The majority of magmatic platinum-group element (PGE) deposits in layered mafic/ultramafic intrusions are characterized by visible Cu-Ni Sulfide mineralization, often located in the lower parts of a magmatic stratigraphy that show evidence of multiple magma injections and crustal contamination. The Platinova Reef, hosted by the 55 Ma Skaergaard intrusion, East Greenland, is an example of a rarer type of PGE deposit, typified by a Pd-Au-Cu-dominant, Ni-Pt-poor, Sulfide assemblage present in the upper parts of the host intrusion. Such deposits are generally considered to form through prolonged fractional crystallization of the magma, with magnetite crystallization playing a role in late-stage S saturation. In the case of the Skargaard intrusion, the crystallization of the cumulate rocks, and the generation of the mineralization hosted within them, occurred within a closed system, from a single, homogenized batch of magma.
The Platinova Reef is characterized by offsets in the stratigraphic position of individual metal peaks; an anomalously low Sulfide content (<0.01 wt % S in bulk rock); a mineralized zone with high precious metal tenor sulfides overlain by more abundant, but much lower tenor sulfides; and lateral variation in the thickness of the mineralized interval. Using new data obtained from recent drilling of the Platinova Reef, we divide the mineralized units into five zones based on the presence of Pd, Au, and Cu mineralization: the lower Subzone, the Pd zone (Pd >1 ppm), the Intermediate zone (Pd 0.1–1 ppm), the Au zone (Au >1 ppm), and the upper Cu zone (Cu >200 ppm with precious metals <1 ppm). The mineralization beneath the Cu zone is associated with very low volume sulfides with calculated Pd and Au tenors of 103 to 104 ppm, low Cu/Pd ratios, and a remarkable upward decrease in Pd/Au ratio from 40 to 1, whereas the sulfides in the Cu zone are lower tenor (103 ppm Pd), higher volume, and have high Cu/Pd ratios and Pd/Au <1. The relationship between Pd and Cu/Pd ratio indicates that sulfides from beneath the Cu zone formed at a very high R factor, with variable, but low volumes of sulfides, whereas those in the Cu zone formed at a lower R factor from a PGE-depleted magma. A broad correlation between V and Pd through the Subzone, Pd, Intermediate, and Au zones indicates that the crystallization of magnetite may be linked to Sulfide saturation.
To explain these features, we present a multistage model of preconcentration, dissolution upgrading, and further metal enrichment. This involves initial S saturation of the Skaergaard magma at a late stage in the crystallization history. The Sulfide liquid produced scavenged metals from the entire volume of the chamber as it sank but was then dissolved as it came into contact with hotter, Fe-rich and S-undersaturated magma at the bottom of the chamber, producing a highly metal rich magma layer at the bottom of the chamber. This layer then became S saturated as a result of continued magnetite crystallization, with the tiny sufide droplets that segregated into this precious metal-enriched magma layer becoming highly enriched in PGE themselves. These were then trapped in situ by the crystallizing cumulate pile forming the Subzone, Pd zone and Intermediate zone. sulfides formed in the rest of the chamber, which was PGE depleted at this stage would have been of low tenor. These Sulfide droplets grew and settled, concentrating any remaining Au when they encounter the floor to form the Au zone and formed the low tenor sulfides of the Cu zone above this.
We propose that the Platinova Reef exhibits an unusual example of a style of magmatic precious metal mineralization formed through Sulfide dissolution upgrading in the latter stage of crystallization of a closed system. The offsets in individual metal peaks mineralization demonstrate the relative partitioning of Pd(+Pt) > Au > Cu into a Sulfide liquid from a silicate magma and demonstrates the importance of partitioning behavior in low-volume magmatic Sulfide melts. This feature can lead to the characteristic stratigraphic offsets in peak metal abundances. The role of magnetite crystallization appears to be an important control on S saturation in such systems, and thus may be an important indicator of the location of mineralized horizons. In addition, in such deposits, indicators of crustal contamination by external S and multiple injections of magma, common features of more Ni rich PGE reefs, are less likely to be present.