Geochemistry and Pb-Pb geochronology of the Neoarchean Nkout West metamorphosed banded iron formation, southern Cameroon

摘要

The Nkout West iron deposit of the northern edge of the Congo craton in southern Cameroon forms part of the northwest plunging isoclinal mega fold composed of metamorphosed banded iron formations (BIFs), which have been deformed and metamorphosed, resulting in recrystallized magnetite, quartz and silicates. Here, we present whole-rock geochemistry together with Pb-Pb isotopic compositions and dating of metamorphosed BIF, with the aim to constrain the genesis and elucidate the metamorphic resetting age. Based on the mineral composition, two types of BIF (magnetite BIF, garnet BIF) have been distinguished. The magnetite BIF shows high SiO2 and Fe2O3 and low Al2O3, TiO2, HFSE and Sigma REE contents similar to detritus-free chemical sediments. In contrast, their high concentrations in garnet BIF indicate clay-rich-contaminated input. The Co/Zn ratios are consistent with trace metals of hydrothermal sources (0.14 for magnetite BIF) or with minor hydrogenous component (0.27 for garnet BIF). The PAAS-normalized REE-Y patterns of the studied metamorphosed BIFs show LREE depletion relative to HREE and positive Eu anomalies. The magnetite BIF exhibits positive Y anomaly (Y/Y*(PAAS)=1.3), which is the signature inherited from sea water, but its Y/Ho (39.93) ratio suggests a mixture of hydrothermal solution with sea water. The garnet BIF shows positive Eu, negative Y anomalies and chondritic Y/Ho ratio (27.49), which is consistent with hydrothermal solution with significant clastic contamination. Whole-rock Pb concentration is roughly on average of 2.0-6.0ppm, suggesting a hydrothermal Pb source. The Pb-Pb dating yielded an age of 2679Ma, interpreted as the Neoarchean metamorphic event in the Ntem complex.