A geochemical and stable isotope study of the Panguna porphyry copper deposit, Bougainville

Abstract

Geochemical and stable isotope studies, when placed in the contextnof plate tectonic models, impose important constraints on the ultimatenorigin of the Panguna intrusives (and the ore metals), theirnrelationship with contemporaneoxis volcanics on Bougainville, thenevolution of hydrothermal fluids, and the nature of mineralization-alterationnprocesses.nnnnnnnnnn The extrusive and intrusive rocks of Bougainville are of related,nbut fundamentally different origins. A model is suggested in which thenPanguna intrusives are progressively derived by high-level crystalnfractionation from a water undersaturated, K-poor, and Cu-rich melt,nformed by mixing of partially melted lower crust and hot (~1300dC),nalmost anhydrous, Benioff-zone derived magmas, at depths of 20-30 km.nThis qhybridq magma utilized established eruptive conduits, and wasnprobably emplaced beneath a complex stratovolcano that had reached anlate evolutionary stage. Several aspects of the Mount Balbinstratovolcanic complex, in northern Bougainville, suggest that it maynoverlie a modem porphyry copper system. The lower arc crust isnproposed as the source of most of the metals and volatiles. There isnan apparent relationship between high plate consumption rates, intensified arc volcanism, and crustal melting processes, which arenfavoured by the evolved, thickened crust of Bougainville, and enhancednthermal gradients due to long-continued magmatism.nnnnnnnn Mineralogical evidence implies that Cu was quantitatively removednfrom a convecting subjacent magma at the time of emplacement of thenmineralizing mine porphyries, and that these porphyries evolved severalnhydrothermal phases. Gain-loss data show that these highly saline fluidsncontained significant levels of Si02, K2O, Rb, and S, as well as the orenmetals, as these components were added to K-silicate alteration zones.nThe ore fluids mobilized Ca, Na, Mg, Mn, Sr, Pb, Zn, and Zr from potassicnassemblages, but redeposited many of these elements in the propyliticnzone. Element zonations are thus similar to, although less pronouncednthan, those evident in other porphyry copper deposits.nnnnnnnnn The alteration pattern at Panguna is rationalized in terms of annoverprinting of three broad alteration types - amphibole-magnetite,nK-silicate, and feldspar-destructive alteration - which resulted from anprogressive collapse of the hydrothermal system, due to decliningntemperatures and the increasing importance of externally derived groundwaters, Propylitization proceeded concurrently with amphibole-magnetitenand K-silicate alteration by groundwater-wallrock interactionnat moderate temperatures (~? 300dC) , in the fringes of the deposit.nnnnnnn The early amphibole-magnetite alteration is restricted to thenPanguna Andesite, where innermost assemblages formed at temperaturesnabout 600-650dC, from magmatic-hydrothermal fluids (ᵟD a -40 to -55d /oo,nᵟ018a 9.5d/oo).nnnnnnnn Mineralization was spatially and temporally associated with mainstagenK-silicate alteration, and the dominant control on the mineralizationalterationnzonation was an outward temperature decline (~30dC/100 m).nAdjuncts to sulphide deposition may have included an outward decrease innC1- activity of the ore fluids, neutralization of the ore solutions, andnthe formation of hydrothermal biotite. Several lines of evidence suggestnaverage temperatures for mineralization-quartz veining-biotitizationnprocesses of 500dC; related hydrothermal fluids had ᵟD = -20 to -45 /oonand ᵟ0 - 6 to 7d /oo. Although fluids of this composition could, innprinciple, represent either magmatic-hydrothermal solutions or 018 -shiftedngroundwaters, independent evidence, coupled with a detailed considerationn18nof 60 values of Panguna whole rocks, indicates that the ore fluids werenlargely magmatic-hydrothermal. There is some evidence suggestingninteraction between these solutions and 018 -poor groundwaters in the outernpart of the K-silicate zone.nnnnnnnnn The absence of specific evidence indicating formation temperatures ofnthe chlorite-sericite alteration within the Biotite Granodiorite meansnthat the isotope data are consistent with either a magmatic-hydrothermalnor lower temperature meteoric-hydrothermal origin for this assemblage.nSome sericitization definitely post-dated K-silicate alteration and formednat lower temperatures; fluids causing sericitization at temperatures innthe vicinity of 300dC had ᵟDa -30 to 50d/oo, ᵟ018 - 3.5d/oo, and werenproduced by 018 shifting of groundwaters during the evolution of thenpropylitic zone.nnnnnnnnn Consideration of the geologic characteristics of other southwesternnPacific deposits, especially those of the Philippines and the New Guinea-nSolomons region, suggests that genetic conclusions presented for thenPanguna deposit are of application to other island arc porphyries.n