Direct crystallization of refractory platinum‐group element alloys from boninitic magmas: Evidence from western Tasmania

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Abstract
Alluvial occurrences of Os‐Ir‐Ru alloys are known from several localities in western Tasmania, and are spatially associated with Cambrian mafic‐ultramafic complexes. A compilation of analytical data from this and previous investigations indicates that the Tasmanian alloys are predominantly Ru‐poor, Pt‐poor iridosmine with minor osmiridium. These compositions are similar to Os‐Ir‐Ru alloys from placers that are associated with ophiolites, and contrast with the more Pt‐rich alloys known from Alaskan‐type complexes. Most of the alluvial alloys contain silicate inclusions, comprising Mg‐rich olivine (Fo89–93), Mg‐ and Al‐rich orthopyroxene (En84.8–98.2) and non‐stoichiometric, Al‐rich calcic pyroxene (endiopside and augite). The chemistry of the inclusions is consistent with a model in which the Os‐Ir‐Ru alloys crystallized within the lithospheric upper mantle. Osmium, Ir and to a lesser degree Ru, behaved compatibly during the crystallization of the parental magmas for the Heazlewood River complex. These elements are concentrated within the most Mg‐rich cumulates (dunite), that occur at the base of the cumulate succession. Boninites from the Heazlewood River complex are believed to represent the parental liquids from the cumulate sequences, and are strongly depleted in Ir relative to Pt, Pd and Au. These observations suggest that the Tasmanian Os‐Ir‐Ru alloys crystallized before the emplacement of the boninitic parental magmas into crustal magma chambers. Early crystallization of the Os‐Ir‐Ru alloys may have promoted their concentration into magmatic schlieren along the base of intrusion. It appears that cumulates derived from boninitic magmas (including the cumulate portion of many ophiolite complexes) are prospective for primary Os, Ir and Ru alloy mineralization, and that the unusual composition of boninites favours the direct crystallization of these alloys.

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