Mineralogical and geochemical studies of the Emperor gold telluride deposit, Fiji

The Emperor gold telluride deposit consists of a system of quartz-filled fractures lying on the margin of a caldera in the Mba Volcanics, of Miocene age, in the north of Viti Levu, Fiji. The veins filled fractures that developed mainly in precaldera basalts. The veins are both pre-and postfaulting, contain vugs, and show evidence of repeated opening. They are dominantly quartz with minor pyrite, tellurides, gold, sphalerite, arsenopyrite, chalcopyrite, tetrahedrite, galena, dolomite, calcite, and mica (part V rich). Flanking wall rocks show inner K-feldspar-mica-quartz alteration and outer chlorite-carbonate alteration.Crustification and replacement textures allow establishment of a generalized paragenetic sequence dominated by five stages of quartz, two of ore minerals without quartz, and late carbonate. Pyrite was among the first and the last of the minerals to precipitate, and is the most common sulfide. Within each ore mineral stage there are small-scale cycles of sulfide deposition indicating widely fluctuating fluid conditions. The first of the two ore mineral stages is dominated by calaverite, krennerite, sylvanite, and native tellurium; the second, by petzite, hessite and native gold, indicating a decline in tellurium activity with time. A third of the Emperor gold is thought to be in solid solution in pyrite and arsenopyrite.Fluid inclusions in quartz indicate that the early stages formed at temperatures ranging from 300 degrees to 250 degrees C and later stages, from 250 degrees to 160 degrees C. The presence of coexisting vapor and liquid-vapor inclusions in much of the vein system indicates that the solutions were boiling. Leachate analyses and freezing point depressions of fluid inclusions yield an average fluid composition of m (sub Na (super +) ) = 0.32, m (sub K (super +) ) = 0.25, m (sub Mg (super +2) ) = 0.005, m (sub Cl (super -) ) = 1.0, and I (ionic strength) = 1.0. The five quartz stages (except possibly the first) appear to have crystallized in isothermal conditions during periods of shallow sealing of the hydrothermal system and low mass fluid flux. No homogenization data are available for the ore mineral stages, but a similar temperature range to that in adjacent quartz is assumed, together with high mass flux to inhibit quartz precipitation. Initial fluid conditions in the first ore stage, assuming equilibrium at 250 degrees C, are pH = 5.5, m (sub Sigma S) = 10 (super -3) and m (sub Sigma C) = 10 (super -2) to 10 (super -3) , a (sub O 2 ) = 10 (super -40) , a (sub Te (sub 2 (sub (g) ) ) ) = 10 (super -7.8) , and a (sub H (sub 2 (sub (g)) ) ) = 10 (super -0.4) . Sulfide, telluride, and gold precipitation probably resulted largely from boiling and cooling along the boiling point depth curve. Changes in the fluid as a direct result of boiling include loss of H 2 , H 2 S, CO 2 , and H 2 Te, with loss of H 2 S the main cause of gold deposition. Assuming only partial equilibrium in the fluid indicates a more reduced fluid with a higher tellurium content.Carbon isotope ratios of carbonates and sulfur isotope ratios of sulfides indicate a dominantly sedimentary source for carbon and sulfur, believed to be carbonate-bearing sediments (the Vatukoro Formation) underlying the host basalts (Mba Volcanics). The delta 18 O and delta D analyses indicate that the ore fluid was mainly composed of magmatic water and seawater with a possible meteoric component. The seawater was probably resident in the Vatukoro Formation prior to cauldron formation. Carbon in late carbonate coating quartz may be of magmatic origin. Mineralization occurs largely outside the cauldron walls and may have preceded the final stages of cauldron development.