotite and al- kali feldspar ; still others are composed principally of phlogopite. Diopside is notably lacking. A representative specimen of the schist that separates the commercial talc bodies from wall rock contains ap- proximately 60 percent tremolite, 20 percent phlogopite, and 20 percent alkali feldspar. Some of the tremolite blades are corroded by aggregates of finer grained phlog- opite and feldspar. Much, if not all, of the feldspar is albite or sodic oligoclase. Met amor phism. Because individual metasedimentary units of the Silver Lake mine area cannot be traced lat- erally into their less metamorphosed equivalents, the origin of the hornfels member as well as the other meta- sedimentary units is somewhat obscure. The bodies of hornfels, quartzite, and talc-tremolite rock* which are stratiform and contrast in bulk chemical composition, apparently reflect differences, chemical, physical, or both, in original sedimentary layers. The small scale textural laminations seem best attributable to metamorphic dif- ferentiation, but most of the gross layering, involving laterally persistent units several or more feet thick, is sedimentary in origin. Although widespread metasomatism is indicated in the alteration of forsterite to tremolite, serpentine and talc, of tremolite to serpentine, carbonate and talc, and of diopside to tremolite and serpentine, none of the con- tacts between the larger metasedimentary bodies is clearly of metasomatic origin, nor are they related in space to contacts with granitic rocks. Mineral assemblages identical with or similar to those of the hornfelsic rocks of the Silver Lake area generally have been attributed to the metamorphism of marly and arenaceous dolomites under conditions characteristic of the amphibolite facies. Hornfelses of this origin were de- scribed in 1914 and 1915 by Eskola in his classic ac- counts of the rocks of the Orijarvi region of Finland. If isochemical reconstitution is assumed for the horn- fels of the Silver Lake area, the diopside-plagioclase- microcline-calcite-quartz assemblage, which composes most of the truly hornfelsic rock of the hornfels member, would have been derived from original impure dolomite rich in alumina, silica, potash and soda. At least a local introduction of potash and alumina is indicated by the textures in which microline and quartz appear to have formed at the expense of diopside, and by the pres- ence in tremolite bodies of late-stage phlogopite, occur- ring as fracture-controlled veinlets and bordering gra- nitic dikes. The preponderance of sodic over calcic feldspar in a calcite-bearing rock, however, need not necessarily be at- tributed to the metasomatic introduction of soda, but could well have been caused by high pressure and shear- ing stress. Such conditions are known (Turner, 1933) fet restrict the lime content of plagioclase. The formation o , microcline in preference to muscovite, probably was con! trolled by the excess of alkalies; whether of sedimentan! or hydrothermal origin. As indicated by Turner (1948)1 in all but highly magnesian rocks, potash feldspar in ai environment of calcite will alwavs crvstallize in prefer! . . D t . , CaO + K 2 + Na 2 CJ ence to mica if the weight ratio n~7\ 1 AI2U3 exceeds unity. The presence of diopside-bearing replacement veinlet;] in forsteritic marble suggests at least some of the diop side in the hornfels is of hydrothermal origin. As tht diopside in the hornfels, however, is not veinlet-forming it probably was derived mostly from original constitul ents. The thorough dissemination of quartz suggests thai silica also was originally abundant. In a consideration of the physical conditions undei which the hornfels formed, the following mineralogic features are significant: (1) the preponderance of diop- side over amphibole and the post-diopside age of most or all of the amphibole, (2) the abundance of quartz and the absence of forsterite, and (3) the widespread quartz^ calcite association and the absence of wollastonite. Bowen (1940, p. 245), in his classic discussion of the progressive metamorphism of siliceous limestone and dolomite, notes that diopside first appears in the fourth of 13 steps that mark increasing decarbonation with! rising temperature. This step, which is indicated by the reaction 2CaC0 3 + 3CaMg 3 (Si0 3 )4 ^ calcite tremolite 5CaMgSi 2 6 + 2Mg 2 Si0 4 + 2C diopside forsterite is defined by a P-T curve showing the temperature which the reaction can proceed at a given pressure. At higher temperatures for a given pressure, calcite and tremolite cannot coexist in equilibrium. Bowen 's (p. 256) schematic representation of this curve suggests, for example, that the reaction could occur at about 470 de- grees C. and 200 atmospheres pressure, and at about 600 degrees C. and 1000 atmospheres pressure. Bowen (p. 260) also has arranged the following sequence of minerals in the order of their production with