GOLD MINERALIZATION AT THE EMPEROR MINE, VATUKOULA, FIJI

The Emperor Mine, located at Vatukoula in northern central Viti Levu, is the only operating mine in Fiji. The deposit is on the western margin of the Tavua Caldera, which is central in the Tavua Volcano, the largest of a series of Tertiary shoshonitic shield volcanoes which occur within a northeast-trending extensional fault zone across the northern portion of Viti Levu. Gold mineralisation at Emperor is low sulphidation in style and is confined to a series of steeply dipping faults and dykes, flat-dipping (less than 45°) normal faults (flatmakes), and complex intersection zones of two or more of the above structures (shatters). Epithermal mineralisation at Emperor is regionally associated with higher-temperature alteration zones near the centre of the caldera, 3 km east of Vatukoula, and with low-temperature high-level epithermal features, such as chalcedonic veining and sinters, some 4 km to the northeast of Vatukoula. Gold, gold-silver tellurides, quartz, carbonate and adularia occur as vein filling, and are concentrated in dilated structural intersections. Wall-rock alteration to quartz, carbonate, adularia, pyrite, roscoelite, sericite and smectite clays is of variable intensity and not necessarily associated with high-grade mineralisation. Isotopic and fluid-inclusion data indicate that gold was probably introduced by a mixed magmatic, seawater and meteoric fluid of 5.5% wt.% NaCl at temperatures up to 300°C, and averaging 200°C, through the Nasivi Shear Zone and other steep faults. An hypothesis that gold deposition was caused by fluid boiling at dilated structural intersections is proposed, with the boiling a result of depressurisation of hydrothermal fluid in open structures. Gold deposition may in part have resulted from the destabilisation of the gold-bisulphide complex through the loss of H2S in the gaseous phase during boiling. Auriferous pyrite associated with structurally constricted lodes probably resulted from sulphidation of iron-bearing phases in the wall rock. Fluid mixing may be an important ore depositing mechanism, but compelling evidence for this is lacking at this time. © 1990.