Keystone Mine

Site Name

Primary: Keystone Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Amador
District: Amador City

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Land Status

Land ownership: Private
Note: the land ownership field only identifies whether the area the mine is in is generally on public lands like Forest Service or BLM land, or if it is in an area that is generally private property. It does not definitively identify property status, nor does it indicate claim status or whether an area is open to prospecting. Always respect private property.
Administrative Organization: Amador County Planning dept.

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Holdings

Not available

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Workings

Not available

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Ownership

Not available

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Production

Not available

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Deposit

Record Type: Site
Operation Category: Past Producer
Deposit Type: Hydrothermal vein
Operation Type: Underground
Discovery Year: 1851
Years of Production:
Organization:
Significant: Y

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Physiography

Not available

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Mineral Deposit Model

Model Name: Low-sulfide Au-quartz vein

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Orebody

Form: Tabular, pinch and swell

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Structure

Type: L
Description: Melones Fault zone

Type: R
Description: Bear Mountains Fault zone, Melones Fault zone

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Alterations

Alteration Type: L
Alteration Text: Wall rocks hydrothermally altered, having been partially to completely converted to ankerite, sericite, quartz, pyrite, arsenopyrite, chlorite, and albite. Locally, greenstone bodies adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute low-grade ore.

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Rocks

Name: Greenstone
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

Name: Slate
Role: Host
Age Type: Host Rock
Age Young: Late Jurassic

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Analytical Data

Not available

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Materials

Ore: Gold
Ore: Pyrite
Ore: Arsenopyrite
Gangue: Quartz
Gangue: Slate
Gangue: Calcite

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Comments

Comment (Economic Factors): Clark (1970) reported that the Keystone Mine ultimately produced $24 million. The mine was noted for its good ore, which averaged about $12 to $18 to a depth of about 900 feet, after which ore as low as $2.40 per ton was mined. The very best ore assayed $40 - $100 per ton.

Comment (Geology): REGIONAL GEOLOGY The Keystone Mine is located within the Sierra Nevada foothills, where bedrock consists of north trending tectonostratigraphic belts of metamorphosed sedimentary, volcanic, and intrusive rocks that range in age from late Paleozoic to Mesozoic. Locally, the Mesozoic rocks are capped by erosional remnants of Eocene auriferous gravels and once extensive volcanic rocks of Tertiary age. The structural belts, which extend about 235 miles along the western side of the Sierra, are flanked to the east by the Sierra Nevada Batholith and to the west by sedimentary rocks of the Cretaceous and Jurassic Great Valley sequence. In Amador County, the structural belts are internally bounded by the Melones and Bear Mountains fault zones. Schweickert and others (1999) provide one interpretive overview of the regional geology of this part of the Sierra Nevada. Gold deposits in the Plymouth - Jackson district occur within the north and northwest trending mile-wide Mother Lode Belt, which is dominated by gray to black slate of the Upper Jurassic Mariposa Formation and associated greenstone and amphibolite schist bodies assigned to its Brower Creek Volcanics member. In Amador County, the Mother Lode Belt approximately parallels Highway 49 southeastward from Plymouth through the town of Jackson. The geology of this segment has been mapped by Zimmerman (1983) and Duffield and Sharp (1975). The lode gold deposits along this stretch are responsible for most of the gold production in the county, which has been reported to be 7.68 million ounces (Koschman and Bergendahl, 1968). Clark (1970) placed the value of this production at $180 million. The Amador County portion of the belt was one of the most productive gold mining areas in the United States, and the Plymouth - Jackson district in Amador County was the most productive part of the belt. The Mariposa Formation contains a distal turbidite, hemipelagic sequence of black slate, amphibolite, schist, and fine-grained tuffaceous rocks, and volcanic intrusive rocks. The thickness of the Mariposa Formation is difficult to ascertain due to structural complexities, but is estimated to be about 2,600 feet thick at the Cosumnes River. Massive greenstone of the Upper Jurassic Logtown Ridge Formation lies west of the Mother Lode Belt. The contact between the Logtown Ridge and Mariposa Formation is generally gradational (Zimmerman, 1983). The Logtown Ridge Formation consists of over 9,000 feet of volcanic and volcanic-sedimentary rocks of island arc affinity. These rocks are mostly basaltic and include flows, breccias, and a variety of layered pyroclastic rocks. Metasedimentary rocks, chiefly graphitic schist, metachert, and amphibolite schist of the Calaveras Complex (Carboniferous to Triassic) are to the east. Mother Lode Gold Quartz Veins Mother Lode-type veins fill voids created within faults and fracture zones. The Mother Lode Belt consists of a vein system ranging from a few hundred feet to a mile or more in width. The vein system consists of a fault zone containing several parallel veins separated by hundreds of feet of highly altered country rock containing small quartz veins and occasional bodies of low-grade ore. Veins are generally enclosed within numerous discontinuous fault fissures within Mariposa Formation slate, associated greenstone, amphibolite schist, or along lithologic contacts. Mineralized fault gouge is abundant.

