Petrological, mineralogical, fluid inclusion and stable isotope characteristics of the Tuvatu gold-silver telluride deposit, upper Sabeto River area, Fiji
The Tuvatu gold-silver telluride deposit (300,000 oz) is located along the >250 km northeast trending Viti Levu lineament, and is spatially and genetically related to low grade porphyry copper mineralization and to the emplacement of the alkaline 4.85 Ma Navilawa Monzonite. It is hosted in sub-vertical, N-S and NNE-SSW trending veins. Four hypogene (late magmatic, potassic, propylitic, and phyllic) and one supergene stages of vein mineralization were identified. The phyllic stage 4 is the important ore-bearing stage which contains native gold, electrum, precious and non-precious metal tellurides (calaverite, krennerite, sylvanite, petzite, stutzite, hessite, altaite, and coloradoite). Stage 1 fluids were hot (>4500C), boiling and hypersaline (>50 wt % NaCl equiv) that formed at [Difference symbol] 3100C. Stage 2 fluids also contained traces of an unidentified immiscible organic fluid. Stage 3 propylitic alteration formed at [Difference symbol] 3000C from fluids that generally ranged in composition from 1 to 10 wt percent NaCl equiv. However, stage 3 fluids were also locally boiling and very saline (up to 37 wt% NaCl equiv). Stage 4 non-boiling, moderately saline fluids (mean = 8.4 wt % NaCl equiv) formed between approximately 3250 and 1000C (mean = 2590C) and accompanied telluride and base metal sulfide deposition. Values of [Delta symbol]34S for sulfides in the porphyry and epithermal veins range from -15.3 to -3.6 per mil and reflect an increase in [Sigma symbol]SO4/[Sigma]H2S ratio of a boiling magmatic fluid. Oxygen isotope compositions for water in equilibrium with stage 2 and stage 3 minerals range from 4.4 to 10.2 per mil whereas values of [Delta symbol]18O for water in equilibrium with stage 4 minerals range from 7.8 to 11.5 per mil. These values, coupled with [Delta symbol]18O and [Delta symbol]D values of fluids in equilibrium with phlogopite of 6.5 to 9.8 per mil and -25.9 to -9.9 per mil, respectively, are consistent with a magmatic source and permissive of a meteoric water contribution. Ore-forming components, such as Au, Ag, Te, V, and S, were most likely derived from the Navilawa Monzonite.