Origin of ultra-nickeliferous olivine in the Kevitsa Ni-Cu-PGE-mineralized intrusion, northern Finland

• Published in: Contributions to Mineralogy and Petrology
• Main Authors: Yang, Sheng-Hong, Maier, Wolfgang D., Hanski, Eero J., Lappalainen, Markku, Santaguida, Frank, Maatta, Sanna
• Format: Article in Journal/Newspaper
• Language: unknown
• Published: Springer 2013
• Subjects: QE Geology; Northern Finland; Lapland
• Online Access: https://orca.cardiff.ac.uk/id/eprint/56704/; https://doi.org/10.1007/s00410-013-0866-5

The 2,058 ± 4 Ma mafic–ultramafic Kevitsa intrusion is located in the Central Lapland greenstone belt, northern Finland. It is hosted by a Paleoproterozoic volcano–sedimentary sequence that contains komatiitic volcanic rocks and sulfide- and graphite-rich black schists. Economic Ni–Cu–(PGE) sulfide mineralization occurs in the middle part of the ultramafic lower unit of the intrusion. Two main types of ore are distinguished, “normal” and “Ni–PGE” ores. The normal ore is characterized by ~2 to 6 vol% disseminated sulfides and average Ni and Cu grades of 0.3 and 0.42 wt %, respectively (Ni/Cu < 1). The Ni–PGE ore has broadly similar sulfide contents, but a higher Ni grade and lower Cu grade. As a result, the Ni/Cu ratio reaches 15, much higher than in the normal ore. The Ni–PGE ores occur as irregular, discontinuous, lense-like bodies in the ultramafic rocks. Notably, the olivines in the Ni–PGE ore contain extremely high Ni contents of up to 14,000 ppm, which is significantly higher than the Ni content of olivine in other mafic–ultramafic igneous rocks globally (up to ~5,000 ppm) and in harmony with the associated Ni-rich sulfide assemblage containing pentlandite, millerite and pyrite. Microprobe mapping of olivine from the Ni–PGE ore suggests relatively low and homogeneous S contents and homogeneous distribution of Ni, Mg, Fe, which is inconsistent with the presence of sulfide inclusions in the olivine grains, or diffusion of Ni from interstitial sulfides into the olivine grains. We therefore conclude that Ni substitutes for Mg in the olivine lattice. The clinopyroxenes from the Ni–PGE ore also have unusually high Ni concentrations reaching 1,500 ppm and show a positive correlation with the nickel content of the associated olivine. The Nicpx/Niolivine is ~0.1 to 0.2 corresponding to high T partitioning of Ni between clinopyroxene and olivine. K D of 20 can account for the partitioning of nickel between olivine and the sulfide phase, consistent with magmatic equilibration. These data suggest that the olivine, ...