Abstract

Equigranular monzonite of the Bingham stock, the main host for the Bingham porphyry copper deposit, was an augite-actinolite-biotite monzonite before alteration, the same composition as the largely unaltered and unmineralized Last Chance stock adjacent to the mine. Mineralogy and chemistry of the alteration is quantified in a 31-sample traverse which illustrates changes in rock composition that occur over a distance of 7,000 feet outward from the center of the orebody. Plagioclase becomes more soda rich, orthoclase becomes richer in potassium, and trioctahedral mica becomes richer in magnesium toward the center of the deposit. Plagioclase is progressively replaced by K-feldspar and augite is replaced by actinolite which is in turn replaced by phlogopite. Trends in whole-rock chemical composition are congruous with these mineral trends. Based on mineral abundance, two main zones are recognized in pervasive hydrothermal alteration, a central quartz-orthoclase-phlogopite zone corresponding to potassium silicate alteration, and an outer actinolite-chlorite-(epidote) propylitic zone.

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