UTLER ESTATE CHROMITE MINE California Division of Mines and Geol Special IZefwtt 7? Cover photo: View southeast toward Butler Estate chromite mine. The pit is 850 feet long, 550 feet wide, and 250 feet deep. GEOLOGY OF THE BUTLER ESTATE CHROMITE MINE SOUTHWESTERN FRESNO COUNTY, CALIFORNIA By ROBERT A. MATTHEWS Mining Geologist California Division of Mines and Geology ecial Report 71 DIVISION OF MINES AND GEOLOGY FERRY BUILDING, SAN FRANCISCO, 1961 STATE OF CALIFORNIA Edmund G. Brown, Governor THE RESOURCES AGENCY William E. Warne, Administrator DEPARTMENT OF CONSERVATION DeWift Nelson, Director DIVISION OF MINES AND GEOLOGY Ian Campbell, State Geologist SPECIAL REPORT 71 Price $1 CONTENTS Page Abstract 5 Introduction 7 History and production . 8 General geology . 9 Rock units _._ 1 1 Rocks of the Franciscan formation 13 Rocks of the Panoche formation 13 Quaternary landslides — . 1 3 Structure , 13 Economic geology . . 1 5 Chromite . 1 5 Asbestos material 17 References _— — 19 ILLUSTRATIONS Plate 1. Geologic map of Butler Estate chromite mine, and vertical cross section through the pit In pocket Figure 1. Index map showing location of Butler Estate mine.. 7 Photo 1. View of Butler Estate chromite mine in 1954 7 Photo 2. View of Holman mill 8 Photo 3. View of Chambers and Russell mill 9 Photo 4. Distant view of Butler Estate chromite mine, 1958 _ 10 Photo 5. "Bouldery" serpentine in west wall of mine 11 Photo 6. Fault contact between serpentine and shale in Corbett-Byles pit . . . 12 Photo 7. Fault contact between serpentine and shale in Butler Estate pit 14 Photo 8. Ore body exposed in Butler Estate pit 15 Photo 9. Platy and flaky "mountain leather" material 16 Photo 10. Close-up of altered serpentine "boulder" in the Corbett-Byles pit 18 Table 1 . Production of chromite in Fresno County 1 1 Table 2. Chemical analysis of "mountain leather" material 19 (3) Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/geologyofbutlere71matt ABSTRACT The Butler Estate chromite mine is in the New Idria quicksilver district in southwestern Fresno County, some 30 miles northwest of Coalinga. It is situated on the southeast flank of a large elliptical serpentine mass that has intruded sedimentary rocks of the Franciscan formation and Panoche formation. Along the northeastern and western flanks, the serpentine is bounded by thrust faults, but elsewhere the contacts are steeply dipping normal faults. The Butler Estate mine, an open pit located in serpentine adjacent to the Panoche- serpentine contact, has been the largest producer of chromite in Fresno County and was one of the largest producers in the state. The ore mined consisted of high-grade disseminated chromite and minor lenses or pods of massive chromite. At the cessation of mining in 1958, disseminated ore that averaged 25 percent chromite was left exposed in the pit. No exploration data are available to indicate the extent of the ore body below this exposure. The disseminated ore yielded concentrates with an average Cr 2 3 content of 46 percent and chromium-to-iron ratio of 2.8:1. (5) GEOLOGY OF THE BUTLER ESTATE CHROMITE MINE SOUTHWESTERN FRESNO COUNTY, CALIFORNIA By ROBERT A. MATTHEWS INTRODUCTION The Butler Estate chromite mine, formerly known as the Mistake mine and the James-Thickstun mine, is about 30 miles northwest of Coalinga, California, and about 10 miles southeast of the New Idria quicksilver mine. It is accessible from Coalinga by the Hernandez Road, a black-topped road, and about 7V 2 miles of rough but passable unsurfaced road. The mine consists of a large open pit located in section 34, T. 18 S., R. 13 E., and section 2, T. 19 S., R. 13 E., at an elevation of about 3,800 feet near the crest of the Diablo Range. The climate of the area, typical of the Diablo Range, is hot and dry in the summer and cold and wet in the winter. Low- growing shrubs, scrub oaks, and a few scattered pine trees are the principal vegetation of the area. Figure 1 (below), chromite mine. Index map showing location of the Butler Estate • ' Photo 1. Butler Estate mine in 1954 (view southeast). An ore body (black) is just above and to the right of the power shovel. An exposed ore body is also visible on the same level as the shovel. Photo by