Abstract

A copper, gold, and silver deposit of the porphyry type occurs adjacent to an altered granodiorite at Copper Canyon, in Lander County, Nevada. The ore is predominantly hypogene disseminated chalcopyrite-pyrite-pyrrhotite in the wall rock of the granodiorite; ore also occurs in veins. The total volume of the adjacent 1,500-meter-wide granodiorite is small, and its shape is approximately laccolithic. Three primary biotite separates from the granodiorite yielded an approximate age of 38 million years (Theodore et al., 1973). The fluids that deposited the ore were chiefly brines with salinites of about 40 weight percent NaCl equivalent. A light stable isotope study of quartz and biotite (both primary and secondary) was undertaken together with standard heating- and freezing-stage studies of fluid inclusions. Fluid inclusion water in quartz yielded delta D values that range from --102 to --76 per mil. The calculated delta D values of water in equilibrium with biotite at temperatures indicated by the fractionation of 18 O between quartz and biotite ranged from --99 to --76 per mil. The calculated delta 18 O values of water in equilibrium with quartz ranged from +2.7 to +9.2 per mil. Calculated delta 18 O values of water in equilibrium with biotite range from +5.8 to +10.4 per mil. These data suggest that the ore-forming fluids were most likely composed of magmatic water that had mixed with some meteoric water during metallization.

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