Mobilization of Tungsten in Greenstone Belts of the Archean Kaapvaal and Singhbhum Cratons

Abstract

Due to the inherently fluid‐mobile nature of W, the 182W record of the early Earth may have been obscured by fluid‐induced mobilization of W. To investigate W mobilization in Archean greenstone sequences, we analyzed 182W isotope systematics and major and trace element concentrations in samples from the 3.53 Ga old Onverwacht Group of the Kaapvaal Craton (South Africa) and the >3.51 Ga old Badampahar Group of the Singhbhum Craton (India). Our results for mafic and ultramafic metavolcanic rocks show W/Th ratios significantly higher than primary magmatic values, which suggests fluid‐induced W enrichment. Samples least affected by secondary W enrichment (W/Th < 0.26) show no resolvable W isotope anomalies from modern mantle values in both cratons. Samples from the Kaapvaal Craton with elevated W/Th exhibit deficits in 182W as low as −8.1 ± 4.3 ppm compared to the modern mantle. Covariations of μ182W with W/Th, and Ce/Pb suggest that negative isotope signatures were introduced during secondary fluid‐mediated processes. The enrichment of W is most evident in altered ultramafic rocks comprising serpentine, resulting in additional covariations between MgO, LOI, and W/Th. The W isotope composition of serpentinized komatiites reflects the composition of younger intruding granitoids. We therefore infer the latter as a possible source of W‐rich fluids. The Badampahar Group samples exhibit little W isotope variability. A well‐resolved 182W deficit of −6.2 ± 2.9 ppm was determined in a single komatiite sample, which indicates an unknown fluid source, currently not represented in any other unit of the Singhbhum Craton.