Magmatic-hydrothermal evolution of the Tuvatu alkalic epithermal Au-Telluride deposit, Viti Levu, Fiji

Abstract

The Tuvatu deposit is a high-grade alkalic epithermal Au deposit in Fiji that consists of steeply dipping lode veins, flat lying lodes, and breccias hosted in alkalic intrusive and volcanic rocks. This study performed drill core logging of veins and alteration, vein sample petrography, and geochemical microanalysis of biotite, pyrite, and marcasite to reconstruct the evolution of the magmatic-hydrothermal system and to identify the relative timing of Au mineralization. Eight vein types have been identified, including four porphyry-style veins which formed at higher temperatures and larger pressures, and four epithermal-style veins associated with shallow Au mineralization. The two environments were found to be genetically distinct. Secondary hydrothermal and primary magmatic biotite grains were analyzed with an electron microprobe and are found to contain different major element ratios. Hydrothermal biotite at Tuvatu contains element ratios associated with the porphyry environment and high temperature potassic alteration. Quartz in epithermal veins is shown to have textures indicating recrystallization from originally noncrystalline silica, which is interpreted to have deposited through flashing. Ore- stage pyrite and marcasite show distinct enrichments in As. Laser ablation-ICP-MS analysis revealed high As and Au incorporation in a specific stage of arsenic-rich pyrite which contains Au as a lattice-bound ion. This study emphasizes the potential importance of hydrothermal fluid flashing to high-grade Au deposition in the epithermal environment and the contribution of invisible Au in Fe sulfides to the total Au contained in alkalic epithermal deposits.