THE PLATINUM-GROUP MINERALS IN THE UPPER SECTION OF THE KEIVITSANSARVI Ni Cu PGE DEPOSIT, NORTHERN FINLAND

Abstract

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), b ut 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, mauch erite 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 th e 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, low-grade disseminated Ni–Cu ore formed by the intercumulus crystallization of a small fraction of immiscible sulfide melt. Serpentinization caused Ni enrichment of sulfides and preserved the original PGE concentrations of the magmatic mineralization. Later, coeval with greenschist-facies metamorphism, PGE and some As (together with other semimetals) were leached out from other mineralized zones by hydrothermal fluids, probably transported in the form of chloride complexes, and precipitated in discrete Ni–Cu–PGE-rich horizons, as observed in the upper part of the deposit. Metamorphism also caused partia l dissolution and redistribution of the sulfide (and arsenide) aggregates, contributing to a further Ni enrichment in the sulfide ores.