Platinum-group elements and gold distributions in peridotites and associated podiform chromitites of the Manipur Ophiolitic Complex, Indo-Myanmar Orogenic Belt, Northeast India

Abstract

The distribution patterns of platinum group of elements (PGE) and gold (Au) in peridotites and associated podiform chromitites of the Manipur Ophiolitic Complex (MOC), Indo-Myanmar Orogenic Belt, Northeast India were discussed to elucidate the nature of the upper mantle of the MOC. Total PGE (∑PGE) concentrations of chromitites are low, ranging from <100ppb to 544ppb. The high-Cr chromitites have higher content of ∑PGE (186.2–543.9ppb) as compared to the high-Al chromitites (∑PGE=81.2–184.5ppb). Au concentration is quite variable in both the high-Al chromitite (5.8–32.4ppb) and high-Cr chromitite (10.3–42.3ppb). Their chondrite-normalised PGE patterns display a flat trend between Os and Ru with negative Ir anomaly, a negative slope between Ru and Pt, and a positive trend between Pt and Pd. Enrichment in the IPGE (Os, Ir, Ru) over the PPGE (Rh, Pt, Pd) in these chromitites is typical for ophiolitic-hosted mantle chromitites. Their low PGE content may reflect a lack of sulphur saturation of the parental magma during an early stage of their crystallisation. The PGE diversity between the high-Al and high-Cr chromitites in the MOC may be attributed to the differences in chemistry of the magma involving in the chromitite formation and geodynamic setting. It also further indicates that the high-Cr chromitites formed in a supra subduction environment have higher contents of PGE as compared with PGE contents in the high-Al chromitites generated in a mid-oceanic ridge setting. The total PGE contents of peridotites vary between 28.0 and 79.3ppb and concentrations ranging from 0.008 to 0.024 times chondrites. Two types of PGE distribution patterns have been observed in the mantle-normalised PGE plot. The type-I display flat patterns from Os to Pt, and positive slope from Pt to Au whereas the type-II samples have Os, Ru, Rh and Pd enrichment relative to Ir and Pt. Their distinct mantle-normalised PGE patterns with the Pd/Ir values (1.7–11.4) and Pt/Pt* values (0.2–1.4) are affinity to the characteristic of residual mantle material and suggest that the suite of mantle peridotites of MOC derivation by varying degree of partial melting rather than magmatic fractionation.