u GEOLOGY G*Z- Jlsdc* 7f ?f Geology of the Little Antelope Valley Clay Deposits Mono County, California J . The cover photo: Hot Creek, in southeast part of Little Antelope Valley area. Cover and frontispiece photos by Mary Hill. GEOLOGY OF THE LITTLE ANTELOPE VALLEY CLAY DEPOSITS MONO COUNTY, CALIFORNIA By GEORGE B. CLEVELAND, Geologist California Division of Mines and Geology SPECIAL REPORT 72 CALIFORNIA DIVISION OF MINES AND GEOLOGY FERRY BUILDING, SAN FRANCISCO, 1962 STATE OF CALIFORNIA EDMUND G. BROWN, Governor THE RESOURCES AGENCY WILLIAM E. WARNE, Administrator DEPARTMENT OF CONSERVATION DeWITT NELSON, Director DIVISION OF MINES AND GEOLOGY IAN CAMPBELL, Chief Special Report 72 Price $1.00 CONTENTS Page Abstract 5 Introduction 7 General geology _ 9 Little Antelope Valley area 9 Volcanic rocks 9 Older rhyolite 1 Tuff breccia 1 Younger rhyolite 1 3 Olivine basalt 1 3 Lacustrine rocks 1 3 Older lacustrine deposits 13 Younger lacustrine deposits 15 Surficial deposits 1 5 Age of the rocks 15 Thermal activity 1 5 Clay 18 Occurrence 18 Mineralogy and properties 1 8 Origin 20 Mines and prospects 22 Little Antelope mine 22 Casa Diablo Kaolin deposit 26 Prospects 27 Guides to prospecting 27 References cited 28 (3) ILLUSTRATIONS Page Plate 1. Geologic map and cross sections of the Little Antelope Valley area, Mono County, California In pocket Plate 2. Summary of general geology, Little Antelope Valley area In pocket Frontispiece. Photo of Casa Diablo "geyser" 6 Figure 1. Index map showing location of Little Antelope Valley area 8 Figure 2. Photo of sample of older rhyolite.._ 9 Figure 3. Photo of sample of tuff breccia 10 Figure 4. Photo of erosional remnant of tuff breccia 1 1 Figure 5. Photo of accidental boulder of older rhyolite _ _ 12 Figure 6. Photo of accidental boulder of granitic rock 12 Figure 7. Photo of sample of coarse-grained tuff 13 Figure 8. Photo of sample of tuff showing surface weathering 13 Figure 9. Photo of "geyser" at Casa Diablo Hot Springs 14 Figure 10. Photo of Hot Creek __. 16 Figure 11. Photo of hot springs along Little Hot Creek 17 Figure 12. Photo of partly altered tuff 21 Figure 13. Photo of Huntley Industrial Minerals Inc. mill _ 22 Figure 14. Geologic map of Little Antelope clay mine 23 Figure 15. Photo of Little Antelope clay mine 24 Figure 16. Differential thermal analysis curves, Areas B-F, Little Antelope Valley __ _ 26 (4) ABSTRACT The Little Antelope Valley area, in southwestern Mono County, contains mineable deposits of clay which were unknown until recent years. From 1952 through 1958 about 16,300 tons of clay valued at about 92,000 dollars was mined. The Little Antelope mine, which has been worked almost continuously, has yielded nearly all of the clay mined in the area. The clay is in a terrane of faulted volcanic and lacustrine rocks that range in age from middle Pliocene? to Pleistocene. The volcanic rocks are mainly rhyolite flows and tuff breccia, but they also include minor flows of basalt. Although the lacustrine deposits consist largely of light-colored tuff, fresh-water diatomaceous earth overlies the tuff locally. Both the volcanic and lacustrine rocks have been altered to clay where hydrothermal fluids have risen along normal faults, probably during late Pleistocene time. Because of their high porosity and relatively low chemical stability, the lacustrine rocks contain the largest and purest clay deposits. Along the trace of the faults the hydrothermal fluids formed elongate alteration halos which are in some places several thousand feet in greatest dimension. In the alteration zones kaolinite, opal, quartz, and rarely alunite and cristobalite occur together. Some of the zones are bounded by massive deposits of opal that yield to a peripheral band of partly altered rock rich in iron oxides and gradationally to unaltered country rock. The nature of the thermal fluids, which are presently being emitted in the area, as well as the chemical composition and mineralogy of the alteration products, suggest that the clay was formed by sulfuric acid-rich waters at or near the surface. The commercial clay material is unusually white, burns white and is refractory. It has been used mainly as a filler in paints, plastics, rubber, and paper, and in smaller amounts in ceramics, stucco mixes, and as a whitener. The value of the processed clay averaged about 30 dollars per ton in 1958. (5) Frontispiece. Casa Diablo "geyser" in Casa Diablo Hot Spring basin. Mono County, California. GEOLOGY