Pacific Quartz Mine

Site Name

Primary: Pacific Quartz Mine

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Commodity

Primary: Gold
Tertiary: Silver

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Location

State: California
County: El Dorado
District: Placerville District

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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: El Dorado 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: 1852
Years of Production:
Organization:
Significant: Y
Deposit Size: S

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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

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Structure

Type: L
Description: Melones Fault Zone

Type: R
Description: Melones Fault Zone

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Alterations

Not available

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Rocks

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: Arsenopyrite
Ore: Pyrite
Gangue: Slate
Gangue: Talc
Gangue: Muscovite

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Comments

Comment (Geology): REGIONAL GEOLOGY The Pacific Quartz Mine is located in 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 auriferous gravel deposits and 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. The structural belts are internally bounded by the Melones and Bear Mountains fault zones and are characterized by extensive faulting, shearing, and folding (Earhart, 1988). In the El Dorado County area, gold deposits occur in the West Belt, the Mother Lode Belt, and the East Belt. The Mother Lode Belt is responsible for most of the gold produced in the county. There has also been substantial gold produced from the West Belt and East Belt (Clark and Carlson, 1956). The West Belt consists of widely scattered gold deposits located west of the Mother Lode vein system. Gold occurs in irregular quartz veins in schist and granitic rocks, altered mafic rocks, and as gray ore in greenstone. The West Belt has been further divided by some authors into an eastern component composed of an ophiolitic melange and a western component composed of Jurassic rocks of the Copper Hill volcanics (Duffield and Sharp, 1975; Saleeby, 1982; Clark, 1964). The Copper Hill volcanics consist of mafic to felsic flows and pyroclastic rocks that are metamorphosed to greenschist and amphibolite facies. The Bear Mountains Fault Zone separates the melange from the Copper Hill volcanics. The Mother Lode Belt consists of the Upper Jurassic Logtown Ridge and Upper Jurassic Mariposa formations. The Logtown Ridge Formation consists of about 6,500 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. The overlying Mariposa Formation contains a distal turbidite, hemipelagic sequence of black slate, fine-grained tuffaceous rocks, and subvolcanic 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 (Earhart, 1988). Mother Lode Belt mineralization is characterized by steeply dipping gold-bearing quartz veins that traverse western El Dorado County. The belt trends north through Nashville, northeast through Placerville, and northwest to Garden Valley. At Garden Valley, the Mother Lode Belt splits. The west branch extends northwest through Greenwood and includes the veins of the Pacific Quartz Mine. The east branch extends north through Georgetown to the Georgia Slide area (Busch, 2001). The Mother Lode veins are generally enclosed in Mariposa Formation slate with associated greenstone. The vein system ranges from a few hundred feet to a mile or more in width. Within the zone are numerous discontinuous or linked veins, which may be parallel, convergent, or en echelon. The veins commonly pinch and swell. Few can be traced more than a few thousand feet. Mother Lode type veins fill voids created within faults and fracture zones and consist of quartz, gold and associated sulfides, ankerite, calcite, chlorite, and sericite (Clark and Carlson, 1956).

Comment (Development): The Pacific Mine was discovered in 1852. It was operated from 1852-1889 and again during 1914-1915. By 1854 it had a small 2-stamp mill. From 1854 to 1861 the mine expanded and produced $480,000, the mill having been increased to 4, 10, and ultimately 20 stamps. Production up to 1883 was about $1,000,000 and some production was made up to 1889 when $6,000 production was reported. The mine then lay idle for 20 years. Efforts to reopen the mine in 1910 were suspended before operations could commence. In 1914, a 5-stamp mill was built, and small tonnages of ore that yielded about $5 per ton were crushed then and in 1915. This was partly from the dump and partly from the orebody (Logan, 1934). The Pacific Quartz Mine became one a large group of lode and placer mining claims owned by Placerville Gold Mining Company and extending for about 3 miles along the Mother Lode and comprising 1,400 acres.

Comment (Economic Factors): Clark (1970) reported that the Pacific Mine produced $1,486,000.

Comment (Identification): The Pacific Quartz Mine is located in the famous Mother Lode Gold Belt in the Sierra Nevada foothills of western El Dorado County. It is the most important of several lode mines in the Placerville District, which is more renowned for it extensive deposits of Tertiary auriferous gravels. Drift and hydraulic mining of the gravels produced approximately $25 million, while lode mines are thought to have produced about $2 million, with the Pacific Quartz mine being responsible for $1,486,000 of that amount. The mine developed a typical Mother Lode quartz vein carrying free milling gold and auriferous sulfides within a narrow band of the Mariposa Formation slate which trends northward through the Placerville district. The principle producing vein is the Pacific vein which was developed by 700 foot shaft and a 1,365 foot winze sunk from the 700 foot level before the mine was idled in 1915.

Comment (Location): Location selected for latitude and longitude is the plotted map location in an unpublished CGS field report.

