Natural and synthetic phase relations of cooperite, braggite and vysotskite

Abstract

In this thesis, results of experimental investigations in the dry system PtS-PdS-NiS at 1200°C, 11 OO°C, 1 OOO°C, 900°C, 800°C, and 700°C are presented. Experimental results are compared with analyses of naturally occurring cooperite (ideally PtS), braggite ((Pt,Pd)S), and vysotskite (ideally PdS) of this study and with analyses reported in the literature. Synthetic cooperite is stable at temperatures above 1200°C, whereas synthetic braggite at temperatures below 1100°C, and synthetic vysotskite below 1000°C. Below 900°C in the system PtS-PdS-NiS, Ni-saturated synthetic cooperite, -braggite, and -vysotskite coexist with Ni1_xS, At 900°C only Ni-saturated, Pd-free synthetic cooperite coexists with Ni1_xS, and synthetic braggite and Pd-containing synthetic cooperite coexist with a Pd-Ni-Pt - sulphide melt. At temperatures > 1000°C all Ni-saturated synthetic cooperite and -braggite coexists with a Pd-Ni-Pt - sulphide melt. There is a clearly defined miscibility gap between cooperite and braggite, but no gap was observed between braggite and vysotskite. There are well defined, positive, relationships between the Pd-contents of synthetic cooperite, -braggite, and -vysotskite and the Pd-content of the coexisting melt or that of the coexisting Ni1_xS. The Ni-content of synthetic braggite and -vysotskite with varying PtlPd ratios in a Ni-saturated environment is a function of temperature; a higher Ni-content resulting from a lower equilibration temperature. In synthetic cooperite, a similar relationship between the Ni-content and the temperature of formation or equilibration is applicable between 1200° and 900°C. At 800°C, the Ni-content is higher than that at 700°C. The variation in the Ni-content in synthetic cooperite is more restricted than that for synthetic braggite and -vysotskite. Sulphur fugacity has an effect on the system Pt-Pd-Ni-S. If formed under higher S-pressure, synthetic braggite synthesized at 1000°C contains higher amounts of Ni than formed at lower S-pressure at given PtlPd ratios in the initial charge. At constant bulk PtlPd ratio, the PtlPd ratio of the synthetic braggite decreases with increasing sulphur fugacity. The inter-element relationships for synthetic braggite are not as pronounced for cooperite synthesized at 1000°C. Compositions of cooperite, braggite, and vysotskite have the potential to indicate at which stage of the development of an ore deposit these phases were exsolved or equilibrated. Comparison of the experimental results with analyses of natural cooperite, braggite, and vysotskite of this study and from the literature, indicate that: (i) Cooperite and braggite from the Merensky Reef have equilibrated below 900°C. (ii) Analyses of cooperite, braggite, and vysotskite from the UG-2, coexisting with pentlandite follow the trend similar to that experimentally established for temperatures < 700°C and where coexisting with millerite, the trend established for temperatures between 800°C and 700°C. (iii) Temperatures of formation or equilibration of cooperite, braggite, and vysotskite from the Sompujävi Reef of the Penikat Intrusion and of Placer Deposits, as well as cooperite from the Stillwater Complex cannot be estimated with certainty. (iv) Braggite and vysotskite from the Stillwater Complex may have formed at temperatures below 700°C. (v) Cooperite-, braggite-, and vysotskite analyses from Noril'sk indicate formation temperatures of < 700°C.