ZONED Zn-RICH CHROMITE FROM THE NÄÄTÄNIEMI SERPENTINITE MASSIF, KUHMO GREENSTONE BELT, FINLAND

JUSSI P. LIIPO, JOUNI I. VUOLLO, VESA M. NYKÄNEN and TAUNO A. PIIRAINEN
Department of Geology, University of Oulu, SF-90570 Oulu, Finland

Abstract

Chromite from the Archean Näätäniemi serpentinite massif, Kuhmo greenstone belt, Finland, exhibits high concentrations of Zn, up to 4.4 wt% ZnO. The chromite is zoned, optically and chemically; the best-preserved chromite cores can be divided into two groups on the basis of their composition. Type-I chromite is interpreted as a relict core, with Cr/(Cr + Al) values from 0.58 to 0.76 and Mg/(Mg + Fe2+) values from 0.53 to 0.60; the core of type-II chromite has been altered to "ferritchromit"; Mg/(Mg + Fe2+) values range from 0 to 0.37. In the core of type-I and -II chromite, Zn shows no correlation with the other divalent cations (Mg, Mn and Fe2+). On the basis of their Zn content, type-I and type-II chromite resemble those examples reported from metamorphosed volcanic-type iron<0150>nickel sulfide ores from Western Australia, though the suite of type-I chromite from the Näätäniemi serpentinite massif displays higher Mg (12.1 wt% MgO) and Al (20.9 wt% Al2O3) contents.

Keywords: chromite, zoning, serpentinite, Archean, Kuhmo greenstone belt, Finland.