rising temperature: (1) tremolite, (2) forsterite, (3) diopside, (4) periclase, (5) wollastonite, (6) mon- ticellite, (7) akermanite, (8) spurrite, (9) merwinite, (10) larnite. He cautions, however, that the presence of any one of these minerals can be used as an indicator of metamorphic temperature only if the original mate- rial was a siliceous dolomitie limestone, and only when such a rock was sufficiently immobile to prevent the rapid elimination of carbon dioxide from the system. Bowen (p. 258) also notes that "the amount of alumina present (or added) might * * * be so great as to prevent the formation of some of the reference phases in all stages of metamorphism." He believes, however, that in the contact metamorphism of a siliceous dolomite a high con- centration of carbon dioxide is generally maintained, and that the index minerals, as listed, can be used with reasonable certainty. In the hornfelsic rocks of the Silver Lake mine area no pre-diopside tremolite or forsterite was observed. ; Silver Lake Talc Deposits 25 Tremolite appears in the first of the thirteen steps which is represented by the reaction 3CaMg(C0 3 ) 2 + 4Si0 2 ^± dolomite quartz CaMj? 3 (Si0 3 ) 4 + 2CaC0 3 + 4C0 2 tremolite calcite jprovided an anhydrous formula for tremolite is used. \ Ja.s indicated by Bowen, the hydrous character of tremo- lite introduces a complicating factor, because a partici- pating liquid phase must be present both above and ' below the reaction temperature. ' ' If the rock ' boils dry ' : below the reaction, no tremolite is formed; if it 'boils pry' at the reaction point, some tremolite forms." : j(Bowen, 1940, p. 241) In the known examples of thermal metamorphism of 'siliceous dolomites tremolite, as a mineral phase, is com- monly absent, and forsterite is ordinarily believed to be the first phase formed. Because forsterite is unstable in the presence of quartz, the forsterite-quartz association lis commonly cited as an example of disequilibrium. The extent of circulating solutions, during the forma- tion of diopside in the hornfelses of the Silver Lake area, : bannot be demonstrated with certainty. As noted above, the oldest of the intrusive rocks are basic and that much, ] if not all, of the metasomatism is probably related to the granitic rocks. It is possible, therefore, that the high- jest temperatures were reached in a relatively dry envi- ronment before the granitic rocks were emplaced, 1 perhaps during the formation of the earlier mafic bodies. If circulating waters were present in negligible I amount, or, if abundant, were able to maintain a high concentration of carbon dioxide, during the formation of jche diopside, several factors could account for the gen- eral absence of tremolite and forsterite. Water may not have been available to enter into the tremolite-forming jreaction or an abundance of alumina may have pre- sented the appearance of tremolite altogether. If tremo- lite did exist and equilibrium was attained upon the appearance of diopside, the instability of the association fcalcite-tremolite would have led to the disappearance of '(tremolite as a mineral phase. The absence of forsterite Ipould be attributed to either an abundance of alumina or ! |an attainment of equilibrium wherein forsterite was '■instable in the presence of quartz. The production of wollastonite by the reaction of ■quartz and calcite, as shown by the equation CaC0 3 + Si0 2 ^ CaSi0 3 + C0 2 , calcite quartz wollastonite [is commonly cited as a reliable indicator of temperature land pressure during metamorphism. The association of Iquartz and calcite in equilibrium is likewise used to ■indicate that, at a given pressure, the temperature that ■would permit the reaction has not been reached. This ■reaction is the sixth of the thirteen steps mentioned labove. Its P-T curve, as shown by Goldschmidt (1912), includes, for example, points at about 650 degrees C. and 1200 atmospheres pressure, and at about 780 degrees C. land 2000 atmospheres pressure. If the presence of diopside and the association of calcite and quartz can be taken as reliable indicators [jof metamorphic temperature and pressure in the development of the hornfels, the maximum temperature 'would lie in the area between the curves of step 4 and step 6 as shown by Bowen. If, for example, a pressure of 1000 atmospheres is assumed, a temperature in excess of 600 degrees C. would be indicated to assure the formation of diopside; and a temperature of 780 degrees C. probably could not have been greatly exceeded without the appearance of wollastonite. The talc bodies are composed mostly of minerals produced during a period of retrogressive metamorphism and extensive metasomatism. The remnant grains of forsterite, partly to wholly replaced by tremolite, serpentine, and talc, appear to be the only representa- tives of the assemblage that antedated the three later minerals. The association of tremolite