MAGMATIC-HYDROTHERMAL EVOLUTION OF THE TUVATU ALKALIC EPITHERMAL AU-TELLURIDE DEPOSIT, VITI LEVU, FIJI

The Tuvatu deposit is an alkalic epithermal Au-telluride deposit located on the island of Viti Levu in Fiji. The deposit lies in the southwestern corner of a topographic feature known as the Navilawa caldera along the ENE-trending Viti Levu Lineament, which controls the location of a series of precious metal deposits associated with alkalic rocks. The Tuvatu deposit is partly hosted within a ~4.85 Ma monzonite that intruded into andesitic host rocks.

Tuvatu is a telescoped system in which a succession of distinct vein types that overprint each other as the magmatic-hydrothermal system cooled. The host monzonite contains early porphyry-style veins and associated high-temperature alteration, including zones of potassic alteration characterized by the abundant presence of hydrothermal biotite. The early porphyry-style veins and associated potassic alteration are generally barren although veins composed of Cu sulfides such as bornite and chalcopyrite locally crosscut the potassically altered wall rocks. Pervasive patchy epidote alteration is widespread within the monzonite and andesite that host the Tuvatu deposit. A set of distinct veins of epidote, biotite, and magnetite are observed in deeper parts of the system.

Three distinct types of epithermal veins occur. Early veins are rich in base metal sulfides including sphalerite, galena, pyrite, and chalcopyrite. In the deeper part of the deposit, the base metal sulfide veins are overprinted by veins containing quartz and distinctly dark pyrite. This pyrite is characterized by high As concentrations and contains structure-bound invisible gold. The highest gold grades are associated with narrow quartz-roscoelite veinlets. These contain abundant native gold and lesser telluride minerals, primarily as dendrites in quartz that formed through recrystallization of originally noncrystalline silica exhibiting a microspherical texture. The textural evidence suggests that this style of mineralization formed due to flashing of the magmatic-hydrothermal fluids.