Hydrothermal alteration of the Kemi layered intrusion

Abstract

The Kemi chromite deposit is located near the towns of Kemi and Tornio (northern Finland). The mineralization is hosted within the mafic-ultramafic Kemi intrusion (~2.4 Ga) that is part of the Tornio–Näränkävaara belt. The main chromite zone is located in the basal part of the intrusion and is enveloped and intercalated by peridotites and bronzites. Hydrothermal alteration has pervasively affected the intrusion mainly in the lower and upper segments, leaving the middle part relatively unaltered. These lithologies can still be identified due to the preservation of their primary textures. The altered ultramafic lithologies had been previously catalogued as cumulate, chlorite, amphibole, serpentine and pyroxene peridotites, amphibole and talc pyroxenites, and serpentinites. Further petrographic observations suggest that the ultramafic rocks experienced at least six episodes of hydrothermal alteration that lead to the replacement of primary minerals to assemblages of serpentine, chlorite, amphibole, carbonate and talc. Evidences in the petrography and mineral chemistry suggests the alteration fluid evolve towards more silica rich compositions.

The hydrothermal fluids responsible for Kemi intrusion alterations can be related to the metamorphism of the footwall rocks during the Svecofennian Orogeny.

Variations in alteration patterns and mineralogy as well as variations in the mineral chemistry do not correlate with the proximity to the chromitite ore zone.