Platinum precipitation in the Waterberg Deposit, Naboomspruit, South Africa

Quartz veins located northwest of Naboomspruit in the Northern Province of South Africa contain native platinum, Pt-Pd alloys, and at least two Pt-O species. Platinum shows a series of close textural associations with monazite [Ce(La,Y,Th)PO 4 ]. The field relations, gangue mineralogy, and fluid inclusions trapped inside quartz in contact with platinum indicate probable fluid temperatures of 200-300 degrees C and a low-pressure environment. This suggests that the Naboomspruit veins represent a new class of platinum deposit which is broadly comparable with some epithermal gold deposits. The ore fluid was highly oxidized with a low salinity and generated extensive replacement of feldspar in the wall rocks. Increase in pH generated by wall-rock reactions is seen as an important mechanism in Pt precipitation, and the presence of reniform mixtures of platinum, Pt-Pd alloy, and Pt-O species are believed to reflect annealing of metal and alloy from an originally amorphous, hydrated Pt-O precursor (possibly resembling PtO.nH 2 O). The close association between monazite and platinum is obvious but enigmatic. An electronic interaction between Ce and Pt producing a lowering of surface energy on Pt during the early stages of nucleation is suggested as having a role in this association.