Evolution of an Active Ductile to Brittle Shear System Controlling Mineralization at the Chuquicamata Porphyry Copper Deposit, Northern Chile

Abstract

The giant Eocene-Oligocene Chuquicamata porphyry copper deposit in northern Chile illustrates a remarkable degree of structural control from the time of porphyry emplacement through mineralization-alteration and supergene enrichment. The porphyry system developed within a transtensional faultbend related to the > 2000-km-long, trench-parallel shear system known as the Precordilleran Fault System, which has been active in the Precordillera since at least the Eocene. Detailed structural sampling and mapping at scales from 1:10 to 1:1000 within representative mineralized structural domains defined within the Chuquicamata open pit reveals that the bulk of mineralization is contained in fractures with defined preferred orientations that impart an anisotropy to ore grades at certain scales. Cross-cutting relationships in one domain, Estanques Blancos, establish a relative chronology of alteration-mineralization pulses related to an older (34 Ma), deeper potassic and a younger, superimposed (31 Ma), shallower quartz-sericitic event. The timing of these mineralizing events is supported by argon geochronology. Although there is evidence of shear-related ductile deformation from pre-porphyry times, fracture permeability was readily accessible to intrusions, hydrothermal fluids, and supergene waters at this localized extensional site, which may explain the remarkable copper concentration in Chuquicamata. Kinematic indicators are compatible with a local N-S dextral shear system during the bulk of mineralization. Fabrics range from ductile through ductile-brittle and brittle, followed by regional sinistral brittle events within the Domeyko Fault system.