MAGMATIC TO EPITHERMAL EVOLUTION OF THE TUVATU GOLD-SILVER TELLURIDE SYSTEM, UPPER SABETO RIVER AREA, FIJI

The Tuvatu gold system is one of several low-sulfidation epithermal gold systems localized along the >250 km northeast trending Viti Levu lineament, Fiji, which are genetically associated with alkaline magmatism. The Tuvatu deposit (300,000 oz Au), second in size in Fiji to the Emperor gold-silver telluride deposit (11.5 Moz Au), is generally hosted in sub-vertical, N-S and NNE-SSW trending veins as well as shallow S-dipping veins, and appears to be intimately related to porphyry Cu-style mineralization and to the emplacement of the 4.85 Ma Navilawa Monzonite and Sabeto Volcanics. Six vein structures have been identified: H-Lode, Tuvatu, SKL-Nasivi Stockwork, Murau Flatmakes, West, and Upper Ridges Lodes. These veins contain pyrite (FeS₂), marcasite (FeS₂), chalcopyrite (CuFeS₂), sphalerite (ZnS), Se-rich galena (PbS), tennantite ((Cu,Fe)₁₂As₄S₁₃), native gold, native bismuth, and native tellurium, and the following tellurides: calaverite (AuTe₂), petzite (Ag₃AuTe₂), hessite (Ag₂Te), stuetzite (Ag_5-xTe₃), sylvanite ((Ag,Au)₂Te₄)), krennerite ((Au,Ag)Te₂), coloradoite (HgTe), tellurobismuthite (Bi₂Te₂S), bismuthinite (Bi₂Te₃), and altaite (PbTe). Ashley and Andrew's (1989) fluid inclusion studies of apatite, K feldspar, and quartz, proposed an initial stage where fluids boiled at temperatures >550^oC with salinities >50 eq. wt.% NaCl, a second stage where fluid temperatures ranged from 285 to 385^oC with salinities at 4.5-15.1 eq. wt.% NaCl, and a third stage where fluid temperatures varied from 116 to 265^oC with an estimated salinity of 2.4 eq. wt% NaCl. Oxygen isotope values (d¹⁸O_H20) of magnetite, muscovite, K-feldspar, phlogopite, quartz, and chalcedony of Ashley and Andrews (1989; n=12) and this study (n=16) vary from 3.9 to 9.8‰, dD_H20 values of phlogopite range from -17.6 to -1.6‰ (n=4), and d³⁴S values of pyrite and chalcopyrite range from -8.4 to -3.2‰ (n=9). Fluid inclusion values from stage I along with oxygen, hydrogen, and sulfur isotope values are consistent with ore-forming components being derived from a magmatic source. The Tuvatu deposit appears to have originally developed as a porphyry copper system that has been overprinted subsequently by epithermal gold mineralization.