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

Abstract

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 ...

Epithermal Gold-Silver Deposits in Western Java, Indonesia: Gold-Silver Selenide-Telluride Mineralization

“…(Kelly and Goddard, 1969;Saunders and May, 1986;Ahmad et al, 1987). Textural evidence shows that tellurium mineral was formed after sulfide mineral and this concludes that magmatic sulfur was put to the fluid first then followed by input of tellurium.…”

Later stages of evolution of an epithermal system: Au–Ag mineralizations at Apigania Bay, Tinos Island, Cyclades, Hellas, Greece

“…7 Hydrogen versus oxygen isotope diagram and histogram, displaying stable isotope systematics of hydrothermal fluids from Apigania Bay mineralization. The "Meteoric water line" and the "Subductionrelated vapor, arc and crystal felsic magma" (Hedenquist and Lowenstern 1994) are also displayed ficients of dissolved species were derived from Helgeson and Kirkham (1974a, b), Harvie et al (1984), Henley et al (1984), Ahmad et al (1987) and Zhang and Spry (1994). From the reaction (1) log a SiO 2 aq ð Þ values of −1.9, −2.5 and −2.8 were derived at 200°C, 175°C, and 150°C, respectively:…”

“…Experimental studies have showed that magnesium has the capacity to increase the rate of hydrogen dissociation, slow down its adsorption kinetics, and lead hydrogen-bearing gases to condense (Swartz et al 1999). This hypothesis is supported by the presence of magnesium minerals in the most studied Ag-Au-Te ore deposits such as: dolomite-talc-brucite (Panormos Bay; Tombros et al 2004Tombros et al , 2005Tombros et al , 2007, Mg-chlorite (Acupan; Cooke et al 1996;Cooke and McPhail (2001), dolomite (Cripple Creek; Thompson et al 1985;Kelley et al 1998), dolomite and Mg-chlorite (Emperor, Tuvatu; Ahmad et al 1987;Scherbarth and Spry 2006), roscoelite and dolomite (Porgera; Richards and Kerrich 1993;Ronacher et al 2004) and Mg-chlorite and manganese ores (Sunnyside mine; Casadevall and Ohmoto 1977).…”

Calculation of the physicochemical parameters ΣS, ΣC, aH2S, I and of the metal ions content of the hydrothermall fluid that has precipitated Au-Ag-Te ores in Panormos - Liardades Bay area, Tinos Island, Cyclades

“…Από την ισορροπία H 2 S<->2H + +S _ , (Casadevall & Omhotto, 1977, Henley et al, 1984 (Barnes, 1979). Τέλος από την ισορροπία H 2 Te«H + +HTe", (Ahmad et al, 1987), για logK= -5,5, pH=5,6, T=250°C και [H 2 Te] = 10" 6 · 5 , (Ahmad et al, 1987, Me PHail, 1995 για το ρευστό που αποθέτει τα τελλουρίδια, υπολογίζουμε [HTe]=10 39 . Η συγκέντρωση των ιόντων κυμαίνεται στην τιμή logLHTe"= -3,9 και η m LHTe " =9,78 ppb, (Πίνακες 2, 3).…”

“…Από την ισορροπία για τον αρσενοπυρίτη σε ιστολογική ισορροπία με σιδηροπυρίτη, FeAsS+S"V»FeS 2 +2As~, (Cooke et al, 1996) (Ahmad et al, 1987, Cooke et al, 1996, για logK=-8,2, pH=5,6, T=250°C, loga , δηλαδή ^Σ^+=-4,855 και m + =1,51 ppb. Από την ισορροπία για τον καλαβερίτη, Au+Te 2 <-»AuTe 2 , (Ahmad et al, 1987, Cooke et al, 1996, για logK=-9,38, pH=5,6, T=250°C, loga Te2 =-6,7 και [AuTeJ = l, υπολογίζουμε [Au] Στάδιο VII.…”