OF THE LITTLE ANTELOPE VALLEY CLAY DEPOSITS MONO COUNTY, CALIFORNIA By GEORGE B. CLEVELAND, Geologist California Division of Mines and Geology INTRODUCTION The Little Antelope Valley clay deposits, in south- western Mono County, are in an extensive Late Tertiary and Quaternary terrane of cinder cones, acid to basic volcanic flows, and lacustrine deposits. Similar rocks are distributed discontinuously along the Owens Valley trough from Mono Lake southward to Indian Wells Vallev in Inyo County. As the kaolinite deposits in Little Antelope Valley were formed mainly by the percola- tion of hydrothermal solutions into tuffaceous lacustrine rocks, similar conditions elsewhere in the province may have formed additional, but undiscovered, reserves of clay. The existence of commercial bodies of clay in this area was first realized in the early 1930s. Nearly all the clay mined from this area was obtained from the Little Antelope mine which yielded about 16,300 tons of clay valued at about 92,000 dollars between 1952, the first year of production, and 1959. This mine is one of the principal sources of high brightness filler clay in Cali- fornia. The Little Antelope Valley clay deposits lie in a low range of volcanic hills between the Sierra Nevada proper on the west and Long Valley on the east. The mapped area is about 20 miles south of Mono Lake and 35 miles northwest of Bishop, and includes all of T. 3 S., R. 28 E., M.D.M. U. S. Highway 395 cuts diagonally through the southwest part of the area (fig. 1). Most of the area is underlain by rhyolite flows that form a highland of finger-like ridges that rise above the relatively level floor of Long Valley. During Pleistocene time a large fresh-water lake occupied this valley. Sedi- ments deposited in the lake cover the eastern part of the mapped area and overlap onto the eastern and southern margins of the volcanic ridges. Although the hills have a maximum relief of only about 1,500 feet, the surface from which they rise is about 7,000 feet in elevation. Thermal springs and fumaroles, marked by bleached ground, are widespread. Mining activity is restricted, from late fall to late spring, for during that period heavy snows mantle much of the region. Although nearly all of the precipitation falls as snow during the winter months, summer thunderstorms are common. The climate supports a relatively dense conifer forest which is restricted almost exclusively to the rhyolite ridges. Sagebrush, grass, and a few trees grow on the lacustrine rocks. The region is only sparsely inhabitated except near the Mammoth Lakes recreation area; elsewhere cattle ranching is the principal land use. Previous Work. The pioneer work of I. C. Russell (1889), who related the glacial history of the Sierra Nevada to the hydrography of ancient Mono Lake (Lake Russell), provided the foundation for much of the sub- sequent geologic work done in the Mono-Mammoth re- gion. Studies by Woods (1924); Blackwelder (1931); Gilbert (1938, 1941); Mayo (1930,1934a, 1934b); Gale (unpublished); Putnam (1938, 1949, 1950, 1952); and Rinehart and Ross (1956, 1957) have provided data on the regional geology. In the Little Antelope Valley area Chelikowsky (1940) studied the tectonics of the rhyolite and Kesseli (1941) described the morphologic features of the glacial de- posits. The area is included in the Mount Morrison quad- rangle, the geology of which was mapped by Rinehart and Ross (in press, 1960), as part of the cooperative program of the California Division of Mines and the U.S. Geological Survey. Acknowledgments. The writer has benefited from discussion and field direction by Charles W. Chesterman and Lauren A. Wright of the California Division of iYlines and from discussions with C. Dean Rinehart and Donald C. Ross of the U. S. Geological Survey. Pro- fessor Joseph A. Pask of the University of California, Berkeley, made the numerous D.T.A. determinations and interpretations as part of the Division of Mines and Uni- versity of California Cooperative Program. Professor Daniel I. Axelrod of the University of California, Los (7) CALIFORNIA DIVISION OF MINES AND GEOLOGY [Special Report 72 Figure 1. Index map showing location of Little Antelope Valley area. 1962] LITTLE ANTELOPE VALLEY CLAY DEPOSITS 9 Angeles, examined the tree fossils. E .W. Tooker of the U.S. Geological Survey kindly read the manuscript and made many helpful suggestions. GENERAL GEOLOGY Prior to the formation of the clay deposits the Pleis- tocene history of