Microscale geochemical variations in metamorphic-hydrothermal scheelite and insights into trace element sources (Otago Schist, New Zealand)

Abstract

ABSTRACT Scheelite (CaWO4) trace elements are frequently used to fingerprint sources of mineralising fluids or styles of mineralisation. However, the chemistries of scheelite from hydrothermal-metamorphic Otago Schist scheelite-quartz veins have complicated parageneses. Coarse (>1 cm) single grains from three deposits contain zones that fluoresce orange or blue in cathodoluminescence. Laser ablation analysis shows the orange zones to be richer in REE but the blue zones to be richer in Sr. With one exception, which may be due to incorporation of calcite, the internally homogenous in-situ 87Sr/86Sr and low Rb/Sr indicate isotopic equilibrium at the time of scheelite precipitation. Micro-drilled cores extracted, dissolved and measured for trace elements and 143Nd/144Nd isotopic compositions, together with the ingrowth history of 87Sr/86Sr and 143Nd/144Nd in the deposit host rocks, indicate that the scheelite isotopic ratios have distal or multiple sources to the Sr and Nd. Other trace elements therefore also likely have complicated paragenetic histories, which mean that these do not necessarily indicate either the element source or the style of mineralisation. A positive aspect is that the internal Sm/Nd variation means that single-grain isochrons resolve ages of 114 ± 50 Ma (Alta Lode) and 136 ± 45 Ma (Macraes) that overlap published ages.