A chemical study of alteration at the Panguna porphyry copper deposit, Bougainville, Papua New Guinea

Abstract

The mineralization-alteration zonation at Panguna is rationalized in terms of an over-printing of three broad alteration events which resulted from the progressive decline of an inner magmatic-hydrothermal system and its eventual disruption by the influx of heated external groundwaters. These three events are represented by amphibole-magnetite (early), K-silicate (main-stage) and feldspar-destructive (late, retrogressive) assemblages. Mineralization was associated with K-silicate alteration processes, and emphasis throughout the study is placed on main-stage events.Chemical analyses of important rock alteration types and their constituent minerals reveal interesting and useful variations. Mg/Mg + Fe ratios of both hydrothermal amphibole and biotite increase toward the ore zone. Only minor differences in TiO 2 contents and Mg/Mg + Fe ratios exist between texturally magmatic and hydrothermal biotites, and the Mg/Mg + Fe ratios of secondary biotite tend to be lower than in other porphyry deposits. Application of the biotite geothermometer of Beane (1974) yields biotitization temperatures in the range 370 degrees to 540 degrees C. Hydrothermal K-feldspar, Fe-Ti oxides, chlorite, sericite, and epidote show limited compositional variations. Chemical gains and losses observed in specific rock alteration types are consistent with mineralogic changes. The highly saline magmatic-hydrothermal ore fluids added Si, K, Cu, S, and Rb to K-silicate assemblages. They mobilized Ca, Na, Mg, Mn, Sr, Pb, Zn, and Zr from the potassic zone but redeposited most of these elements in the propylitic fringe. The resulting element zonation is similar to that observed in other porphyry deposits, with the mineralized central K-silicate zone coincident with high K, Cu, Cu/S, Rb, and Rb/Sr values, and decreased levels of Ca, Mn, Sr, K/Rb, Zn, and Pb. Although an outward temperature decline was the dominant control on the alteration-mineralization zonation, several additional interdependent factors were probably involved. These include chemical variations in the outward migrating fluids, the mafic wall-rock chemistry, and predominance of hydrothermal biotite.