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- W372649142 abstract "The Waisoi porphyry copper/gold deposits at Namosi, in the Republic of Fiji, arehosted by several units of strongly faulted, fractured and altered volcanic rocks.The lower host volcanic unit consists of coherent and autobrecciated basalticandesite lavas of the Late Oligocene to Late Miocene (32 to 10 Ma), Wainimalabasement (early island arc magmatic affinities), whereas the upper host volcanicunit is dominated by Late Miocene (8 to 5.5 Ma), calc-alkaline andesitic dacitelavas and volcaniclastics of the Namosi Andesite (mature island arc magmaticaffinities). These two units are separated by a +5my time break, with a massflow conglomerate unit (Basal Namosi Conglomerate) marking the beginning ofthe calc-alkaline volcanism. These rocks are intruded (at 5.5 - 6.0 Ma) by daciticquartz diorites of similar chemistry to the Namosi Andesite volcanics. Fluidsassociated with these intrusives caused extensive alteration in the Namosi areaand 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 geologicalfeatures, demonstrated that the paragenetic sequence consists of early,regional, greenschist facies, burial metamorphism and albitisation followed byporphyry-related alteration. The latter (porphyry-related alteration) consists ofearly sericitisation, followed by silicification, before the main mineralising fluidswere introduced. These resulted in a potassic alteration event, during whichmost of the copper and gold was deposited, followed by a chloritisation event,also with significant copper mineralisation. The variable phase mineralisingevent was followed by several carbonate-rich alteration events.Minor copper mineralisation was deposited as chalcopyrite during thesilicification event, but most of the copper and gold was introduced anddeposited as chalcopyrite and bornite during the subsequent mainmineralisation episode. This involved a series of events, from those with highlysaline fluids at temperatures in excess of 500°C, to high salinity (+45 wt.% NaCIequivalent) fluids at temperatures above 380°C, to fluids with lower salinities (10 wt.% NaCI equivalent) and temperatures around 300°C. Minor copper was alsodeposited as chalcopyrite during the early stages of the carbonate alterationevent.Mineralisation distribution is controlled by lithology and structure. Regionalstructure controlled the location of the mineralising porphyries, the fluidpathways and the porosity/permeability of the host rocks, whereas thelithologies controlled the degree of chemical reaction and the style ofmineralisation. The dominant structures controlling the emplacement of theporphyries are northeast-southwest and northwest-southeast trending corridors,with lesser influence of east-west zones. East-west and north-south structurestended 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 theWaisoi mineralisation, but lower erosion at nearby Waivaka has only exposedthe top of a similarly mineralised system." @default.
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- W372649142 date "1968-01-01" @default.
- W372649142 modified "2023-09-27" @default.
- W372649142 title "The Geology and mineralisation of the Waisoi porphyry copper deposits, Namosi Province, Republic of Fiji" @default.
- W372649142 hasPublicationYear "1968" @default.
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