Mineralogical variation in platinum group element within altered chromitite of the Kondapalli layered igneous complex (Southern India): Implication on magmatic evolution and its petrogenetic significance

Abstract

The Kondapalli Layered Igneous Complex (KLIC) is deformed and dismembered stratiform type of mafic-ultramafic complex emplaced into high-grade metamorphic rocks of Eastern Ghat Belt (EGB). Which contains chromite with relatively oxidized compositions that occur as bands, pockets, pods, stringer, and dissemination. This chromite hosts traces of PGM eg. laurite, irarsite [(Ir, Ru, Rh, Pt)AsS], Os-Ir-Ru alloys and Fe–Ni–As–S minerals. The present study helps to understand the changes and/or reworking of chromite and PGM due to the effect of alteration. Petrographic observations and EPMA study indicate that the chromite core is still consistent with magmatic fractionation processes, whereas sequential alteration of the chromite rim suggests upper greenschist to lower amphibolite facies of metamorphism (500° to 550 °C). The platinum-group minerals (PGM), however, display evidence of partial hydrothermal/metamorphic reworking. Especially, laurite and irarsite which occur near the chromite grain boundary. In contrast, the primary ones which probably formed at high-T and low fs2 and occur in the chromite cores are relatively less affected by alteration.The calculated parental magma composition was similar to that of modern primitive boninitic/tholeiitic basalt formed by a high-degree of mantle melting. Present study ascribes the formation of the subduction-generated arc mafic-ultramafic magmatism in the Kondapalli to the closure of the ocean that led to the formation of the suture between Eastern Dharwar Craton (EDC) and EGB. The parent magma of the KLIC is interpreted as mantle-derived arc-related boninites with S-undersaturated nature like other layered intrusions. This is indicating that an initial percolation of sulphur-poor melts for long-period without significant contamination for the genesis of KLIC. The mineral chemistry supports the very limited crustal contamination and non-availability of the sufficient meta-sediments during assimilation precludes the sulphur saturation in the KLIC, which is mainly constrained for the development of potential Ni-Cu-PGE mineralization. As a result, the KLIC fever the deposition of only primary orthomagmatic chromite-PGE mineralization. These findings determined that the investigated KLIC are low-priority Ni-Cu-PGE targets. The compositions of the chromites can be used to determine the petrologic history of the intrusions and may prove to be a useful exploration tool in such mineralised belt.