Contrasting Landscape Evolution and Development of Supergene Enrichment in the El Salvador Porphyry Cu and Potrerillos-El Hueso Cu-Au Districts, Northern Chile
Thomas Bissig, Rodrigo Riquelme, 2009. "Contrasting Landscape Evolution and Development of Supergene Enrichment in the El Salvador Porphyry Cu and Potrerillos-El Hueso Cu-Au Districts, Northern Chile", Supergene Environments, Processes, and Products, Spencer R. Titley
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The landscape from the Central Depression to the eastern Precordillera in the southern Atacama Desert, northern Chile (lat. 26°–27° S), is dominated by multiple pediplains which formed from the late Oligocene through the middle Miocene, followed by canyon incision in the late Miocene. The oldest such planer surfaces are observed in the eastern Precordillera and are represented by the probably late Oligocene Sierra Checos del Cobre and the ≥17.5 Ma Asientos surface. The El Hueso disseminated epithermal Au deposit and La Coya porphyry Au-Cu prospect are hosted beneath the Sierra Checos del Cobre surface. In the western Precordillera and adjacent Central Depression, planar landforms formed throughout the middle Miocene in several stages and gave rise to the Atacama pediplain, which is composite in nature. The El Salvador porphyry Cu deposit and associated exotic mineralization is situated at the back scarp of the Atacama pediplain.
Supergene activity at El Salvador occurred from the late Eocene to late middle Miocene, but, based on published ages, was most important from ~25 to ~19 and ~15 to ~12.5 Ma. The multistage landscape evolution and related supergene activity at El Salvador during the late Oligocene and middle Miocene was favorable for the efficiency of supergene enrichment and exotic mineralization but climate desiccation after 13 Ma led to cessation of supergene processes. In contrast, El Hueso, La Coya, and Potrerillos were situated below a relatively stable landscape throughout most of the Miocene. Published alunite ages from El Hueso indicate that supergene activity took place in the late Oligocene and late Miocene but was absent through most of the early and middle Miocene. Late Miocene supergene processes coincided with a time when the Precordillera was uplifted and attained an elevation sufficient to capture increased orographically controlled precipitation. This increase in rainfall led to the incision of the El Salado and Asientos canyons, which, combined with the increased availability of meteoric water, exposed new hypogene mineralization to oxidation in the late Miocene, at a time when at El Salvador supergene activity had ceased.
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At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.