The origin of high metal tenor in Ni-PGE ores from the Kevitsa Ni-Cu-(PGE) deposit, northern Finland:constraints from an in-situ trace element study of base metal sulphides

The Kevitsa mafic-ultramafic intrusion is located in the Central Lapland Greenstone Belt, in Finnish Lapland, and hosts a large Ni-Cu-PGE sulphide deposit. Since 2012, Kevitsa has been one of Europe’s most important nickel mines with a pre-mine resource of 274.8 Mt @ 0.30 % Ni, 0.41 % Cu, 0.014 % Co, 0.11ppm Au, 0.15 ppm Pd and 0.2 ppm Pt. There are two main ore types that make up the economic resources, named normal ore and Ni-PGE ore, of which the normal ore type comprises 90 vol.%. The normal ore has average Ni and Cu grades of 0.3 and 0.42 wt.%, respectively, with the main ore minerals being pyrrhotite, pentlandite, and chalcopyrite. The Ni-PGE ore consists predominantly of pentlandite, pyrite and millerite and has higher and more variable Ni grades, lower Cu grades (Ni/Cu 1.5–15) and extremely high Ni tenors, up to 40 wt.%. The Ni-PGE ore has a high PGE content ranging from >1 ppm to 26.8 ppm, much higher than that of the normal ore (0.5 to 1 ppm of combined Pt, Pd and Au). The uniqueness of the Ni-PGE ore type is amplified further by the high Ni contents of its cumulus silicates, as attested by Ni contents of olivine that reach up to 1.4 wt.%. In addition to the aforementioned ore types, there is an uneconomic type, called false ore, which consists of pyrrhotite, with rare chalcopyrite and pentlandite and generally has a low Ni content (1 ppm Ru, Ir and Os, and up to 51 ppm Pd. Although Pd is found in all of the sulphide phases, pentlandite is the richest, with Pd contents ranging from 0.3 ppm in the false ore to tens of ppm in the Ni-PGE ore. Millerite and pyrite are much lower in PGEs than pentlandite, and pyrrhotite and chalcopyrite record the lowest levels. Platinum contents are low throughout, with the majority of analyses falling below the detection limit (<0.001ppm).