The Penikat Intrusion, Finland: Geochemistry, Geochronology, and Origin of Platinum– Palladium Reefs W
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J OURNAL OF Journal of Petrology, 2018, 1–40 doi: 10.1093/petrology/egy051 P ETROLOGY Advance Access Publication Date: 25 May 2018 Original Article The Penikat Intrusion, Finland: Geochemistry, Geochronology, and Origin of Platinum– Palladium Reefs W. D. Maier1*, T. Halkoaho2, H. Huhma3, E. Hanski4 and S.-J. Barnes5 1School of Earth and Ocean Sciences, Cardiff University, Cardiff, UK; 2Geological Survey of Finland, Kuopio, Finland; 3Geological Survey of Finland, Espoo, Finland; 4Oulu Mining School, Oulu, Finland; 5Universite´ du Que´bec a` Chicoutimi, Quebec, Canada *Corresponding author. E-mail: [email protected] Received November 21, 2016; Accepted May 8, 2018 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 intru- sion, 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 corre- lations 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 in- compatible 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 sort- ing, consistent with bonanza-style PGE grades in large potholes. Key words: platinum-group elements; chromite; Palaeoproterozoic; layered intrusion; Finland INTRODUCTION relatively little studied, with few contributions having Layered intrusions host the bulk of global resources in appeared in the literature since the early 1990s, follow- platinum-group elements (PGE) that are crucial in the ing the discovery of the mineralization in the late 1980s. production of vehicle exhaust catalytic converters and In this study, we first review the geology and PGE min- many other applications in, for example, the chemical eralization styles of the Penikat intrusion. We then pre- and electrical industry. However, at present only four sent new U–Pb zircon data to refine the age of the intrusions globally are mined for PGE as the main prod- intrusion using in situ laser ablation inductively coupled uct, namely Bushveld (South Africa), Great Dyke plasma mass spectrometry analysis, and new whole- (Zimbabwe), Stillwater (USA) and Lac des Iles (Canada). rock major and trace element as well as Nd isotopic Among sub-economic intrusions, the Penikat intrusion data to propose a new model for the origin of the PGE is perhaps the most richly mineralized, yet it remains reefs. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: [email protected] 1 Downloaded from https://academic.oup.com/petrology/advance-article-abstract/doi/10.1093/petrology/egy051/5004663 by Cardiff University user on 03 August 2018 2 Journal of Petrology, 2018, Vol. 0, No. 0 2Á44–2Á50 Ga layered intrusions in the and greenstone belts in northeastern Finland, closely Fennoscandian Shield associated with the coeval 2Á44 Ga Koillismaa and The northeastern part of the Fennoscandian Shield Akanvaara layered intrusions. Magmatism was trig- hosts more than two dozen Palaeoproterozoic layered gered by plume-driven aborted break-up of the mafic–ultramafic intrusions (Fig. 1). Uranium–lead zircon Archaean Kenorland supercontinent (Amelin et al., and Sm–Nd whole-rock data have yielded a mean age 1995; Hanski et al., 2001; Hanski & Huhma, 2005) and for the Finnish intrusions of 2440 Ma (Huhma et al., was concentrated in linear belts tracing intracratonic rift 1990, 2018). In Russia, the ages of the intrusions span a zones (Fig. 1). In Finland and neighbouring Sweden, wider range, from c. 2450 to 2500 Ma (for a summary, several intrusive belts or clusters can be distinguished see Hanski & Melezhik, 2012), possibly reflecting two (Alapieti et al., 1990; Iljina & Hanski, 2005), including distinct mantle plume events, analogous to the 2445– those in central Lapland (Koitelainen and Akanvaara 2490 Ma Matachewan and 2510 Ma Mistassini events of intrusions), northwestern Lapland (Tsohkoaivi, the Superior craton (Ernst & Bleeker, 2010), which was Kelottija¨ rvi, Kurkovare, Keukiskero), and in north–cen- probably attached to Fennoscandia at the time. tral Finland. The last is the most mineralized, compris- The intrusions were formed by bimodal igneous ac- ing an 300 km long, west–east-trending belt named tivity, resulting in mafic and felsic, plutonic and extru- the Tornio–Na¨ra¨ nka¨ vaara belt (TNB), which includes sive rocks in Finland and NW Russia. Magnesian Penikat, the target of this