Telluride mineralogy of the low-sulfidation epithermal Emperor gold deposit, Vatukoula, Fiji

Summary

¶The epithermal, low sulfidation Emperor gold telluride deposit in Fiji, hosted by Late Miocene-Early Pliocene shoshonitic rocks, is spatially related to a low-grade porphyry Cu system on the western flank of the Tavua Caldera. Gold is largely in the form of “invisible” gold in arsenian pyrite but 10 to 50% of gold is in the form of precious metal tellurides. Gold mineralization occurs in steeply dipping dikes and faults, flat-dipping structures (<45°), referred to locally as “flatmakes,” and at the intersection of two or more structures referred to as “shatter zones.” Petrographic, electron microprobe, and scanning electron microscope analyses of ores from some of the more recently discovered orebodies, Matanagata, Matanagata East, and R1 reveal that tellurium-bearing minerals, sylvanite, calaverite, krennerite, petzite, hessite, coloradoite, melonite, native tellurium, and rare benleonardite, formed during various hydrothermal stages, hosted in quartz, and to a lesser extent arsenian pyrite and tetrahedrite group minerals. Sylvanite followed by krennerite are the two most common tellurides in these orebodies. These tellurides show no systematic spatial distribution within flatmakes but there appears to be a higher concentration of tellurides where the flatmake intersects steep structures. Gold-rich tellurides preceded the formation of silver-rich tellurides and were constrained at 250 °C in log f S₂ and log f Te₂ space at −12.7 to −10.1 and −9.4 to −7.8, respectively, based on sulfide and telluride stabilities, and the composition of sphalerite. Ore forming components, such as Au, Ag, Te, Cu, V, and S, were likely derived from Late Miocene-Early Pliocene monzonites in and adjacent to the Tavua caldera.