Little Antelope Valley was marked first by the ejection and widespread disposition of the Bishop tuff, and then by the formation of montane glaciers on the eastern slope of the Sierra Nevada. The distribution of the tuff and the accumulation of meltwater from the glaciers led to the formation of a lake in Long Valley. The Bishop tuff crops out several miles north, east, and southeast of the Little Antelope Valley area, but not in the mapped area. According to Gilbert (1938, p. 1860) the tuff is middle Pleistocene in age and prob- ably originated from vents in the vicinity of Long Val- ley. The present Long Valley basin was formed when the tuff blocked a northwest extension of Owens Valley (Rinehart and Ross, 1957). The basin was inundated by glacial meltwater, which formed a lake (ancient Long Valley Lake) that covered an area of at least 80 square miles, probably during the Tahoe glacial stage, which is the next to youngest of the four stages recognized by Blackwelder (1931) in the Sierra Nevada (Mayo, 1934b, p. 96; Rinehart and Ross, 1957). The Bishop tuff is estimated to have a total volume of about 35 cubic miles. Gilbert (1938, p. 1833, 1860) has suggested that the Bishop tuff was derived from a magma chamber beneath Long Valley and that the removal of the magma probably caused large-scale faulting and sub- sidence of the basin floor. Continuing structural adjust- ment along these zones of weakness may have opened fissures in the Little Antelope Valley area through which hydrothermal solutions later reached the lacustrine rocks and altered them to clay. LITTLE ANTELOPE VALLEY AREA The rocks in the Little Antelope Valley area are divisi- ble into two main groups on the basis of lithology: the volcanic rocks, which include older rhyolite, tuff breccia, younger rhyolite, and olivine basalt; and the lacustrine rocks, which comprise older and younger lacustrine deposits. (See pi. 2.) Volcanic Rocks The most abundant volcanic rocks in the area are dense to pumiceous rhyolitic flows, tuff breccia of rhyo- litic composition, and flows of basalt. The rhyolitic rocks are the oldest and most widely distributed. They appear to have originated from vents in or near the area because they lie almost entirely within its boundaries; the pumice- ous flows can be traced to a vent in section 4 (pi. 1). All the rhyolitic rocks are similar in chemical com- position and are probably of nearly the same age. The composition of these rocks is given in table 1; the partial composition shown for tuff, obsidian, and tuff breccia was determined by the refractive index method of George (1924). The basalt flows are present only along the southern margin of the area and are younger than the other vol- canic rocks. Figure 2. Sample of older rhyolite showing flow banding. Photo by Mary Hill. 10 CALIFORNIA DIVISION OF MINES AND GEOLOGY [Special Report 72 Table 1. Composition of rhy otitic rocks in the Little Antelope Valley area. 1 2 3 4 Si0 2 74.2 14.3 1.2 ) 0.26/ 0.16 0.79 3.7 5.2 0.18 0.05 0.02 0.50 0.05 75. 0.2 0.2 1. 4.2 78. 1.5 0.2 0.9 4.4 73. A1 2 3 Fe 2 3 FeO 3. MgO CaO Na 2 K-.0 0.4 1.7 4.1 Ti0 2 P 2 5 MnO Li 2 S0 3 B,0, H,0+ H-0~ C0 2 T.)tal 101. Sample 1. Chemical analysis of older rhyolite from top of high ridge NEX sec. 17. Data provided by U. S. Geological Survey, Petrology and Geochemistry Branch. Analysts: Katrine E. White, Paul L. Elmore, Paul W. Scott. Approximate partial composition of the following samples determined by refractive index method (George, 1924); average percentage error of oxides Si0 2 , 2.0; Fe>0 3 + FeO, 1.4; MgO, 0.52; CaO, 0.62; K>0, 1.1 2. Obsidian, from near main road, NE^ sec. 20. 3. Tuff, from north slope of Little Hot Creek, SWJ< sec. 11. 4. Tuff breccia, from cliff on east side of Little Antelope Valley, center of sec. 15. Older Rhyolite The older rhyolite, a pale red purple (5RP 6/2) 1 to pale reddish brown (10R 5/4) flow rock, forms the core of the low range of hills in the west half of the area, where it crops out over 15 square miles. The rhyolite has an exposed thickness of about 1,000 feet. The vertical 1 All rock color designations from Rock-Color Chart Committee (1948). distance between the lowest and highest exposures is abouc 1,400 feet, but part of this relief is attributable to normal faulting. The rock is characteristically dense, but a thinly banded variety is common (fig. 2). The rock locally is also glassy and consists of pitch-like black obsidian and medium light-gray (N6) to black