Zeila Mine

Site Name

Primary: Zeila Mine
Secondary: Coney Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Amador
District: Jackson - Plymouth

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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: 1860
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: R
Description: Bear Mountains Fault zone, Melones Fault zone

Type: L
Description: Melones Fault zone

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Alterations

Not available

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Rocks

Name: Amphibole Schist
Role: Host
Age Type: Host Rock
Age Young: Permian
Age Old: Carboniferous

Name: Slate
Role: Host
Age Type: Host Rock
Age Young: Permian
Age Old: Carboniferous

Name: Greenstone
Role: Host
Age Type: Host Rock
Age Young: Permian
Age Old: Carboniferous

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Analytical Data

Not available

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Materials

Ore: Gold

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Comments

Comment (Identification): The Zeila Mine is located just east of Jackson in the famous Mother Lode Gold Belt in the Sierra Nevada foothills of western Amador County. The mine is included in the Jackson - Plymouth district which was the most productive district of the Mother Lode belt with an estimated total production of about $180 million (Clark, 1970). The Zeila Mine alone produced about 250,000 ounces of gold (Moore, 1968) valued at $5+ million (Clark, 1970).

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

Comment (Workings): The Zeila Mine was never a large or complex mine. It was developed by a main 1700 foot main inclined shaft at an angle of 65? and a second shaft for which little information is available (ventilation shaft?) On the 1570-foot level a drift was driven 3000 feet north from the shaft, at which point a winze was sunk on the vein to a depth of 458 feet. One hundred and fifty seven feet below the 1570 level, a 352 foot drift was driven north on the vein from the winze. Two hundred ninety five feet below the 1579 foot level, a 450 foot drift was driven north (Carlson & Clark, 1954). The country rock in the Zeila Mine was heavy and expensive to hold. Filling was necessary in working this mine. Fill material was obtained from the vein and also from chambers cut into the hanging wall (Storms, 1900). Extensive timbering was also required. At its peak, the mine had a 40 stamp mill, chlorination plant, and canvas plant. Details of the milling and processing operations are described in Storms (1900).

Comment (Commodity): Ore Materials: Free milling gold in ribboned quartz structures and auriferous sulfides

Comment (Commodity): Commodity Info: Free gold constitutes only 35% of the gold. Sulfides contribute the most gold with concentrates averaging $100 per ton. Pyrite is the principal sulfide with lesser amounts of arsenopyrite and small quantities of galena and sphalerite.

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 (Geology): REGIONAL GEOLOGY The Zeila 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 (Deposit): The Zeila Mine produced from mesothermal gold quartz stringers within slates and schist of the Calaveras Formation. veins. Unlike the other Mother Lode veins within slates and greenstones of the Mariposa Formation, the Zeila mine is generally considered to lie within a different vein system east of the . The Mother Lode proper. The general strike of the productive vein is N 27? W. It dips between 60?- 65? east. The vein averages 20 feet wide but has been 40-50 feet thick in some stopes. Unlike other Mother Lode mines, most of the gold occurs in the sulfides rather than free gold. Free gold constitutes only 35% of the values and is very rarely visible in ore samples. Sulfides, average $100 per ton. Pyrite is the principal sulfide with lesser amounts of arsenopyrite and small quantities of galena and sphalerite. Ore is low grade averaging less than $4 per ton.

Comment (Development): The Zeila Mine was first worked in the 1860s when it was known as the known as the Coney Mine (Logan, 1927). By 1867, it had a 16 stamp mill and chlorination plant treating the concentrates. The mine closed in 1875 but reopened again in 1880 and operated continuously thereafter until 1914 when it finally closed down. By that time the Zeila Mine produced over $5 million in gold. After the mine's closure it was purchased by the Kennedy Mining and Milling Company for $35,000.

Comment (Economic Factors): Clark (1970) reported that the Zeila Mine produced $5+ million. The ore is low grade averaging less than $4 per ton. At the time of closing it has been stated that 360,000 tons of ore averaging less than $4 per ton remained (Logan, 1927).

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 Published reports on the Zeila Mine geology are generally meager and vague. Ore occurs as quartz stringers on the contact of greenstone and Calaveras Formation slate and amphibolite schist. Unlike the other principal mines in the Jackson Plymouth district which lie along the narrow axis of the Mother Lode within slates and greenstones of the Mariposa Formation, the Zeila mine is generally considered to lie within a different vein system than the Mother Lode proper. The general strike of the productive stringer vein is N 27? W. It dips between 60?- 65? east. The vein averages 20 feet wide but has been 40-50 feet thick in some stopes. Gouge occurs only on the footwall of the vein and ranges from a few feet to 50 feet in thickness. Gold occurs as free milling gold and in sulfides with the highest-grade ores occurring in ribboned quartz structures (Carlson & Clark, 1954). However, the mine is not noted for being a rich free milling gold quartz mine. Most ore is low grade averaging less than $4 per ton. As opposed to other Mother Lode mines, the Zeila Mine is known for its ore carrying most of its gold values in the sulfides rather than free gold (Logan, 1927). Free gold constitutes only 35% of the values and was very rarely visible in ore samples. Sulfides, averaged $100 per ton (Storms, 1900). Pyrite is the principal sulfide with lesser amounts of arsenopyrite and small quantities of galena and sphalerite. Calcite is abundant, both as stringers and crystallized with the quartz.

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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): Storms, W.H., 1900, The Mother Lode region of California: California Mining Bureau Bulletin 18, p 45-49.

Reference (Deposit): Tucker, W.B., and Waring, C.A., 1916, Mines and mineral resources of El Dorado, Placer, Sacramento, and Yuba counties: California State Mining Bureau, 15th Report of the State Mineralogist, p. 293-295.

Reference (Deposit): Additional information on the Zeila Mine is available in file no. 322-2949 (CGS Mineral Reources Files, Sacrament).

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

Reference (Deposit): Clark, W.B. and Carlson, D.W., 1956, Mines and mineral resources of El Dorado County: California Division of Mines, California Journal of Mines and Geology, v. 52, p. 195.

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. 85-88.

Reference (Deposit): Logan, C.A., 1927, Amador County, Zeila Mine: California State Mining Bureau, 23nd Report of the State Mineralogist, p.184-185.

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.

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