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 ele...
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