and forsterite, known to occur only in metamorphosed carbonate rocks, indicates the original sediment to have been carbonate- rich. In contrast with the enclosing wall-rocks of horn- fels, the talc bodies contain only one or two percent each of alumina, potash, and soda. That the original sedimentary rock was comparably poor in these con- stituents, is probably a valid assumption. If equilibrium can be assumed at the time of the production of the forsterite, a deficiency of silica would thereby be indicated. That the original rock was silica-deficient is also suggested by an apparently com- plete absence of quartz in the talc bodies, except in granitic veinlets. The lenses of various schistose rock types in the hornfels member generally appear to represent zones of stress that parallel the planar features of the member. The phlogopite and tremolite, so common in the schistose layers, are probably both younger than the diopside in the enclosing rocks. Although diopside is notably absent in the larger schist bodies, it is in association with amphibole (tremolite or actinolite) along the schist borders and along contacts between hornfels and granitic dikelets. Where age relations between diopside and the amphibole were observed, the amphibole was consistently later. A diopsidic border zone along a granite veinlet in tremolite rock provided the only observed exception to this statement. That phlogopite commonly occurs in tremolite rock as veinlets and as borders of late-stage granitic dikelets, shows that much, if not all, of the phlogopite in the mine area was late-forming. Quartz- Biotite Schist Member Petrographic Features. In thin section, a specimen of a biotitic phase of the quartz-biotite schist member is shown to contain about one-half quartz, one-fourth microcline, one-fourth mica, and minor amounts of albite, apatite, and opaque grains. The quartz and microcline form a mosaic with grains as much as 5 mm in diameter. Most of the microcline, however, is con- centrated in what seems to be a migmatitic layer. The quartz grains show undulatory extinction and a tendency toward elongation parallel with the schistosity. About three-fifths of the mica is biotite ; the remainder is muscovite and sericite. Most of the mica shreds are in general alignment and are clustered in discontinuous parallel layers. Biotite grains are as much as 3 mm in length; the muscovite grains are ordinarily smaller. Some of the muscovite is in large grains, but most is in sericitic aggregates that have partly to wholly replaced biotite. The microcline is slightly sericitized. 26 Special Report 38 Metamorphism. The unmigmatitized parts of the quartz-biotite schist member, consisting predominantly of dimensionally oriented, elongate quartz grains and biotite shreds, seem best attributed to the metamorphism of an impure, carbonate-free quartzite under conditions of high directional pressure. That biotite, rather than muscovite, is the principal mica, indicates iron- and magnesia-rich impurities indigenous to the original sedi- ment in the form of chloritic or perhaps mafic tuffaceous material. The abundance of biotite in and near the migmatite zone in the west part of the mine area could, perhaps, be cited as a basic front in which iron and magnesia had been introduced in advance of a granitization wave (see, for example, Reynolds, 1947). Several factors, however, point against the presence of biotite-rich basic fronts in the mine area. The distribution of biotite in the meta- sediments appears to be largely, if not entirely, a stratigraphic feature. Well-defined, stratigraphic planes separate the quartz-biotite schist member from the under- lying biotite-poor hornfels member and the overlying biotite-free quartz-muscovite schist members. None of the metasedimentary rocks that normally contain little or no biotite show the development of the mineral in the vicin- ity of granitic contacts. Although magnesia and iron metasomatism is indicated in the apparent tremolitiza- tion of carbonate rock and the development of iron-rich minerals along granitic dikes, the hornfels member, the iron and magnesia, in general, were deposited indepen- dently at different times, in different places, and did not lead to the development of biotite. It would seem, there- fore, that little, if any, of the biotite in the quartz-biotite schist developed during migmatitization. Instead the more biotitic and, consequently, schistose phases of the member appear to have offered the most favorable en- vironment for migmatitization. A degree of impoverish- ment in iron and magnesia and enrichment in potash is indicated by a progressive decrease in biotite with in- creasing intensity of migmatitization. This trend is shown microscopically by the partial sericitization of the biotite. The reconstitution of initially solid rocks to migmatites commonly has been described as occurring in the pres- ence of silicate