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 s pecies. Platinum shows a series of close textural associations with monazit e [Ce(La,Y,Th)PO4]. The field relations, gangue mineralogy, and fluid inclu sions trapped inside quartz in contact with platinum indicate probable flui d temperatures of 200 - 300 degrees C and a low-pressure environment. This suggests that the Naboomspruit veins represent a new class of platinum depo sit which is broadly comparable with some epithermal gold deposits. The on fluid was highly oxidized with a low salinity and generated extensive repla cement 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 p resence of reniform mixtures of platinum, Pt-Pd alloy, and Pt-O species are believed to reflect annealing of metal and alloy from an originally amorph ous, hydrated Pt-O precursor (possibly resembling PtO.nH(2)O). The close as sociation between monazite and platinum is obvious but enigmatic. An electr onic interaction between Ce and Pt producing a lowering of surface energy o n Pt during the early stages of nucleation is suggested as having a role in this association.