Abstract

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.

Early-stage alteration was the product of nonboiling, cool (trapping temperature ≤ca. 380°C), low-salinity (≤8 wt % NaCl equiv) fluids which added substantial K, Na, Mg, Fe2+, Cl, F, and water to the host andesite. The initial Main-stage fluids, boiling at a paleodepth of ca. 2.5 to 3.0 km, were up to 160°C hotter (≤544°C) and highly saline (≤52 wt % NaCl equiv). As these fluids rose, they cooled to ≤320°C, were diluted (to ≤37 wt % NaCl equiv), deposited sulfides, and leached K, Na, Ca, Mg, Fe2+ ,and Cl from the host rocks, yielding diverse, broadly contemporaneous, intermediate and advanced argillic alteration facies. Pressure estimates require that low-density (≤0.3 g/cm3), and thus more acidic, fluids were primarily responsible for the formation of the quartz-sericite-clay and shallow advanced argillic alteration. Subsequent phyllic alteration was similarly caused by boiling fluids which were hot (≤486°C) and saline (≤47 wt %) at depth but cooler (≤334°C), dilute (≤8 wt %), and vapor dominated at shallower levels. Even Terminal-stage pyrite veins formed at temperatures as high as 450°C, albeit from low-salinity (≤8 wt %) fluids.

Following a major prograde thermal transition from the Early to the Main stage, each sulfide-depositing alteration episode at Cerro Colorado was generated by a pulse of high-temperature fluid which cooled and diluted as it rose. Such changes in fluid characteristics, temperature, and alteration relationships are well documented in numerous geothermal fields, where potassic alteration generally develops at ca. 270° to 350°C and are likely to occur at an early stage in any hydrothermal system in which magmatic fluids, exsolving at relatively high pressures, ascend into the near-surface environment. The initial alteration at Cerro Colorado plausibly developed under conditions similar to those in nonexplosive geothermal systems, in which hydrothermal fluids cool and disperse laterally at shallow depths. Extensive zones of biotite-rich alteration, generally barren and with a hornfelsic appearance, occur in many andesite-hosted porphyry copper deposits, but few data are available elsewhere for mineralogical, pressure, temperature, or metasomatic exchange relationships in such systems. Cerro Colorado may, however, be representative of a subclass of porphyry copper deposits exhibiting unusually close analogies with geothermal systems.

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