Extreme sulfur isotope fractionation of hydrothermal auriferous pyrites from the SW fringe of the Taupo Volcanic Zone, New Zealand: Implications for epithermal gold exploration

Abstract

The Taupo Volcanic Zone and its environs, New Zealand, represent an ideal place for epithermal gold exploration. We have documented structurally-bound gold in low temperature hydrothermal pyrite veins from the Erua Basin area on the outside fringe of the Taupo Volcanic Zone. Extreme ³⁴S/³²S fractionation of 122.2‰ from auriferous pyrite is documented in hydrothermal veins hosted in alluvial deposits from the Erua Basin. We document three generations of pyrite based on petrographic and Nano-SIMS microstructural analysis. First generation pyrite (Py assemblage -1) has extremely high δ³⁴S values of +70.3 to +100.0‰ probably resulting from sequential-cyclic bacterial reduction and Rayleigh fractionation of limited sulfate. Second generation pyrite (Py assemblage -2) has relatively lower δ³⁴S values of -39.8 to -3.3‰, interpreted to reflect the influx of magmatic-related oxidized fluids mixed with remobilized, reduced sedimentary sulfides that then formed the host calcite-pyrite veins. Third generation pyrite (Py assemblage -3) incorporated much heavier ³⁴S resulting in positive δ³⁴S values ranging from +6.8 to +72.2‰, indicating greater reduction of sulfate to H₂S in an anoxic (reducing) environment. One single composite ~ 10 um pyrite grain encompassing all three types of subgrains has a large δ³⁴S range from -22.2‰ along its rim and +100.0‰ from its core (122.2‰ for a single grain, and 139.8‰ for the system). This is the largest known variation in δ³⁴S ever reported in a single pyrite grain from any system on Earth, but is comparable to δ³⁴S ranges in pyrite systems of up to 186‰ reported from microbial sulfate reduction in deep bedrock fracture systems in Sweden. The growth stages from Py-2 to Py-3 indicate a transformation of the environment from an open to a closed (highly reducing) system, and it is these late-stage vein pyrites that are slightly auriferous. This process is interpreted to be induced by the precipitation of sulfides perhaps near the margin of the hydrothermal system or closed growth of pyrite in hydrothermal veins, which both restrict circulation and convection of sulfate within the veins. Our results suggest that the Erua Basin may sit on the fringe of a low-temperature epithermal gold system; other “low-temperature” areas around the TVZ may also preserve similar, potentially enriched zones on the edge of an epithermal Au belt.