Geology and Geochemistry of Ultramafic-Mafic Rocks from Antarghatta Belt, Western Dharwar Craton, Karnataka: Implications for PGE Mineralization and Future Targets

Abstract

The mafic-ultramafic (MUMF) suite of rocks of Archaean age occur as discontinuous pods north of Arsikere up to south of Vedavathi river in western Dharwar Craton, Karnataka. The mafic-ultramafic enclaves consist of meta-peridotites, tremolite-actinolite schists, serpentinites, amphibolites and metabasalts with gabbro dykes. The rocks have undergone green schist to amphibolites facies metamorphism. Our studies have shown nodular and ocellar, pillows, jack straw textures in these ultramafic-mafic suite (the talc-serpentine rocks). Talc-serpentine and tremolite-actinolite schists contain chromite and titanomagnetite concentrated at the grain boundaries. The overall major element composition of these rocks shows a high MgO (22-40 %) and total iron (8-10 %) contents and some samples also exhibit komatiitic affinity. The distribution of chromite grains in amphibole and associated matrix is also noticed. High contents of Ni and Cr (∼3000, 2800 ppm) were obtained in some of these rocks. The REE data show slightly fractionated patterns with enriched LREE (LREE/HREE ∼5.0) and negative europium anomalies. The reported Pt and Pd values are 6500 and 190 ppb. The element relationships indicate that different types of hydrothermal fluids re-distributed Ni, Cu, and (Pt+Pd) in different ways, which may have led to local concentration of Pt and Pd. The overall geochemical data and (Pt+Pd) contents imply that, at Antarghatta ultramafic belt, different genetic types of (Pt+Pd) enrichment processes were active. (1) Small amounts of immiscible sulphide melt collected relatively high amounts of PGE (relatively high (Pt+Pd) at low Ni and Cu contents) and represent a low sulphur type silicate PGE mineralization, but no major sulphide liquation event may have happened that would lead to a potential economic mineralization. (2) A hydrothermal Ni-enrichment trend (at low Cu) is reflected by the presence of millerite, which enriches the rocks beyond the Ni in the silicate minerals and the slightly higher values of Ni where elevated chromium values occur. (3) Where Cu (at low Ni contents) gets enriched by fluids, often (Pt+Pd) is enriched as well. Thus, the Antarghatta ultramafic belt warrants a multi-disciplinary approach to understand the critical parameters and processes involved in the genesis of Ni-Cu-(PGE) associated with these komatiites.