S. J. Rice. The Butler Estate mine is the largest of the more re- cently developed chromite mines in California. Three gravity concentrating mills which processed the dissem- inated ore are located along White Creek and the San Benito River. The Holman and the Chambers and Russell mills are located on White Creek about 3 miles southwest and 5 miles southwest of the mine, respectively. The James mill, which is about 10^; miles west of the mine, processed several tons of chromite ore for Holman. Chambers and Russell, prior to 1955, contract-mined and (7) 8 California Division of Mines and Geology [Special Report milled ore for Holman. After 1955 Chambers and Russell mined and milled all of the ore from the Butler Estate mine. Concentrates, fines, and lump ore from these mills were shipped to General Services Administration Chro- mite Purchase Depot at Grants Pass, Oregon. Field work for this report was begun in December 1958, after mining operations had ceased. The open pit area was mapped by plane table method at the scale of 40 feet to the inch. J. L. Burnett of the Division of Mines assisted in the preparation of the plane table map. The writer wishes to acknowledge cooperation of A. Thickstun, original operator, J. Holman, who de- veloped the mine, E. F. Chambers, last operator of the mine, and E. Cullity, consulting mining engineer. C. W. Chesterman and S. J. Rice of the Division of Mines offered many valuable suggestions both during field work and in the preparation of this report. HISTORY AND PRODUCTION As early as 1853, chromite was reported in the New Idria district of Fresno County, and prior to World War I chromite was reported from the area of the Butler Estate mine. During World War I, L. Rhodes of Coa- linga shipped several tons of high-grade float collected in the general vicinity of the present Butler Estate mine. In 1930 about 2 tons of chromite float averaging 55 percent Cr 2 3 , were shipped from this same general area to a gear-manufacturing company in Los Angeles. World War II passed with little mining activity in this area. Late in 1951, stimulated by the recently inaugurated domestic chromite purchasing program of the General Services Administration, J. James and A. Thickstun, upon the advice of L. Rhodes, began prospecting the area where the Butler Estate property is located and opened a prospect about 200 feet long, 40 feet wide, and 35 feet Photo 2 (below). View south toward the Holman mill on White Creek. Photo by S. J. Rice. - - ~v>4 .„*» W :' i. '■' * 71—1962 Butlf.r Estate Chromite Mine Photo 3. Chambers and Russell mill on White Creek. Stock pile of chromite concentrates in foreground. deep. The development exposed some good chromite. In 1952, James and Thickstun shipped 25 long tons of lump ore to the Government Purchase Depot at Grants Pass, Oregon, and stockpiled the disseminated ore. Later they leased the prospect to J. Holman, who began strip- ping operations that uncovered additional stringers of massive and disseminated ore. Subsequent stripping re- vealed a large deposit of disseminated ore. As a result, Holman constructed a mill on White Creek in sections 4 and 9, T. 19 S., R. 13 E. to process this ore. Chambers and Russell (Southwest Oil Company) proc- essed a portion of Holman's disseminated ore at their mill located on White Creek. During the later days of Hol- man's operation of the mine, some of the disseminated ore was processed at the James mill located along the San Benito River in section 21, T. 18 S., R. 12 E. In 1955 the mine property went into litigation. Cham- bers and Russell began operating the mine, and changed its name from the "Mistake mine"— as it had been named by Holman in 1952— to the Butler Estate mine. The mine and the Chambers and Russell mill were operated up to the closing of the Government chromite purchasing pro- gram in June 1958. By that time the mine pit had ex- panded from the original prospect to a large open pit 850 feet long, 550 feet wide, and 250 feet deep. All ore produced, including fines and lump ore, from 1952 to 1958, was trucked to Coalinga and transferred to rail cars for shipment to the Government Purchase Depot. The total production of concentrates, fines, and lump ore from this mine