perlite. Shattered out- crops on the crests of ridges and talus on their slopes composed of this material (pi. 1) appear to represent a rapidly cooled surficial phase of the rhyolite. Tuff Breccia Tuff breccia includes all older pyroclastic rocks, and ranges from lapilli tuff breccia (consisting largely of pumice fragments), to vitric tuff of rhyolitic composition (fig. 3). They are all light colored, and range from white to yellowish gray (5Y 8/1) to dark yellowish orange (10YR 6/6). They are generally porous and therefore correspondingly light in weight, except where they have been permeated by silica-rich thermal solu- tions. The silicified tuff breccia is commonly tinted gray- ish green (5G 5/2). A fresh sample taken on the east slope of Little Antelope Valley contained about 10 to 30 percent void space. This rock is mainly on the ridges in thin dusty patches that resemble clay deposits, but on protected slopes it may crop out as high, tower-like bodies. The exposed thickness of tuff breccia ranges from a few inches to about 30 feet. In the southeast quarter of section 9 the rock is an ash- to lapilli-tuff which has been eroded to unique beehive-shaped out- crops (fig. 4). The tuff breccia overlies the older rhyolite and is overlain by the older lacustrine rocks. Accidental frag- ments of older rhyolite and granitic rocks are present in the tuff breccia in the northeast quarter of section 32 Figure 3. Sample of tuff breccia of rhyolitic composition showing angular frag- ments of pumice in a matrix of lapilli and ash. Photo by Mary Hill. 19621 LITTLE ANTELOPE VALLEY CLAY DEPOSITS 11 Figure 4. Erosional remnant of tuff breccia showing beehive-shaped outcrop. 12 CALIFORNIA DIVISION OF MINES AND GEOLOGY Special Report 72 \ ■ * x ■ mm ^4 ■•*#■> " Si i j >} (.">' Figure 5. Accidental boulder of older rhyolite in tuff breccia. Photo by Charles W. Chesferman. Figure 6. Accidental boulder of granitic rock in tuff breccia. Photo by Charles W. Chesfermon. ££^' f ^ 5^E o o X — \Z ° 18 CALIFORNIA DIVISION OF MINES AND GEOLOGY [Special Report 72 Pool, in the northwest quarter of section 35, is the largest in the area— a shallow, azure-blue pool about 100 feet in diameter. It lies along a northwest-trending fault zone. Within the same zone are several aligned deposits of opaline silica that were deposited by hot springs. The thermal waters commonly contain sulfur and chlo- rine compounds, and emit hydrogen sulfide and carbon dioxide. Partial analyses of water from the Little Ante- lope Valley area are given in table 2. The deposition of quicksilver by these springs was reported by Stearns and others (1935, p. 127), but was not noted by White (1955, p. 126). Table 2. Partial chemibal analyses of waters in the Little Antelope Valley area (in ppm). 1. 2. 3. 4. 5. Si Fe Ca Mg Na K SO, CI B As F 11. 12. 23. 8. 11. 0.45 11. 6. 119. 31. 65. 2.96 56. .04 13. 4.8 69.0 6.3 21.0 37.0 1.84 .10 1.7 7.25 11.9 15.60 Total 65.45 234.96 210.78 19.15 15.60 From Gale (unpublished) 1. Hot Creek at Fish Hatchery 2. Boiling Pot in Hot Creek Gorge 3. Hot Creek at County Road (NEX sec. 19, T. 3 S., R. 29 E., M.D.M. one mile outside the area) 4. Hot Springs on Little Hot Creek 5. Casa Diablo Hot Springs CLAY OCCURRENCE Most of the clay deposits found to date are at and near the Little Antelope mine, in the eastern half of section 15 and in the western half of sections 14 and 23. Near Casa Diablo Hot Springs, in the northern half of section 32, other deposits of clay also have been worked in the past. Of these, the Casa Diablo Kaolin deposit is the largest. Small deposits of clay also are known in the central part of the south half of section 28; the southwest quarter of section 30; the southeast quarter of section 10; and on the north and south slopes of Little Antelope Valley. The known clay deposits have a combined area of expo- sure of about one square mile. The clay deposits are generally in elongate zones of alteration. These zones are aligned along or within a few hundred feet of normal faults; but not all of the normal faults in the area are associated with clay bodies. The individual clay deposits are irregular in shape and range from a few feet to several thousand feet in maximum exposed dimension. The deposits of clay are almost en- tirely hidden by younger rocks. Only where thermal solutions have bleached the younger rocks, or where the overburden has been removed, is there surface evidence of clay. The clay, however, generally lies only a few feet