PGE enriched magnetite of the Union Bay Alaskan-type intrusive complex and its hydrothermal origin

摘要

PGE concentrations of 18 g/t occur in the pyroxene rich rocks at the Union Bay Alaskan-type complex. This PGE enrichment is hosted in magnetite veins. These veins cross magmatic layering and have an irregular morphology in both outcrop and thin section. Fe-Ti oxide geothermometry suggests that this mineralization took place between ～700-400°C. A variation in the amount of ilmenite and Mg-Al spinel exsolu-tion in magnetite correlates with temperature estimates, lower temperature magnetite has less exsolution. Magnetite veins with a formation temperature of <575°C are surrounded by an alteration envelope of hydrous silicates that vary with temperature and contain appreciable amounts of Cl. Interaction of a PGE-Fe-Cl rich fluid with clinopyroxene increased the fluids pH and decreased PGE and Fe solubility depositing a PGE enriched magnetite within the Pyroxenites and Olivine Pyroxenites of the Union Bay complex.