Abstract

Mineralogy and chemical composition of the altered Commercial limestone were investigated in surface rocks over a 1,500-meter interval outward from the monzonite stock which contains the Bingham porphyry copper deposit. Skarn alteration is divided into six mineralogical zones: (1) marble, (2) wollastonite, (3) cristobalite, (4) garnet-clay, (5) garnet-quartz, and (6) amphibole-epidote, in order approaching the intrusion. Outside the boundary of economic copper mineralization, alteration is dominantly controlled by faults, fractures, and sedimentary features. Within the copper orebody, coinciding with the area of most intense silication, relative proximity to the quartz monzonite porphyry is the most important factor in formation of mineralogical zones. These zones develop at the expense of adjacent outer zones, leaving evidence of overprinting at the periphery of each zone. Comparison of bulk chemical compositions in unaltered and altered limestone indicates that alteration was accompanied by overall addition of SiO 2 , Al 2 O 3 , MgO, and MnO and depletion of CaO and CO 2 ; addition of TiO 2 , BaO, Cu, Pb, and Zn in the marble and wollastonite zones; and addition of Cu, Mo, Zn, and Ag in the ore-grade zones.

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