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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 Cr-magnetite 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 Cr-magnetite 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. [ABSTRACT FROM AUTHOR]

Several komatiite-hosted Ni-Cu-PGE deposits occur in Archean and Paleoproterozoic Greenstone Belts in Finland. Some of these deposits are enriched in platinum-group elements, especially in Pd and Pt. The Lomalampi PGE-(Cu-Ni) deposit is associated with a peridotitic cumulate body of the Sattasvaara Formation in the Paleoproterozoic Central Lapland Greenstone Belt. The sulfides in the deposit occur in disseminated form. Whole rock sulfur contents are 0.4-2 wt%, and Ni contents are <0.5 wt% and Cu <0.4 wt%, while PGE contents exceed 500 ppb. The sulfides consist of magmatic pentlandite, pyrrhotite, and chalcopyrite and have not been substantially modified by metamorphic processes. Palladium minerals are associated with sulfides and silicates, but the only Pt-bearing phase, sperrylite, occurs mainly within silicates. The host rock of the deposit is a chromite undersaturated Al-undepleted high-Mg basalt or low-Mg komatiite. In contrast to most other komatiite-hosted Ni-Cu-PGE deposits world-wide that have Pt/Pd around 0.5, the Lomalampi deposit is enriched in Pt over Pd (Pt/Pd = 2). Only a weak contamination signal in the host-cumulate is evident in REE data, but a strong signal is evident in S-isotope ratios (δS + 10 ‰ to +15 ‰), which differ substantially from the mantle value (0 ± 2 ‰). Geochemical characteristics (e.g., PGE enrichment) and R-factor modeling indicate certain similarities between Lomalampi and the Raglan Ni-Cu-PGE deposits of Canada. The combined data suggest that Lomalampi formed through contamination of a PGE-rich magnesian magma with S rich country rocks. This suggests that the extensive Central Lapland Greenstone Belt is favorable for komatiite-hosted Ni-Cu-PGE deposits, which are substantially enriched in platinum and palladium. [ABSTRACT FROM AUTHOR]

The Hietaharju Ni–Cu–PGE deposit occurs in the Archean Suomussalmi greenstone belt of eastern Finland and is associated with high-Mg basalts and low-Mg komatiitic rocks. It represents a PGE-enriched type among komatiite-related Ni–Cu deposits. The Hietaharju deposit consists of several mineralized lenses in highly altered olivine–pyroxene cumulates (i.e., talc–carbonate–chlorite rocks) and contains disseminated, massive to semi-massive and brecciated sulfides. Post-magmatic processes have not significantly changed the primary magmatic sulfide assemblage (i.e., pyrrhotite–pentlandite–chalcopyrite), but has modified sulfide textures and possibly remobilized and reconcentrated the sulfides. Though the deposit is currently subeconomic, it has high PGE contents in the sulfide fraction, averaging 5.7 ppm Pd and 2 ppm Pt (average of all ore types), being highest in disseminated sulfides. The parental magma represents an intermediate type between Al-undepleted and Al-depleted komatiites and differs from most other komatiitic rocks in the Tipasjärvi–Kuhmo–Suomussalmi greenstone belt complex, which are normally of the Al-undepleted type. The relatively fractionated nature of the komatiitic basalt parental magma of the Hietaharju deposit is consistent with the low Ni/Cu (< 4) and Ni/Co (< 18) and high Pd/Ir (< 25) ratios of the sulfide fraction, similar to the values in the Paleoproterozoic Raglan belt deposits, Canada. Use of REE for constraining crustal contamination is hampered by the highly altered nature of komatiitic rocks in the Hietaharju area. Sulfides in the country rock metasediments that host the Hietaharju Ni–Cu–PGE deposit show clear mass-independent sulfur isotope fractionation (MIF-S) consistent with their age and published S isotope data from the Suomussalmi belt. In contrast, sulfides of the Hietaharju Ni–Cu–PGE deposit only possess a small degree of MIF-S, providing limited direct evidence for the role of local metasedimentary sulfur in ore formation. Archean base metal sulfide mineralization is among the potential sources of sulfur, however, and, as shown by the epithermal Taivaljärvi Au–Ag–Zn deposit, can have mass-independent sulfur isotope fractionation that is similar in magnitude and sign to that of the Hietaharju Ni–Cu–PGE deposit. [ABSTRACT FROM AUTHOR]