was more than 23,000 long tons, worth more than 2 million dollars. Of this, fines and lump ore totalled only about 1,000 long tons (see table 1). During the General Services Administration Chrome Purchase Program, when the Butler Estate mine was operating, lump ore containing 48 percent Cr 2 3 with 3:1 chromium to iron ratio was sold to General Services Administration for more than $115 per long ton. Several hundred tons of this premium ore was sold from the Butler Estate mine. In June, 1959 price quotations from E. & M. J. Metal and Mineral Market ranged from $35 to $36 per long ton for Rhodesian metallurgical grade chro- mite ore containing 48 percent Cr 2 3 , with 3: 1 chromium to iron ratio. GENERAL GEOLOGY The Butler Estate mine is on the southeast border of a large northwest-trending mass of serpentine which crops out along the crest of the southern Diablo Range. Eckel and Myers (1946, pp. 87-91), Anderson and Pack (1915, pp. 36-46), Mielenz (1939), and Phillips (1939) have described the geology of the area. 10 California Division of Mines and Geology [Special Report '5. o *8 - o 5 S a O O . I I 8 o — I? 71—1962] Butler Estate Chromite Mine Table 1. Production of chromite, 1 Fresno County, California. 11 Yea/- Chromite (long tons) Cr 2 3 (percent) Cr/Fe ratio Remarks Pre 1915 4,685 3 30-45 2.0-2.5:1 Major production from eastern Fresno County. 1915-18 13,018 2 - 3 30-45 2.0-2.5:1 Major production from eastern Fresno County. 1919-45 2,350 2 30-45 2.0-2.5:1 Major production from eastern Fresno County. 1946-51 No recorded production 1952 25* Lump ore from southwest Fresno County. 1953-58 23,705* Av. 46.3 Av. 2.82:1 Production principally from the Butler Estate (Mistake mine). Total production 43,783 long tons (approx.) Av. Cr 2 3 42 (approx.) Av. Cr/Fe 2.6: 1 (approx.) 1 Chromite includes — lump ore, fines, and concentrates. 2 Production statistics taken from Table 1, Chromite production from Fresno County 1915, in Calif. Div. Mines Bull. 134, Pt. Ill, Chap. 3, pp. 68-69 (1945) (Note: Production figures prior to 1941 from U.S. Bureau of Mines and Calif. Div. Mines approximate.) No accurate statistical figures prior to 1919 are available. 3 Includes small tonnages from San Benito County (Calif. Div. Mines Bull. 134, Pt. II, Chap. 2). 4 Unpublished U.S. Bureau of Mines statistics. The serpentine mass, elliptical in shape, is approxi- mately 12 miles long and 4 miles wide. It is flanked by rocks of the Franciscan formation, of Late Jurassic to Early Cretaceous age, and sedimentary rocks of the Panoche formation of Late Cretaceous age. Several small intrusive masses of soda syenite are exposed in the ser- pentine, but none is in the immediate area of the chromite mine. Much of the bedrock geology is obscured by both old and recent landslides and slumped ground. According to Eckel and Myers (1946, pp. 91-96), the general structure suggests that the serpentine occupies the crest of an asymmetrical anticline. On the northeast and western boundaries of the mass, serpentine is in thrust contact with Franciscan and Panoche rocks; ser- pentine and Franciscan rocks having been thrust over the overturned Panoche strata. All other contacts of the ser- pentine mass with the enclosing rocks are also faults, but along these contacts the faults are steeply dipping and the sedimentary rocks apparently all dip away from the cen- tral part of the serpentine mass. The general trend of the serpentine mass and enclosing Franciscan, Panoche, and Tertiary rocks is northwest and parallels the general trend of the Diablo Range. Serpen- tine in the chromite mine area, as well as in many other areas of the mass, is highly sheared and deuterically altered. Greater than normal concentrations of chryso- tile, talc, magnesite, and hydromagnesite, as well as the high degree of shearing, impart a light color and a sheet- like appearance to much of the rock. Miners and local residents have used the term "mountain leather" to de- scribe this highly sheared and altered serpentine. ROCK UNITS Serpentine The rock referred to in this report as serpentine was derived by the alteration of periodotite and pyroxenite. These original intrusive rocks were almost completely serpentinized, and most of the serpentine has in turn been highly sheared and altered to a flaky and platy material rich in particles of matted chrysotile, antigorite, magnesite, and talc. Hydromagnesite is locally abundant near the surface as fissure fillings and replacement nodules in the altered serpentine. Photo 5. Boulder-like masses of serpentine in highly sheared "mountain leather" serpentine, in the west wall of the Butler Estate chromite mine. This zone of rock is bounded by two apparently parallel vertical faults (outlined in black). '*!