Hypogene texture and mineral zoning in a copper-bearing granodiorite porphyry stock, Santa Rita, New Mexico

The distribution of hypogene mineralization and alteration around the Santa Rita stock shows that volatiles released on cooling of themagma caused the mineralization. The volatiles migrated outward through cracks in the cooler stock margin. Successive zones, outward from the center, show increasing sulfide content and increasingly destructive alteration of feldspar: (a) montmorillonite and biotite after plagioclase; (b) montmorillonite-kaolinite after plagioclase; (c) kaolinite after plagioclase; (d) quartz-sericite after plagioclase and also after orthoclase. Pyrite, the predominant sulfide, is concentrated, with chalcopyrite, in the outermost zone, at the periphery of the stock. The sequence is attributed to temperature gradient and possibly mixing with ground water.

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