Stratiform platinum-group element mineralizations in the ultramafic cumulates of the Leka ophiolite complex, central Norway

Abstract

Three types of stratiform platinum-group element (PGE) enrichment have been documented within olivine cumulates of the layered series in the Leka Ophiolite Complex: (1) Os-Ir-Ru enrichments associated with chromite-rich rocks, (2) Pt-dominated enrichments, and (3) Pd-dominated enrichments associated with discrete 10- to 50-cm-thick sulfide-rich horizons. Drill cores across the Pt- and Pd-enriched horizons have yielded 0.5 and 1 m averages of ca. 1 ppm PGE + Au, and hand samples from one of the horizons yielded 3 to 4 ppm PGE. The horizons can be traced laterally for more than 1.5 km. beta autoradiography of core samples shows that the enrichments may be only a few cm thick and that they form well-defined horizons rich in discrete PGE-bearing minerals (mainly alloys of Pt-Fe-Ni and Cu-Au-Pd, tellurides, and antimonides).The PGE enrichments are associated with the bases of macrorhythmic units and seem to have formed during, or shortly after, the influx of primitive magma into the chamber. The Pt-Pd-enriched horizons are displaced relative to chromite horizons, thus explaining why stratiform chromitites in ophiolites, in contrast to chromitites in many intracratonic layered intrusions, are depleted in Pt and Pd relative to Os, Ir, and Ru. The study demonstrates that ophiolites should not be disregarded as potential sources for Pt and Pd.