Geology and Isotope Geochemistry of the Wainaulo Cu-Au Porphyry Deposit, Namosi District, Fiji

Abstract

The late Miocene, calc-alkalic, Wainaulo Cu-Au porphyry deposit of the Namosi district, Fiji, hosts distinct styles of alteration and mineralization that overlapped to produce a substantial porphyry Cu-Au resource. The early stages produced medium-grade Cu, low-grade Au and concentric calc-potassic to propylitic alteration that is zoned around the early-stage diorite intrusions. Discrete zones of high-grade Cu and Au and calc-sodic alteration were then superimposed during the intrusion of the main-stage quartz diorites and the formation of quartz-sulfide and epidote-sulfide veins. As the magmatic-hydrothermal system waned, lower Cu and Au grades, with a weaker intensity of calc-sodic alteration and lower density of veins, were produced coincident with emplacement of subsequent quartz diorite intrusions. Late-stage anhydrite-pyrite veins and chlorite-illite alteration overprinted the quartz diorite intrusive complex. The final hydrothermal event consisted of argillic alteration that was concentrated in and around steeply dipping, ENE-trending shears. These structures appear to have controlled the emplacement of the quartz diorite complex and distribution of high-grade Cu-Au mineralization, suggesting they were active during the pre- and synmineralization stages. Stable and radiogenic isotopic data provide evidence for direct seawater contributions to the magmatichydrothermal system. Measured δ34Ssulfide (–5.0 to 3.8‰) and δ34Ssulfate (9.0–16.8‰) values are consistent with a predominantly magmatic source, whereas an elevated bulk sulfur composition (6.7‰) suggests mixing with an isotopically heavy fluid (e.g., seawater). Estimates of δDfluid derived from epidote (–9.1 to 11.3‰) and δ18Ofluid from epidote and anhydrite (–0.2 to 4.7‰) approach that of Vienna standard mean ocean water, and the initial Sr isotope ratios of epidote (0.70364–0.70378) suggest a component of seawater Sr ranging from 3.2 to 5.8%. These results are consistent with the inferred submarine paleogeographic setting and may explain the abundance of albite- and epidote-rich alteration assemblages at Wainaulo.