| Summary: | The Keivitsansarvi deposit, in northern Finland, is a low-grade dissemination of Ni–Cu sulfides containing 1.3–26.6 g/t PGE. It occurs in the northeastern part of the 2.05 Ga Keivitsa intrusion and is hosted by olivine wehrlite and olivine websterite, metamorphosed at greenschist-facies conditions. The sulfide-mineralized area shows variable bulk S, Ni, Co, Cu, PGE, Au, As, Sb, Se, Te and Bi contents. S and Au tend to decrease irregularly from bottom to top of the deposit, whereas Ni, Ni/Co, PGE, As, Sb, Se, Te and Bi tend to increase. Thus, the upper section of the deposit has low S (<1.5 wt.%) and Au (160 ppb on average), but elevated levels of the PGE (2120 ppb Pt, 1855 ppb Pd on average). Sulfides consist of intergranular, highly disseminated aggregates mainly made up of pentlandite, pyrite, and chalcopyrite (all showing fine intergrowths), as well as nickeline, maucherite and gersdorffite in some samples. Most platinum-group minerals occur as single, minute grains included in silicates (57%) or attached to the grain boundaries of sulfides (36%). Only a few PGM grains (6%) are included in sulfides. Pt minerals (mainly moncheite and sperrylite) are the most abundant PGM, whereas Pd minerals (mainly merenskyite, Pd-rich melonite, kotulskite and sobolevskite) are relatively scarce, and most contain significant amounts of Pt. Whole-rock PGE analyses show a general Pd enrichment with respect to Pt. This discrepancy results from the fact that a major part of Pd is hidden in solid solution in the structure of gersdorffite, nickeline, maucherite and pentlandite. The mineral assemblages and textures of the upper section of the Keivitsansarvi deposit result from the combined effect of serpentinization, hydrothermal alteration and metamorphism of preexisting
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