melts. Eskola (Barth, et al., 1939), for example, limits the term "migmatite" to rocks of this origin. Such melts have been indicated variously as true magmas, as highly fluid magmas, and as liquids produced by differential fusion. The development of rocks with migmatitic textures has also been ascribed to metasomatic replacement involving an aqueous pore solution, and to diffusion in the solid state. Few geologists insist, how- ever, that all migmatitic rocks must develop by any one of these processes. Turner (1948, p. 305) has stated that the presence of a silicate melt during the development of a migmatite is indicated by a "general abundance of pegmatitic, aplitic and other igneous veins, lenses, and streaks"; whereas quartzose veins prevail in strictly metamorphic rocks. If this criterion is valid the migma- tite zones of the mine area were of the silicate melt variety. Megascopic observations and detailed mapping show that the migmatites have developed with no apparent displacement or shouldering aside of the enclosing schists. Indeed the rocks of entire metasedimentary sequence appear to remain undeformed by the emplacement of granitic rocks and to retain their precise stratagraphic position regardless of the size and shape of the granitic bodies with which they are associated. Quartz- Muscovite Schist Member Petrographic Features. A thin section of a specimen, typical of the more schistose facies of the quartz-musco- vite schist member, is composed of approximately three- fourths quartz and one-fourth muscovite. It also con- tains one or two percent of iron oxide and a minor amount of sphene and rutile. Most of the quartz grains have lengths from 1| to 3 times their widths and are dimensionally oriented par- allel with the schistosity. The grains are generally less than 0.3 mm in long dimension ; only a few exceed 0.5 mm. They show no undulatory extinction. The rutile, which occurs as numerous needle-like inclusions in the quartz, is also oriented parallel with the schistosity. Some of the muscovite is in shreds as much as 1 mm in length, but most of it is in aggregates of very minute sericitic shreds. The aggregates are peripheral to the quartz grains, and form thin layers that are the prin- cipal cause of the schistosity. The iron oxide grains are dark red and opaque to translucent. Microscopic examination of a specimen from one of the marble lenses shows that it is an ophicalcite composed of about three-fourths carbonate, (mostly calcite) one- fourth chrysotile, and a few percent of antigorite talc and opaque material. Most of the carbonate is in grains that average about 0.2 mm in diameter. The grains are locally stained- with iron oxide, but are generally un- clouded. The serpentine is in evenly disseminated grains that average less than 0.1 mm in maximum dimension, Some of the antigorite shreds have been partly to wholly replaced by talc. The opaque material is in very fine grains that occur mostly as inclusions in the equant serpentine grains. The chrysotile grains are similar in shape, abundance, and distribution to the forsterite grains in marble of the overlying member. Though nc forsterite was observed in the thin section, the chrysotik grains undoubtedly are completely serpentinized for- sterite. The presence of both forsterite and humite else-i where in the marble lenses was noted in the inspection! of mineral fragments in immersion media. Thin section studies show that the amphibolite schisl layers are composed almost entirely of quartz and fer- ruginous amphibole in approximately equal proportion The amphibole is strongly pleochroic in shades of pah yellow to medium green and is either actinolite or horni blende. Very minor amounts of sphene and opaque ma terial were the only accessories noted. Most of both th( quartz and amphibole is in grains that range from O.i to 1 mm in long dimension. The schistosity is caused bj a dimensional parallelism of the amphibole grains and by elongation of the quartz grains. Metamorphism. The predominant rocks of the quartz muscovite schist member, consisting essentially of quartz subordinate muscovite, and minor amounts of ferric ox ide, rutile, and sphene, most certainly developed froir the metamorphism of a somewhat impure sandston< notably lacking in magnesian and carbonate material Although the proportion of muscovite varies from layei to layer, its otherwise even distribution throughout th< Silver Lake Talc Deposits 27 'ertical and lateral extent of the member strongly sug- gests that much, if not all of the material from which t has formed was present in the original sediment. This naterial could have been sericite, argillaceous material, )r a mixture of the two. The flattening of the quartz irrains and planar disposition of the muscovite add to he evidence for an environment of high directional jressure and stress. The thin laminations in this rock, is well as those in the other