below the surface. Although each of the principal rock types has been locally altered to clay, the largest and most uniform clay bodies are in the tuffaceous beds of the older lacustrine sequence. Where alteration has been most complete, the clay deposits have formed at the expense of two or more host rocks. At the Little Antelope mine, for example, the clay bodies extend uninterrupted from the lacustrine rocks into the older rhyolite below. .Elsewhere most of the larger clay deposits are confined to the more easily altered tuffaceous strata. The clay deposits that are solely in the older rhyolite are narrower and more closely aligned with the traces of the fault zones than are the deposits formed in lacustrine rocks. In general, the altera- tion zones consist of a core of clay material, composed of various proportions of clay and other silicates and sulfates. The core grades outward to silicified country rock or partly altered rock brightly tinted with iron oxides. The deposition of silica by the altering solutions has caused local contamination of the clay. At some places the silicified zones measure several tens of feet in greatest dimension. At still other localities the tuffaceous lacus- trine beds are saturated solely with opaline silica. Cap rock composed of opaline silica has formed above clay deposits at several localities. The largest and best exposed caps are at the Little Antelope mine and along the fault that forms the eastern limit of a shallow graben south of the mine. Similar cap rock above the clay was noted by Kerr and others (1957) in the Marysvale area, Utah. MINERALOGY AND PROPERTIES The principal clay mineral in the Little Antelope Valley deposits has been identified as kaolinite by X-ray (G. A. Uman, personal communication, 1956) and dif- ferential thermal analyses methods (see fig. 16). How- ever, a small pit near Casa Diablo Hot Springs contains both kaolinite and montmorillonite. Thin layers of swell- ing bentonite (principally montmorillonite) are exposed in twb pits east of the Little Antelope mine. The kaolinite is commonly associated with opal, quartz, and hydrated iron oxides, and less commonly with alunite and cristobalite. The presence of poorly crystalline and amorphous material was indicated by D.T.A. analyses of some clay samples. As euhedral crystals of alunite and cristobalite line microscopic voids within kaolinite ag- gregates, they post-date the kaolinite. The position of 1962] LITTLE ANTELOPE VALLEY CLAY DEPOSITS 19 Table 3. Partial chemical analyses of natural clay from the Little Antelope Valley area. 1. 2. (percent) (percent) Si0 2 59.88 53.01 AI2O3 28.67 31.13 Fe 2 3 FeO 0.01 0.48 MgO 0.01 1.37 CaO 0.01 0.11 K 2 0.01 0.06 Na 2 0.01 0.59 Ti0 2 0.20 S0 3 0.0 0.57 Total 88.8 81.52 1. Sample from Little Antelope mine. Analysis by Smith-Emory Com- pany, Los Angeles. 2. Sample from Casa Diablo Kaolin deposit. Analysis by Smith-Emory Company, Los Angeles. opal in the mineral sequence was not determined petro- graphically; however, veins of opal cut pre-existing kaoli- nite at many localities. This relationship is best illustrated at the Little Antelope mine. As the iron minerals, in general, have been deposited at the periphery of the al- teration halo, their relationship to the other minerals is not clear. The clay is generally pale yellowish orange (10YR 8/6) to light red (5R 6/6), less often light gray (N7) to white. The white clay has unusual natural brightness and when milled and classified has a brightness that ranges from 86 to 92.5 (G. E. equiv.) 2 . The clay material ranges 1 Brightness equated to standard scale of meter manufactured by the General Electric Company. from hard and massive to friable or punky. It commonly contains megascopic to microscopic cavities. It has an earthy luster; a specific gravity of about 2.5; a powder pH of about 7.8; a refractive index of about 1.56; and it ranges in hardness from 1 to 4. The chemical composi- tion of the clay is given in table 3. The host rocks in the Little Antelope Valley area generally have yielded similar products when hydrother- mally altered. This is a common condition in environ- ments of intense alteration elsewhere (Schwartz, 1950, p. 201). Although most of the host rocks are rhyolitic and would be expected to form similar alteration prod- ucts, clay material derived from the older rhyolite ap- pears to be of somewhat higher commercial quality than that found in tuff or tuff breccia. It is composed largely of kaolinite, and