Magma chamber processes in the formation of the low-sulphide magmatic Au-PGE mineralization of the platinova reef in the skaergaard intrusion, East Greenland

Abstract

The Platinova Reef comprises a series of platinum group element (PGE)-and Au-rich layers that contain precious metals intimately associated with magmatic Cu-rich sulphides. This study presents new PGE, Au, S, Se and Te data for samples collected along a stratigraphic reference section from the base of the Lower Zone up to the Sandwich Horizon of the Skaergaard intrusion, and seeks to address the magma chamber processes that led to the formation of the Platinova Reef. The majority of the Skaergaard rocks have low S contents (<500 ppm), with the S contents of those within and below the Platinova Reef being especially low (<100 ppm). The very low S contents of these rocks are due in large part to the low S content of the initial Skaergaard magma; there is no evidence for any post-magmatic S loss in this part of the stratigraphy. Rayleigh fractionation modelling of the variations in metal concentrations of samples from this stratigraphic reference section indicates that the initial Skaergaard magma contained 240ppm Cu, 89ppm S, 4·0 ppb Au, 18·7 ppb Pd, 9 ppb Pt, 90 ppb Se and 5·7 ppb Te. The high Pd/Pt ratio of the Skaergaard magma indicates that it had undergone a considerable amount of differentiation prior to its entry into the Skaergaard magma chamber. Precious metal enrichment commenced at a stratigraphic level ~300m below the Platinova Reef owing to saturation of the magma in Au-PGE-rich Cu sulphides. Although only tiny amounts of sulphides were initially formed, precious metal enrichment increased rapidly upwards to culminate in the formation of the Platinova Reef. Sulphide saturation of the magma was initially restricted to the boundary layer between the magma and the crystal mush where cumulus Fe-Ti oxides were forming. Although only very small amounts of sulphides were initially formed, the rate of sulphide production increased with time, leading to the entire residual magma in the chamber becoming sulphide saturated immediately after the formation of the Platinova Reef. Sulphide saturation and the eventual formation of the Au-PGE mineralization in the Reef was the product of a number of important factors. These include prolonged fractionation of the magma, which produced a residual melt that was enriched in Au, Pd, Cu, S and FeO and led to the build-up and eventual saturation of the magma in Fe-Ti oxides and Cu sulphides. The formation of cumulus ilmenite and magnetite slowed down the rate of build-up of FeO in the magma and also removed O2 and caused some of the SO42- in the residual magma to be converted to S2-, whereas the high Cu content (621ppm calculated from Rayleigh fractionation modelling) of the residual magma at this stage drove the magma to sulphide saturation, forming very small amounts of immiscible PGE-Au-rich Cu sulphides. Other factors that contributed to sulphide saturation of the magma were decreases in temperature and fO2 of the magma, which were accompanied by an increase in the SiO2 concentrations of the residual melt.