Serpentinised Dunites of the Neoarchean Shimoga Greenstone Belt, Western Dharwar Craton, India: Insights on Ni-PGE Mineralisation and Genesis

Abstract

Ultramafic rocks of the Archean greenstone belts worldwide are potential hosts for Cu-Ni, precious metal deposits like platinum group elements (PGEs) and gold. This study highlights the geochemical evidence and genesis of Ni-PGE mineralisation in the ultramafic rocks of Shimoga greenstone belt of the Western Dharwar Craton, India. Petrographically, the studied rocks are identified to be serpentinised dunites, while their geochemical signatures indicate komatiitic affinity. Presence of disseminated sulphides and pronounced serpentinisation in these rocks suggest a combination of Type II (disseminated sulphides) and Type IV (post-magmatic alterations) komatiite-related Ni–Cu-PGE deposits. Chondritic Al₂O₃/TiO₂ (9.65–16.38) ratios, superchondritic Gd/Yb_N (1.3–1.6), depleted high-field-strength elements (HFSEs) (Zr, Hf), enrichment of LREE over HREE and negative Nb–Ta anomalies reflect the generation of parental melts from plume-sourced Al-depleted komatiites with significant crustal contamination during their emplacement. The major, trace element and PGE relationships (FeO vs. MgO, Cu vs. Pd, Ni/Cu vs. Pd/Ir) infer the origin of sulphur undersaturated primary melts through moderate to higher degrees of partial melting followed by crustal assimilation that led to PGE enrichment. These observations suggest their formation from melts derived from greater depths of the upper mantle (> 400 km) at high pressure (> 10 GPa), wherein, the mantle residue retained majorite garnet. The high Ni (avg. Ni = 6511 ppm) substantiated by high Kambalda ratio ([Ni:Cr] × [Cu:Zn] = 5.6), Ni/Cr ratios (> 1) with high concentration of PGEs (avg. ∑PGE = 3078 ppb) confirm the fertile/mineralised nature of the komatiitic source and potential Ni-PGE mineralization in the ultramafic rocks of the Shimoga greenstone belt.