metasedimentary units ap- >ear to be of metamorphic differentiation. r orsterite Marble Member Petrographic Features. Thin sections of several typi- :al unveined specimens of the marble member indicate a iomposition of approximately three-fourths crystalline iarbonate and one-fourth disseminated magnesium sili- ate grains. The carbonate grains, mostly dolomite, aver- ige about 0.1 mm in diameter and are clouded by very ine-grained, unidentified particles. Of the disseminated nagnesium silicates, forsterite and clinohumite are by ! ar the more common. The grains of both the forsterite and clinohumite are iquant and sub-rounded. Both have maximum diameters )f about 2 mm, but most grains are less than 0.5 mm in liameter, a distinctly smaller size than that of the fors- ;erite grains in the talc-tremolite bodies. Both are similar in appearance and in optical properties ; but the clino- mmite is ordinarily distinguishable by a straw-yellow )leochroism. The forsterite is best distinguished by a partial to complete alteration to chrysotile. The clino- mmite, by contrast, is unaltered. Scattered shreds of the antigorite variety of serpentine, mostly 1 mm or less in length, compose 2 or 3 percent of each thin section. These have been partly to wholly altered to talc. Minute grains of a steel gray opaque mineral (magnetite ?) are com- mon inclusions in the chrysotile derived from forsterite. Green spinel grains are sparsely scattered through sev- eral of the thin sections. An inspection of numerous powdered specimens of the veinlet material showed that serpentine is the most abundant mineral in these bodies, but that diopside lo- cally predominates. In a thin section composed of 90 per- cent diopside, serpentine is the later of the two minerals. The diameters of the diopside grains average less than 0.3 mm, but some are as much as 3 mm. Antigorite shreds are interstitial to the diopside. Both of these minerals are transected by numerous chrysotile veinlets and by less numerous calcite veinlets. Opaque grains are commonly included in the antigorite shreds and are par- ticularly abundant in the chrysotile veinlets. Extended petrographic studies may well show that the serpentine in all of the veinlets has similarly formed at the expense of diopside and antigorite. Metamorphism. The silica deficient assemblages of the forsterite marble member and of the marble lenses in the underlying schist are typical metamorphic deriv- atives of low-silica magnesian limestones and dolomites ; but the paragenesis of the rocks is somewhat clouded by the effects of circulating solutions, the full extent of which cannot be demonstrated. In the absence of con- tradictory evidence, evenly disseminated silicate grains are assumed to reflect the proportion and character of the impurities in the original rock. Much or all of the vein-forming material appears to have been introduced. That hydrous solutions also penetrated the intervein blocks and the unveined lenses is shown by the wide- spread serpentinization of forsterite and alteration of antigorite to talc. The introduction of fluorine is sug- gested by the abundant clinohumite. That the combined proportion of forsterite and clino- humite remains virtually constant throughout these rocks, regardless of the proportions of the two minerals, suggests that the magnesia and silica of both was de- rived largely from the original sediments. Because the rock contains no free silica, it can be reasonably assumed that what silica was present in the original sediment was consumed in the development of the magnesian silicates. The absence of diopside as a disseminated mineral is probably attributable to the early depletion of the avail- able silica. The presence of very thinly disseminated spinel, the only aluminous constituent of the marbles, probably points to the original presence of a correspond- ingly low content of argillaceous material. The metamorphic significance of the disseminated an- tigorite shreds is not clear. If formed at the expense of a pre-existing magnesian silicate, they contain no rem- nants of it, nor is there evidence of pseudomorphism. Quartzite Member Petrographic Features. A thin section of a specimen typical of the quartzite member contains about 90 per- cent quartz, 10 percent feldspar, and 2 to 3 percent mica. The quartz grains, although irregular in outline and having sutured borders, are markedly elongate par- allel with the overall attitudes of the member. The lengths of such grains commonly exceed 5 mm and are ordinarily from 2 to 5 times their widths. The feldspar and mica grains are much smaller ; these occur in the quartz as small inclusions, and also are elongate parallel with the general planar structure of the rock. The lengths of most of the feldspar and mica grains are less than 0.2 mm ; rarely do they exceed 0.5 mm. Although much of the feldspar is highly seri- citized, all appears to be alkalic. Many