Abstract

Post-ore rhyolitic to dacitic dikes up to one hundred feet wide cut the ore-bearing veins in the Butte district. Pyrite-bearing chalcocite ore, typical of Butte's Central Zone, is converted to massive bornite and chalcopyrite adjacent to the dikes. The reaction is interpreted as a duplication in nature of laboratory furnace experiments in which similar products were obtained by heating chalcocite and pyrite together in the absence of excess sulphur.The textural patterns evolved between chalcopyrite and bornite adjacent to the dikes, and between bornite and chalcocite in the lower intensity metamorphosed zone farther from the dikes, must all have resulted through exsolution in the solid state, with possible minor readjustments owing to inversions and recrystallization during the declining thermal history after exsolution.

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