- <>».* ;-# v; **£^$*; 9 & %: v ^ ' / / **S*' ^ 12 California Division of Mines and Geology [Special Report * VJT £* J i '#"- i * , « 71—1962] Butler Estate Chromite Mine 13 The serpentine just northwest of the mapped area is massive, much of the peridotite and pyroxenite texture being preserved. It is blocky, with a rough, weathered surface very typical of Coast Ranges serpentine. The principal rock type in the mine area as illustrated by the geologic map of the mine (pi. 1) is highly sheared and altered serpentine. This flaky material, which is almost white, is sufficiently friable to be crushed easily between the fingers. In most exposures the flaky material encloses large and small boulder-like masses of unsheared serpentine (photo 5). The sheared and altered material is well exposed in the west wall of the mine pit and in bulldozer prospect pits along the road to the New Idria mine. The degree of shearing in the serpentine along the faulted contact zone is not much different than else- where within the highly sheared portions of the main mass. An apple-green serpentinized dunite encloses the chro- mite exposed in the pit area. The dunite is largely mas- sive, as compared to the sheared serpentine rock exposed in most of the pit: Black grains of magnetite and chro- mite are sparsely disseminated through the dunite, and thin veinlets of chrysotile and minor amounts of talc and magnesite are also present. This enclosing serpentinized dunite shows little evidence of shearing in comparison to the main mass of serpentine in the pit. Rocks of the Franciscan Formation No Franciscan rocks were mapped in the immediate area covered by plane-table mapping. However, a few yards north of the mapped area, a small narrow mass of Franciscan graywacke and chert about 100 feet long crops out along the access road to the pit entrance. This small mass is in fault contact with Panoche shale to the east, covered by landslide material to the north, and in fault contact with serpentine to the south and west. Angular grains of plagioclase and quartz, and shale fragments are the principal constituents of the gray- wacke, and it is cut by veinlets of white calcite and quartz. The graywacke is dark brown on weathered surfaces and from a distance it is distinguished with difficulty from younger, sandy shale of the Panoche formation. However, the freshly broken surfaces of graywacke are gray-green and the rock is texturally similar to Francis- can graywackes found throughout the Coast Ranges. The chert is interbedded with the graywacke. This thinly bedded chert is also buff -colored on weathered surfaces but light-colored to colorless on freshly broken surfaces. Rocks of the Panoche Formation The Panoche formation, of Late Cretaceous age, sur- rounds or flanks the elongate serpentine mass and the Franciscan rocks. In the mine area, this formation con- sists of dark, thin-bedded clay shale, silty and sandy shale, and thin-bedded sandstone of marine origin. The layers of soft, thin-bedded clay-shale range in thickness from about 1 inch to about 4 or 5 inches and are com- posed of finely divided fragments of quartz, feldspar, biotite, chlorite, and clayey material. The silty and sandy shale is concretionary and the beds range in thickness from a few inches to 18 inches or more. The mineralogi- cal composition of the silty and sandy shale is essentially the same as the thin-bedded clay-shale. Weathered sur- faces of the shale are gray-brown, while freshly broken surfaces are greenish-gray. Along the fault contact between the Panoche forma- tion and serpentine is a zone of alteration a few feet wide. Here the normally soft shale is well indurated and car- bonatized, and ranges in color from