Chromitites and associated mineralization of the Akkarga ophiolitic massif in the southeastern Urals (Russia)

Abstract

The Akkarga ultramafic massif exposed in the Trans-Uralian megazone represents the mantle section of an ophiolite assemblage that was emplaced into the upper crust during the Permian collision. Ultramafic rocks include harzburgites and subordinate dunites that have undergone complete serpentinization and host podiform chromitite bodies typical of the ophiolite complexes worldwide. Numerous lens-shaped and podiform occurrences of massive and densely disseminated chromitites are surrounded by envelopes of serpentinized dunites whereas nodular chromitites and lenticular bodies of banded disseminated ores are of subordinate importance. Three ore zones are distinguished on the massif, including the Western, Central and Eastern, but only the sites of the latter two are currently accessible for study. Chromite from chromitites of the Central Ore Zone has higher Cr# (Cr/Cr + Al) (0.81–0.83) than that of chromitites of the Eastern Ore Zone (Cr# = 0.67–0.80) and accessory chromite of peridotites (Cr# = 0.52–0.75). Chromites contain mineral inclusions, which are distributed unevenly. The most abundant are inclusions of high-Mg high-Ni olivine (Fo_94–98 and up to 1.5 wt% NiO) and calcic amphibole, while inclusions of pyroxenes and base metal sulfides are less common. Platinum group minerals (PGMs) in chromitites are represented by alloys, sulfides, and sulfoarsenides, which occur in single-mineral and composite inclusions. Ruthenium and Os disulfides typically compose the euhedral single-mineral inclusions in cores of chromite crystals, whereas the composite inclusions, mostly of irregular shapes, are dominated by Ir compounds. PGMs are regularly associated with OH-bearing silicates such as amphibole and, less frequently, chlorite. The setting, morphology and composition of the inclusions seem to support a leading role of subsolidus solid-state exsolution in the formation of primary laurite-erlichmanite mineralization in Akkarga chromitites. Subsequent hydrothermal reworking of podiform chromitites and their ultramafic hosts, which is likely related to the supra-subduction setting, led to the precipitation of more diverse interstitial assemblages, comprising base-metal sulfides, nickeline, sulfoarsenides of the Ir-subgroup platinum group elements (IPGE), REE phosphates, zircon, barite, and baddeleyite. Later granite intrusions likely provided an additional contribution of fluid-mobile incompatible elements to chromitites.