Prograde Evolution and Geothermal Affinities of a Major Porphyry Copper Deposit: The Cerro Colorado Hypogene Protore, I Region, Northern Chile

摘要

The middle Eocene (51.8 ± 0.6 Ma) hypogene protore underlying the supergene orebody of the Cerro Colorado porphyry Cu (-Mo) deposit, I Región, Chile, exhibits features not commonly documented in such hydrothermal systems. Early-stage alteration of Upper Cretaceous plagioclase-phyric andesite generated a sub-horizontal blanket of pervasive, extremely fine grained but texture-preserving biotite (35 modal %)-albite (40%)-magnetite (3%) alteration, 8 km2 in area but lacking sulfide minerals. At least seventy percent of the chalcopyrite > pyrite stockwork mineralization was emplaced during the subsequent Main-stage alteration, which comprises, with decreasing depth, quartz-albite, sericite-chlorite-clay (smectite), quartz-sericite-clay, and andalusite-diaspore-pyrophyllite assemblages. The deposit is apparently unique among documented central Andean porphyry systems in the association of the highest grade copper mineralization with intermediate argillic alteration. The subsequent Transitional-stage phyllic (i.e., quartz-sericite-pyrite ± tourmaline) alteration was associated with the emplacement of molybdenite-rich breccia bodies. The occurrence of undumortieri-tized tourmaline veinlets cutting andalusite-diaspore assemblages confirms that much of the advanced argillic alteration took place during the Main stage.