very light brown through green to black. Chalcedony, calcite, dolomite, and natrolite occur as veinlets in the shale. These very light-colored minerals stand out in sharp contrast to the dark shale. A specimen of shale found in the mine pit below the shale-serpentine contact contains well-formed crystals of natrolite, several of which measure over a half inch in length. Quaternary Landslides Extensive landslides and slope wash material occur throughout the general serpentine area. The area north of the mine is overlain by large slides of loose, broken serpentine and shale. Most of the large slides are found along the southern boundary of the serpentine mass, and one of the largest occurs just southwest of the Butler Estate mine. The unstable conditions of these numerous slide areas will certainly be of concern during the con- struction of roads and in mining operations. Maintenance of roadways will be of greater than normal expense. STRUCTURE The regional structure has been described by previous writers (Eckel and Myers, 1946, p. 91) as consisting of an elongate, asymmetric, dome-like mass of serpentine in fault contact with steep-dipping sedimentary beds, some of which have been overthrust and others over- turned. These sedimentary beds flank the dome-like mass which is an extension of the Coalinga anticline to the southeast. The anticlinal structure is probably closely re- lated to the ultramafic intrusion which forms the pierce- ment core of the dome. The Vallecitos syncline to the north and the White Creek syncline to the south parallel the anticline. The Butler Estate mine is on the southeastern edge of the serpentine mass near its contact with the Panoche shale. The fault contact between serpentine and shale dips steeply to the south and is well exposed in the pit area and in the Corbett-Byles pit a quarter of a mile to the west. Except for a narrow zone of massive rock 50 to 65 feet wide on the west wall of the Butler Estate pit (pi. 1) and another area north of the mine, along the 14 California Division of Mines and Geology [Special Report V • 71—1962] Butler Estate Chromite Mine 15 ERP- >f JmB ^ ,ORE turn "*$ "*->*,,■ ■*>, •». - .^. . - SERP. c .„. Photo 8. View south toward ore body exposed in the Butler Estate pit. The man in the left center of the photo shows the approximate scale. ECONOMIC GEOLOGY road to the New Idria mine, the serpentine is highly sheared and altered. Two apparently parallel faults form the boundaries of this less-sheared and altered serpentine zone exposed in the wall of the pit west of the ore body (photo 5). Field mapping did not determine any pattern of folia- tion or shearing in the serpentine. The general trend of shearing along the borders of the mass appears to parallel the serpentine contact. Good exposures of the fault contact between the Pa- noche formation and serpentine are shown in photos 6 and 7. This fault contact is exposed in the Corbett-Byles pit. The contact shown on both photographs strikes approximately east-west, and has a very steep dip to the south, which is normal in the mine area. Chromite The only ore mineral recovered in the Butler Estate chromite deposit was chromite. Other chromium min- erals occurring with chromite are uvarovite, a green chromium garnet, and kammererite, a peach-blossom red chromium chlorite. These minerals are found as thin in- crustations along shear zones in the ore body and are of no commercial importance. The Ore Body. The shape of the ore body as de- scribed by past operators of the mine would roughly resemble a pipe-like body of coarse-grained disseminated chromite with fingers or lenses of high grade massive chromite and disseminated chromite protruding from the main body. The attitude of the ore body as reported by 16 California Division of Mines and Geology [Special Report Photo 9. A large platy mass and a pile of ''mountain leather" asbestos material from the Butler Estate pit. Length of the platy mass is about 14 inches. Photo by J. Alfon. 