Abstract

Gahnite-bearing rocks are common throughout the Proterozoic Broken Hill domain, New South Wales, Australia, where they are spatially associated with Broken Hill-type Pb-Zn-Ag mineralization, including the supergiant Broken Hill deposit. In the past, such rocks have been utilized as exploration guides to ores of this type, but their presence has had mixed success in discovering new occurrences of sulfide mineralization. Major element chemistry of gahnite has previously been used to define a compositional range associated with metamorphosed massive sulfides deposits, including Broken Hill-type deposits, but it fails to distinguish sulfide-rich from sulfide-poor occurrences. Major and trace element data from LA-ICP-MS and electron microprobe analyses were obtained for gahnite from twelve Broken Hill-type deposits to determine whether or not gahnite chemistry may be used to distinguish prospective exploration targets from nonprospective occurrences. Major and trace element data were discriminated using a principal component analysis, and in a bivariate plot of Zn/Fe versus Ni + Cr + V to distinguish gahnite associated with the Broken Hill deposit from that associated with sulfide-poor lode pegmatite, and sillimanite gneiss. Bivariate plots of Zn/Fe versus trace element contents (e.g., Ga, Co, Mn, Co, Ni, V, Cd) suggest gahnite from the Broken Hill deposit has a relatively restricted compositional range that overlaps with some minor Broken Hill-type occurrences. Based on the ore grade (wt % Pb + Zn) of rocks hosting gahnite at each locality, gahnite in the highest grade mineralization from minor Broken Hill-type deposits possess compositions that plot within the field for gahnite from the Broken Hill deposit, which suggests that major and trace element chemistry (e.g., Zn/Fe = 2–4 vs. Co = 10–110 ppm, Ga = 110–400 ppm, Mn = 500–2,250 ppm; and Co = 25–100 ppm vs. Ga = 125–375 ppm) may be used as an exploration guide to high-grade ore.

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