Original Amador Mine

Site Name

Primary: Original Amador Mine
Secondary: Little Amador Mine
Secondary: Minister?s Claim

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Commodity

Primary: Gold

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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: Hydrothermal alteration of greenstone wall rocks resulting in 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

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Comments

Comment (Location): The location selected for latitude and longitude is the Original Amador Mine shaft symbol as illustrated on Fig. 4 of Zimmerman (1983). The mine is not shown on the USGS 7.5 minute Amador City quadrangle).

Comment (Workings): Early workings of the Original Amador were conducted through five inclined shafts: Old Shaft No. 1, Old Shaft No. 2, North East Shaft, North West Shaft, and the South Shaft which became the main shaft in later operations and reached a final inclined depth of 1238 feet. The two-compartment main shaft was sunk parallel to the dip of the hanging wall to the 850-foot level, at which point the vein flattened and the shaft entered the greenstone foot wall. The first 220 feet of the shaft dipped 82? east. From 220 to 700 feet, it dipped 46?and from 700 feet to the 1200-foot level it dipped 58?. Levels were at 200', 300', 400', 500', 600', 700', 850', and 1200 feet (McDougall, 1929). The mine had a total of 9,719 feet of drifts, 7,023 feet of crosscuts, and 27,465 feet of raises (Logan, 1934). From 1909-1915 the mill had 20 stamps with a capacity of 90 tons a day. The ore crushed during this period averaged $3.44 per ton. In 1915, the capacity of the mill was trebled to a capacity of 300 tons a day with the addition of 2 Hardinage mills, classifiers, etc. The mill made a recovery of 90%.

Comment (Geology): Below 700 feet, however, the gold content declined, similar to other Mother Lode mines. In the quartz and slate ores, most of the gold was free (Logan, 1927). Mill recovery was 90%. Gray ore was characterized by 2% to 4% sulfides, mostly pyrite, which carries most of the gold in this ore. Concentrates formed 1.5%-2.5% of the ore. They ranged from $40 - $70 per ton in value and contained usually from 1/3 to 1/2 of the gold (Logan, 1934).

Comment (Geology): REGIONAL GEOLOGY The Original Amador 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 (Development): The Original Amador Mine has the distinction of being the first gold quartz discovery in Amador County. It was located in on February 1851, by a Baptist preacher. Three other ministers were associated with him and their mine was called the "Ministers Claim". The mine was opened in 1852. Later, it became known as the "Original" Amador Mine to distinguish it from two other early mines named the Amador Number One and the Amador Number Two mines. The Original Amador Mine was purchased by an English Company in 1872 when the mine was at a depth of 365 feet, and a 40-stamp mill was erected. The mill recovery turned out to be less than expected and the mine was idled with the exception of intermittent prospecting work between 1874 and 1898. In 1908, the Original Amador Consolidated Mines Company reopened the mine and sunk the shaft to the 700 foot level and gold production resumed in 1909. Adjacent claims were acquired until the property included the Original Amador, East Amador, Amador Wedge, Eclipse, Eclipse Extension, Last Chance, Great Eastern, and Excelsior claims. From 1910 to 1918, production was between $90,000 and $130,000 per year (Logan, 1934). In 1915, the mill capacity was increased and ball mills were substituted for the stamps. The property remained in operation until to 1918 when extremely high operating costs and labor shortages incidental to WWI caused the mine to shut down. In 1929, the mine was dewatered, the mill repaired and put in condition for operation. Operating only a week, the property was taken over under option by the Stobie Forlong Company who made no attempt to produce it, and was forced into the hands of a receivership in January, 1930. The property lay idle until 1935 when it was reopened under the direction of Hamilton, Beauchamp, and Woodworth. The mill was reconditioned and its capacity increased. Mining operations were finally suspended in 1937, but the mill remained in operation until 1942 milling ore produced from the nearby Keystone Mine (Carlson & Clark, 1954).

Comment (Economic Factors): Moore (1968) reported the Original Amador Mine produced 160,0000 ounces of gold, which Clark (9170) valued at $3.5 million.

Comment (Commodity): Commodity Info: Sulfide concentrates yielded $40 - $70 per ton

Comment (Commodity): Ore Materials: Free milling gold in banded seams of quartz, crushed slate, and about 2% sulfides. Hydrothermally altered greenstone containing disseminated auriferous pyrite and arsenpopyrite

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

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 Original Amador Mine is centrally located on the Mother Lode where the Mariposa slate belt is from 600-1000 feet wide. Two main quartz veins systems traverse the property, the east vein (Original Amador vein) and the west vein. A number of branch veins, isolated ore chimneys, and mineralized country rock (gray ore) zones were also encountered. The principal ore-bearing zone in the Original Amador Mine was within the east vein Almost all development and production was from the east vein with ore shoots opened up at every level down to the 1200-foot level (McDougall, 1929). Near the surface, the vein ranges from 20-50 feet thick (Tucker, 1914). The vein is represented by two branches, separated by greenstone. The footwall branch, which was chiefly worked in the early operations on the upper levels, is thinner and better grade than the hanging wall branch which it joins north of the shaft. It is a stringer lead in slate, 6 to 8 feet wide (Logan, 1934). The thickness of the footwall vein ranges from a thin stringer to 5 feet of solid quartz. The maximum width of the hanging wall branch is 90 feet. The hanging wall is greenstone while the foot wall is slate to a depth of 300 feet. Below the 300-foot level, the vein enters greenstone and assumes a flatter dip. The ore on the hanging wall vein below the 300 foot level was partly from the vein itself and partly altered greenstone lying on the hanging wall side of the quartz (Logan, 1934). Ore was obtained in part from the Original Amador quartz vein and in part from large irregular masses of auriferous greenstone occurring in the hanging wall of the quartz vein (Knopf, 1929). East of this vein, parallel veins of gray ore 12 - 20 feet wide and consisting of quartz stringers and altered greenstone were also produced (Logan, 1934). The east vein contained shoots of excellent commercial ore and was accompanied by a large amount of gouge with sulfides.

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).

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References

Reference (Deposit): Moore, L., 1968, Gold resources of the Mother Lode Belt, El Dorado, Amador, Calaveras, Tuolumne, and Mariposa counties, California: U.S. Bureau of Mines Technical Progress Report 5, p. 1-22.

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): Tucker, W.B., 1914, Amador County, Original Amador Mine: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 39-40.

Reference (Deposit): Additional information on the Original Amador Mine is available in file no. 332-5820 (CGS Mineral Resources 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.

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): Koschman, A.H., and Bergendahl, M.H., 1968, Principal Gold-Producing districts of the United States: U. S. Geological Survey Professional Paper 610, 283 p.

Reference (Deposit): Knopf, A., 1929, The Mother Lode system of California: U. S. Geological Survey Professional Paper 157, 88 p.

Reference (Deposit): Logan, C.A., 1921, Amador County, Central Eureka Mine: California State Mining Bureau, 17th Report of the State Mineralogist, p. 409-410.

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 104-106.

Reference (Deposit): Logan, C.A., 1927, Amador County, Central Eureka Mine: California State Mining Bureau, 23rd Report of the State Mineralogist, p. 178-179.

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. 188-189.

Reference (Deposit): McDougall, B.W., 1929, Preliminary report on the Original Amador and Bunker Hill Mines, Amador County, California, unpublished report, 13 p.

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