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 >20 km of strike, amongst them the SJ reef, which at $3-7 ppm Pt þ 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 (average 2Á6 ppm Pd, 4 ppm Pt over 1Á1 m) and AP1 reef (average 6Á2 ppm Pd, 1Á7 ppm Pt over 0Á7 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 6 8 Ma 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 þ Pd and Os þ Ir þ 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 (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, highmagnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes.

Parental magma, magmatic stratigraphy, and reef-type PGE enrichment of the 2.44-Ga mafic-ultramafic Näränkävaara layered intrusion, Northern Finland

“…6), implying a shared parental magma with its composition controlled by olivine fractionation. The decreasing olivine Fo contents between the reversals suggests progressive fractionation in a parallel staging magma chamber before emplacement in the layered series (similar to Penikat intrusion, Maier et al 2018). The gradual increase in incompatible elements with stratigraphic height from PER-1 to PER-2 to PER-3 ( Fig.…”

“…The parental magmas have been thought to be (boninite-like) siliceous high-Mg basalts (SHMB), usually classified into high-Cr and low-Cr types (Alapieti et al 1990;Iljina & Hanski 2005). The high-Cr and low-Cr magma types may be related by fractional crystallization (Maier et al 2018). Wholerock and isotope geochemical results (average Ɛ Nd of − 2) point to crustal contamination of primary magmas from mantle plume sources (Puchtel et al 1997;Hanski et al 2001;Vuollo & Huhma 2005;Yang et al 2016;Maier et al 2018).…”

“…2) (Alapieti 1982). Modified after Alapieti et al (1990) Iljina 1994; Mutanen & Huhma 2001;Perttunen & Vaasjoki 2001;Maier et al 2018). The layered intrusions of the Fennoscandian shield have been classified into three compositional types: (1) mafic-ultramafic (e.g., Kemi, Näränkävaara), (2) mafic (Akanvaara, Koitelainen, Koillismaa), and (3) megacyclic (Portimo, Penikat) (Alapieti & Lahtinen 2002).…”

“…Within the Fennoscandian shield, the parental magmas of the 2.5-2.4 Ga layered intrusions have been interpreted as siliceous high-Mg basalts, sourced from the asthenospheric mantle, with the magmas having incorporated substantial amounts of felsic continental crust during ascent (Amelin & Semenov 1996;Puchtel et al 1997;Hanski et al 2001;Yang et al 2016;Maier et al 2018). A crustal contamination signature is evident in the intrusions as LREE-enrichment with La N /Yb N of 3-10; and in fractionated PM-normalized trace element patterns, variably enriched in LILE, Th, and U, with prominent negative Nb, Ta, Ti, and P anomalies (Iljina & Hanski 2005;Vuollo & Huhma 2005;Maier et al 2018).…”

Effect of Al₂O₃ Addition on the Phase Equilibria Relations of CaO-SiO₂-5 wt%MgO-Al₂O₃-TiO₂ System Relevant to Ti-bearing Blast Furnace Slag

“…It is well known that vanadium-titanium magnetite is widely distributed all over the world including South Africa, Australia, Russia, and China. 1) Until now, most of the magnetite ore is disposed by high blast furnace smelting process, after which most of the iron and vanadium compounds are reduced into hot metal for the following utilization in the subsequent processes, 2,3) while most of titanium components are transferred into slag phase, which is called Ti-bearing slag. Ti-bearing slag has been considered as a valuable secondary resource due to its TiO 2 content is higher than 20 wt%.…”