Platinum-group mineral inclusions in chromitite from the Osthammeren ultramafic tectonite body, South Central Norway

Abstract

Fourteen different platinum-group minerals (PGM) have been found in massive chromitite from the Osthammeren ultramafic tectonite (serpentinite) body, Norway. The PGM-inclusions occur in two distinctly different textural groups. 1. Os, Ir, Ru and minor Pt occur in euhedral-subhedral, small (< 5–20 , µm), mainly single-phase inclusions of Os-free laurite, Os-laurite, osmiridium and Pt2 (Ir, Os)Fe0,65. They are totally enclosed in fresh chromite. A few PGM-inclusions are however associated with small blebs of Na-bearing hornblende and phlogopite, included in the chromite, indicating that the presence of volatiles at an early stage (i.e. preceding chromite crystallization) may have influenced the formation of these PGM. The occurrence of PGM within unaltered chromite suggests a primary magmatic origin of these. 2. Purely secondary PGM occur as anhedral-(subhedral), texturally often very complex grains or grain-aggregates of varying size (5–70 µm) and consisting of from one to eight PGM-phases plus Ni-sulphide and Ni-arsenide. These PGM always occur within, or in contact with, late formed cataclastic cracks or very fine fissures, in the primary magmatic chromite grains. The finer fissures are generally tightened by ferrit-chromite, a hydrothermal alteration product of chromite. The wider cracks are usually serpentineand chlorite-filled and are rimmed by ferrit-chromite against the chromite. Os, Ir, Ru, Rh, Pt and minor Pd are represented in grains of this group, and the secondary PGM—association found consists of Os-free laurite, Os-laurite, erlichmanite, Ir-rich erlichmanite, native Os, iridosmine, osarsite, irarsite, hollingworthite, Rh-rich platarsite (?), Ru-rich platarsite, sperrylite, (Ir, Rh)SbS, IrSbS, (Ir, Pt, Pb)S2 (new?), Pd-antimonide (probably stibiopalladinite) plus the associated phases pentlandite, heazlewoodite and niccolite.