
Pioneer Mining

Gold Districts - Jackson - Plymouth

Gold Districts of California Gold Districts of California






As mining operations progressed to greater and greater depths, costs increased, especially because in some mines, the grade of ore decreased at depth and it became necessary to mine larger amounts. A number of immense mills were erected, including those at the Kennedy mine, which employed 100 "stamps" each, one at the South Eureka with 80 "stamps" and those at the Argonaut and Oneida, which had 60 "stamps" each. The ground became extremely heavy at depth and required much timbering. As costs continued to increase during the early 1900s and were accelerated during World War I, a number of mines were shut down. The South Spring Hill mine was shut down in 1902, the Lincoln Cons., in 1912, the Oneida and Zeila in 1914, the South Eureka in 1917, and the Bunker Hill and Treasure in 1922. However, the district continued to yield large amounts of gold as the Argonaut, Kennedy, Central Eureka and others increased the size of their operations. The Old Eureka and Central Eureka merged in 1924; the new operation was known as the Central Eureka, for a time as the Hetty Green, as it was controlled by that financier. The district's output increased after the 1934 rise in the price of gold. The veins continued to be developed at greater and greater depths until the Argonaut and Kennedy became the deepest mines in the country. Each has a vertical depth of more than 5900 feet. The Central Eureka, South Eureka, and Plymouth Cons. are more than 4000 feet deep.

All of the mines were shut down soon after the beginning of World War II. The Central Eureka mine was reopened in 1945, but because of greatly increased costs it was shut down again in 1953. This was the last active major gold mine on the Mother Lode.

Jackson-Plymouth was the most productive district of the Mother Lode belt, with a total output estimated by the author at about $180 million. If large-scale gold mining were ever to be done here again, it would be most desirable to consolidate the major mines and operate them as a unit. Some are connected underground and all produced considerable water.

The remaining surface plant of the Kennedy mine is now a museum. The large wooden tailing wheels and the superintendent's office at this mine, long noted landmarks, are historical displays. Several other distinctive old mine buildings have been preserved, including the Keystone mine office, which is a motel, and the Zeila office, which is a private home.

Geology. The gold deposits are in a north- and northwest-trending mile-wide belt of gray to black slate of the Mariposa Formation (Upper Jurassic), with some interbedded coarse and occasionally sheared conglomerate and minor sandy and gritty layers. Massive greenstone of the Logtown Ridge Formation (Upper Jurassic) lies west of the belt of Mariposa Formation slate. Metasedimentary rocks, chiefly graphitic schist, metachert and amphibolite schist of the Calaveras Formation (Carboniferous to Permian) are to the east.