generally contains little or no alunite, quartz, or opal. One sample however, contains about 20 percent quartz. After firing, the material is generally white and well sintered, and shows little shrinkage (table 4). The clay materials derived from the tuff and tuff breccia were similar, but exhibited commercial properties inferior to those of clay material in older rhyolite host rock. The clay has been used principally as a filler, but it has also been used in ceramics. The purest material ranges in Pyrometric Cone Equivalent from about 30 to 34, has a white to cream fired color, and low shrinkage except where montmorillonite is present; but it generally has poor bonding properties. Table 4. Ceramic properties and mineralogy of clay materials in the Little Antelope Valley area} Mineralogy Clay material Ceramic properties (indicated by D.T.A. 2 ) Alteration Sample Maximum fired Condition of fired area 3 number Host rock temperature (°C) Fired color sample B 1 Tuff 1200 White Slightly sintered Kaolinite; alunite (trace) ; quartz (approx. 5%) 2 Tuff 1275 Cream Very well sintered; shrunk Kaolinite; amorphous material 3 Tuff 1250 White Slightly sintered Amorphous material; kaolinite (trace) C 1 Tuff breccia 1300 Pale Green Melted Poorly crystalline material, quartz (trace) 2 Tuff breccia 1410 Pale Green Melted Poorly crystalline material D 1 Tuff breccia 1300 White Slightly sintered Cristobalite; kaolinite; alunite (trace) 2 Older rhyolite 1350 White Very well sintered Kaolinite 3 Older rhyolite 1310 White Very well sintered Kaolinite E 1 Older rhyolite 1325 White Slightly sintered Kaolinite; quartz (about 5%) 2 Older rhyolite 1350 White Well sintered; slightly shrunk Kaolinite; quartz (approx. 20%) 3 Older rhyolite 1275 Cream Very well sintered; shrunk Kaolinite; montmorillonite 4 Tuff 1275 White Well sintered Kaolinite F 1 Older rhyolite 1330 White Sintered Kaolinite; quartz (trace) 1 Tests performed under the Division of Mines-University of California Cooperative Program, by the Ceramic Laboratories, University of 'California, Berkeley. 2 See D.T.A. curves, fig. 16. 3 Location of alteration area and sample points indicated on plate 1. Data collected from alteration area A are reported in the description of Little Antelope mine (table 9). 20 CALIFORNIA DIVISION OF MINES AND GEOLOGY [Special Report 72 Table 5. Specific gravity of fresh rocks in the Little Antelope Valley area. Rock Specific gravity Apparent Sp. Gr. Porosity (percent) Older rhyolite Tuff Clay (after processing) Older rhyolite (obsidian) Tuff breccia - 2.50 1 2.60 2 2.46 2.25* 2.35 2 2.21 1 1.45 11.6 44.2 1 Data provided by the U. S. Geological Survey, Geochemistry and Petrology Branch. 3 Determined by refractive index method of George 0924). ORIGIN The localization of the clay deposits is believed to have been controlled mainly by faulting, by the physical state of the host rocks, and by the chemical nature of the hydrothermal solutions. The close association of zones of faulting and zones of alteration indicates that faulting was the principal physi- cal control in conveying the altering solutions to the host rocks. The role played by the physical properties of the host rocks is shown by the localization of the larger clay bodies in the tuffaceous lacustrine deposits and the lower susceptibility of the flow rocks to alteration. This is mainly attributable to the chemical instability of glass, which is most abundant in the tuffs. It is also controlled in part by their high porosity, which facilitated the movement of the solutions through the rock. In samples tested (table 5) the porosity of the tuff is about four times greater than that of the older rhyolite. Petro- graphic examination of other tuff samples, however, indi- cates a porosity of about 30 percent. The more extensive alteration in tuff than in less porous rocks also was noted by Kerr and others (1957) in the Marysvale area, Utah. Locally, older rhyolite is selectively altered where flow banding is well developed. The rock tends to be fractured along these planes, so that natural openings are provided for thermal solutions to follow. At the Casa Diablo Kaolin deposit, where banded rhyolite is the host rock, petrographic examination shows that the bands con- sist of alternating layers of fine-grained, dense rock, and thinner layers containing abundant voids. In a zone of incipient alteration the rock consists of alternate lamellae of fresh rhyolite and clay, which grade laterally away from the