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QUANTITATIVE MINERALOGIC EVALUATION OF THE “TRANSITION ZONE” ENVIRONMENT: PRELIMINARY OBSERVATIONS FROM THE CERRO COLORADO PORPHYRY COPPER SYSTEM, REGIÓN I, NORTHERN CHILE

The 51.8 Ma Cerro Colorado porphyry copper deposit is located approximately 130 km eastnortheast of Iquique, Región I, Northern Chile. As part of our preliminary study of the “transition zone”, that rock volume comprising the geochemical change from supergene metals accumulation to essentially unoxidized hypogene copper sulfides, fifty ten-meter composite pulp samples were collected from six drill holes representing various ore environments within the Cerro Colorado hydrothermal system. Heavy mineral separates were obtained from all of the ten-meter composite pulp samples, with polished grain mounts of these separates examined using standard reflective light petrographic techniques. For each sample a technique called line integration (Brimhall, G.H, Jr., 1977, Early fracturecontrolled disseminated mineralization at Butte Montana: Econ. Geol. v. 72, p. 37-59 ) was used to determine the relative volume percents of each sulfide mineral present. Preliminary observations of these samples have identified three distinct mineralogic zones: 1) Supergene enrichment zone; 2) Transition zone; and 3) Hypogene protore zone. The supergene enrichment zone is characterized by well-developed chalcocite replacement of pyrite grains and is composed of a pyrite + chalcopyrite + chalcocite mineral assemblage. The “transition zone” is characterized by the incomplete replacement of hypogene bornite, chalcopyrite, and pyrite by chalcocite and covellite and such is composed of a mixed hypogene/supergene mineral assemblage of pyrite + chalcopyrite + bornite + chalcocite + covellite. Mineral ratios of this zone illustrate a general trend, from base of enrichment to hypogene mineralization, of decreasing chalcocite-bornite ratios, increasing pyrite-chalcocite ratios, and an abrupt decrease in the chalcocite-covellite ratio immediately above hypogene mineralization. The partial replacement of hypogene bornite, chalcopyrite, and pyrite by supergene chalcocite and covellite, along with the presence of supergene bornite and chalcopyrite, suggest that the supergene copper-bearing solutions responsible for enrichment could not maintain the low pH and/or copper concentrations need to completely replace these hypogene sulfides. The hypogene zone at Cerro Colorado is characterized by the mineral assemblage pyrite + chalcopyrite + bornite and is interpreted to represent copper sulfides associated with the emplacement of the Cerro Colorado magmatic hydrothermal system.