The petrology of the Chuquicamata Plutonic Complex, Chile

Abstract

The Tertiary Chuquicamata Plutonic Complex, in which the Chuquicamata porphyry copper deposit occurs, is described from petrographical and mineralogical viewpoints. The complex is composed of the Fortuna Monzonite, Chuqui Granite Porphyry and Este Monzogranite. The Chuqui Granite Porphyry, the host rock of the mineralization, is divided into the porphyry I, II and III. Potassic, propylitic and phyllic alteration of varying degrees prevails throughout the complex. Amphibole from the Fortuna Monzonite, the most basic rock type, has Mg/(Mg+Fe*) ratios higher than 0.74 and high Si atoms. Biotites contained in all the rock types exhibit Mg/(Mg+Fe*) ratios higher than 0.58. The high Mg/(Mg+Fe*) ratios of the mafic silicates, high Fe2O3/FeO whole rock ratios, the presence of magnetite and high magnetic susceptibility indicate that the Chuquicamata rocks were formed at high oxygen fugacities between the Qz-Mt-Fa and Mt-Hm oxygen buffer reactions. Two-feldspar geothermometer of Stormer (1975) gave fairly low equilibration temperatures from 550° to 350°C. The lowest temperatures were obtained for the twofeldspar pairs from the porphyry I, which is consistent with the lowest TiO2 contents of their biotites. The early plagioclase crystallization in the Fortuna Monzonite, and the quartz crystallization before biotite in the Chuqui Granite Porphyry imply that the Chuquicamata magmas were not saturated with H2O throughout their entire congelation. Because only a little amount of magmatic fluid is available, the presence of meteoric fluids is speculated for the cause of the hydrothermal alteration and subsolidus equilibration.