Geology of the Namosi Porphyry Cu-Au deposits, Fiji

The Namosi Cu-Au district in southeastern Viti Levu, Fiji, hosts several mineralized porphyry centers. Porphyry emplacement coincided with a time of major tectonic reorganization in the southwest Pacific that resulted in counterclockwise rotation and fragmentation of the Fiji platform, calc-alkaline magmatism, and hydrothermal activity. The principal resources in the district are the Waisoi deposits (Waisoi East and Waisoi West), which contain an estimated combined reserve of 1,791 million tonnes (Mt) at 0.37 % Cu and 0.12 grams per tonne (g/t) Au, ranking them among the largest undeveloped Cu resources in the world. The basement rocks of the Namosi district, the late Oligocene to late Miocene andesitic Wainimala Group, are unconformably overlain by gently dipping late Miocene andesitic volcaniclastic rocks of the Namosi Formation. These successions were intruded by many calc-alkaline diorite to quartz-diorite porphyries and altered to high-temperature mineral assemblages of albite ± biotite ± magnetite ± bornite ± chalcopyrite. Peripheral to the mineralized centers, medium- to low-temperature alteration assemblages are composed of chlorite ± albite ± epidote ± actinolite, and chlorite + magnetite. Chlorite + illite ± pyrite alteration assemblages have locally overprinted the cores and the peripheries of the porphyry deposits. These southwest Pacific Cu deposits, with a central core of dominantly ore bearing sodic-calcic alteration, contrast with the giant Cu-rich Andean calc-alkaline porphyry deposits.