Comment (Geology): Mineralization is characterized by steeply dipping massive gold-bearing tabular quartz veins striking north to northwest and dipping between 50 to 80? east. Veins are discontinuous along both strike and dip, with maximum observed unbroken dimensions of 6,500 feet in either direction (Zimmerman, 1983), but individual veins more commonly range from structures 3,000 feet long and 10 to 50 feet wide to tiny veinlets. In rare instances, veins are known to reach as much as 200 feet thick (Keystone Vein). Veins may be parallel, linked, convergent, or en echelon, and commonly pinch and swell. Few can be traced more than a few thousand feet. At their terminations, veins pass into stringer zones composed of numerous thin quartz veinlets or into gouge filled fissures (Knopf, 1929). Ores consist of hydrothermally deposited minerals and altered wall-rock inclusions. Gold occurs as free gold in quartz and as auriferous pyrite and arsenopyrite. Quartz is the dominant mineral component in the veins, comprising 80-90% or more with ankerite, arsenopyrite, pyrite, albite, calcite, dolomite, sericite, apatite, chlorite, sphalerite, galena, and chalcopyrite in lesser amounts of a few percent or less. Cumulative sulfides generally range 1% - 3% of the rock (Carlson and Clark, 1954; Zimmerman, 1983). Ore grade material is not evenly distributed throughout the veins, but was localized in ore shoots, which tend to occur at vein intersections, at intersections of veins and shear zones, or at points where the veins abruptly change strike or dip (Moore, 1968). Ore shoots generally display pipe-like geometries raking steeply in the veins at 60-90%. Horizontal dimensions of the ore shoots are commonly 200-500 feet, but pitch lengths were often much greater, and often nearly vertical. Pockets of high grade ore are relatively abundant. Single masses of gold containing over 2,000 ounces and single pockets containing more than 20,000 ounces have been found. Silver is subordinate. Gold fineness averages 800. While most of the Mother Lode ore shoots mined have been less than 300 feet in strike length, many have extended down dip for many thousands of feet. In the deeper mines, mining continued to almost 6,000 feet on the dip of the vein with no evidence of bottoming. Cessation of operations in the deep Kennedy (5912') and Argonaut (5570') mines was caused by increasing costs at the greater depths rather than an absence of ore. Milling ore was generally low to moderate in grade (1/7 to 1/3 ounce per ton). Alteration Wall rocks have invariably been hydrothermally altered, having been partially to completely converted to ankerite, sericite, quartz, pyrite, arsenopyrite, chlorite, and albite with traces of rutile and leucoxene (Knopf, 1929). The mineralization is usually adjacent to the veins in ground that has been fractured and contains small stringers and lenses of quartz.. Locally, greenstone bodies (altered volcanic rocks) adjacent to the quartz veins contain enough disseminated auriferous pyrite in large enough bodies to constitute what has been called "gray ore". Altered slate wallrock commonly contains pyrite, arsenopyrite, quartz, chlorite, and sericite with or without ankerite (Zimmerman, 1983). Large bodies of mineralized schist also form low-grade ore bodies throughout the Mother Lode. This ore consists of amphibolite schist which has been subjected to the same processes of alteration, replacement, and deposition that formed the greenstone gray ores. The altered schist consists mainly of ankerite, sericite, chlorite, quartz, and albite. Gold is associated with the pyrite and other sulfides that are present. Pyrite comprises about 8 percent of the rock. The average grade of mineralized schist is about 0.1 oz per ton (Moore, 1968).

