Evolution of the hydrothermal alteration at the Chuquicamata porphyry copper system, northern Chile

Chuquicamata, in northern Chile, is one of the world's greatest natural concentrations of copper. The Chuqui Intrusive Complex (CIC), developed during the Eocene-Oligocene, is composed of the heavily mineralized and hydrothermally altered Este, Oeste and Banco porphyries, and truncated by the West Fault. Juxtaposed across the West Fault lies the unaltered and unmineralized Fortuna Intrusive Complex (FIC).; The initial objective of this study was to unravel the evolution of hydrothermal alteration and its relation to intrusion of the various igneous phases. At the same time it was intended to answer some fundamental questions with practical implications, such as: (1) Is the FIC genetically related to the CIC or an extraneous body? (2) Was Chuquicamata formed by a protracted event or multiple phases of alteration? (3) What was the character of the fluids involved in the development of the main (potassic and quartz-sericitic) alteration zones? (4) What processes formed the conspicuous K-feldspar megacrysts, and important anhydrite veins of Chuquicamata? For this purpose a large sample set was studied using optical microscopy, electron probe microanalysis, lithochemistry and stable isotopes, and the study was developed in parallel to a geochronological study.; Petrographically and geochemically, the FIC is too felsic to be the source of the Este Porphyry magmas. High precision 40Ar/39 Ar dating of hornblende, biotite and K-feldspar indicates the FIC is ∼37.6–35.5 Ma, indicating it crystallized and cooled prior to the emplacement of the CIC (34–33 Ma).; The potassic alteration zone at Chuquicamata affects the Este Porphyry and is characterized by an assemblage of albite, K-feldspar, biotite, quartz and rutile. Similar textures and 40Ar/39Ar ages suggest that the potassic alteration zone did not result from an overprinting by a separate intrusion, but represents a more hydrous development phase of the fresh Este Porphyry. Stable isotope analyses suggest the potassic alteration zone was in equilibrium with magmatic fluids at 535°C, 60°C lower than the fresh Este Porphyry. The lack of Ca-bearing silicate minerals in the potassic zone resulted from high halogen-contents that preferentially partition Ca into the melt and fluid phase. (Abstract shortened by UMI.)...