71—1962] Butler Estate Chromite Mine 17 past operators was quite constant from the upper limits of mining of scattered lenses or pods at the surface down to the floor of the pit some 225 to 250 feet lower. The plunge or dip is to the southeast at a steep angle. Dimen- sions of the ore body exposed in the bottom of the pit are approximately 135 feet by 65 feet. The total depth of the ore body has not been determined, but it has been exposed to a depth of about 250 feet. Subhedral to anhedral crystals of chromite disseminated in serpentinized dunite and occasional high-grade pods and lenses of massive chromite constitute the ore body in the Butler Estate pit. The massive ore contains only minor amounts of gangue minerals. Highly sheared antig- orite, chrysotile, talc, and magnesite occur along shear planes, and drusy calcite, magnesite, kammererite, and uvarovite form thin coatings along fractures in the ore body. Microscopic examination of a sample taken from the ore body, reveals that chrysotile forms rims around chromite grains and occurs in veinlets that transgress chromite grains as well as the main direction of shearing in the ore body. Iron-oxide-stained magnesite replaces portions of these chrysotile veinlets and much of the chrysotile matrix. The concentration of chromite appears to be relatively uniform throughout the ore body except for the few minor high-grade lenses or pods of massive ore. In the disseminated ore, the chromite grains are not concen- trated in high-grade clots or bands, but are rather evenly dispersed through the serpentinized dunite. The highly sheared character of the ore body makes it difficult to determine megascopically the mineralogy of the gangue minerals; however, with the aid of the petrographic microscope, the various minerals can be differentiated. Along fractures in the ore body, chryso- tile occurs also as flaky material and as thick sheets of felted fibers together with other gangue minerals. The sheets of felted fibers are tough and pliable, and range in size from small flakes to large sheets, one of which measured 14 inches by 10 inch by % inch thick (photo 9). Character of Ore. The principal ore in the Butler Estate chromite mine is a disseminated type containing a high proportion of chromite. The average grain size of the chromite is about 5 millimeters. Along shear planes in the ore body, chromite grains have been pulverized to a fine chocolate-brown powder. The interstitial ser- pentine as well as chromite grains have been highly sheared and granulated and this condition accounts for the friable character of the ore. This characteristic of the ore makes it amenable to mining by power shovels or other low-cost mechanical digging methods that re- quire little or no blasting. The flaky gangue material presented serious problems in milling the ore. Mats of felted chrysotile, antigorite, talc, and magnesite had tended to plug mill discharge screens, lower the recovery of chromite, and adversely affected clean separation of the chromite from gangue. According to j. Holman, whose mill processed a great portion of the concentrates produced from this mine, it was possible to overcome these problems by the installation of a 1-inch pre-crusher screening system which permitted the fines to be proc- essed separately. The installation not only eliminated most of the problems mentioned above, but also increased the capacity of the mill. No chemical analyses of mill feed were available to the writer other than estimates collected by the Bureau of Mines from operators of the mine. The average chro- mite content of mill feed as computed from the United States Bureau of Mines statistics is about 25 percent. Fines and lump ore shipped to General Services Admin- istration Stockpile, Grants Pass, Oregon, averaged 46.6 percent Cr 2 3 with a chrome-to-iron ratio of 3:1. The concentrates averaged 46 percent Cr 2 3 with a chrome- to-iron ratio of 2.8:1. Reserves. Since 1952 approximately 63,000 long tons of chromite ore have been mined from this deposit. Chromite-bearing reserves, computed from the ore body dimensions noted above, total 8,775 cubic feet per foot of depth. Unbroken ore of this grade approximates 10 cubic feet or less per ton which amounts to about 900 tons per foot of depth of the ore body. Thus, projecting the ore body along the known dimensions, roughly 900 tons per foot of depth of chromite ore averaging about 25 percent chromite may be predicted. Drilling of the ore