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

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 mineralised layered intrusions on Earth, containing at least six platinum-group element (PGE) enriched horizons exposed along >20 km of strike, amongst them the SJ reef, which at ∼ 3-7 ppm Pt+Pd over a width of ∼ 1-2 m is surpassed by few other PGE reefs globally in terms of its endowment in PGE. 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 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 ± 8 Ma for the intrusion. We show that the PGE reefs formed by predominantly orthomagmatic processes as, e.g., 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 6 cyclic units in the intrusion, displaying a progressive upward decrease in differentiation indexes Mg# and Cr/V. 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 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, high-magnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices) http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Journal of Petrology Oxford University Press

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

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Publisher
Oxford University Press
Copyright
© The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com
ISSN
0022-3530
eISSN
1460-2415
D.O.I.
10.1093/petrology/egy051
Publisher site
See Article on Publisher Site

Abstract

Abstract The Palaeoproterozoic Penikat layered ultramafic-mafic intrusion in northern Finland is one of the most richly mineralised layered intrusions on Earth, containing at least six platinum-group element (PGE) enriched horizons exposed along >20 km of strike, amongst them the SJ reef, which at ∼ 3-7 ppm Pt+Pd over a width of ∼ 1-2 m is surpassed by few other PGE reefs globally in terms of its endowment in PGE. 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 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 ± 8 Ma for the intrusion. We show that the PGE reefs formed by predominantly orthomagmatic processes as, e.g., 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 6 cyclic units in the intrusion, displaying a progressive upward decrease in differentiation indexes Mg# and Cr/V. 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 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, high-magnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes. © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)

Journal

Journal of PetrologyOxford University Press

Published: May 25, 2018

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