Geochemistry of Archean carbonaceous cherts deposited at immature island-arc setting in the Pilbara Block, Western Australia

Abstract

Abstract Archean carbonaceous meta-cherts (upper-greenschist to amphibolite facies) from the Nickol Well Unit, the Cleaverville Group (∼3.2Ga) in the Roebourne district in West Pilbara are characterized by variable Al2O3/TiO2, Zr/TiO2 and Cr/Th values and by high concentrations of carbon and some trace metals. Distribution patterns of the cherts on a newly proposed diagram (Al2O3/TiO2–element/Al2O3) indicate that concentrations of Cr, V and Ni can be explained by contributions of clastic components, whereas concentrations of Cu, Zn and Pb are much higher than those expected solely from clastic contributions. Detrital material in the cherts was derived from a wide variety of volcanic sources (mafic–ultramafic–felsic sources) consistent with the inferred geological setting of the Cleaverville Group (immature island arc setting). Excess Cu, Zn and Pb are probably from hydrothermal solutions, as suggested by positive Eu anomalies and enrichment in heavy rare-earth elements (REE) in chondrite-normalized patterns. High concentrations and light isotopic values of C (−17 to −24 per mil) in the cherts indicate that the precursory sediments were enriched in organic materials. Although it is unclear whether the organic material was derived from organic particles that settled from the ocean surface or benthic microbes, in either case, it is possible that this material played an important role in the accumulation of heavy metals (Cu, Zn and Pb) in the sediments.