APPLICATION OF LA-ICP-MS ANALYSES TO THE FORMATION OF NI-SULFIDE ORE DEPOSITS

Over the past 10 years LA-ICP-MS has developed from being a research tool only available in specialized centers for fundamental research to the level of being widely available and accessible. We have been using LA-ICP-MS to determine the concentrations of PGE, Au, Ni, Cu, Co and other chalcophile elements in base metal sulfides (BMS) of magmatic ore deposits. We have also investigated trace elements in the silicate and oxide minerals associated with these deposits. The aims of our studies are: a) a mass balance of the economically important elements; b) investigating whether the BMS have characteristic PGE patterns; c) to apply these observations to models of Ni-sulfide ore deposits. All BMS PGE patterns show negative Pt anomalies indicating the Pt is not present in the BMS in solid solution. Pyrrhotites (Po) are enriched in Re, Os, Ir, Ru and Rh relative to Pt, Pd and the whole rock. Pentlandites (Pn) are enriched in all PGE and Re (except Pt) relative to the whole rock and Pd is enriched relative to the IPGE. Chalcopyrites (Ccp) are depleted in all PGE, but the patterns overall are enriched in Pd relative to Ir. The Po patterns are consistent with a model of monosulfide solid solution crystallization from sulfide liquid followed by exsolution of Po and Pn. The Pn patterns are also consistent with this, except for the Pd enrichment. The strong Pd enrichment implies that Pd has diffused into the Pn, i.e. Pd is mobile at fairly low temperatures. Platinum is largely present as PGM inclusions in the BMS. Either the Pt exsolved from the BMS or it crystallized directly from the sulfide liquid. The contrast in mass balance calculations on BMS from rapidly cooled unmetamorphosed deposits to slowly cooled unmetamorphosed deposits to metamorphosed deposits indicates that initially all of the PGE partitioned in the sulfide liquid and the PGE (except Pt) partitioned into the BMS during crystallization. In rapidly cooled BMS the process ended here. In slowly cooled intrusions PGM from the BMS but remained in contact with or within the BMS. In metamorphosed BMS the low percentage of PGE present in solid solution and the isolated PGM grains indicates that the degree of exsolution PGM is more advanced and in some cases S loss has resulted in residual isolated PGM amongst the silicate grains and possibly the mobility of Pd.