A mixed seawater and hydrothermal origin of superior-type banded iron formation (BIF)-hosted Kouambo iron deposit, Palaeoproterozoic Nyong series, Southwestern Cameroon: Constraints from petrography and geochemistry

摘要

The Kouambo iron deposit contains banded iron formations (BIFs) and is located in the northwestern margin of the Congo craton. The BIFs are hosted in Palaeoproterozoic Nyong series, a dominantly metasedimentary formations, which were metamorphosed into greenschist to granulite facies. The Kouambo BIFs are medium-to coarse grained banded rocks consisting of alternation of Si-rich and Fe-rich mesobands, and belong to oxide fades iron formations, Geochemistry analyses reveal that these iron formations are composed of >96 wt% Fe2O3 and SiO2 and have low concentrations of Al2O3, TiO2 and trace HFSE, suggesting chemical precipitates of silica and iron. Moreover, these BIFs have low concentrations of Al2O3, TiO2 and trace HFSEs (high field strength elements, e.g., Zr, Hf, Ta, Pb and Th), suggesting that terrigenous detrital materials contributed insignificantly to the sedimentation. The Post-Archean Australian Shale (PAAS)-normalized REE-Y patterns display seawater-like profile: minor LREE depletion and HREE enrichment, positive Y anomalies. However, they display positive Eu and negative Ce anomalies, and low Y/Ho ratio (average 29), which suggest the influence of the hydrothermal fluids. The weak positive Eu/Eu*((PAAS)) ratio (average 1.5), associated with the low V (17.5 ppm), Co (6.1 ppm) and Ni (27.5 ppm) contents similar to other Superior-type BIFs worldwide, are consistent with the deposition of the Kouambo BIFs in continental marginal sea or back-arc basin environment. In summary, the Kouambo BIFs show a seawater-like REE + Y signature, however, the positive Eu anomalies and reduced Y/Ho ratios relative to seawater indicates a possible mixing with hydrothermal fluids (similar to 0.5%). (C) 2016 Elsevier B.V. All rights reserved.