Comment (Identification): The Keystone Mine is located within the city limits of Amador City in the famous Mother Lode Gold Belt in the Sierra Nevada foothills of western Amador County. While the mine is technically in the smaller Amador City district, the uniform nature of gold mineralization with neighboring districts has causes some authors to consolidate the smaller neighboring districts into Jackson - Plymouth district (Clark, 1970). The Jackson - Plymouth district was the most productive district of the Mother Lode belt with an estimated total production of about $180 million (Clark, 1970). The Keystone mine itself is credited with $24 million and was one of the most profitable Mother Lode mines.

Comment (Development): Most of the important lode gold deposits in Amador County Mother lode were discovered in the 1850s while rich Tertiary placer deposits were being worked. The Keystone mine grew up around the mining locations of the Granite Ridge and Pleasant Ridge companies, themselves formed from several claims located in 1851. The Granite State claims were the third group of claims made on that section of the lode. There were six original locators forming the company, each claim being 60 feet wide by 120 feet long (Logan, 1934). One claim showed particularly rich outcrops and four arrastras were built to crush selected ore which paid $100 per ton for a few months (Logan, 1934). A one-half interest in the claims was then given in exchange for a 12 stamp mill, but within 2 years the project failed, and the claims were purchased at a Sheriff's sale by A.H. Rose of the Pleasant Ridge Company and the Keystone Mine came into being in 1853 (Logan, 1934). In 1854, the mine produced 2,204 tons of ore that yielded $21/ton, or a gross of $46,284, and monthly dividends of $200 per share were declared. In the early 1860's, the mine was sold to Gashwiler and McDonald for $102,000 and like many early gold mines the Keystone remained a small operation with only a partial record of results published. Some of these were: From December 8, 1865 to December 21, 1866: production $135,000, dividends $51,300, average recovery $16 per ton. For the year ended July 31, 1868: production $154,355, dividends $75,000, average yield of $12.86 per ton. For the fiscal year 1879: estimated $25,000 gross and $15,000 net monthly production. For fiscal year 1871: estimated gross output $300,000, dividends for calendar year 1871 $20,000. For 1872: from $35,000 to $40,000 gross per month. For 1873: dividends of $185,000. In 1874: 25,146 tons of ore yielded $452, 507 or $18 per ton at a cost of $7.16 a ton for mining and milling. In 1875 it was said that since 1870, the production of the Keystone had averaged $1,000 for each working day (Logan, 1934). By 1875, the Keystone Mine had only been opened to a depth of only 750 feet and for a length of 900 feet, but was already considered the leading mine on the Mother Lode. Keystone Mill contained 40 stamps and had a capacity of 90 tons a day. The good ore grade for which the mine was noted gave out at about 900 feet when the Keystone vein entered greenstone and by 1888, most of the high-grade ore had been exhausted. From 1911 to 1919, the Keystone Mining Company operated the mine, after which it was idled. From early 1915 to closing, 40 stamps were crushing about 7,000 tons of ore each month. During that time, ore averaged only $2.90 per ton, being mostly gray ore where the concentrate carried about 80% of the gold (Logan, 1927). Margins were especially thin. The filling of the tailings pond and lack of additional tailings space, as well as rising costes, hastened the mine's closing. Buy 1919, total production for the Keystone Mine was estimated at $17 million (Logan, 1927). In 1933, the adjoining Spring Hill Mine to the east was acquired (Carlson and Clark, 1954) and the Keystone Mine was reopened by the Keystone Mines Syndicate. Production was underway by 1935 and continued until the fall of 1942 when the mine was closed down for WW II. At the time of closure, about 100 men were employed at the mine and mill (Carlson and Clark, 1954). The Keystone Mine never reopened, but the Keystone Mines Syndicate continued to do a small amount of work until 1952 in the Wonder shaft of the Spring Hill properties

