South Eureka Mine

Site Name

Primary: South Eureka Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Amador
District: Sutter Creek

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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: 1891
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

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Comments

Comment (Geology): REGIONAL GEOLOGY The South Eureka 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 (Commodity): Ore Materials: Free milling gold in quartz with auriferous pyrite and arsenopyrite Disseminated auiferous pyrite and arsenopyrite in greenstone

Comment (Development): The South Eureka Mine was originally developed in 1891 and during much of its period of operation was one of the largest producers in the country. Its later discovery was the result of the absence of any quartz outcroppings, the claims being capped by ancient stream gravels. The sinking of the shaft was a speculative venture based on the position of the claims between known producers on the well-defined Mother Lode trend. An inclined shaft was started in black slate which was soft, crushed and heavy (Logan, 1934). It proved to be expensive requiring frequent timbering and realignment. Cross cuts were run to the hanging wall greenstone at 500', 600', and 900' inclined depth. From 1895 to 1908, the property was only a moderate producer from the small ore bodies of the hanging wall vein. This ore ranged for $2 to $6 a ton in yield and the total output was about $1.2 million for this period (Logan, 1934). During this time the 2740 level had been reached and 58 assessments had been levied to continue prospecting. A 20-stamp mill was in use part time. In 1908, crosscut to the footwall discovered the foot wall vein system. This ore shoot was subsequently developed by crosscuts from the higher levels, and was stoped up to about the 2000-foot level. The mill was increased to 80 stamps and the property for several years was one of the largest producers in the county employing from 200-250 men (Logan, 1934). From 1908 until the mine closed in 1917, over 980,000 tons of ore yielding about $4.1 million was produced (Carlson & Clark, 1954). Total production of the South Eureka was $5.3 million from which about $1 million in dividends was paid (Carlson & Clark, 1954).

Comment (Economic Factors): Clark (1970) reported that the South Eureka Mine produced $5.3 million.

Comment (Commodity): Commodity Info: Concentrates formed 1.7% - 2% of the ore and carried form $60 - $72 a ton in gold and 40 to 65 cents in silver

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

Comment (Workings): The South Eureka Mine was developed through a 2875-foot inclined (67?) 3-compartment shaft (north shaft), and a 600-foot deep shaft located 880 feet to the southeast (south shaft). Crosscuts were run east and west from the main shaft to the ore bodies. A winze was sunk to the 2900-foot level but no ore was found. When the Central Eureka Mining Company had the property under option they prospected much of the deeper ground between 3500 and 4100 feet by extending several of the deeper levels of the Central Eureka Mine into the South Eureka property. The 3350 south level drift of the Central Eureka Mine was extended to a point 120 north of where the South Eureka shaft would be if continued. A west crosscut from the Central Eureka was run in greenstone near the north end line of the South Eureka on this level. On the 3900-foot level, the drift was run south 1180 feet from the Central Eureka shaft, extending into the South Eureka ground (Logan, 1934). These efforts resulted in the discovery of no new commercial ore bodies. Ore was milled in an 80-stamp mill with a capacity of 400 tons per day. The ore was crushed, run through a 24-mesh screen, amalgamated, and concentrated on 48 Frue vanners. Mill recovery ranged from 85 to 91 percent. At full production, 200 to 250 men were employed at the mine and mill (Carlson & Clark, 1954).

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 At the South Eureka Mine, the Mother Lode system occurs in and along a belt of Mariposa slate, which is several hundred feet wide and bounded on both sides by greenstone. Three quartz veins were developed; the hanging wall vein, the foot wall vein, and the "middle vein" (Tucker (1914). A little ore also occurred in hanging wall greenstone. Form the surface to a depth of 2000 feet, the hanging wall vein is a contact vein on the slate-greenstone contact, below which the vein passes into the greenstone. To a depth of 900 feet, the fissure was filled principally with crushed slate and gouge, containing only a little quartz.. The vein was highly disturbed by faulting with ore occurring in scattered bunches within the slaty gouge filling the fissure. This ore, however, did not turn out to be extensive and was not very profitable (Logan, 1934). On the footwall side of the vein there was also a rich pay streak from 1 -10 inches wide with heavy sulfides. A crosscut on the 900-foot level also revealed a five-foot wide ore body in mineralized streaks of greenstone within the hanging wall. In the middle and foot wall veins ore occurs in pockets, in greenstone streaks, and at the junction of the foot wall and middle veins below the 2000-foot level, where they merged into a single ore body. Most of the pay ore in the lower levels is from this ore body. The foot wall vein was the principal ore body, and was found by following a spur vein from the hanging wall contact vein about 250 feet west until it joined the foot wall vein. It was followed upward from the 2750-foot level to above the 2000-foot level where it split into the foot wall and middle veins. The chief mine production was from ore bodies where the veins split and in the foot wall vein between the 2000-foot and 2750-foot levels (Carlson & Clark, 1954). In winzes and drifts below the bottom of the shaft, the ore body was only 60 feet long on the 2900-foot level where it raked north into the adjoining Central Eureka Mine's 2700-foot level (Logan, 1934). Ore is free milling gold in quartz with auriferous pyrite, arsenopyrite, and minor amounts of sphalerite and galena. Concentrates formed 1.7% - 2% of the ore and carried form $60 - $72 a ton in gold and 40 to 65 cents in silver (Logan, 1934).

Comment (Identification): The South Eureka Mine is located one-quarter mile east of Sutter Hill in the famous Mother Lode Gold Belt of the Sierra Nevada foothills. While the mine is technically in the smaller Sutter City district, the uniform nature of gold mineralization with neighboring districts has caused some authors to consolidate the smaller neighboring districts into the 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 South Eureka Mine itself is credited with $5.3 million.

Comment (Environment): The South Eureka Mine produced from typical Mother Lode type mesothermal gold quartz veins. The producing veins are part of a NNW-SSE striking, steeply dipping vein system that extends northward from the Kennedy and Argonaut mines through the South Eureka Mine and into the Central Eureka Mine to the north. Three quartz veins were developed; the hanging wall vein, the foot wall vein, and the "middle vein". The hanging wall, or contact vein, consisted primarily of crushed slate and gouge containing only a little quartz. Ore bodies were small and scattered and moderately profitable. The principal ore bodies occurred at and below the 2000-foot level where the middle and foot wall veins joined to become a large orebody which was stoped from the 2750-foot level to the 2000-foot level. Ore is free milling gold in quartz with auriferous pyrite, arsenopyrite, and minor amounts of sphalerite and galena. Concentrates formed 1.7% - 2% of the ore and carried form $60 - $72 a ton in gold and 40 to 65 cents in silver (Logan, 1934).

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References

Reference (Deposit): Tucker, W.B., 1914, Amador County, Oneida Mine: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 46.

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.B., 1954, Mines and mineral resources of Amador County: California Division of Mines, California Journal of Mines and Geology, v. 50, p. 192-193.

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): Logan, C.A., 1927, Amador County, Lincoln Consolidated Mines: California State Mining Bureau, 23nd Report of the State Mineralogist, p. 180-182.

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 112-114.

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): Storms, W.H., 1900, The Mother Lode region of California: California Mining Bureau Bulletin 18, p 63-64.

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California Gold

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