Comment (Geology): The Melones Fault zone separates the Mother Lode Belt from the East Belt. The East Belt lies in the south central part of El Dorado County approximately 15 miles east of the Mother Lode belt. The East Belt traverses the county from the southern county line, north through Omo Ranch and Grizzly Flat, and apparently terminates near the Hazel Creek Mine east of Jenkinson Reservoir (Busch, 2001). The Eastern Belt is dominantly argillite, phyllite and phyllonite, and chert of Paleozoic age. Other rocks in the Eastern Belt include a Jurassic granodiorite pluton near the Cosumnes River and small bodies of Jurassic serpentinite, gabbro, diorite, and limestone. The Paleozoic metasedimentary rocks of the Eastern Belt have been assigned to the Shoo Fly Complex by most investigators. Lode deposits of the East Belt consist of many individual gold-bearing quartz veins enclosed in metamorphic rocks of the Shoo Fly complex, or in granitic rocks. Most of the veins trend northward and dip steeply. An east-west set of intersecting faults may be a controlling factor in controlling deposition of ore. Ore deposits of the East Belt are smaller and narrower than those of the Mother Lode, but commonly are more chemically complex, and richer in grade. Gold is usually associated with appreciable amounts of pyrite, chalcopyrite, pyrrhotite, galena, sphalerite, and arsenopyrite (Clark and Carlson, 1956) LOCAL GEOLOGY The Placerville District includes both Mother Lode quartz lode mines and extensive Tertiary placer deposits. The geology of the placer deposits of the district are not discussed here, but are addressed elsewhere in the Mineral Resources Data System (MRDS) database under the "Placerville District". A narrow belt of Mother Lode Mariposa Formation slate bedrock, approximately one to two miles wide, extends northward through the district separating schist and slate of the Calaveras Complex to the east from greenstone and amphibolite on the west (Clark, 1970). The principle productive vein was the Pacific vein within a zone of talc and mariposite in Mariposa Formation slate on the west side of the Mother Lode. Peripheral serpentinite bodies are nearby. The vein strikes N 25? W and dips 70? NE. In much of the mine, the hanging wall and footwall are composed of black slate. At 1,600 feet, the vein was in an ankerite zone and was ribbon rock in places colored by mariposite (Logan, 1934). A number of ore shoots several hundred feet long and as wide as 12 feet were developed. The ore yielded $6-$18 per ton in gold, and contained considerable amounts of pyrite, arsenopyrite, and silver (Clark and Carlson, 1956). Sulfide concentrates made up 0.5 percent of the ore and assayed $85 per ton in gold and $2 per ton in silver (Logan, 1934).

Comment (Workings): The Pacific Quartz Mine was developed by the 700-foot Pacific shaft with levels at 100, 200, 300, 400, 500, 600, and 700. A 1,365 foot winze at 70? incline sunk from 200 feet north of the shaft on the 700-foot level. Drifts: On 300-foot level - N 200 feet On 500-foot level - N 1,000 feet & S 1,500 feet On 700-foot level - N 1,200 feet & S 250 feet Crosscut from shaft on 700-foot level to Pacific vein a distance of 80 feet E. Crosscut from shaft on 700-foot level to black slate hanging wall a distance of 300 feet. Ore was stoped form the 500-foot level to the surface. This vein was thought to have been lost by being faulted eastward between the 300 and 400-foot levels. Later work in that direction failed to reveal any ore. From the 700-foot level 200 feet north of the Pacific shaft, a winze was sunk at an angle of 70? for an inclined depth of 1365 feet. This winze encountered the footwall slate at 1,600 feet in depth and continued in it to the bottom (Logan, 1934). Extensive diamond drilling was done from the 1,700-foot and 2,000-foot levels, but over 8,000 feet of hole failed to show ore.

Comment (Commodity): Commodity Info: Quartz ore yielded between $6 - $18/ton Sulfides comprised less than 1% of the ore and assayed $85/ton

Comment (Commodity): Ore Materials: Free-milling gold and auriferous arsenopyrite and pyrite

Comment (Commodity): Gangue Materials: Slate, talc, mariposite

Comment (Deposit): The Pacific Quartz Mine produced from typical Mother Lode-type low-sulfide mesothermal gold-quartz veins. The principal vein was the Pacific vein striking N 25? W and dipping 70? NE. Ore consisted of free gold and auriferous pyrite within discontinuous ore shoots enclosed within a zone of talc and mariposite within black slates of the Upper Jurassic Mariposa Formation. Quartz ore generally yielded between $6 - $18 per ton. Sulfide concentrate comprised less than 1% of the ore and consisted of pyrite and arsenopyrite assaying $85 per ton gold and $2 per ton silver.

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References

Reference (Deposit): Busch, L.L., 2001, Mineral land classification of El Dorado County, California: California Geological Survey Open-File Report 2000-03.

Reference (Deposit): Clark, L.D., 1964, Stratigraphy and structure of part of the western Sierra Nevada metamophic belt, California:

Reference (Deposit): U.S. Geological Survey Professional Paper 410, 70 p.

Reference (Deposit): Clark, W. B., 1970, Gold districts of California: California Divisions of Mines and Geology Bulletin 193, p. 107-108.

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. 422.

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): Earhart, R.L., 1988, Geologic setting of gold occurrences in the Big Canyon area, El Dorado County, California: U.S. Geological Survey professional Paper 1576, 13 p.

Reference (Deposit): Irelan, W., Jr., 1888, El Dorado County, Church Mine: California State Mining Bureau, 8th Annual Report of the State Mineralogist, p. 183-185.

Reference (Deposit): Logan, C.A., 1926, El Dorado County: California State Mining Bureau, 22nd Report of the State Mineralogist, p.417-418

Reference (Deposit): Logan, C.A., 1934, Mother Lode gold belt of California: California Division of Mines Bulletin 108, p. 35, 52.

Reference (Deposit): Logan, C.A., 1938, Mineral resources of El Dorado County: California Division of Mines, 34th Report of the State Mineralogist, p. 266.

Reference (Deposit): Saleeby, J., 1982, Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development: Journal of Geophysical research, v. 87, n0. 8, p. 1803-1824.

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 Pacific Quartz Mine is contained in File No. 322-5974 (CGS Mineral Resources Files, Sacramento)

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