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 preand 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-feldsparmica-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, sylvanitc, and native tellurium; the second, by petzite, hessRe 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 ø to 250øC and later stages, from 250 ø to 160øC. 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 mNa+ = 0.32, mK+ = 0.25, mMg+• = 0.005, mcl-= 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 250øC, are pH = 5.5, mzs = 10 -3 and mzc = 10 -a to 10 -3, ao• = 10 -4ø, aTe• = 10 -*'8, and a•i• = 10 -ø'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 Ha, H2S, COa, and HaTe, with loss of H2S 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 b•SO and bD analysesindicate that the ...

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The Rosebery pyritic zinc-lead-copper-silver-gold orebody occurs in dominantly felsic volcanic rocks of Cambrian age in western Tasmania. Ore formation occurred in a marine environment following eruption of a thick pyroclastic unit consisting mainly of welded, ashflow tuffs, and subsequent subsidence. The main orebody consists of a discontinuous massive sulfide horizon and in the southern part of the mine there is a generalized metal zonation with a lower and central Cu-Fe-rich zone surrounded and overlain by Zn-Pb-Ag-rich ore. Barite-sulfide ore occurs as separate lenses higher in the sequence separated from the sulfide orebody by unmineralized, cleaved siltstone. The orebody is underlain by an extensive zone of alteration marked by depletion of Na and Sr and enrichment in Rb, K, Mg, Mn, and H20. The pyrite and chalcopyrite content of the altered footwall tuff and the Co content of the pyrite is highest beneath the Fe-and Cu-rich zones in the sulfide orebody and a feeder is inferred in this area.The •a4S values for sulfides in the sulfide orebody are higher in the richest Zn-Pb-Ag ore (>14.1%0) than the Fe-Cu ore (>7.8•), a pattern paralleled by a decrease in the Fe/Fe + Mg + Mn ratio of chlorite and a decrease in the FeS content of sphalerite. The •$48 values for sulfides in the barite ore are even higher (14.5 to 19.8%0) and the FeS content of sphalerite lower. Barite throughout the barite and sulfide orebodies is fairly constant between 84.6 and 41.2 per mil, but that in the sulfide ore appears to be epigenetic. The/i34S values for baritesulfide pairs indicate a temperature range of 255 ø to 298øC for the barite orebody, but the temperature of barite ore formation was probably about 250øC.The use of $a4S values for sulfides as stratigraphic markers indicates that the sulfide orebody is diachronous. It is possible to account for the metal and $a4S distribution by assuming that the ore solutions are buoyant on reaching the sea floor during the Fe-and Cu-rich phases but show reversing buoyancy during the PboZn-rich phase.The coexisting arsenopyrite-pyrite-chlorite-quartz assemblage in the sulfide orebody defines a range of aO2-T conditions using the six-component solid solution chlorite model of Walshe and Solomon (1981). The maximum indicated temperature of $00øC is consistent with the minimum temperature required by iron and copper solubilities. However, the chlorite compositions may be postdepositional, as seems to be the case for the FeS contents of sphalerite. The $a4Szs for the sulfide orebody is probably derived by almost complete reduction of Cambrian seawater sulfate mixed with sulfur leached from underlying rocks. The/ia4S values for the barite orebody may represent partial reduction of seawater sulfate.The ore fluid is supposed to have been generated by convective circulation of seawater in a rock pile heated by a Cambrian granitoid pluton.

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