By A. H. KOSCHMANN and M. H. BERGENDAHL - USGS 1968
Modoc County produced about 14,400 ounces of gold from 1880 through 1959. The High Grade district, in the northeast corner of the county, has been the only important gold-mining area.
Gold was first discovered in the district in 1870, but it was not until 1909 that any great interest was shown in prospecting the area (Hill, 1915, p. 38). Apparently the district was never developed on a large scale, and it is doubtful that the total production by 1959 exceeded 11,000 ounces of gold.
The country rock in the district consists of a series of andesite, rhyolite, and basalt lava flows that have been displaced and tilted by movement along generally north-trending normal faults. The ore deposits are in quartz veins and mineralized breccia zones in rhyolite and andesite. The vein minerals are quartz, adularia, and small amounts of pyrite and gold (Hill, 1915, p. 40-47).
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