body would aid in determining the presence or absence of additional ore. Asbestos Material The highly sheared serpentinized mass comprising the bulk of the rock exposed in the Butler Estate pit con- tains abundant felted sheets resembling mountain leather, a mixture of antigorite, chrysotile, and talc. The chryso- tile occurs primarily as slip-fiber, felted with varying proportions of talc and antigorite. These tough flakes and sheets, that make up the bulk of the serpentine in places are matted along closely braided shear planes and have the characteristic "soapy" feel of talc. This matted ma- terial is flexible, and can be separated with the fingers to reveal the fibrous character of chrysotile. However, indi- vidual fibers are very difficult to separate, contrary to chrysotile with the more common cross-fiber habit. The highly sheared fiber-bearing rock sloughs easily and forms talus slopes. The very light color of such slopes of asbestos-bearing material stands out and from a dis- tance resembles the large dumps seen around chromite mines. The dry surface of these slopes is white, but below the surface a slightly darker greenish color is revealed. V^A&^'jdC 71—1962] Butler Estate Chromite Mine 19 Even when in place, this material is so highly sheared and loose that mining could be accomplished by power shovels or other mechanical digging equipment, without blasting. The rock having the greatest number of shear planes apparently contains the highest fiber content. As a possible aid to prospecting for asbestos along this large serpentine mass, those areas having blocky or massive, fresh, bouldery serpentine would be less favorable areas of prospecting than those areas where stionger shearing and alteration of serpentine is evident. Table 2. Chemical analysis of ground sample of '''mountain leather" material.* Percent Silica 36.21 Alumina 0.38 Ferric oxide - _... _ ._ .... 3.42 OilcMim oxide .. 0.21 Magnesium oxide Ignition loss ..... 44.50 _. 15.07 99.79 Fiber content of coarse rock sample is 59.80%. Note: The high ignition loss is due to combined water plus the carbon diox- ide present in the magnesite. * Sample of "mountain leather" asbestos from section 30, northwest of the Butler Estate Pit. Analysis by Abbott A. Hanks, Inc. (Courtesy of E. Cullity). The "boulder" material enclosed in the flaky fiber-rich rock, discussed earlier in the section on serpentine rocks of this report, is generally much darker-colored massive serpentine, and contains little or no fibrous material. These "boulders" range from 5 or 6 inches to over 15 feet in diameter. Hydromagnesite is a common and locally abundant secondary mineral in this material. It is a white mineral of mineralogical and potential industrial interest, and occurs abundantly as hard concretionary masses and as powdery veinlets, particularly near the surface. Magne- tite and chromite are present in small amounts as minute black grains in the matted material. Hydrous iron oxides form yellow-brown stains in some places, principally near the surface. It is difficult to estimate reserves of this asbestos-bearing material in the Butler Estate pit area. During chromite mining operations, much of this high-fiber material was disposed of in dumps which covered undisturbed asbes- tos-bearing material as well as other rock types. Exposures left in the walls of the Butler Estate pit show approxi- mately 500 feet of length, 450 feet of width, and 250 feet of depth of asbestos-bearing material. The fiber content and grade would have to be determined by experimenta- tion in mill-run lots. A laboratory analysis of a bulk sample of material very similar in character to fibrous material found in the pit is given in table 2. The chryso- tile fiber content of this particular sample ran 59.8 per- cent. Exposures in the Butler Estate pit and in other numer- ous prospect pits along this approximately 12- by 4-mile ultramafic mass indicate a huge supply of asbestos-bearing material which can be measured in the magnitude of square miles. Assuming an average depth of 100 feet of asbestos material in this ultramafic mass, approximately 110 million tons of ore would be available for open pit mining. As of January 1960, extensive prospecting and laboratory work was being done by several companies toward developing the asbestos potential of the area. The Mike-Ron Corporation has constructed a plant in Coa- linga, California, to process the asbestos material.