Platinum group element enriched hydrothermal magnetite of the Union Bay Alaska-type ultramafic intrusion, Southeast Alaska

Abstract

"The Union Bay Alaskan-type Complex represents a completely different type of platinum group element (PGE) concentration from the classic Alaskan-type Complex PGE mineralization model, with PGEs localized in vein/veinlet magnetite, commonly within the olivine + pyroxene portions of the body. PGE concentrations of 18 g/t occur in magnetite veins and pods which cross magmatic layering and have an irregular morphology in both outcrop and thin section. Fe-Ti oxide geothermometry indicates that this mineralization formed at temperatures between 675-475°C. A variation in the amount of ilmenite and Mg-Al spinel exsolution in magnetite correlates with temperature estimates, lower temperature magnetite has less exsolution. Magnetite veins with a formation temperature of less than 600°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 and olivine increased the fluids pH and decreased PGE and Fe solubility depositing a PGE enriched magnetite within the wehrlite and clinopyroxenite of the Union Bay Alaskan-type complex. Potentially this fluid remobilized the PGE from disseminated euhedral magnetite of the peripheral units to form PGE enriched magnetite veins in the near-central portion of the complex"--Leaf iii