The Geology and mineralisation of the Waisoi porphyry copper deposits, Namosi Province, Republic of Fiji

The Waisoi porphyry copper/gold deposits at Namosi, in the Republic of Fiji, are hosted by several units of strongly faulted, fractured and altered volcanic rocks. The lower host volcanic unit consists of coherent and autobrecciated basaltic andesite lavas of the Late Oligocene to Late Miocene (32 to 10 Ma), Wainimala basement (early island arc magmatic affinities), whereas the upper host volcanic unit is dominated by Late Miocene (8 to 5.5 Ma), calc-alkaline andesitic dacite lavas and volcaniclastics of the Namosi Andesite (mature island arc magmatic affinities). These two units are separated by a +5my time break, with a mass flow conglomerate unit (Basal Namosi Conglomerate) marking the beginning of the calc-alkaline volcanism. These rocks are intruded (at 5.5 - 6.0 Ma) by dacitic quartz diorites of similar chemistry to the Namosi Andesite volcanics. Fluids associated with these intrusives caused extensive alteration in the Namosi area and led to formation of the 930 million tonne Waisoi porphyry mineralisation, averaging 0.43% copper and 0.14 ppm gold. Petrological, mineralogical, alteration and fluid inclusion investigations, combined with field observations of temporal relationships between geological features, demonstrated that the paragenetic sequence consists of early, regional, greenschist facies, burial metamorphism and albitisation followed by porphyry-related alteration. The latter (porphyry-related alteration) consists of early sericitisation, followed by silicification, before the main mineralising fluids were introduced. These resulted in a potassic alteration event, during which most of the copper and gold was deposited, followed by a chloritisation event, also with significant copper mineralisation. The variable phase mineralising event was followed by several carbonate-rich alteration events. Minor copper mineralisation was deposited as chalcopyrite during the silicification event, but most of the copper and gold was introduced and deposited as chalcopyrite and bornite during the subsequent main mineralisation episode. This involved a series of events, from those with highly saline fluids at temperatures in excess of 500°C, to high salinity (+45 wt.% NaCI equivalent) fluids at temperatures above 380°C, to fluids with lower salinities (10 wt.% NaCI equivalent) and temperatures around 300°C. Minor copper was also deposited as chalcopyrite during the early stages of the carbonate alteration event. Mineralisation distribution is controlled by lithology and structure. Regional structure controlled the location of the mineralising porphyries, the fluid pathways and the porosity/permeability of the host rocks, whereas the lithologies controlled the degree of chemical reaction and the style of mineralisation. The dominant structures controlling the emplacement of the porphyries are northeast-southwest and northwest-southeast trending corridors, with lesser influence of east-west zones. East-west and north-south structures tended to occur later, and disrupt the mineralisation on a series of block faults. The high erosion rate of the Waisoi area has resulted in the exposure of the Waisoi mineralisation, but lower erosion at nearby Waivaka has only exposed the top of a similarly mineralised system.