Copper mineralization at Panguna occurs associated with a complex of Pliocene diorite/granodiorite intrusives into the hornfelsed Panguna Andesite. Four separate intrusive rock units are recognized: These are the mainly equigranular Kaverong Quartz Diorite, and the Stage I, Stage II and Stage III porphyries. Chalcopyrite and bornite, with associated pyrite, magnetite and minor molybdenite, occur as vein fillings, within selvages of wallrock alteration adjacent to fractures, and disseminated in zones of pervasive alteration. The distribution of copper mineralization shows a close spatial relationship to the Stage II porphyry. The period of introduction of copper is shown to directly span the period of intrusion of the Stage II porphyry. At least four generations of quartz veining occur, but only those vein sets which postdate the Stage I and pre-date the Stage III porphyry contain significant copper mineralization. Extensive hydrothermal alteration occurs. Five main alteration types, namely biotite, chlorite-sericite, sericite-clay, chlorite-epidote, and clay-calcite alteration are recognized. The alteration pattern is strongly affected by rock unit interfaces and structural conditions within the individual rock units. Two sequences of alteration occur : a regressive alteration sequence where biotite alteration gives way through chlorite-epidote alteration to unaltered rock, and a progressive alteration sequence in which biotite alteration grades through chlorite-sericite alteration to sericite-clay alteration. The regressive sequence shows a decrease in the degree of alteration with declining temperature of alteration. The progressive sequence shows an increase in the degree of alteration with declining temperature of alteration. There is a close relationship between silicate alteration types and coexisting sulfide species. In general terms the extent of vein development controls the copper grade, and the host rock unit controls the mode of occurrence of the mineralization. The preservation of vesicles in part of the Stage II porphyry indicates that boiling off of a fluid phase from the ascending magma, as postulated by Burnham (1967), has occurred. At least one generation of fracturing at Panguna has been shown to result from the breakout of these magmatic fluids. © 1973 Society of Economic Geologists, Inc.
