NANOMETER-SIZED PLATINUM-GROUP MINERALS (PGM) IN BASE METAL SULFIDES: NEW EVIDENCE FOR AN ORTHOMAGMATIC ORIGIN OF THE MERENSKY REEF PGE ORE DEPOSIT, BUSHVELD COMPLEX, SOUTH AFRICA

摘要

We report new results of a combined focused ion beam and high-resolution transmission electron microscopy (FIB/HRTEM) investigation of platinum group element-rich (PGE) samples from the Merensky Reef of the Bush-veld Complex in South Africa. Chromitites and pegmatitic melanorite from the Northam and Rustenburg platinum mines contain Ru-Rh-Pt arsenide nanocrystals (<50 nm in size) as inclusions in pyrrhotite and pyrite, as well as Ru-Rh-Os, Ru-Rh-Pt-Ir, and Ru-Rh-Pt sulfide nanocrystals in pyrrhotite and pentlandite. Chalcopyrite did not incorporate nanometer-sized PGE-bearing minerals (PGM). These nano-scale PGM inclusions form a new population that was hitherto unknown from the Merensky Reef. The PGM inclusions are found as idiomorphic, plate-like nanocrystals, which are differently oriented with respect to the host base metal sulfide (BMS). In one TEM foil we observed Ru-Rh-Pt-sulfide nanocrystals exsolved from a pentlandite matrix with a common crystallographic orientation that is visible in TEM dark-field images. However, this phenomenon is only patchily developed and ahead of the exsolution front the pentlandite still contains Ru-Rh in solid solution.