PGE-enrichment with late-stage Fe-Ti oxide crystallisation observed in the Dufek-Forrestal layered mafic intrusion, Antarctica

With an areal extent of at least 6,600 km2 and an estimated thickness of ~ 8 to 9 km, the Jurassic Dufek-Forrestal layered mafic intrusion in the Pensacola Mountains, Antarctica, is believed to be one of the largest bodies of its kind in the world. The layered sequence of mainly gabbroic cumulates with minor interlayers of anorthosites, pyroxenites and magnetitites is exposed in two petrologically related sections in the Dufek Massif and in the Forrestal Range (FR). To better understand the magmatic evolution of this well-differentiated intrusive body, we investigate the low-pressure fractionation behaviour of the platinum-group elements (PGE) by studying their stratigraphic distribution and their mineralogical siting throughout the exposed profile. Selected samples of all lithologies available were analysed for their bulk-rock Ir, Ru, Rh, Pt and Pd concentrations by ID-ICP-MS after NiS fire assay. Considering the wide lithological and geochemical variations, the rocks also exhibit strongly variable PGE compositions; obviously, only the magnetite-rich gabbros and magnetitites from the lower part of the FR section show significantly elevated PGE concentrations reaching up to ~800 ppb. In this 300 m thick cyclic sequence of magnetite-bearing gabbros and anorthosites, thin layers, laminae and lenses of magnetitite represent the most prominent Fe-Ti oxide accumulation found within the intrusion so far. However, these magnetite-rich rocks exhibit also highly variable PGE totals and ratios as well as S and Cu concentrations. Their sulphide assemblage is composed of mainly chalcopyrite, pyrrhotite and pentlandite. Based on electron microprobe analysis (EMPA), we currently identified a variety of well-known as well as a number of unnamed platinum-group minerals (PGM) found as small inclusions (< 1 to 25 μm) either hosted by base-metal sulphides, Fe-Ti oxides or silicates or occurring in altered areas as well as in cracks and along rims of primary Fe-Ti oxides. PGM are either Pt-rich or Pd-rich alloys with semimetals such as As, Sb, Bi, and Te and a few mixed Pt-Pd phases. PGE-rich sulphides are notably absent and there are no discrete phases of Rh, Ir, Os and Ru. Common base-metal sulphides do not incorporate PGE detectable by EMPA, except of cobalt-rich pentlandite that contains up to 1.44 wt% Rh and 0.75 wt% Pd. The PGE variations within the Dufek-Forrestal intrusion are ascribed to an extensive differentiation history under mainly S-undersaturated conditions resulting in PGE-enrichment until the onset of late-stage Fe-Ti oxide crystallisation. This was accompanied by a delayed saturation and exsolution of sulphide melt scavenging the PGE from the highly evolved tholeiitic magma. However, there is evidence that the PGE in most of the studied rocks are not solely controlled by base-metal sulphides but by a variety of PGM indicating that both high-temperature and subsolidus processes influenced the PGE fractionation behaviour and caused the highly variable PGE totals and ratios throughout the exposed profile as well as the formation of the PGM assemblage identified.