MINERALOGICAL AND GEOCHEMICAL EVOLUTION OF THE POTASSIC ALTERATION ZONE IN THE CHUQUICAMATA PORPHYRY COPPER SYSTEM, CHILE

The giant Chuquicamata porphyry copper deposit, Chile, formed in association with the granodioritic Chuquicamata Intrusive Complex of Eocene-Oligocene age, composed of the Este, Oeste and Banco Porphyries. Potassic hydrothermal alteration in this system, which coincided with a first pulse of copper mineralization, represents a more evolved hydrous stage in the evolution of the Este Porphyry magma. The potassic alteration zone is essentially the same age as the fresh Este Porphyry but the solidus temperature was depressed by the abundant volatiles. The most conspicuous effect of this evolution is the albitization of the plagioclase, which in the fresh Este Porphyry is oligoclase-andesine. K-feldspar in the potassic alteration zone exhibit magmatic features such as perthitic lamellae and crosshatch twinning but only locally displays a regular concentric barium zonation observed in the K-feldspar of the fresh Este Porphyry. The lack of this barium zonation is concluded to be the result of deuteric remobilization of elements during cooling. Potassically altered (phlogopitic) biotite has a higher Mg:Fe ratio, higher F-values and lower Ti-concentration than biotite in the fresh Este Porphyry. The removal of calcium during the potassic alteration event resulted in the replacement of amphibole by biotite and titanite by rutile. The lack of calcium in the potassic zone results from deuteric fluids enriched in halogens leaching the calcium from the Ca-bearing silicates. Calcium was replaced by potassium and sodium in the feldspars and amphiboles when Cl and F-rich fluids preferentially complexed with the calcium. A structurally-controlled and younger quartz-sericite hydrothermal alteration phase overprinted the potassic zone, and added a second pulse of copper mineralization to the system.