COMPOSITIONAL VARIATION OF COOPERITE, BRAGGITE, AND VYSOTSKITE FROM THE BUSHVELD COMPLEX

The compositions of coexisting and individual cooperite (ideally PtS) and braggite (ideally (Pt,Pd)S) grains from the Merensky Reef of the B ushveld Complex, as well as cooperite, braggite and vysotskite (ideall y PdS) grains from the UG-2 of the Bushveld Complex were investigated. There is a clearly defined miscibility gap between cooperite and brag gite, but no evident gap between braggite and vysotskite. Partition co efficients between cooperite and braggite are determined on coexisting phases. The K(D)braggite/cooperite in atomic ratios are estimated to be 0.54 for Pt, 15.81 for Pd and 5.93 for Ni. For Rh and Co the K(D)co operite/braggite are estimated to be > 1.40 and > 1.46 respectively. N o systematic behaviour is detected for Fe and Cu. Coupled substitution s of Pd + Ni for Pt in cooperite and braggite/vysotskite are indicated . Within the cooperite of the Merensky Reef, the Pd:Ni ratio is approx imately 9:11. The substitution trend in braggite, which extends to vys otskite in the UG-2, is dependent on the base-metal sulphide (BMS) ass ociation. If pentlandite is the dominant Ni-bearing BMS, the Pd:Ni rat io is about 7:3 in the Merensky Reef and in the UG-2. Millerite as the dominant Ni-bearing BMS in the UG-2 changes this ratio to 3:1. It is concluded that the Ni-content in braggite/vysotskite from BMS assembla ges does not depend on the NiS activity, but rather on temperature of formation.