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We have studied the Re-Os isotope systematics of komatiite-related Ni-Cu-PGE sulfide deposits in Finland, including the Archean Ruossakero Ni-(Cu), Vaara Ni-(Cu-PGE), Tainiovaara Ni-(Cu-PGE) deposits and Ni-PGE sulfide mineralization related to a dunitic body at Tulppio, and the Paleoproterozoic Lomalampi PGE-(Ni-Cu) deposit. We analyzed chromite separates from both mineralized and non-mineralized komatiitic rocks, bulk-sulfide separates, and country-rock black schist from one deposit, and for comparison, chromite separates from non-mineralized Archean komatiitic rocks from the Kovero and Kuhmo greenstone belts. We have observed open-system behavior of sulfides in three deposits, with their Re-Os isotope compositions indicating resetting at ca. 1.83-1.87 Ga by metamorphic events related to the Svecofennian orogeny. Most of the chromite separates, especially those with >10 ppb Os, retain their primary isotope compositions with near-chondritic initial 187 Os/ 188 Os ratios, indicating that their Os was derived from a mantle source with a chondritic Re-Os isotope evolution. These include the Vaara deposit, which yielded a chromite Re-Os isochron age of 2719 ± 140 Ma and low initial γOs of-1.2 ± 0.02, in spite of the evidence for the presence of mass-independent S isotope fractionation in this deposit, suggesting a low-Os, sulfur-bearing contaminant. In the case of the Lomalampi deposit, chromites yield an isochron with an age of 2058 ± 93 Ma and initial γOs of +0.9 ± 0.02, consistent with earlier isotope data from Central Lapland komatiites. However, one chromite from a mineralized cumulate gave a γOs(2050 Ma) value of +24 ± 18, indicating a minor/moderate degree of crustal contamination involving material from country rock black schists. This is also supported by earlier S isotope data. In the Archean Tainiovaara deposit, chromite also suggests crustal contamination (γOs +13 ± 1), but country rocks are Os-poor Archean gneisses, leaving the source of radiogenic Os unclear, though it is possible that contamination occurred at a greater depth. Our results also provide evidence that the Re-Os age of the Ruossakero deposit is ca. 2.9 Ga, with the deposit likely representing komatiitic magmatism that is older than the komatiitic magmatism elsewhere in Finland. Re-Os results also confirmed that the mineralized Tulppio Dunite is Archean in age. Keywords: Re-Os isotopes; Ni-Cu-PGE sulfide and chromite; komatiite; Archean; Paleoproterozoic; Finland to basaltic (Luolavirta et al., 2017). Another deposit close to Kevitsa, the Sakatti Cu-Ni-PGE sulfide deposit, which is related picritic or komatiitic magmatism (Brownscombe et al., 2015; Makkonen et al., 2017), has a great potential to become economically viable. Experimental work and empirical observations have shown that magmatic sulfide deposits are the products of segregation of an immiscible sulfide-rich liquid from a silicate melt (e.g., Naldrett, 2004, and references therein). The sulfide liquid sequesters chalcophile metals from their parental silicate melt, which may le...

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Using electron probe microanalyzer (EPMA) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), we analyzed major and trace element compositions of iron oxides from Ni-Cu-PGE sulfide deposits hosted by mafic-ultramafic rocks in northern Fennoscandia, mostly focusing on Finland. The main research targets were the Archean Ruossakero Ni-(Cu) deposit; Tulppio dunite and related Ni-PGE mineralization; Hietaharju, Vaara, and Tainiovaara Ni-(Cu-PGE) deposits; and Paleoproterozoic Lomalampi PGE-(Ni-Cu) deposit. In addition, some reference samples from the Pechenga (Russia), Jinchuan (China), and Kevitsa (Finland) Ni-Cu-PGE sulfide deposits, and a barren komatiite sequence in the Kovero area (Finland) were studied. Magnetite and Crmagnetite show a wide range of trace element compositions as a result of the variation of silicate and sulfide melt compositions and their post-magmatic modification history. Most importantly, the Ni content in oxide shows a positive correlation with the Ni tenor of the sulfide phase in equilibrium with magnetite, regardless of whether the sulfide assemblage is magmatic or post-magmatic in origin. The massive sulfide samples contain an oxide phase varying in composition from Cr-magnetite to magnetite, indicating that Crmagnetite can crystallize directly from sulfide liquid. The Mg concentration of magnetites in massive sulfide samples is lowest among the samples analyzed, and this can be regarded as a diagnostic feature of an oxide phase crystallized together with primitive Fe-rich MSS (monosulfide solid solution). Our results show that magnetite geochemistry, plotted in appropriate discrimination diagrams, together with petrographical observations could be used as an indicator of potential Ni-(Cu-PGE) mineralization.