Alexandra Arnott

Instructor, Earth and Environmental Sciences, Dalhousie University

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: I) 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 40 Arl'9 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 40 Ar/39 Ar 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.

The Banco Porphyry was intruded into a cooled and potasically altered Este Porphyry, and has preserved igneous intermediate plagioclase (oligoclase-andesine), in contrast with tbe latter, which contains only albite, indicating that it was not affected by potassic alteration-mineralization. Therefore, the Banco Porphyry was not the source of the potassic alteration, as has been suggested by other workers.

The Quartz-sericite (Qser) alteration zone is characterized by an assemblage of muscovite, quartz, rutile and pyrite. The Qser alteration zone formed at -3 I Ma by reaction with a reducing and highly acidic fluid. Stable isotope studies indicate the fluids responsible for the alteration event were a magmatic-meteoric mixture and yield temperatures of -335° to 400°C. The magmatic component of the Qser alteration event is interpreted to reflect an intrusion at depth, not yet exposed nor recognized in drilling.

Anhydrite veining occurred late in the Qser alteration event. The anhydrite veins are the result of bot, CaCJ+-rich magmatic fluids from depth mixing with meteoric water. The source of the Ca is interpreted to be magma at depth, and Sr isotope data suggest there was no contamination by relatively radiogenic upper crustal rocks.

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Pages: 474
Supervisors: Marcos Zentilli