Geochemical and temperature controls on ore mineralization at the Emperor gold mine, Vatukoula, Fiji

Emperor is a large epithermal gold-tellurium deposit which occurs at the margin of the Tavua caldera and is associated with high-level mineralized tuffs, breccias and silicious sinters. The deposit consists of mainly narrow, steep “shear” and shallow dipping “flatmake” mineralized vein structures. Mineralization spanned a period during which different veins and dyke intrusions occurred so that some early structures are offset by later ones. High Au values and high Au/Ag ratios, corresponding to the intersection of shear and flatmake structures, generally correlate with high Ag, Te and Ba values in some structures (Crown Shear) and high Hg in others (166 Flatmake). Other elements commonly anomalous in the high-grade ore zones are As, Mo and Cu. Thallium, Se and Sb are commonly high below ore zones while Hg may be high above (e.g., Crown Shear), in (166 Flatmake) or even below (e.g., Prince of Wales Shear) ore zones.Gold precipitated in a temperature interval of 180–210°C; higher temperatures commonly occur at depth in some flatmakes (166 N, 608) and lower temperatures nearer the surface. In the two steep shears analyzed, higher temperatures correspond to where they are intersected by flatmakes. Temperatures may decrease below such intersections in which case Hg, Sb, etc., commonly increase with depth.Evidence of boiling, characterized by coexisting gas- and liquid-rich fluid inclusions, is minor and spatially sporadic. Quartz and carbonate found in ore containing bonanza concentrations of Au contain few to no gas-rich fluid inclusions. The intensity of wall-rock alteration is unrelated to gold grades.Ore deposition is inferred to have been caused mainly in response to fluid mixing rather than extensive boiling and/or wall-rock alteration, although these processes occurred. The source of the Au, Ag, Te, etc. is inferred to be near a neutral, relatively reduced, bisulphide-rich, ore solution at near 300°C and derived in some way from the shoshonitic volcanics or associated monzonitic intrusions. The second non-ore solution necessary for mixing, is inferred to have been an acid, oxidized solution at 150°C and having a large meteoric component. The result of fluid mixing was ore genesis, and a telluride, minor sulphide and very minor sulphate (barite) and anhydrite ore assemblage.