This district is in southwestern Tuolumne County and northwestern Mariposa County. It is that portion of the Mother Lode gold belt that extends from the vicinity of the McAlpine mine southeast through Penon Blanco and the town of Coulterville to Virginia Point, a distance of about 10 miles.
The surface portions of the veins and the streams were worked during the gold rush. The town was named for George Coulter, who opened a store there in 1849. Quartz-mining began about 1852 with the discovery of the Malvina and Mary Harrison veins. Considerable lode mining was done from the 1860s through the 1890s, when many of the mines belonged to the Cook estate. There was some activity from the early 1900s until 1942. In recent years some work has been done at the Virginia, McAlpine and a few other mines.
In this district, the Mother Lode system is associated with an extensive northwest-trending lenticular body of serpentine. In a number of places the serpentine has been hydrothermally altered to mariposa-Quartz-ankerite rock. Slate and greenstone lie to the wesr, and amphibolite schist and greenstone are to the east. Also present are smaller amounts of chlorite schist, phyllite, and metadiorite and aplitic dikes.
The are deposits consist of pyrite-bearing bodies of mariposite-quartz-ankerite rock, with numerous parallel quartz stringers and adjacent massive quartz veins. The gold occurs in the free state or with the pyrite and usually is found in the quartz stringers, although in places there are values in the massive veins. A number of high-grade pockets have been found in this district, some of which came from the gossan that overlies some of the deposits. Tellurides also have been found here. Milling are usually ranges from 1/5 to 1/2 ounce of gold per ton. The are shoots had stoping lengths of up to 400 feet and were mined to inclined depths of more than 1200 feet. The massive veins form prominent ridges; Penon Blanco Ridge is visible for many miles.
Mariposa County: Adelaide, Big Lode, Champion I $150,000 to $200,000, Flyaway group, Louisa $100,000, Malvina $1 million, Mary Harrison $1.5 million, Midas, Oro Rico, Potosi, Tyro $110,000+, Virginia $824,000. Tuolumne County: McAlpine $100,000+
Bowen, O. E., Jr., 1957, Mariposa County, Virginia Mine: California Jour. Mines and Geology, vol. 53, nos. 1-2, pp. 176-179.
Castello, W.O., 1921, Mariposa County, Coulterville district: California Min. Bur. Rept. 17, pp. 92-93.
Logon, C. A., 1935, Mother Lode gold belt-Moriposa County: California Div. Mines Bull. 108, pp. 180-190.
Lowell, F. lo, 1916, Mariposa County, Mary Harrison quartz mine: California Min. Bur. Rept. 1.4, p. 588.
Ransome, F. lo, 1900, Mother Lode district folio: U. S. Geol. Survey Atlas of the U. S., folio 63, 11 pp.
Storms, W. H., 1900, The Mother Lode region-Mariposa County: California Min. Bur. Bull. 18, pp. 142-147.