Hydrothermal Attributes of the Union Bay Platinum Prospect

<400°C. Platiniferous magnetite at Union Bay occurs in veins and irregular pods in both outcrop and thin section. Magnetite-rich veins cut across layering in ultramatfic rocks and commonly parallel diopside veins. The latter veins are compositionally distinct from the magmatic clinopyroxene and apparently represent an early stage of hydrothermal alteration in olivine-clinopyroxene rocks. PGM-rich zones are zones of abundant magnetite veins which can be traced along vein strike but cut across different ultramafic host rock types. Temperature estimates from PGE bearing magnetite present in clinopyroxene rich rocks vary accordingly with changes in the alteration envelopes, and the amount of spinel and ilmenite exsolution. As the amount of exsolution in magnetite decreases so does the magnetite-ilmenite based temperature estimates. Alteration minerals surrounding magnetite change from hornblende to chlorite with decreasing temperature. Multiple PGMs have been identified in this magnetite and include Pt-Fe alloys, Erlichmanite (OsS2), Hollingworthite (RhAsS), native Os, Os-Ir alloys, Ir-Pt alloys, Rh-Fe alloys, PtSb, and a possible Ptlr sulfide. Microanalysis is being conducted on select samples to quantitatively identify the alloys and antimonides. Hydrothermal pyroxene at Union Bay is nearly pure diopside, in contrast with the Fe-Al-bearing magmatic clinopyroxenes. Hydrothermal hornblende is richer in Na and Fe3+, and poorer in than magmatic hornblende. Both Phlogopite and Hornblende associated with hydrothermal magnetite contain significant CI", suggesting equilibrium with a moderately CI" rich fluid. Hydrothermal chlorite varies significantly in optical properties and composition. In particular chlorite associated with lower-temperature magnetite deposition has different Mg/Fe ratios than that which overprints alteration hornblende. A likely scenario for deposition of magnetite and PGE at Union Bay involves a quartz-undersaturated fluid with moderately high CI" contents, a moderate pH, and a moderate f0 2. Reaction of such a fluid with olivine and clinopyroxene rich rocks would produce a reaction envelope of amphibole and (or) chlorite, depending on temperature. In either case, reaction with the magmatic silicates causes an increase in solution pH, which would cause both magnetite and PGE deposition from chloride complexes in solution. The origins of this fluid are unknown, but the absence of quartz deposition