| Summary: | The 2444 Ma Penikat layered ultramafic-mafic intrusion in northern Finland contains at least six platinumgroup element (PGE) enriched horizons, including the SJ, AP and PV reefs, each with 4 to >10 ppm Pt+Pd over a width of between <1 and > 10 m. The mineralisation formed by orthomagmatic processes as indicated by strong positive correlations between Pt and Ir contents. Subdued stratigraphic variation in incompatible trace element ratios and Nd isotope ratios indicates that mixing of magmas of distinct lineage, or in situ contamination with country rocks, was not required to form the PGE reefs. There is also no evidence for addition of external sulphur to the magma, based on S/Se ratios at, or below, primitive mantle levels. Instead, sulphide melt saturation was likely reached in response to silicate fractionation of a siliceous highmagnesium basalt. We propose that the SJ reef formed through hydrodynamic sorting of crystal mushes, hereas the AP and PV reefs formed through tectonically induced injection of evolved, PGE- and volatile-rich, plagioclasecharged melt into bedding-parallel dilational zones within the semi-consolidated cumulate pile.
|
|---|