Longwood Range Platinum, Southland

Abstract

The Longwood Range includes extensive outcrops of the Hekeia Gabbro, a Permian mafic-ultramafic layered plutonic complex that was intruded into Permian Brook Street Terrane volcanogenic sediments. Analyses of core from drill holes in the Hekeia Gabbro show that there is localised enrichment in platinum group elements (PGE), but no PGE minerals have yet been identified. The most PGE-enriched rocks are gabbros and troctolites, rather than the most ultramafic rocks, which are plagioclase wehrlites. Cr-spinels and magmatic sulfides (pyrrhotite, chalcopyrite, pentlandite) are rare and show no evidence of accumulations in association with PGE enrichment zones. The Hekeia Gabbro is extensively altered in places to amphibolite facies and/ or greenschist facies assemblages. There has been minor remobilisation and localised enrichment of Pt, Pd and/ or Cu in association with some of this alteration. Primary igneous magnetite was replaced during alteration, and alteration magnetite has formed in some zones, especially in association with serpentinised olivine. Detrital isoferroplatinum, sperry lite, Os-Ru-lr alloys and cooperite-braggite solid solution minerals occur in palaeoplacers on the southern slopes of the Hekeia Gabbro. The cooperite-braggite solid solution minerals have low Ni, typical of Ural-Alaska complexes, and the Os-Ru-Ir alloy particles are only incipiently rounded, suggesting that they may have been sourced nearby, from the Hekeia Gabbro. The Hekeia Gabbro, and other similar mafic-ultramafic bodies in the Brook Street Terrane, and their correlatives in Australia, are apparently fertile sites for PGE enrichment.