Constellation, le dépôt institutionnel de l'Université du Québec à Chicoutimi

Résumé

The Palaeoproterozoic Penikat layered intrusion in northern Finland contains at least six platinum-group element (PGE) enriched horizons exposed along >20 km of strike, amongst them the SJ reef, with ̴3-7 ppm Pt+Pd over a width of ̴1-2 m. Important PGE enrichments also occur in the PV reef (avg. 2.6 ppm Pd, 4 ppm Pt over 1.1 m) and AP1 reef (avg. 6.2 ppm Pd, 1.7 ppm Pt over 0.7 m0. The PGE reefs formed by predominantly orthomagmatic processes. Late-magmatic fluids played no significant role in concentrating PGE. Subdued stratigraphic variation in incompatible trace element ratios (Ce/Sm mostly 5-10) and Nd isotope compositions (eNd -3 to -1) indicate that mixing of distinct magmas, or in situ contamination with country rocks, was not required to form the reefs. There is also no evidence for addition of external sulphur, based on S/Se ratios at, or below, primitive mantle levels. Instead, we propose that sulphide melt saturation was reached in response to magma fractionation, and the sulphides were concentrated through hydrodynamic phase sorting.