Bunker Hill Mine

Site Name

Primary: Bunker Hill Mine
Secondary: Rancheria Mine
Secondary: Ranchoree Mine
Secondary: South Mayflower Mine

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Commodity

Primary: Gold
Secondary: Silver

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Location

State: California
County: Amador
District: Amador

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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 Department

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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: 1852
Years of Production:
Organization:
Significant: Y
Deposit Size: M

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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: Slate
Gangue: Calcite
Gangue: Quartz

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Comments

Comment (General): Additional information on the Bunker Hill Mine is available in file no. 331-3639 (CGS Mineral Resources Files, Sacramento).

Comment (Workings): The Bunker Hill Mine was developed through the Bunker Hill shaft, a two-compartment 2800-foot inclined shaft (2382 feet vertical) sunk at an average angle of 58?. A 640 foot inclined winze (53?) was sunk 1020 feet north of the 2800-foot level shaft station to a depth of 3440 feet. Total vertical depth was 2893 feet. Levels were run at inclined depths of 200', 300', 400', 500', 600', 800', 1200', 1400', 1550', 1750', 1950', 2200', 2400', 2600', 2800', 3200', and 3400 feet. The shaft was 550 feet north of the south claim line and work in both directions was carried far enough to connect with the adjoining mines, the Treasure on the north and the Mayflower (before its acquisition by the Bunker Hill) on the south. The greatest distances drifted north were 1750 feet on the 2400-foot level and 1778 feet on the 2600-foot level. To the south drifts extended 550 feet to the end line. Numerous crosscuts were run (Logan, 1927). On the Bunker Hill vein, square set timbering and stopes were used followed by waste-rock filling. In greenstone ore bodies, shrinkage stoping was employed, using some stulls and butt caps. A double drum hoist hoisted ore in 2.5-ton skips. Milling practices when at full capacity in 1915-1916 included primary crushing in a Knight crusher, crushing by forty 1050-lb stamps, and amalgamation, followed by concentration on Deister tables after which the sands were ground in the 5' x 6' Hendy ball mill and the entire pulp was concentrated on the Frue vanners (Logan, 1934). The recovery was 80% to 82% before the ball mill was added to regrind sand tailings. For some years, a cyanide plant was operated on tailings, but the saving with this went as low as 25 cents a ton (Logan, 1935).

Comment (Geology): In the slate gouge ores of the Bunker Hill vein most of the gold is free gold (Logan, 1927). The gray ore was formed by the hydrothermal alteration of the greenstone and contained between 2% an 4% sulfides, principally pyrite and arsenopyrite. About two thirds of the gold recovered from the gray ore is from concentrates which yielded an average of $77 a ton. Gray ore furnished the bulk of profitable tonnage during the final operations of the mine (Logan, 1934).

Comment (Geology): REGIONAL GEOLOGY The Bunker Hill 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 (Commodity): Commodity Info: Shallow ores yielded as much as $50 - $75 a ton decreasing to less than $4 a ton below 2400 feet. Gray ores generally yielded $4 to $6 a ton.

Comment (Commodity): Ore Materials: Free milling quartz and slate fault gouge and mineralized greenstone ?gray ore?. Gray ore contained 2%-4% auriferous sulfides, principally pyrite and arsenopyrite.

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

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 principal vein is the Bunker Hill vein or the hanging wall vein. This vein strikes N 25? W and dips 58?east and averages 6 feet wide. Lesser veins including the "gouge vein" and the Last Chance vein lie in the footwall. Most of the richest ore occurred in the Bunker Hill vein. The vein consisted of a zone of crushed slate gouge and quartz stringers with some large bodies of low-grade massive quartz extending from the surface to the lowest workings at 3200 feet. While most of the gold in the ore is free gold, the slate gouge is highly mineralized with auriferous pyrite and arsenopyrite and is high-grade in the upper workings declining to non-commercial below the 2400-foot level. By the 2200-foot level, the Bunker Hill vein attained a thickness of 30 feet of barren quartz with only 4 feet of ore on the footwall. Between the 2400-foot and 3400-foot levels, the vein consisted of low-grade or barren slate gouge with quartz stringers ranging from a few inches to 30 feet in width. The largest ore body was a maximum length of 800 feet and a maximum width of 10 feet, averaging 5-6 feet (Logan, 1934). From the surface to the 200-foot level the vein followed a Mariposa slate-greenstone contact. Shallow ores along this contact yielded as much as $50-$75 per ton. Between the 200-foot and 1400-foot levels, the vein is enclosed in slate, and below the 1400-foot level the vein crosses interlayered bands of slate and greenstone. Most of the higher grade Bunker Hill vein ore was worked out by the 800-foot level. Mineralized greenstone ore ("gray ore") bodies were encountered in the mid to lower levels of the mine. One ore body of low-grade auriferous greenstone was found on the 1400-foot level 40 feet west of and parallel to the Bunker Hill vein. The ore lenses occupied a wedge shaped section, entering the hanging wall on the north. It had a maximum length of 850 feet and an average width of 30 feet, but was in places 125 feet wide. It was worked down to the 1950-foot level with average values of $4 a ton or less. Another gray ore body was encountered between the 3200-foot and 3400-foot levels approximately 8690 feet north of the shaft. This large ore body occupied the wedge between the Bunker Hill vein and a quartz and slate gouge spur vein.