Comment (Commodity): Commodity Info: the best ores ranged from $40 - $100 per ton

Comment (Commodity): Ore Materials: Free milling gold in quartz and mineralized greenstone containing auriferous pyrite and arsenopyrite

Comment (Commodity): Gangue Materials: Quartz, slate, greenstone, calcite,

Comment (Deposit): The Keystone Mine produced from Mother Lode mesothermal gold-quartz veins. The producing veins are part of a narrow NNW-SSE striking, steeply dipping vein system that extends southward from Plymouth through the towns of Amador City and Sutter Creek, and includes the prolific Keystone, Kennedy, Argonaut, and Lincoln Consolidated mines. The Keystone Mine was located in 1851 and developed the thick Keystone vein which occupied a reverse fault fracture cutting Mariposa Formation black slate and greenstone. The Keystone vein ranges from 12 to 200 feet wide and dips 35? to 65? east. The massive vein material was generally low grade, the best ore occurring in ore shoots localized where side stringers intersect the main vein from the foot wall slate. Low grade "gray ores" ranging from less than $3 per ton to over $8 per ton were also produced. Gray ore consisted of mineralized greenstone formed by hydrothermal alteration (ankeritization) of the greenstone in the hanging wall side of the Keystone vein. Mineralization is disseminated auriferous sulfides, principally pyrite.

Comment (Location): Location selected for latitude and longitude is the Keystone Mine shaft symbol on the USGS 7.5 minute Amador City quadrangle.

Comment (Workings): The Keystone Mine was worked through the main Patton shaft, which was 2689 feet deep on a 52? incline. The shaft was located between the Keystone and Spring Hill veins. A south shaft, 1024 feet on the incline (789 feet vertical), was used as an air and water shaft. Both shafts are timbered throughout. Levels were driven at the 300, 400, 600, 700, 800, 1000, 1200, 1400, 1800, 2100, and 2600 foot levels. Cross sections through the Patton shaft workings are provided by Logan (1934) and Zimmerman (1983) (fig. 18). The mill contained 40 stamps. Vanners were used for concentrates and no tailings were saved (Logan, 1934). Irelan (1888) provides a detailed description of the milling operations during the late 1880s.

