THE RELATIONSHIP BETWEEN INTRUSION AND ALTERATION AT THE CHUQUICAMATA PORPHYRY COPPER SYSTEM, CHILE

Mineralogical and geochemical evidence suggests a complex history of intrusion and alteration/mineralization at the Eocene-Oligocene Chuquicamata porphyry copper deposit, Chile. The granodioritic Chuquicamata Intrusive Complex (CIC), consisting of the Este, Oeste, and Banco porphyries, is truncated by the West Fault. The CIC is overprinted by multiple phases of alteration (e.g. potassic, quartz-sericite) and mineralization. Juxtaposed across the West Fault is the relatively unaltered and unmineralized Fortuna Intrusive Complex (FIC).

The potassic alteration affecting the Este Porphyry is the result of a late hydrous phase of the same magma rather than an overprinting by a separate intrusion. 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 is unaffected by the potassic Ca-leaching event giving a firm relative age of intrusion.

The Qser, formed from a reduced and highly acidic fluid of magmatic-meteoric origin, overprints the potassically altered Este Porphyry and the Banco Porphyry.

Petrography, geochemistry, and geochronology indicate the FIC is unrelated to the juxtaposed CIC. However, the FIC may have shared a common source, thus explaining their similarities.