Tapio A.A. Halkoaho

The mechanism of charge compensation in Cu-Fe-PGE thiospinels from the Penikat layered intrusion, Finland

Abstract Thiospinels of Cu-(Fe) and platinum-group elements (PGE) are relatively abundant in the Kirakkajuppiua PGE deposit of the Peuikat layered complex. Finland. In actinolite-clinochlore rock that is nearly base-metal sulfide-free and relatively poor in chromite, the thiospinels occur as subhedral or anhedral grains (up to 0.4 mm). They are members of the cuprorhodsite-ferrorhodsite and cuprorhodsite-malanite series, relatively poor in cuproiridsite. and display considerable grain-to- grain variations in Cu. Fe. Pt. and Rh. Strong negative Fe-Cu. Pt(+Ir)-Fe. Rh-Cu. and Rh-Pt and strong positive Pt(+Ir)-Cu and Rh-Fe correlations in these thiospinels are indicative of a coupled substitution: Fe-for-Cu substitution in the tetrahedral (A) sites causes an excess in formal positive charge, which is compensated by Rh-for-(Pt+Ir) substitution hi the octahedral (B) sites. Probable valence states in the Fe-free and Fe-rich end-members of the solid-solution series at Penikat are Cu+[Rli3+(Pt,Ir)4+] S4- and (FefoCu0.5+)Rh2S42-. and these suggest the heterovalent substitution scheme AFe3+ + 2 BRh3+ → ACu+ + 2 BPt4+(+2 Ir4+) to incorporate Fe in the ferrorhodsite-rich end member.

Erratum to: The temporal variation of Mesoarchaean volcanism in the Suomussalmi greenstone belt, Karelia Province, Eastern Finland

and the Eastern Finland Office of the Geological Survey of Finland are acknowledged for providing resources for the field and laboratory work. The Nordsim facility is financed and operated under an agreement between the research councils of Denmark, Norway and Sweden, the Geological Survey of Finland and the Swedish Museum of Natural History. This is Nordsim contribution no. 448. The staff of the Nordsim laboratory, Martin Whitehouse, Kerstin Lindén and Lev Ilyinsky, and the staff of the Isotope laboratory of the Geological Survey of Finland, Hugh O’Brien, Yann Lahaye and Leena Järvinen, are thanked for the sample preparation and technical assistance. Tom Andersen is acknowledged for providing program to handle the LAMC-ICPMS data. Comments from reviewers Stefan Claesson and Mike Fowler and editorial work by Victoria Pease are highly appreciated. Erratum to: Int J Earth Sci (Geol Rundsch) DOI 10.1007/s00531‐016‐1327‐y

The tectonomagmatic significance of Neoarchaean variably alkali-enriched gabbro and diorite intrusions of the western Karelia Province

Abstract This study describes a group of Neoarchaean alkali enriched gabbros and diorites from the western Karelia Province of the Fennoscandian Shield. We provide new field observations, petrography, whole-rock chemical data and additional whole-rock Sm–Nd and O-isotope data from these Neoarchean rocks. Compositionally, the rocks can be classified as shoshonitic with elevated rare earth element, K2O, Ba and Sr contents together with variable MgO, Ni and Cr contents. The MgO, Ni and Cr depletion observed especially in one of the intrusions could have been caused by fractionation of olivine and clinopyroxene from the system. Zircon O-isotope values from one of the intrusions (δ18O=7.34±0.10‰) indicate the involvement of material that had undergone low-temperature fractionation of oxygen in the genesis. Samarium–Nd data imply contribution from older material in the petrogenesis of these rocks. The above-mentioned characteristics can be explained with a magma source in the mantle that was heterogeneous owing to the variable degrees of metasomatism. The alkali-enriched gabbros and diorites provide additional evidence for magmatism derived from heterogeneously enriched mantle during the Neoarchaean in the Karelia Province and associated with the cratonization of the area. Supplementary material: Table detailing oxygen-isotope analyses from this study is available at https://doi.org/10.6084/m9.figshare.c.3466575

The Penikat Intrusion, Finland: Geochemistry, Geochronology, and Origin of Platinum–Palladium Reefs

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 >20u2009km of strike, amongst them the SJ reef, which at ∼3–7u2009ppm Ptu2009+u2009Pd over a width of ∼1–2u2009m 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 2·6u2009ppm Pd, 4u2009ppm Pt over 1·1u2009m) and AP1 reef (average 6·2u2009ppm Pd, 1·7u2009ppm Pt over 0·7u2009m). 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 2444u2009±u20098u2009Ma for the intrusion. We show that the PGE reefs formed by predominantly orthomagmatic processes as, for example, reflected by well-defined positive correlations between Ptu2009+u2009Pd and Osu2009+u2009Iru2009+u2009Ru 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, high-magnesium basalt, and that the sulphides were concentrated through hydrodynamic phase sorting, consistent with bonanza-style PGE grades in large potholes.