* REFERENCES Anderson, R., and Pack, R. W., 1915, Geology and oil resources of the west border of the San Joaquin Valley north of Coalinga, California: U. S. Geol. Survey Bull. 603, 220 pp. Eckel, E. B., and Myers, W. B., 1946, Quicksilver deposits of the New Idria district, San Benito and Fresno Counties, California: California Jour. Mines and Geology, vol. 42, no. 2, pp. 81-124. Mielenz, R. C, 1939, The geology of the southwest part of San Benito County, California: Univ. Calif, unpublished thesis, 295 pp. Phillips, R. M., 1939, The general geology of a part of the Priest Valley quadrangle, California: Univ. Calif, unpublished thesis, 76 pp. Some of the information in this report has also been drawn from unpublished records at the California Division of Mines and Geology and U.S. Bureau of Mines. * As of July 1961, Johns-Manville and Kern County Land Com- pany have awarded a contract to construct an asbestos mill near Coalinga, California. The expected completion date is October 1961 with operations beginning in January 1962. The expected capacity is 12,500 to 25,000 tons of fiber per year. This fiber will be utilized primarily in the manufacturing of asphalt-asbestos and vinyl-asbestos floor tiles. The proposed mill will also include a laboratory with equipment for precise quality control methods. CHROMITE BULLETIN AVAILABLE Deposits of chromite in California are described in California Division of Mines Bulletin 134, Geological Investigations of Chromite in California, prepared cooperatively by the Division and the U.S. Geological Survey, and published under the direction of Olaf P. Jenkins. Most of the chapters listed below can still (as of June 1962) be obtained from the California Division of Mines and Geology, Ferry Building, San Francisco 11, California. PART I— KLAMATH MOUNTAINS Chapter 1, Chromite deposits of Del Norte County, California, by Francis G. Wells, Fred W. Cater, Jr., and Gam A. Rynearson, pp. 1-76, pis. 1-11, 9 figs., 1946. Includes a topographic map of Del Norte County, scale 1" = 2 mi., showing distribution of peridotite and location of chromite deposits. Price 75?. Chapter 2, Chromite deposits of Siskiyou County, California, by Francis G. Wells and Fred W. Cater, Jr., pp. 77-127, pis. 12-18, 4 figs., 1950. Includes topographic map of Siskiyou County west of meridian 122° 10', scale 1" = 2 mi., showing distribution of known peridotite and location of chromite occurrences. Price $1.00. PART II— COAST RANGES Chapter 1, Chromite deposits of the northern Coast Ranges of California, by D. H. Dow and T. P. Thayer, pp. 1-38, 2 figs., 1946. Price 250. Chapter 2, Chromite deposits of the southern Coast Ranges of California, by George W. Walker and Allan B. Griggs, pp. 39-88, 8 pis., 5 figs., 1953. Price $1.25. PART III— SIERRA NEVADA Chapter 1, Chromite deposits of Tuolumne and Mariposa Counties, California, by Fred W. Cater, Jr., pp. 1-32, pis. 1-4, 2 figs., 1948. Price 350; Chapter 2, Chromite deposits of Calaveras and Amador Counties, California, by Fred W. Cater, Jr., pp. 33-60, pi. 5, fig. 1, 1948. Price 350. Chapter 3, Chromite deposits of Tulare and eastern Fresno Counties, California, by Garn A. Rynearson, pp. 61-104, pis. 6-7, 7 figs., 1948. Price 500. Chapter 4, Chromite deposits of El Dorado County, California, by Fred W. Cater, Jr., Garn A. Rynearson, and Donald H. Dow, pp. 105-167, pis. 8-15, 6 figs., 1951. Price 900. Chapter 5, Chromite deposits in the northern Sierra Nevada, California (Placer, Nevada, Sierra, Yuba, Butte, and Plumas Counties), by Garn A. Rynearson, pp. 169-323, pis. 12-15, 10 figs., 1953. Price $2.00. A61696 4-62 3,500 lltlnltd 111 CALIFOtNIA 1TATI PUNTING OFP1CI Pa'm I 5'.° N ° F MINES flND GEOLOGl IAN CAMPBELL, STATE GEOLOGIST STATE OF CALIFORNIA THE RESOURCES AGENCY DEPARTMENT OF CONSERVATION SPECIAL REPORT 7 I PLATF I Fault showing dip (Dashed where approximate) Slnke and dip of beds 60 120 CONTOUR INTERVAL 50 FEET DATUM ASSUMED GEOLOGIC MAP AND SECTION OF THE BUTLER ESTATE MINE FRESNO COUNTY, CALIFORNIA