Abstract

Most of the porphyry copper deposits are characterized by a similar pattern of lateral zoning involving sulfides and silicate alteration assemblages. A central zone of biotite-orthoclase alteration is surrounded by a partial to complete halo of quartz-sericite alteration. Propylitic alteration commonly forms a further halo if rocks of appropriate composition are present. The hypogene sulfides also are zoned, from a central zone with a low pyrite/chalcopyrite ratio and generally low total sulfides, through a zone of moderate sulfides with dominant chalcopyrite, then to a halo of high pyrite/chalcopyrite and high total sulfides which grades outward to lower total sulfides. The highest copper content normally occurs within the biotite-orthoclase zone or at its outer borders with quartz-sericite alteration. The silicate and sulfide zoning is generally centered on a granitic porphyry stock.Lead and sulfur isotope data, as well as the zoning, suggest that the ore fluid traveled outward from the porphyry, starting at high temperatures and relatively high K/H ratios in the orthoclase field, thereby forming the biotite-orthoclase alteration. In order to form the strong sericite alteration, cooling of the fluid is concluded to be the most probable means of changing mineral equilibria. A model calculation, for a large porphyry copper ore body, of heat losses to wall rock during outward flow of the solution shows that this mechanism cannot be the major means of cooling. The calculation also demonstrates that mineral deposition from solutions as dilute as 1-100 ppm base metals encounters grave difficulties in cooling from temperatures of 500 degrees C or above, because of the huge volumes of solution involved to form a porphyry copper orebody, and the resulting huge amount of heat that must be dissipated. Copper concentrations in the ore fluid on the order of 1,000 ppm or higher are therefore suggested. Cooling by expansion or by mixing with cool ground water seems an attractive means of bringing the high temperature ore fluid into the muscovite stability field. These processes are probably also instrumental in causing metal sulfide precipitation.

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