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Pacific Quartz Mine (Pacific Mine), Placerville (Hangtown), Placerville District, Mother Lode Belt, El Dorado Co., California, USA

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A former lode Au-Ag-As occurrence/mine located in sec. 7 & in the NE¼ sec. 18, T10N, R11E, MDM, 0.9 km (0.5 mile) WSW of Placerville proper (S part of the city within the city limits), on private land. Discovered in 1852. Owned by the Placerville Gold Mining Co. (1956) . MRDS database stated accuracy for this location is 500 meters.

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 (period values), the mill having been increased to 4, 10, and ultimately 20 stamps. Production up to 1883 was about $1,000,000 (period values) and some production was made up to 1889 when $6,000 (period values) 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 ore body (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.

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 (period values), with the Pacific Quartz mine being responsible for $1,486,000 (period values) 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.

Mineralization is a vein 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 Late Jurassic Mariposa slate. The ore body is tabular in form, strikes N25W and dips 70NE. The vein is in a talc and mariposite zone in slate W side of the Mother Lode. Serpentine bodies are nearby. A number of ore shoots are several hundred feet wide and up to 12 feet thick. Controls for ore emplacement included open fracture filling. Local rocks include Jurassic marine rocks, unit 1 (Western Sierra Nevada and Western Klamath Mountains) and/or Mesozoic volcanic rocks, unit 2 (Western Sierra Foothills and Western Klamath Mountains).

Local and regional geologic structures include the Melones Fault Zone.

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).

Workings include underground openings comprised of a 700 foot shaft with a 1,365 foot winze.

The Pacific Quartz Mine was developed by the 700-foot Pacific shaft with levels at 100, 200, 300, 400, 500, 600, and 700. Drifts were 200 feet N on the 300 foot level; 1,000 feet N & 1,500 feet S on the 500 level; and, 1,200 feet N & 250 feet S on the 700 level. A Crosscut was driven from shaft on 700-foot level to Pacific vein a distance of 80 feet E. A crosscut was driven from the 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 degrees 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.

Production data are found in: Clark & Carlson (1956).

The ore yielded $6 to $18 (period values) Au/ton.

Mineral List



8 entries listed. 5 valid minerals.

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References

Trask, John Boardman (1856), Report on the geology of northern and southern California: California Legislature, 7th. sess., Ap. to Jour. S. Doc. 14, 66 pp.: 28.

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

Lindgren, Waldemar (1911), The Tertiary gravels of the Sierra Nevada of California: USGS Professional Paper 73, 226 pp.: 173.

Knopf, Adolf (1929), The Mother Lode system of California: USGS PP 157, 88 pp.; […(abstract): Engineering & Mining Journal: 128: 24 (1929); […Geol. Zentralbl., Band 41: 364-367 (1930)]: 37.

Logan, Clarence August (1938), Mineral resources of El Dorado County: California Journal of Mines and Geology, California Division Mines (Report 34): 34(3): Pl. II.

Clark, Wm. B. & D.W. Carlson (1956), Mines and mineral resources of El Dorado County, California: California Journal of Mines and Geology (Report 52): 52(4): 422, 524.

Murdoch, Joseph & Robert W. Webb (1966), Minerals of California, Centennial Volume (1866-1966): California Division Mines & Geology Bulletin 189: 268.

Clark, Wm. B. (1970a) Gold districts of California: California Division Mines & Geology Bulletin 193: 107.

Pemberton, H. Earl (1983), Minerals of California; Van Nostrand Reinholt Press: 18 (map 2-7), 434.

Earhart, R.L. (1988), Geologic setting of gold occurrences in the Big Canyon area, El Dorado County, California: USGS Professional Paper 1576, 13 p.

USGS (2005), Mineral Resources Data System (MRDS): U.S. Geological Survey, Reston, Virginia, loc. file ID #10007059, 10211728 & 10310663.

U.S. Bureau of Mines, Minerals Availability System (MAS) file #0060170693.

California Geological Survey Mineral Resources files, Sacramento, California, file No. 322-5974.

 
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