Platinum Group Minerals and Silicate Inclusions In Chromitite From the Naga-Manipur Ophiolite Complex, Indo-Myanmar Orogenic Belt, Northeast India

Abstract

Abstract This study reports the first documented occurrence of platinum group minerals (PGM) and silicate inclusions in the podiform chromitites of the Naga-Manipur ophiolite complex, Indo-Myanmar orogenic belt, northeastern India. The studied chromitites were divided into three groups, high-Cr (Cr# = 0.72 to 0.80), intermediate-Cr (Cr# = 0.66 to 0.68), and high-Al (Cr# = 0.46 to 0.47). The bulk PGE content is low and varies in the range of 57–162 ppb. The chondrite normalized PGE patterns show that the studied chromitites are enriched in IPGE (Os, Ir, Ru) over PPGE (Rh, Pt, Pd), which in general is consistent with podiform chromitites hosted in a mantle sequence of ophiolites. All the PGE patterns are characterized by a consistent positive anomaly in Ru and some samples display a slight positive anomaly in Pt, compared with Rh and Pd. Most of the PGM occur in fresh chromite crystals consisting of single or polyphase grains composed of different PGM associated with Ni and Cu sulfides and silicates. The primary PGM paragenesis consists of minerals of the laurite (RuS 2 )–erlichmanite (OsS 2 ) series with minor osmium, iridium, and ruthenium. An unidentified Ir–Ni sulfide mineral, probably kashinite, was also observed. Based on their assemblage, we can argue that the PGM discovered in the Naga-Manipur chromitites are magmatic in origin and formed at relatively high sulfur fugacity, as testified by the presence of Os-rich laurite, erlichmanite, and kashinite. However, the relatively low PGE content of the investigated chromitites suggests that sulfur saturation was not achieved in the parental magma during the early stage of their crystallization. The presence of heazlewoodite in chromite fractures and in the altered matrix reflects its secondary origin, suggesting that it formed progressively during the serpentinization process. The presence of both Cr-rich and Al-rich chromitite suggest that the Naga-Manipur chromitite formed from separate intrusions of magmas derived from different depleted mantle sources ( e.g ., MORB versus boninitic) during evolution of the oceanic lithosphere from the MOR (Al-rich chromitite) towards a supra subduction zone (SSZ). Abundant hydrous silicates inclusions were found exclusively in the Cr-rich chromitites, whereas the Al-rich chromitites contain only clinopyroxene, confirming the proposed geodynamic evolution.