Cooperite and braggite from the Bushveld Complex: implications for the miscibility gap and identification

Abstract

Analyses of the Pt-Pd±Ni sulphides cooperite, braggite, and vysotskite reported from worldwide occurrences seem to imply a continuum of compositions between vysotskite and cooperite, with no obvious miscibility gap. This is contrary to the experimentally confirmed miscibility gap between cooperite and braggite, and the established compositional gap between co-existing cooperite and braggite from the Merensky Reef. Although the only unambiguous way of distinguishing between cooperite and braggite is to obtain structural information through X-ray diffraction or equivalent techniques, most identification of Pt-Pd±Ni sulphides is based on microanalytical techniques due to the small grain size of most platinum-group minerals. Ni contents also have to be considered because a classification based on the Pt/Pd ratio alone can be very misleading. Naming of Pt-Pd±Ni sulphide compositions with high Pt contents based on qualitative or semiquantitative analyses should be avoided. Natural PtPd±Ni sulphides which project into the compositional gap (established by experimental work) cannot be named without supporting structural information. Compositions of grains, which plot inside the gap, are considered to be metastable and to result from the loss of Pd through interaction with hydrothermal fluids.Keywords Braggite AE Cooperite AE Pt-Pd-Ni-S AE Miscibility gap AE Bushveld AE South Africa Recent experimental investigations of the system PtS- PdS-NiS (Verryn and Merkle 2000, 2002) confirmed the presence of a miscibility gap between cooperite and braggite and a continuous solid solution series between braggite and vysotskite. Contrary to the experimentally confirmed miscibility gap between cooperite and braggite, a compilation of analyses reported in the literature from worldwide occurrences seems to imply a continuum of compositions between vysotskite and cooperite with no obvious gap (Fig. 1). This contribution focuses on the reasons for the discrepancy between experiments and natural occurrences. The question also arises of which criteria to use for identification of minerals inside the experimentally established miscibility gap. The data baseA compilation of published analyses from the literature (for sources see caption of Fig. 1) was used to evaluate the miscibility gap between cooperite and braggite and the problem of identifying some natural Pt-Pd-Ni sulphides. The database of 592 microprobe analyses from Miner Deposita (2003) 38: 381-388

40 Ar/ 39 Ar age constraints on ore deposition and cooling of the Bushveld Complex, South Africa

“…Thus any important changes in composition of platinum-group minerals should occur mainly above the biotite closure temperature and less so under a slower cooling regime. This hypothesis is in agreement with the recent work by Merkle & Verryn (2003), which demonstrated that minerals such as braggite and cooperite equilibrate and change in composition above the blocking temperature of biotite. The clarification of the Bushveld Complex's lower-temperature thermal history would benefit from the application of (U þ Th)/He dating on apatite for example, recently shown (Min et al 2003) to be applicable to rocks more than twice as old as the Bushveld Complex.…”

Detrital Platinum-Group Minerals in Rivers Draining the Great Dyke, Zimbabwe

“…Cooperite and braggite are dealt with together, as these minerals are hard to distinguish microscopically and they form a chemically nearly continuous series (Cabri 2002, Merkle & Verryn 2003. Notably, cooperite was found in the Makwiro and Umtebekwe Rivers, and braggite in the Makwiro River alone.…”

Platinum-group minerals of the F and T zones, Waterberg Project, far northern Bushveld Complex: implication for the formation of the PGE mineralization

“…The braggite intergrowth suggests a secondary origin, although the chemistry of a precursor is difficult to constrain. It is known that cooperite is the highest-temperature Pt sulfide in the Bushveld reefs and it may be replaced or overgrown by later braggite at the late magmatic stage (Merkle and Verryn, 2003; Yudovskaya et al ., 2017). The Ni-rich braggite and vysotskite (Pd,Ni)S are not widespread in the Bushveld reefs, rather they are known to be formed under postmagmatic hydrothermal conditions, for example, in massive sulfides ores of the Noril'sk deposit (Genkin et al ., 1981).…”