Original Amador Mine (Eclipse Mine; Little Amador Mine; Original Amador claim; East Amador claim; Amador Wedge claim; Great Eastern claim; Eclipse Extension claim; Last Chance claim), Amador City, Jackson-Plymouth District, Mother Lode Belt, Amador Co., California, USA
|Latitude & Longitude (WGS84):||38° 25' 23'' North , 120° 49' 34'' West|
|Latitude & Longitude (decimal):||38.42306,-120.82611|
|Köppen climate type:||Csa : Hot-summer Mediterranean climate|
A former lode Au mine located in sec. 36, T7N, R10E, MDM, 0.5 km (0.3 mile) NNW of Amador City, on the lower SE slope of Bunker Hill (between the Bubnker Hill Mine on the N and the Keystone Mine on the S), on private land. Discovered in 1852. The mine is a consolidation of 6 claims. Operated by J. W. Bullock C/O C. E. Crandall. This mine was worked during the period 1852 to 1874 and was prospected intermittently until 1898. Between 1898 and 1918, it was operated by the Original Amador Consolidated Mines Co. and from 1910 to 1918, it produced between $90,000 and $130,000 (period values) per year. It was idle until early 1935, when it was reopened and worked until the middle of 1937. MRDS database stated accuracy for this location is 500 meters.
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).
Mineralization is a vein Au deposit (Mineral occurrence model information: Model code: 273; USGS model code: 36a; Deposit model name: low-sulfide Au-quartz vein; Mark3 model number: 27) hosted in rocks of the Franciscan Complex (Late Jurassic Mariposa Formation greenstone & slate). The ore body is tabular and pinch & swell in form at a thickness of 15. 24 meters. Ore shoots in quartz veins are lenticular in shape. Black slate occurs in the upper levels of the workings and passes into greenstone at depth s the quartz vein flattens. A contact is exposed a short distance W of the main shaft collar. Auriferous greenstone on the hanging wall where the quartz vein branches. Crosscut veins, rare in other Mother Lode mines are particularly well exposed at the 700 level, and their courses are irregular. hese veins range from 40 to 90 feet and cross stratigraphic contact without any displacement. Local alteration includes places on the vein where the quartz is commonly splotched with aggregates of muscovite formed by hydrothermal alteration of enclosed filaments of schistose greenstone. Hydrothermal alteration of greenstone wall rocks resulting in disseminated auriferous pyrite in large enough bodies to constitute low-grade ore. Local rocks include Jurassic marine rocks, unit 1 (Western Sierra Nevada and Western Klamath Mountains).
Geologic structures include: Regional: the Bear Mountains Fault zone; the Melones Fault zone. Local: the Melones Fault zone. Local structures include a reverse faulting along the vein fissure. It is probably more clearly and accurately measurable in the Original Amador Mine than any other mine on the mother lode; it amounts to 180 feet.
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. 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).
Workings include underground openings. The mine was opened by an inclined shaft 1,240 feet deep with 9,720 feet of drifts, 7,025 feet of crosscuts and 27,450 feet of raises. The shaft is on the vein from surface to the 300 foot level, where the vein passes into the footwall.
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%.
The total production is estimated at $3,500,000 (period values). The ore was obtained from the Original Amador quartz vein and from large, irregular masses of auriferous greenstone.
Commodity Info: Sulfide concentrates yielded $40 - $70 per ton (period values). Free milling gold occurs in banded seams of quartz, crushed slate, and about 2% sulfides.
Hydrothermally altered greenstone containing disseminated auriferous pyrite and arsenpopyrite
Gangue Materials: Quartz, slate, greenstone
Moore (1968) reported the Original Amador Mine produced 160,0000 ounces of gold, which Clark (9170) valued at $3.5 million (period values).
Mineral ListMineral list contains entries from the region specified including sub-localities
5 valid minerals.
This geological map and associated information on rock units at or nearby to the coordinates given for this locality is based on relatively small scale geological maps provided by various national Geological Surveys. This does not necessarily represent the complete geology at this locality but it gives a background for the region in which it is found.
Click on geological units on the map for more information. Click here to view full-screen map on Macrostrat.org
|Late Jurassic - Triassic|
145 - 252.17 Ma
|Jurassic marine rocks, unit 1 (Western Sierra Nevada and Western Klamath Mountains)|
Age: Mesozoic (145 - 252.17 Ma)
Stratigraphic Name: Agua Fria Formation; Colfax Formation; Cosumnes Formation; Galice Formation; Hunter Valley Cherts; Mariposa Formation; Merced Falls Slate; Salt Spring Slate; Jasper Point Formation
Comments: Western Klamath Mountains, western Sierra Nevada. Primarily slate and metamorphosed graywacke; minor siltstone, conglomerate, chert, and volcanic rocks. Mainly Late Jurassic in age, but also includes some Early Jurassic or older rocks
Reference: Horton, J.D., C.A. San Juan, and D.B. Stoeser. The State Geologic Map Compilation (SGMC) geodatabase of the conterminous United States. doi: 10.3133/ds1052. U.S. Geological Survey Data Series 1052. 
|Jurassic - Triassic|
145 - 252.17 Ma
|Mesozoic sedimentary and volcanic rocks|
Age: Mesozoic (145 - 252.17 Ma)
Reference: Chorlton, L.B. Generalized geology of the world: bedrock domains and major faults in GIS format: a small-scale world geology map with an extended geological attribute database. doi: 10.4095/223767. Geological Survey of Canada, Open File 5529. 
Localities in this Region
- Amador Co.
- Mother Lode Belt
- Amador Co.