Geological characteristics and environment of some porphyry copper occurrences in the southwestern Pacific

Abstract

The discovery of economic porphyry copper mineralization during the mid-1960s on the islands of New Guinea and Bougainville established the potential of this type of copper mineralization on certain southwestern Pacific islands. Continued exploration has resulted in the discovery of numerous porphyry copper systems. However, economic evaluation of these bodies is still pending or incomplete at this time. Only two of about 25 occurrences are presently in production.Considerable scientific interest attends the study of these deposits as they are geologically youthful and appear to be genetically related to the histories of old or active island arcs, or continental edges. Although detailed resolution of the times of certain events in the history of the Pacific Basin is lacking, there does appear to be correspondence in a general way of the probable age of those events with the ages of certain deposits. As a working hypothesis, it is suggested that times of increase of convergence rates of plates, which appear to be those times of porphyry copper evolution, are also those times of uplift and extension of the preore rock systems.Variations in the type of porphyry copper deposits known are the same as those recognized in deposits of the western hemisphere. They are composed of large volumes of rock, either porphyry or porphyry-associated, reflecting large sulfide systems with at least a hundredfold increase in the Clarke of copper. Supergene enrichment of a few has upgraded primary concentration another two to five times. Porphyritic rock types appear to be commonly quartz diorite or dacite. Alteration ranges from pyrometasomatic in carbonate wall rocks through potassic, phyllic, and propylitic alteration of intrusive and extrusive igneous rocks. Zoning of alteration seems less discrete than that generally recognized in some western hemisphere bodies and in many instances appears to be telescoped.