Abstract

Three distinctive chromitite occurrences are recognized from the Cambrian mafic-ultramafic Heazlewood River Complex, Tasmania. Type I chromitite represents very thin schlieren comprising Cr-rich spinel and is hosted by primitive, olivine-rich cumulates. Type II chromitite occurs as thicker schlieren containing more aluminous spinel and is hosted by an interlayered sequence of cumulate dunite and peridotite. Type III chromitite is more massive, occurring as irregular stringers, pods, and layers containing chromite having intermediate Cr/Al ratios. It is spatially associated with gabbroic dikes that intrude dunite-harzburgite-orthopyroxenite cyclic units and is interlayered with or enclosed by an unusual xenolith-bearing gabbronorite. Both type I and II chromitites are interpreted to have precipitated directly from a primitive magma and may reflect periods of magma replenishment. Type III chromitite was formed in response to mixing between ultramafic magma and volatile-rich intercumulus liquids, producing hybrid magmas having chromite as the only liquidus phase.All of the chromitites display enrichment in platinum-group elements relative to the cumulate ultramafic sequences. The maximum total platinum-group element tenor in the chromitites is 6 ppm, with average grades between 0.2 and 1 ppm. The chromitites are richest in ruthenium, platinum, and rhodium, reflecting the presence of discrete platinum-group minerals. Laurite (RuS 2 ) is the most abundant platinum-group mineral, occurring as isolated inclusions in chromite. A wide variety of Pt and Rh minerals are contained in the chromitites. These include alloys, arsenides, and sulfarsenides, the majority of which occur as altered, anhedral grains in the silicate matrix.The platinum-group mineral assemblages recognized in this investigation are similar to bimodal platinum-group mineral suites documented in chromitites from ophiolites (Legendre and Auge, 1986) and stratiform intrusions (von Gruenewaldt et al., 1986). These include early crystallizing Os, Ir, and Ru (Ir-group elements) phases and later formed Rh, Pt, and Pd minerals (Pd-group elements, Barnes et al., 1985). The Ir-group elements display a preference for octahedrally coordinated sites in magmas, similar to Cr (Murck and Campbell, 1986) and, like Cr, have a low solubility in primitive magmas. The unusually low Ir tenor of type I chromitites from the Heazlewood River Complex may reflect the prior removal of Os-Ir alloys from the parental liquids, which would account for the known occurrence of osmiridium in the complex. Localized magma mixing associated with the development of the type III chromitites led to increasing a (sub S 2 ) and a (sub A s ) , but not necessarily to S saturation, in the resultant hybrid liquid. Under these conditions, laurite was precipitated as an early formed cumulus phase, and the more soluble Pd-group elements crystallized as intercumulus sulfarsenides and arsenides.

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