alteration zone into fresh, banded rhyolite. Hydrothermal alteration in the Little Antelope Valley area has yielded impure clay material. In each of the main areas of alteration the material is characterized mainly by the presence of kaolinite, opal, quartz and alunite (table 4). This association is typical of alteration by sulf uric- acid-rich waters as described by Day and Allen (1925), Allen and Day (1935), Anderson (1935) and White (1957). In this alteration process, H 2 S is oxidized to sulfuric acid near the surface. In the presence of surface water the acid becomes diluted; but if little or no water is available thick, "syrupy films" of concentrated acid form. The products of complete alteration of wall rocks by concentrated sulfuric acid are silica and aluminum sulfate; dilute solutions yield the intermediate products kaolinite and alunite, and free silica. Nature of the Altering Solutions. As kaolinite is now being formed locally in hot-spring waters in the Little Antelope Valley area, the nature of these waters is the Table 6. Properties of the thermal springs. Springs Properties 1 2 3 4 5 Temperature °C (1959) 94.0 90 about 63 about 15 5.2 51.0 46-86 none (?) 6.8 4.4 92.0 92 none 15 2.9 75.0 49-82 none none 7.0 77.0 Waring (1915) Gale (Unpublished).- ... _ .. 83.4 Flow, gal./min. (1959) Estimated to be Waring (1915) greater than 50 pH (1959) 7.0 Mayo (1934a) 1. "Geyser" at Casa Diablo Hot Springs. 2. Casa Diablo Hot Springs (north of the "geyser"). 3. Mud pots at the Casa Diablo Kaolin deposit. 4. Casa Diablo Hot Pool. 5. Springs on Little Hot Creek (NW^ sec. 13). 1962] LITTLE ANTELOPE VALLEY CLAY DEPOSITS 21 best evidence of the character of the hydrothermal solu- tions that might have produced the clay deposits. The hot-spring waters can be tentatively grouped with the acid-sulfate-chloride type according to the classification by White (1957, pp. 1647-1649; 1651-1652). Some, how- ever, are sufficiently poor in chloride to be classed as acid-sulfate waters. Most of the waters are neutral, or only slightly acid, and little or no clay is forming near them (table 6). At the Casa Diablo Kaolin deposit, where small quantities of kaolinite are being formed, the pH is 2.9. Silica, in the form of opal or quartz, is not abundant in alteration zones surrounding the hot springs nor is it being carried in appreciable quantities by the waters themselves. The abundant silica commonly associated with the main clay deposits suggests that the pH of the altering solutions was lower than the average of the present hot-spring waters. As low pH solutions are more potent chemically than high pH solutions in this environ- ment they carried the alteration process further in the main clav deposits where opaline silica is a chief product (fig. 12)'. The chemical composition of clay forming at the Casa Diablo Kaolin deposit is much like that formed by the altering solutions at the Little Antelope mine. A com- parison of two clay samples from these deposits suggests that leaching of iron, magnesium, and alkalies apparently was more complete at the Little Antelope mine (table 4). Boron is common in many of the springs in the Mam- moth region, and, in the Little Antelope Valley area, the concentration reaches a maximum of about 16 ppm in the hot spring waters at Casa Diablo (table 2). Boron was present in concentrations of .01 percent (100 ppm) in three samples from the Little Antelope mine. The con- Figure 12. Partly altered tuff. Massive opal in the upper part of the specimen has been deposited by silica-rich solutions. Photo by Mary Hill. centration of boron in the clay material is relatively high and suggests that the solutions which formed the clay may also have been rich in this element. Alteration by sulfuric acid is carried on above the water table at temperatures of 100°C or less (Schmitt, 1950, p. 211; White, Brannock, and Murata, 1956, p. 54). The temperature of most of the hot spring waters in the Little Antelope Valley area is of this order, although each is below 100°C because of the low atmospheric pressure at the elevation of the springs. Furthermore, the presence of opaline silica in the main clay bodies indicates that temperatures were relatively low during the altera- tion process, and that the site of formation was near the surface. White, Brannock and Murata (1956, p. 57) have shown that at a temperature of 140°C or more, opal is slowly converted to chalcedony. The presence of cristo- balite in the clay might