Hydrothermal arsenic enrichment in an active mountain belt: Southern Alps, New Zealand

Abstract

Torlesse terrane greywackes and hydrothermal calcite associated with active fault systems in the Main Divide Fault Zone of New Zealand's South Island exhibit anomalous As concentrations relative to background values of similar rock types. Mass-balance analysis of greywacke compositional data from the upper Wilberforce valley of the Main Divide region demonstrate an average mass gain of As on the order of a factor of 6 above typical Torlesse greywackes (Wilberforce: mean=35.3 ppm; σ=31.3 ppm; n=44; Torlesse: mean=5.6 ppm; σ=4.4; n=31). In addition, hydride generated ICP-MS analyses of the weak acid soluble fraction of individual calcite-rich veins and fault gouges in the Main Divide Fault Zone exhibit As concentrations (mean=81.2 ppm; σ=113 ppm; n=17) in excess of a factor of 100 over hydrothermal calcite from other fault systems (mean=1.7 ppm; σ=4.0; n=29). Stable isotopic analyses of these calcite veins show a wide range for both oxygen ( δ 18O=9.9‰ to 25.6‰) and carbon ( δ 13C=−12‰ to −1.1‰). This observation suggests that the hydrothermal fluid source of these veins was of a mixed meteoric and highly exchanged fluid over a wide range of temperatures. We suggest that the presence of high As concentrations in young hydrothermal calcite veins in the Main Divide region results from active transport of As-rich deep crustal fluids upward along the fault zone. This interpretation is consistent with both fluid flow/tectonic models and geophysical observations, which suggest that expansion in the Main Divide Fault Zone provides a conduit for the escape of deep exchanged fluids into the upper crust.