A systematic rare-earth elements and yttrium study of Archean cherts at the Mount Goldsworthy greenstone belt in the Pilbara Craton: Implications for the origin of microfossil-bearing black cherts

A systematic REE + Y study of Archean (ca. 3.0 Ga) cherts from the Mount Goldsworthy greenstone belt in the northeastern Pilbara Craton, Western Australia was performed in order to understand their origin and depositional environment. Analyzed samples include microfossil-bearing black cherts from the Farrel Quartzite and a black vein chert from the underlying Warrawoona Group, and laminated to banded chert including carbonaceous chert, jaspilite and banded iron-formation from the overlying, deepening-upward Cleaverville Formation. Laminated to banded cherts from the Cleaverville Formation show a clear stratigraphic trend upsection of increasing Y/Ho and positive Eu-anomalies, with HREE-enrichment and positive La-anomalies. The data comprise a mixing array between two end-member components on a newly proposed Y/Ho-Eu-anomaly diagram. One end-member is interpreted to be Archean seawater, with a super-chondritic Y/Ho ratio (similar to 100) and a weak positive Eu-anomaly (similar to 3). The other end-member with a chondritic Y/Ho ratio and a negligible Eu-anomaly is assumed to be non-marine water such as continental run-off, ground water, or geothermal water. Black cherts containing microfossils in the Farrel Quartzite are characterized by a positive La-anomaly, HREE-enrichment, negligible to a slight positive Eu-anomaly, and a chondritic to slightly super-chondritic Y/Ho ratio. They are distinct from vein cherts with a distinct MREE-enrichment and contemporaneous hydrothermal cherts with pronounced Eu-anomaly, and plot close to the supposed non-marine end-member component on the Y/Ho-Eu-anomaly diagram. The black cherts were thus precipitated from a water mass influenced significantly by for example continental run-off, ground water and/or geothermal water, but not from high-T hydrothermal solution, increasing the credibility of microfossils contained in them. (C) 2009 Elsevier B.V. All rights reserved.