of the less altered feldspar grains are albite. The mica is predomi- nantly biotite, but the section contains a few scattered grains of muscovite. Minute rutile needles, oriented in seemingly random directions, are common in the quartz grains. Other relatively abundant accessories are sphene, apatite and opaque grains. Metamorphism. The minerals of the quartzite sug- gest little other than the reerystallization, under condi- tions of high directional pressure, of a sandstone con- taining subordinate amounts of aluminous material. The sericitization is the only apparent metasomatic effect. Basic to Intermediate Intrusive Rocks An examination of several thin sections of the horn- blende kersantite shows a moderate range in mineralogy, textural features, and degree of alteration. The smaller bodies appear consistently to contain about 70 percent andesine, 15 percent hornblende, and 15 percent biotite. A specimen typical of the medium-grained parts of the larger bodies is somewhat more mafic in that it contains about 60 percent andesine, 35 percent hornblende, and 5 percent biotite. Sphene and opaque grains are very abundant accessories; apatite is also common. 28 Special Report 38 The rocks are hole-crystalline and of relatively uniform grain size. The minerals of the smaller bodies are mostly in grains that range from 0.2 to 1 mm long. Average grain sizes of more than 2 mm are common in the larger bodies. The grains of all three principal minerals show a degree of dimensional parallelism in all of the thin sections examined. Much of the feldspar is partly to thor- oughly sericitized. Otherwise, the kersantite shows very little alteration. A thin section of a typical specimen of dacite porphyry shows a ground mass composed of approxi- mately 45 percent calcic andesine, 15 percent quartz, 25 percent biotite, 10 percent hornblende, and 5 percent opaque grains. Plagioclase phenocrysts form about 10 percent of the section. These appear to be labradorite, but they are highly sericitized and difficult to identify in thin section. The andesine and quartz of the ground mass are mostly in grains that range from 0.1 to 0.3 mm in length ; the biotite and hornblende grains are gen- erally two to three times longer. Most of the plagioclase phenocrysts are 1 to 2 mm in diameter. The grains of the ground mass are dimensionally aligned to form a pronounced schistosity. Tonalite Thin sections of specimens of the tonalite unit gathered at widely spaced localities show that, in spite of the range in the ratio of potash feldspar to plagioclase, the textural and structural features of the unit are per- sistent. Most of the plagioclase and quartz are in grains that range from 1 to 5 mm in maximum dimension. The potash feldspar is predominantly microcline, the grains of which are commonly much larger than those of plagioclase and quartz. Orthoclase is less abundant and in smaller grains. The felsic minerals form a typical granitic mosaic, that in some places contains the larger microcline crystals. Feldspar forms from five-eighths to three-fourths of the rock. The quartz content ranges from one-fifth to one-third. In general, the specimens gathered in the vicinity of large bodies of lamprophyre and in the sill-like bodies within the metasediments, are the richest in plagioclase. The specimens gathered near masses of microcline- quartz-mica gneiss are the richest in potash feldspar. The plagioclase ranges from calcic oligoclase in the granite phase of the unit to andesine in the typical tonalite. The potash feldspar is predominantly micro- cline. Microperthitic grains are common as are micro- graphic inter growths of quartz in orthoclase. Mica, consisting of biotite and very subordinate muscovite, ordinarily forms from 3 to 10 percent of the rock. The mica is mostly in shreds from 0.5 to 2 mm long, and is corroded and transected by quartz and feldspar. Opaque accessory grains are commonly associ- ated with the biotite. In some sections, apatite is also a common accessory. Zircon is present, but less abundant. Some of the feldspar has been partly sericitized, but the rock as a whole is relatively unaltered. Minute frac- tures are common, however, and the quartz grains show undulatory extinction. Microcline-Quartz-Mica Gneiss In thin section a typical microcline-quartz-mica gneiss specimen, contains about 45 percent microcline, 35 per- cent quartz, 10 percent albite, 5 percent orthoclasi 5 percent biotite and muscovite and a minor amount c] opaque grains. The quartz-microcline mosaic is coiri posed of irregular grains that are as much as 4 mm il long dimension. The albite and orthoclase grains arj much smaller. The microcline is slightly perthitic. It also containl numerous rounded grains of quartz and orthoclasi! Micrographic intergrowths of orthoclase and quartz arl common. Mica shreds are irregular and as much as 3 mi J in length. They commonly occur as aligned residu