Geochemical and stable isotope studies, when placed in the context of plate tectonic models, impose important constraints on the ultimate origin of the Panguna intrusives (and the ore metals), their relationship with contemporaneoxis volcanics on Bougainville, the evolution of hydrothermal fluids, and the nature of mineralization-alteration processes.
The extrusive and intrusive rocks of Bougainville are of related, but fundamentally different origins. A model is suggested in which the Panguna intrusives are progressively derived by high-level crystal fractionation from a water undersaturated, K-poor, and Cu-rich melt, formed by mixing of partially melted lower crust and hot (~1300°C), almost anhydrous, Benioff-zone derived magmas, at depths of 20-30 km. This "hybrid" magma utilized established eruptive conduits, and was probably emplaced beneath a complex stratovolcano that had reached a late evolutionary stage. Several aspects of the Mount Balbi stratovolcanic complex, in northern Bougainville, suggest that it may overlie a modem porphyry copper system. The lower arc crust is proposed as the source of most of the metals and volatiles. There is an apparent relationship between high plate consumption rates, intensified arc volcanism, and crustal melting processes, which are favoured by the evolved, thickened crust of Bougainville, and enhanced thermal gradients due to long-continued magmatism.
Mineralogical evidence implies that Cu was quantitatively removed from a convecting subjacent magma at the time of emplacement of the mineralizing mine porphyries, and that these porphyries evolved several hydrothermal phases. Gain-loss data show that these highly saline fluids contained significant levels of Si02, K2O, Rb, and S, as well as the ore metals, as these components were added to K-silicate alteration zones. The ore fluids mobilized Ca, Na, Mg, Mn, Sr, Pb, Zn, and Zr from potassic assemblages, but redeposited many of these elements in the propylitic zone. Element zonations are thus similar to, although less pronounced than, those evident in other porphyry copper deposits.
The alteration pattern at Panguna is rationalized in terms of an overprinting of three broad alteration types - amphibole-magnetite, K-silicate, and feldspar-destructive alteration - which resulted from a progressive collapse of the hydrothermal system, due to declining temperatures and the increasing importance of externally derived groundwaters, Propylitization proceeded concurrently with amphibole-magnetite and K-silicate alteration by groundwater-wallrock interaction at moderate temperatures (~? 300°C) , in the fringes of the deposit.
The early amphibole-magnetite alteration is restricted to the Panguna Andesite, where innermost assemblages formed at temperatures about 600-650°C, from magmatic-hydrothermal fluids (ᵟD ≈ -40 to -55° /oo, ᵟ018≈ 9.5°/oo).
Mineralization was spatially and temporally associated with mainstage K-silicate alteration, and the dominant control on the mineralizationalteration zonation was an outward temperature decline (~30°C/100 m). Adjuncts to sulphide deposition may have included an outward decrease in C1- activity of the ore fluids, neutralization of the ore solutions, and the formation of hydrothermal biotite. Several lines of evidence suggest average temperatures for mineralization-quartz veining-biotitization processes of 500°C; related hydrothermal fluids had ᵟD = -20 to -45 /oo and ᵟ0 - 6 to 7° /oo. Although fluids of this composition could, in principle, represent either magmatic-hydrothermal solutions or 018 -shifted groundwaters, independent evidence, coupled with a detailed consideration 18 of 60 values of Panguna whole rocks, indicates that the ore fluids were largely magmatic-hydrothermal. There is some evidence suggesting interaction between these solutions and 018 -poor groundwaters in the outer part of the K-silicate zone.
The absence of specific evidence indicating formation temperatures of the chlorite-sericite alteration within the Biotite Granodiorite means that the isotope data are consistent with either a magmatic-hydrothermal or lower temperature meteoric-hydrothermal origin for this assemblage. Some sericitization definitely post-dated K-silicate alteration and formed at lower temperatures; fluids causing sericitization at temperatures in the vicinity of 300°C had ᵟD≈ -30 to 50°/oo, ᵟ018 - 3.5°/oo, and were produced by 018 shifting of groundwaters during the evolution of the propylitic zone.
Consideration of the geologic characteristics of other southwestern Pacific deposits, especially those of the Philippines and the New Guinea- Solomons region, suggests that genetic conclusions presented for the Panguna deposit are of application to other island arc porphyries.