A geologic and microscopic study of a magmatic copper sulphide deposit in Plumas County, California, and its modification by ascending secondary enrichment
Economic Geology and The Bulletin of the Society of Economic Geologists
The copper deposits described in this paper are situated in Plumas County, California, on the west slope of the east range of the Sierra Nevada to the northeast of Indian Valley and are embraced in the claims of the Engels Copper Mining Company. The nearest railroad point is Keddie on the Western Pacific R. R., about thirty miles distant by wagon road. The location is shown on the accompanying sketch map (Fig. 9o) . The general geology of the region is described in Bulletin 353 of the U.S.
... gical Survey by J. S. Diller, which, comains two geological maps. In that portion of the district northeast of Indian Valley, with which we have to do, the prevailing rock is granitic in character and is designated granodiorite by Diller, for mapping purposes, although in the neighborhood of the mines all the dioritic rocks are basic and 'contain but little free quartz, as will appear in the detailed descriptions of Professor Rogers. This dioritic rock is intrusive in Jura-Trias and Carboniferous sediments and associated meta-andesites and meta-rhyolites, but all the ore bodies referred to in this paper are in the dioritic series. The copper deposits of the main Engels mine occur in a fresh massive dioritic rock in which most of the fractures, except those referred to under pegrnatites, are distinctly post-mineral. The most abundant metallic minerals are magnetite, bornite and chalcopyrite: ' Pyrite is conspicuously absent. These metallic • Publ, ished b•r 'permission of Mr. E. H.