Abstract

The Santa Rita intrusive of Southern New Mexico consists of highly altered and strongly fractured granodiorite porphyry. Minor fracturing of the ingneous body is probably the combined effect of localized tectonic activity and stress developed in the crystallized shell of the cooling mass by continued movement of the magma. Previous studies indicate four stages in the hydrothermal alteration of the porphyry, starting with nearly fresh rock and ending with a highly siliceous and sericitized counterpart. Chemical changes of wall-rock alteration indicate a loss of Na2O, CaO, MgO, and SiO2, with a relative gain in water of hydration and sulfur in argillized rock. Loss of Na2O, CaO, MgO, and FeO continues during sericitization, while potash, sulfur, and probably silica increase. Copper mineralization consists of disseminated supergene chalcocite and pyrite coated with chalcocite. Chalcocite also coats quartz-sericite-pyrite veinlets. Chalcopyrite and bornite are hypogene. Intergrowths of chalcopyrite in bornite suggest that some of the hypogene copper mineralization took place at 475° C or higher. Primary copper minerals seem to antedate the quartz sericitization of the Santa Rita porphyry. The concentration of supergene chalcocite in stage 3 seems to be due partly to the relatively greater porosity and higher pyrite content of this rock. The high porosity and pyrite content of sericitized porphyry may similarly render stage 4 favorable to economic chalcocite mineralization. Stage 3 or argillization of granodiorite appears to be the stage of alteration most favorable to ore, for, besides having a porosity and pyrite content high enough to facilitate noteworthy secondary enrichment, this stage of hydrothermal alteration also furnished the hypogene copper minerals which constitute the protore of the deposit.

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