Challenges to the study of PGE distributions in ores and products

Category Other
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Cabri, Louis
Holding Date 04 October 2008

Platinum-group elements (PGE) are dispersed as rare constituents in nature; occurring either as trace concentrations in relatively abundant sulphide minerals or as trace discrete platinum-group minerals (PGM). Exceptions to this bimodal distribution are found in certain placer deposits where large Pt-rich alloys have been winnowed and concentrated from large volumes of igneous rocks.
The first challenge to understanding the distribution of the PGE is to determine the proportion occurring as trace constituents of minor or major minerals and that occurring as discrete PGM.
A recent review of sample preparation and bulk analytical techniques for PGE gives a good summary of the status of this rapidly evolving field and the precautions one must apply for different sample types/elements (Hoffman and Dunn, 2002). Exploration and mining geologists use bulk methods extensively, as do those studying PGE deposit genesis through modelling. However, one cannot but wonder at understanding partitioning of PGE without knowing how they occur. In contrast, it is accepted that extraction of PGE from PGE-bearing ores requires good understanding of the mineralogical distribution of the PGE, i.e., determination of the amount occurring as structural constituents of minor minerals such as sulphides, and that occurring as PGM. In the latter case, determining their grain size and mineral associations is also important.
Because PGM tend to occur as very small and rare constituents, the mineralogist is faced with two related challenges: finding the proverbial needle in a haystack, and once found, characterizing PGM. The first challenge has been greatly advanced by applying the new Russian technology of hydroseparation/HS (e.g., Rudashevsky et al., 2002) to concentrate PGM into few polished sections making it easier (and cheaper) to find PGM. Further, liberating PGM using electric pulse disaggregation (EPD) followed by HS, increases the amount of PGM, besides producing pristine crystals (e.g., Oberthür et al., 2007; Cabri et al., 2008; Zaccarini et al., 2008).
The next step is to fully understand the mineralogy of PGM. In spite of the literal explosion of new and unnamed PGM found due to application of electron probe microanalyses in the early 1960’s, by 2002 it was reported that of 109 accepted PGM species the crystal structure of ~95% had not been determined (Cabri, 2002). Additionally, problems that need to be solved include an exact understanding of several solid solution series between PGM (e.g., sperrylite-platarsite, braggite-vysotskite). The second challenge is to determine trace concentrations of PGE in different minerals using in situ micro-beam methods. The method of choice is LAM-ICPMS because of the availability of new standards developed as result of a MUN–CANMET collaboration (e.g., Sylvester at al., 2005). Better understanding of PGE distributions can be achieved by refining our understanding of PGM and applying the above-mentioned techniques.