Abstract
The Platinova Reef, in the Skaergaard Intrusion, east Greenland, is an example of a magmatic Cu–PGE–Au sulfide deposit formed in the latter stages of magmatic differentiation. As is characteristic with such deposits, it contains a low volume of sulfide, displays peak metal offsets and is Cu rich but Ni poor. However, even for such deposits, the Platinova Reef contains extremely low volumes of sulfide and the highest Pd and Au tenor sulfides of any magmatic ore deposit. Here, we present the first LA-ICP-MS analyses of sulfide microdroplets from the Platinova Reef, which show that they have the highest Se concentrations (up to 1200 ppm) and lowest S/Se ratios (190–700) of any known magmatic sulfide deposit and have significant Te enrichment. In addition, where sulfide volume increases, there is a change from high Pd-tenor microdroplets trapped in situ to larger, low tenor sulfides. The transition between these two sulfide regimes is marked by sharp peaks in Au, and then Te concentration, followed by a wider peak in Se, which gradually decreases with height. Mineralogical evidence implies that there is no significant post-magmatic hydrothermal S loss and that the metal profiles are essentially a function of magmatic processes. We propose that to generate these extreme precious and semimetal contents, the sulfides must have formed from an anomalously metal-rich package of magma, possibly formed via the dissolution of a previously PGE-enriched sulfide. Other processes such as kinetic diffusion may have also occurred alongside this to produce the ultra-high tenors. The characteristic metal offset pattern observed is largely controlled by partitioning effects, producing offset peaks in the order Pt+Pd>Au>Te>Se>Cu that are entirely consistent with published D values. This study confirms that extreme enrichment in sulfide droplets can occur in closed-system layered intrusions in situ, but this will characteristically form ore deposits that are so low in sulfide that they do not conform to conventional deposit models for Cu–Ni–PGE sulfides which require very high R factors, and settling of sulfide liquids.
Highlights
The Skaergaard Intrusion in east Greenland is host to the Platinova Reef, which is an example of a rare subclass of magmatic sulfide deposit, manifested as stratiform PGE reef deposits in the upper parts of the host intrusions (Brooks 1989; Bird et al 1991; Andersen et al 1998; Nielsen et al 2005, Miller and Andersen 2002; Godel et al 2014; Holwell and Keays 2014; Nielsen et al 2015)
Laser Ablation-ICP-MS of sulfide droplets from Platinova Reef confirms a significant shift in trace element concentrations of bornite–digenite/chalcocite droplets across the Au zone
The S/Se ratios are the lowest recorded in any magmatic sulfide deposit, and mineralogical evidence implies this is not due to any significant post-magmatic hydrothermal S loss, but is rather a function of magmatic processes
Summary
The Skaergaard Intrusion in east Greenland is host to the Platinova Reef, which is an example of a rare subclass of magmatic sulfide deposit, manifested as stratiform PGE reef deposits in the upper parts of the host intrusions (Brooks 1989; Bird et al 1991; Andersen et al 1998; Nielsen et al 2005, Miller and Andersen 2002; Godel et al 2014; Holwell and Keays 2014; Nielsen et al 2015). While there are a number of common characteristics to all of these deposits, in detail, there are a great number of geochemical, mineralogical and stratigraphic variations and there is no current consensus on the magmatic and hydrothermal processes operating to explain all of these (e.g., Prendergast 2000; Andersen 2006; Nielsen and Bernstein 2009; Nielsen 2013; Holwell and Keays 2014; Godel et al 2014; Nielsen et al 2015; Keays and Tegner 2015)
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