Abstract

The shoshonitic Tavua caldera in Viti Levu, Fiji, defines the volcanic center of the Tavua volcano. The caldera occurs at the intersection of the northeast-trending Viti Levu lineament and a northwest-trending lineament, locally manifested by the Nasivi shear zone. Fault-related subsidence, which contributed to the evolution of the Tavua caldera, the central Inner caldera, and the structural preparation for mineralization, occurred on these two regional fault systems and local concentric normal faults.A continuum of mineralization and alteration styles is evident along the Nasivi shear zone, caused by a northwesterly evolution and migration of intrusion and hydrothermal activity. Porphyry-style, subeconomic copper-gold mineralization and associated alteration occurred at depth in the Nasivi shear zone at the center of the caldera, approximately contemporaneously with the development of the Inner caldera. Acid sulfate-style epithermal alteration and minor gold mineralization immediately postdated and were superimposed on this porphyry-style alteration. Farther northwest along the Nasivi shear zone, transitional ac{d sulfate to adularia-sericite epithermal alteration and minor gold mineralization occur. Variation in hydrothermal style is considered to reflect an increasing distance from magmatic source, combined with an increasing proportion of meteoric water mixing with the hydrothermal fluid.Northwest of this system, at the intersection of the Nasivi shear zone and the Viti Levu lineament, the 4.4 M oz adularia-sericite-style epithermal Emperor gold mine occurs. High-level epithermal mineralization and alteration occur 4 km northeast of the Emperor gold mine, in an area which has a demonstrable present-day hydraulic connection with the mine. Variations in fluid inclusion homogenization temperatures from the mine indicate that fluids entered the structurally prepared mine area via the Nasivi shear zone. Emperor epithermal mineralization postdated magmatism by approximately 0.4 Ma. Stable isotope data, the presence of albitic alteration beneath mineralization, and high salinities within the mine indicate that the Emperor-Northern prospects hydrothermal system is related to an intrusion into the Nasivi shear zone beneath the mine. This is consistent with a demonstrable northwesterly migration of intrusions with time along the Nasivi shear zone.Albitic, phyllic, and propylitic assemblages comprise the main porphyry-style alteration types in the caldera center. Calculated temperatures of formation of chlorite in these assemblages indicate that these three alteration facies formed in the temperature range 270 degrees to 340 degrees C, in good agreement with available fluid inclusion data and the stability ranges of the mineral assemblages. Kubler Indices of illite likewise imply that these assemblages formed at relatively high temperature. Epithermal lode alteration-mineralization at Emperor is shown by fluid inclusion data to have formed around 200 degrees C. Kubler indices of illite confirm that this alteration formed at significantly lower temperatures than did porphyry alteration and that chlorites in the Emperor mine have calculated temperatures of formation mostly below 200 degrees C. High-level epithermal alteration occurring northeast of Emperor is inferred to have formed at even lower temperatures.

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