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Petrographic characterization and evolution of the alkalic-type epithermal Tuvatu Au-Te deposit, Fiji

Schmidt, Daniel E.
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2023
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Abstract
The Tuvatu alkaline epithermal gold deposit is located on the island of Viti Levu in the Republic of Fiji. This world class gold telluride system is part of a sub-class of low-sulfidation epithermal systems which are hosted in alkaline igneous rocks. These alkaline systems are genetically similar to high-sulfidation epithermal systems in that they have a direct connection to magmatic hydrothermal fluids, however due to the difference in composition of the source magma, alkaline epithermal deposits more closely resemble low-sulfidation epithermal systems in their alteration styles and overall mineralogy. Therefore, from an exploration perspective, it is important to treat these alkaline systems as genetically distinct. The Tuvatu deposit is hosted within the variably altered Navilawa Monzonite and is spatially and genetically associated with the Navilawa Caldera, an eroded shoshonitic volcano. Mineralization occurs in high grade, relatively narrow sub-vertical north-south trending veins as well as flat-lying veins, faults, shatter zones and hydrothermal breccias. A significant portion of the gold within the deposit occurs as free gold, with additional mineralization in the form of precious metal tellurides and base-metal sulfides. Forty-two samples of high-grade ore were analyzed using optical microscopy, mapping μ-XRF, various SEM-based techniques, fluid inclusion analysis and cathodoluminescence imaging. An early high temperature alteration stage and three subsequent stages of epithermal mineralization have been identified. An early high-temperature alteration style of the host monzonite is characterized by porphyry-style alteration in which pervasive potassic alteration, containing hydrothermal K-feldspar overgrowths lining undulose fractures, and secondary shreddy biotite was locally overprinted by later propylitic alteration. This first epithermal alteration stage observed is base metal sulfide-rich and shows quartz-sericite-pyrite alteration and potassium metasomatism halos the veins. Base metal veins cross-cut previous potassic alteration and contain large pyrite grains up to 5mm which are coated with dendritic or euhedral overgrowths of chalcopyrite, sphalerite, galena, and minor amounts of native gold. The dendritic shape of the sulfides indicates that they formed under far-from-equilibrium conditions, which suggests precipitation due to rapid boiling (flashing) of the hydrothermal fluids. Quartz is the principal gangue mineral and forms fine-grained aggregates with abundant recrystallization and mosaic textures under cross-polarized light, presumably the result of the recrystallization of non-crystalline silica. The second epithermal mineralization stage is host to the majority of the native gold and precious metal tellurides. This stage can be characterized by the presence of quartz veins with adularia rims, quartz-sericite-pyrite alteration and abundant potassium metasomatism around vein walls. Within the veins is dendritic native gold, roscoelite, and gold-tellurides. Although this quartz is recrystallized to a high degree, relic microspherical textures were observed within the highest-grade portions of these veins, suggesting that silica in this stage was deposited as Opal-AG. Observations from modern day hydrothermal systems suggest that opal-AG forms in response to near instantaneous vaporization (flashing) of hydrothermal fluids at temperatures of 200°C to 250°C and at sub-hydrostatic conditions. The third and final stage of epithermal mineralization consists of barren carbonate veins which cross-cut or reopen previous veins. The margins surrounding these carbonate veins sometimes show albite and chlorite overprinting previous potassium metasomatism and quartz-sericite alteration. Based on these observations, an updated deposit model was formulated in which host rocks were subjected to high-temperature porphyry-style alteration at depths of >1.5km below the paleosurface under lithostatic conditions. This was followed by unroofing and erosion of the Navilawa over a period of up to a million years. After unroofing, epithermal mineralization was driven by periodic flashing of hydrothermal fluids within the system. Epithermal mineralization likely occurred relatively close to the paleosurface at sub-hydrostatic conditions. Due to this inferred time gap, it is unlikely that early porphyry-style mineralization and subsequent epithermal mineralization were fed by the same causative intrusion. Instead, the later epithermal mineralization was fed by fluids from a younger and deeper intrusion which utilized the same plumbing system provided by faults associated with the Navilawa caldera and structures associated with the Viti Levu Lineament. Evidence presented herein for the deposition of high-grade ore minerals due to fluid flashing has important implications for how these alkalic low-sulfidation epithermal systems form, as well as providing valuable information which can aid future exploration.
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