Origin of Ni-Cu-(PGE + Au) sulfides in late-Archean komatiitic suite of rocks in the Shankaraghatta belt, Western Dharwar Craton (India)

Abstract

The Shankaraghatta ultramafic–mafic suite of rocks (12 × 0.3 km) within the late ArcheanShimoga greenstone belt (Western Dharwar Craton, southern India) is constituted of sulfide-bearing serpentinized dunite flanked by metabasaltic schist rocks, which are probable analogues of the adcumulate-mesocumulate sheet flows in a komatiitic sequence, flanked by komatiitic basalts. Owing to limited field exposures in this study, the bulk concentrations of Ni, Cr, Ti, Fe, and Mg were used to classify the rocks into different volcanogenic facies observed in a komatiitic flow. The mineralized mesocumulate (serpentinized dunites) flanking its S-poor adcumulate counterpart host 5–8 modal % disseminated sulfide globules formed by an assemblage of pentlandite-millerite-pyrite-chalcopyrite. Millerite is the dominant sulfide mineral, whereas coarse euhedral pyrite grains are abundant in the serpentinites. The sulfide-bearing serpentinites have higher platinum-group element (PGE) abundances than those lacking visible sulfides (∑PGEsulfide-bearing samples = 440–1848 ppb; ∑PGEsulfide-lacking samples = 4.8–69 ppb) and show a strong PPGE (Pd, Pt, Rh) enriched trend with distinct negative Pt-anomalies in the primitive-mantle normalized PGE diagram. The mineralized serpentinites have higher Ni and Cu concentrations (0.5–2.2 wt% and 29–201 ppm, respectively) compared to their unmineralized counterparts (0.1–0.6 wt% and 22–72 ppm, respectively). Major element data indicate a komatiitic composition for the serpentinites, whereas the schists are komatiitic basalts. Chondrite-normalized REE plots for the serpentinites and schists show LREE-enriched patterns (La/SmNserp = 1.8–3.5; La/SmNschist = 2.5), with almost flat to slightly depleted HREEs (Gd/YbNserp = 1.1–1.7; Gd/YbNschist = 1.8). The rocks show prominent negative anomalies of Zr and Hf, which along with the subchondritic Al2O3/TiO2 ratios for serpentinites (~9.9), indicate derivation of the komatiitic magma from a plume that originated from a deep mantle source (>400 km) with the mantle residue retaining a majorite garnet phase. The komatiites and komatiitic basalts from which the serpentinites and schists were derived have a common mantle source. They are connected by fractional crystallization as understood from collinear and coherent patterns in bivariate plots between major oxides such as MgO, and alteration resistant immobile trace elements such as Nb, that are plotted with other major and trace elements, as well as from the coherent patterns observed in the chondrite-normalized REE and multi-element spider diagrams. The serpentinites and schists show signatures of crustal contamination as defined by negative anomalies of Nb, Ta, and Ti,high La/Sm ratios, along with higher concentrations of trace elements like La and Th, and lower Nb/Th ratios(Nb/Thavgserp = 6.6; Nb/Thavgschist = 3.7). AFC (Assimilation Fractional Crystallization) models indicate that the schists underwent a greater extent of crustal contamination (50% AFC) relative to the serpentinites (12% AFC). Sulfide saturation was achieved late, after emplacement of the komatiitic magma and crystallization of the olivine adcumulates, when the melt was slightly more differentiated to form the mesocumulates. The sulfides are later overprinted by low-temperature alteration processes, as evident from the significant enrichments in serpentinites for Au (AuNavg: 2.1), As (AsNavg: 20), Bi (BiNavg: 8.2), and Te (TeNavg: 1.5), which occurred perhaps during the replacement ofpentlandites by millerites.