study, as well as the Tornio, basaltic to dacitic volcanic rocks are exposed in the Kemi, Portimo, Koillismaa and Na¨ra¨ nka¨ vaara intrusions. Vetreny belt, whereas felsic volcanic rocks (Hanski, The compositions of the intrusions vary from ultramafic 2012) and rapakivi-type granite batholiths (Ra¨mo¨& dominated (Tornio and Na¨ra¨ nka¨ vaara) to mafic–ultra- Luukkonen, 2001) occur in the Imandra Varzuga belt mafic interlayered (Kemi, Penikat, Portimo), to Fig. 1. Simplified geological map of part of the Fennoscandian Shield, highlighting location of 2Á44–2Á5 Ga layered intrusions. Figure modified after Alapieti & Lahtinen (1989). Downloaded from https://academic.oup.com/petrology/advance-article-abstract/doi/10.1093/petrology/egy051/5004663 by Cardiff University user on 03 August 2018 Journal of Petrology, 2018, Vol. 0, No. 0 3 predominantly mafic (Koillismaa). Based on similarities (Fig. 3). As a whole, the intrusion is relatively mafic, in their stratigraphy (Iljina & Hanski, 2005; Karinen with ultramafic rocks (i.e. those containing >90% dark et al., 2015), some of the intrusions, or parts of them, minerals) making up <10% of the stratigraphy (Fig. 3). may represent dismembered fragments of one or sev- Most of the rocks are gabbronorites with about 40–60% eral larger magmatic bodies. The footwall to most plagioclase and 10–20% orthopyroxene and clinopyrox- of the intrusions consists of Archaean granite ene each, as well as accessory Cr-spinel, magnetite, gneiss basement rocks, whereas the hanging-wall quartz, and phlogopite. Trace phases include sulphides, rocks consist of Palaeoproterozoic metavolcanic rocks K-feldspar, apatite, loveringite and others (Alapieti & or younger supracrustal sequences deposited on a Lahtinen, 1989; Halkoaho, 1993). The paucity of ultra- Palaeoproterozoic erosional unconformity (e.g. Alapieti mafic rocks constitutes a major difference from the et al., 1989, 1990; Karinen, 2010). This may imply a rela- Kemi intrusion, located immediately to the SW, which tively shallow emplacement depth followed by tilting, contains c. 1000 m of massive lherzolite, plagioclase- faulting, rapid uplift and partial erosion of the intru- bearing websterite–olivine websterite and plagioclase- sions. Gravity and seismic surveys indicate that the bearing clinopyroxenite, in addition to gabbroic rocks intrusions plunge underneath the Proterozoic cover (Alapieti et al., 1986, 1989). Further major differences rocks for several kilometres (Kukkonen et al., 2010; Iljina from Kemi include the relatively low abundance of chro- & Salmirinne, 2011). mitite seams, measuring centimetres to decimetres at The Finnish layered intrusions contain a wide range Penikat versus tens of metres at Kemi, and the relative of PGE mineralization styles, including narrow silicate- enrichment in PGE (up to tens of ppm at Penikat versus hosted reefs at Penikat and Portimo that are analogous <500 ppb at Kemi; Linkermann, 2011). In comparison to the Merensky Reef of the Bushveld Complex, wide with many other layered intrusions such as Bushveld contact-style reefs at Portimo and Koillismaa, analo- and Stillwater, anorthosites are rare at Penikat and gous to the Platreef of the Bushveld, PGE-bearing chro- mostly consist of thin, centimetre- to decimetre-wide mitite layers at Kemi, Koitelainen, Akanvaara and bands. However, these can be traced along the entire Penikat, analogous to those in many other layered intru- strike of the intrusion. Massive ilmenomagnetite or sions, and an unusual style of vein-hosted footwall min- apatite-rich seams are absent. eralization (i.e. the Kilvenja¨ rvi offset veins at Portimo; Based on CIPW norms, the intrusion has been subdi- Andersen et al., 2006), that has rarely been identified in vided into five megacyclic units (MCU I–V) that can be other layered intrusions. None of the intrusions is cur- traced across most blocks, with the exception of MCU III rently exploited for PGE, but an exploration programme (discontinuous in the Ala-Penikka block), and MCU V in the Suhanko block of the Portimo intrusion is at an (largely absent in the Keski-Penikka and Sompuja¨ rvi advanced feasibility stage (Puritch et al., 2007). blocks, probably owing to erosion) (Fig. 2). All units are characterized by relatively pyroxene-rich rocks at the base and gabbroic rocks in their upper portions (Fig.