Comment (Identification): The Bunker Hill Mine is located one-quarter mile north of Amador City in the famous Mother Lode Gold Belt of the Sierra Nevada foothills. It is 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 Bunker Hill Mine itself is credited with $5.1 million. (Clark, 1970).

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

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 (Deposit): The Bunker Hill Mine produced from the Bunker Hill vein and mineralized greenstone ("gray ore") bodies within the Mother Lode gold belt. The Bunker Hill vein strikes N 25? W and dips 58?east and averages 6 feet wide. Vein ore consists of crushed slate gouge and quartz stringers containing free gold and auriferous sulfides, principally pyrite and arsenopyrite. Shallow ores were very rich yielding $50 to $75 per ton to depths of about 800 feet. Ore quality declined with depth to $4 a ton or less below 3000 feet. The mine also produced low-grade "gray ore" consisting of hydrothermally altered and mineralized greenstone wall rock containing 2% to 4% sulfides. Gray ore could yield an average of $77 a ton. Sulfides were principally pyrite and arsenopyrite.

Comment (Development): Bunker Hill Mine was first worked in 1852 as the Rancheria Mine. Early operations included an open cut, then later a south shaft only a short distance from the adjoining Original Amador Mine. In 1863, the Bunker Hill Quartz Mining Company was organized and during the next few years the south shaft was sunk to 450 feet on the incline (Logan, 1934). Soon after, a 12-stamp mill was built. Later, a north shaft was started 360 feet from the first and both were in use for many years. By the 1860s and 1870s, ore yielding from $50 to $75 per ton was being processed (Logan, 1934). In 1880, a 40-stamp mill and chlorination plant were built. By 1888, the north shaft reached a depth of 800 feet on the incline (680 feet vertical) and the mill was crushing an average 99 tons a day. By 1893, the mine had been idle long enough that the main shaft had caved and the plant had deteriorated. In that year, the Mayflower and Nevada claims were added to the Bunker Hill claim and with 2587 feet on the lode, the mine was reopened as the South Mayflower Mine. In 1894, the mine produced $30,000 and in 1895 a claim in the hanging wall greenstone known as the East Mayflower yielded $5,000. In 1899, the Bunker Hill Consolidated Mining Company was organized and operated the mine as the Bunker Hill Consolidated Mine until 1922 during which time the company paid dividends of about $1,000,000 on an original capitalization of $200,000. During this period, 887,585 tons of ore were produced which yielded $3,834,550. By 1916, the ore being mined averaged about $4 a ton while costs kept increasing. From May 1917 to February 1920, mill operation was at half capacity and ceased altogether in 1920 due to poor economics. When the mine property was passed to the new owner in 1922, little milling was done and most attention was given to a search for new ore bodies in the black slate, but none were found (Logan, 1927). The Bunker Hill Mine produced a total of $5,154,382 during its lifetime.

Comment (Economic Factors): Clark (1970) reported that the Bunker Hill Mine produced $5.1 million.

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References

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

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): Clark, W.B., 1970, Gold districts of California: California Division 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: California Journal of Mines and Geology, 50th Report of the State Mineralogist, p. 173.

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

Reference (Deposit): Koschmann, 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): Logan, C.A., 1921, Amador County, Bunker Hill Mine: California State Mining Bureau, 17th Report of the State Mineralogist, p. 408-409.

Reference (Deposit): Logan, C.A., 1927, Amador County, Bunker Hill Mine: California State Mining Bureau, 23rd Report of the State Mineralogist, p. 157-158.

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 72-74.

Reference (Deposit): Storms, W.H., 1900, The Mother Lode region of California: California Mining Bureau Bulletin 18, p. 80.

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, Bunker Hill Mine: California State Mining Bureau, 14th Annual Report of the State Mineralogist, p. 20-22.

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