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Bibhuranjan Nayak ¹, Franz Michael Meyer ²

Affiliations
1 Mineral Processing Division, CSIR-National Metallurgical Laboratory, Jamshedpur 831 007, IN
2 Institute for Mineralogy and Economic Geology, RWTH Aachen University, D-52056 Aachen, DE

Abstract

Manganilmenite is found to be associated with the magnetite ore body of Pokphur area in the Nagaland ophiolites, North East India. There is perhaps no earlier description of the mineral from the Indian subcontinent. It occurs as an accessory mineral with magnetite and Fe-chlorite (chamosite). Electron probe micro-analytical data reveal that the mineral contains 5.6-8.5 wt% MnO and traces of MgO, ZnO and Cr₂O₃, while the TiO₂ content remains within narrow limits of 50-53 wt%. The calculated pyrophanite end-member varies from 13% to 18%. Although the magnetite body of Pokphur has been reasonably proved to be a hydrothermally altered product of basic and ultrabasic igneous rocks, and most of the minerals in the magnetite body are supergene in nature, different end-member compositions of mangan-ilmenite indicate that it has originally crystallized with the basic suite of rocks and has survived the alteration process with only marginal effects. Since manganilmenite has been considered as a diamond indicator mineral and ophiolites are a newly documented host of microdiamonds elsewhere in the world, the presence of manganilmenite in the Pokphur magnetite hints towards occurrence of microdiamonds in the ophiolite suite of rocks of the Indo-Myanmar ranges.

Keywords

Diamond Indicator Mineral, Ophiolite, Magnetite Ore Body, Manganilmenite.

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Rojalin Debata ¹, Bibhuranjan Nayak ¹

Affiliations
1 CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751 013, India; Academy of Scientific and Innovative Research, Ghaziabad 201 002, India

Abstract

Platinum group minerals (PGMs) belonging to laurite (RuS2) – erlichmanite (OsS2) solid solution series and Os–Ir–Ru alloys generally occur as minute inclusions (<10 mm) in both stratiform and podiform chromitite deposits. In a recent study of chromitite samples from Bangur chromite deposit in the Boula–Nuasahi ultramafic complex, Odisha, India, we found a relatively coarse-grained (~30 mm) laurite crystal in addition to other PGMs. This laurite occurs as an included crystal within a ferritchromite grain and has a chemical composition (wt%): Ru ~ 43–45, Os ~ 12–15, S ~ 35, Ir ~ 2–3 and As ~ 0.2, as measured by electron probe micro-analyzer. Due to the high osmium content, it is named as Os-laurite. The mode of occurrence, texture, structure and chemical composition of laurite support a magma­tic origin for this mineral that might have crystallized at a temperature slightly lower than 1200°C. The unusual and complex oscillatory zoning (i.e. Os-rich and Os-poor bands) observed within the laurite could have developed below 1200°C (but above 1000°C) in a late magmatic stage due to local fluctuations in the surrounding thermodynamic conditions, primarily the sulphur fugacity

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