Genesis of peridotite-hosted podiform chromite ore and associated PGE, southern Manipur Ophiolite Complex, northeast India

The present study comprehensively describes the composition of key minerals in chromite ore and associated peridotite and the distribution and fractionation of platinumgroup element (PGE) including Au in peridotite and chromite ore from the southern Manipur Ophiolite Complex of the Indo-Myanmar Orogenic Belt (IMOB), northeast India. The investigated area is composed of a well-preserved mantle sequence consisting of mantle-derived peridotite associated with podiform chromite ore, mafic rocks, plagiogranite, and oceanic pelagic sediments. The composition of Cr-spinel from the host peridotite and the chromite ore clearly demonstrates the diverse sources, composition, and tectonic set-up responsible for the formation of these two rocks. The peridotite was formed in Mid Oceanic Ridge (MOR) setting whereas the chromite ore was formed in Supra Subduction Zone (SSZ) setting. The peridotite shows consistent PGE abundance ( n-ary sumation PGE=73-142 ppb) with slight enrichment of PPGE (39-76 ppb) over IPGE (34-66 ppb). In contrast, the chromite ore shows a wide range of PGE budgets and is highly fractionated ( n-ary sumation PGE=993-5547 ppb) and characterized by strong enrichment of IPGE (963-5477 ppb) over the PPGE (30-83 ppb). The high fractionation pattern of IPGE with strong positive and negative anomalies of Ru and Pt are indicative of crystallization from a Ru-rich parental magma source. Thus, it is proposed that the PGE concentrations in peridotites have been affected by the magmatic differentiation process with a low degree of partial melting and the melt originated in the MOR environment. On the other hand, the PGE content of the chromite ore was controlled by the high degree of the partial melting process during the ore formation in a later SSZ tectonic environment. Both methods confirm the SSZ origin of the chromite ore.