The Penikat intrusion, Finland: Geochemistry, geochronology, and origin of platinum- palladium reefs

Abstract

The Palaeoproterozoic Penikat layered ultramafic-mafic intrusion in northern Finland is one of the most richly mineralized layered intrusions on Earth, containing at least six platinum-group element (PGE) enriched horizons exposed along > 20km of strike, amongst them the SJ reef, which at ~3-7ppm Pt+Pd over a width of ~1-2m is surpassed by few other PGE reefs globally in terms of its endowment in PGE. Important PGE enrichments also occur in the PV reef (average 2·6ppm Pd, 4ppm Pt over 1·1m) and AP1 reef (average 6·2ppm Pd, 1·7ppm Pt over 0·7 m). Here we present new major and high-precision trace element and Nd isotope data from a traverse across the intrusion, and a new U-Pb age of 2444±8Ma for the intrusion. We show that the PGE reefs formed by predominantly orthomagmatic processes as, for example, reflected by well-defined positive correlations between Pt+Pd and Os+Ir+Ru contents. Late-magmatic fluids played no significant role in concentrating PGE. There are at least six cyclic units in the intrusion, displaying a progressive upward decrease in differentiation indices Mg# and Cr/V. Subdued stratigraphic variations in incompatible trace element ratios (Ce/Sm mostly 5-10) and Nd isotope compositions (εNd -3 to -1) indicate 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, we propose that sulphide melt saturation at Penikat was reached in response to fractionation of a siliceous, highmagnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes.