Magma Chamber Processes in the Formation of the Low-sulphide Magmatic Au-PGE Mineralization of the Platinova Reef in the Skaergaard Intrusion, East Greenland

• Published in: Journal of Petrology
• Main Authors: Keays, Reid R., Tegner, Christian
• Format: Text
• Language: English
• Published: Oxford University Press 2015
• Subjects: Original Papers; Greenland; East Greenland
• Online Access: http://petrology.oxfordjournals.org/cgi/content/short/56/12/2319; https://doi.org/10.1093/petrology/egv075

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 240 ppm Cu, 89 ppm 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 ∼300 m 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 ...