Comment (Geology): Ore Genesis Several mechanisms have been suggested as the source of the Mother Lode gold deposits. The most widespread belief is that plutonic activity magmatically differentiated vein constituents or provided the heat to circulate meteoric fluids or to metamorphose the country rocks to liberate the vein constituents. Knopf (1929) proposed that carbon dioxide, sulfur, arsenic, gold, and other constituents were emitted from a crystallizing magma but the components were carried by meteoric water in a circulation system driven by plutonic heat. Most theories suggest that gold deposits formed at temperatures of 300 to 350 degrees centigrade with a possible magmatic or metamorphic origin. Zimmerman (1983) proposed that the Mother Lode veins were generated by and localized near a major late Nevadan shear zone, the mechanism of ore genesis being the shearing and redistribution of mass within a major fault zone. He suggested that the early reverse faults had strike slip component, which is evident in the correlation of expected strike-slip dilatant zones with the geometries and steeply raking attitudes of the ore shoots. Fault movement and shearing would cause recrystallization of the rocks within the fault zone, releasing the more mobile elements including gold and most of the other vein constituents. Moreover, the heat generated by shearing would contribute to the metamorphism of the rocks in the fault zone and cause fluid circulation in the fault zone. Mineral laden auriferous fluids generated by this shearing channeled into the fault fracture system into dilatant zones, which represented avenues of increased flow and lower strain LOCAL GEOLOGY The main Keystone or contact vein varies from 12 to 200 feet wide and occurs on the contact between Mariuposa Formation slate foot wall and altered greenstone hanging wall. The vein dips 35? to 65? east. The massive vein occurs on the hangingwall side of an important fault that has been traced 2000 feet. The vein is composed of crushed quartz and greenstone. In many places the quartz has been reduced to almost powder. The Keystone vein is generally low grade with good paying ore shoots localized where side stringers intersected the main vein from the foot wall slate (Logan, 1934). The quartz stringers dipped 47? east. Most of the production to an inclined depth of 900 feet came from this kind of ore. East of the Keystone vein is the Spring Hill or East vein. This vein was originally worked in the Spring Hill Mine which was acquired by the Keystone in 1933. Ore values in both veins varied a great deal. Ore was free gold in quartz associated with pyrite and arsenopyrite and small amounts of stibnite (Carlson and Clark, 1954). The best ore was in the deeper section of the Keystone vein within slate wall rocks. Here the vein was 3-40 feet wide and yielded as much as $40 per ton. It carried from 1.5-1.74% sulfides, principally pyrite and arsenopyrite, with concentrates yielding $110 per ton. In 1906 at a depth of 1000 feet, in ore that yielded $2.43 per ton there was a 2% concentrate worth $63 per ton and carrying 52% of the gold and silver. In 1917, with ore of nearly identical yield, sulfides formed 4.3% of ore, yielded $53 per ton and contained 95% of the gold and silver (Logan, 1934) A gray ore body was found on the 1200-foot level, 800 feet north of the main crosscut. This gray ore is mineralized formed by hydrothermal alteration (ankeritization) of the greenstone in the hanging wall side of the Keystone vein. Mineralization is disseminated auriferous sulfides, principally pyrite (Logan, 1934). This ore body measured about 130 feet long by 8 feet wide and assayed $8 or more per ton. The ore body pitches south being about 400 feet north of the Patton shaft on the 1400-foot level and 600 feet south of it on the 2100-foot level where the ore body was 100 feet long and was mined for a width of 18 feet (Logan, 1934).

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References

Reference (Deposit): Schweickert, R.A., Hanson, R.E., and Girty, G.H., 1999, Accretionary tectonics of the Western Sierra Nevada Metamorphic Belt in Wagner, D.L. and Graham, S.A., editors, Geologic field trips in northern California: California Division of Mines and Geology Special Publication 119, p. 33-79.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 69-76.

Reference (Deposit): Carlson, D.W., and Clark, W.H., 1954, Mines and mineral resources of Amador County, California: California Division of Mines and Geology, 50th Report of the State Mineralogist, p. 173-177.

Reference (Deposit): Clark, W.B. and Carlson, D.W., 1956, Mines and mineral resources of Amador County: California Division of Mines, California Journal of Mines and Geology, v. 52, p. 422.

Reference (Deposit): Duffield, W.A. and Sharp, R.V., 1975, Geology of the Sierra foothills melange and adjacent areas, Amador County, California: U.S. Geological Survey Professional Paper 827, 30 p.

Reference (Deposit): Irelan, W., Jr., 1888, Amador County, Keystone Consolidated Mining Company: California State Mining Bureau, 8th Annual Report of the State Mineralogist, p. 63-66.

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 92-95.

Reference (Deposit): Logan, C.A., 1921, Mines and mineral resources of Amador County, Keystone Mine: California State Mining Bureau, 22nd Report of the State Mineralogist, p.411.

Reference (Deposit): Storms, W.H., 1900, The Mother Lode region of California: California Mining Bureau Bulletin 18, p 77-78.

Reference (Deposit): Tucker, W.B., 1914, Mines and mineral resources of Amador County, Keystone Mine: California State Mining Bureau, 14th Report of the State Mineralogist, p. 34-36.

Reference (Deposit): Additional information on the Keystone Mine is available in file no. 339-1454 (CGS Mineral Reources Files, Sacramento).

Reference (Deposit): Zimmerman, J.E., 1983, The Geology and structural evolution of a portion of the Mother Lode Belt, Amador County, California: unpublished M.S. thesis, University of Arizona, 138 p.

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