ordinarily indicate a high tem- perature of formation; however, cristobalite is known to form from metastable solutions at relatively low temper- atures in the hot spring environment (White, Brannock, and Murata, 1956, p. 56). Time of Alteration. Although limited alteration is going on at the present time, the principal period of alteration, that which yielded the clay derived from older lacustrine deposits in the vicinity of the Little Antelope mine, ended prior to the deposition of the younger lacustrine deposits in late Pleistocene to Recent time. These deposits overlie the alteration zones, but are not altered themselves. The youngest rocks clearly af- fected by the thermal solutions are the older lacustrine deposits of Tahoe? age. The principal period of intense hydrothermal activity appears to have been during late Tahoe? time. 22 CALIFORNIA DIVISION OF MINES AND GEOLOGY [Special Report 72 !HW *Z JsT-' •*f"r- -v n. mn ^ ■ >»•••.■-* .*■ ,-**-• Figure 13. Mill of Huntley Industrial Minerals Inc. in the southeastern part of the Little Antelope Valley area. Clay material is coarse-ground and calcined here prior to further treatment at the company's plant near Bishop. MINES AND PROSPECTS The earliest commercial development of the Little Antelope Valley clay deposits was by the California Kaolin Company in the late 1930s. Their activities cen- tered around Casa Diablo Hot Springs. This company began developing the Casa Diablo Kaolin deposit about 1937. A small quantity of clay was sold in 1940, but the property was abandoned in 1941. In the late 1940s, clay was discovered about 4 miles northeast of Casa Diablo Hot Springs at the present site of the Little Antelope mine. Five association placer claims, the Little Antelope numbers 1, 3, 4, 5, and 6, were filed by W. M. Bathrick and others. The claims were subsequently leased by Huntley Industrial Minerals Inc. and commercial mining of clay began in the early 1950s at the Little Antelope mine. The Huntley organization enlarged their holdings to 46 placer claims in 1954 and completed a drilling pro- gram with the Georgia Kaolin Company to determine the extent of the Little Antelope deposit and to find additional clay bodies nearby. Numerous prospect pits were dug throughout the property and some new reserves were found. Exploratory work was also done in the Casa Diablo Hot Springs area, but water-saturated ground forced the company to abandon the area (W. H. Hunt- ley, personal communication, 1955). In 1957 a mill was built in section 35 south of the Little Antelope mine to handle coarse milling and calcining, formerly done at the company's plant at Laws, north of Bishop (fig. 13). LITTLE ANTELOPE MINE The Little Antelope mine, in the eastern part of the mapped area, is the largest mine developed, and the only one that has been in sustained operation. It lies on the west slope of a gently east-tilted fault block. The core of the block is composed largely of older rhyolite partly capped by a thin veneer of older lacustrine deposits. The bounding fault trends northeast and dips steeply toward the west. Vertical displacement on the fault has amounted to about 120 feet since deposition of the older lacustrine beds. The mine area is cut by a parallel fault of smaller displacement a few hundred feet east of the Little Ante- lope pit. Both of these fault zones acted as conduits through which thermal solutions flowed into the host rocks. The solutions carried by the main fault appear to have altered the host rocks more thoroughly than did those following the other fault. The clay zone is exposed along the east side of the main fault for about 2,000 feet northward from the Little Antelope pit, hut is buried by loose ash south of the pit. West of the fault, the clay zone is buried below younger lacustrine deposits. East of the pit the alteration zone is about 700 feet wide and extends for about a mile to the center of section 14. The locus of intense alteration and the place where the best quality clay has been found is along the southern trace of the main fault at the Little Antelope pit. Toward the north, alteration becomes grad- uallv less intense, and near Little Hot Creek a zone of 1962] LITTLE ANTELOPE VALLEY CLAY DEPOSITS 23 ~ o CO Q CO V . * .* ,~ ~ ~ '.<*yr- >* O o^ E >* o .V" Q o * J 4) ~ -C C O w » - ~ o -T3 « ,_ O => 0) <-> TjTJ «> — C -O O O O O Z s ~ elow oca lit tr 6 E o 3 u u o CO c CO -O — 24 CALIFORNIA DIVISION OF MINES AND GEOLOGY [Special Report 72 1, .;»-» ; ,<4 *X .v