Mineralogy of Platinum Group Elements in Explosive Breccias of the Poperechnoe Deposit (Malyi Khingan, Russia)

Abstract

The results of detailed study of the platinum-group minerals (PGM) from explosive breccias of the Poperechnoe Fe-Mn deposit (the Lesser Khingan Range, Russian Far East) are presented. Native PGMs are dominated by isoferroplatinum; rutheniridosmine, native iridium, platinum, and osmium are less com-mon. Micron-sized segregations of laurite, bowieite, сuproiridsite, cuprorhodsite, hollingworthite, as well as new mineral phases of (Ir,Rh,Os)7 (S,As)13 , (Rh,Ir,Ru) 7(S,As) 13, and Pd 3(Sb,As) were recognized as micro-inclusions in isoferroplatinum and on the surface of its grains. It is shown that the studied PGMs are derived from rocks of ultramafic formations: (1) vein pyroxenites, harzburgites, and dunites of metamorphic and cumulative complexes of the most depleted peridotite varieties of the suprasubduction wedge of island-arc ophiolites and (2) vein pyroxenites of cumulative high-pressure ultramafic complexes of the basement of the ensialic island arc and products of evolution of suprasubduction mantle melts. In terms of PGE contents, the compositions of isoferroplatinum from explosive breccias are divided into four groups: group I: isoferroplatinum of fluid-metamorphogenic genesis from harzburgite, group II: isoferroplatinum of fluid-metamorphogenic genesis from dunites and magmatogenic-fluid-metasomatic genesis from vein pyrox-enites; group III: isoferroplatinum of magmatogenic genesis from chromitites of dunites of the cumulative complex; and group IV: isoferroplatinum with an elevated Pd content, which is likely derived from the melt formed by explosive breccias. Three scenarios of PGM occurrence in the fluid-saturated andesite-dacite melt of explosive breccias at the Poperechnoe deposit are discussed: (1) directly from early ultramafic com-plexes in suprasubduction settings; (2) from the reservoir of ancient platinum placer deposits; and (3) from a mantle wedge above the Mesozoic subduction zone during the generation of initial island-arc melts. The first two petrogenetic models suggest «rejuvenation» of the 190Pt-4He age of isoferroplatinum grains as a result of the thermal effect of the andesite-dacite melt to the age of 125 ± 21 Ma. The third geodynamic scenario suggests that the Lower Cretaceous age of isoferroplatinum marks the processes of metamorphogenic-metasomatic transformation of the dunite-harzburgite mantle wedge and probably corresponds to the age of the subduction magmatism at the Poperechnoe Deposit in particular and within the Lesser Khingan terrane as a whole. PGM associations in andesite-dacite breccias of the Poperechnoe Deposit are a new type of potentially commercial noble-metal mineralization in the Russian Far East, while explosive breccias (fluidoliths) can be used as an exploration guide to discover PGE lode and placer deposits of volcanogenic-explosive genesis in the Russian Federation.