A supergene-hydrothermal origin of the itabirite-hosted high-grade iron ores in the Mbarga prospect, Mbalam iron ore district, southern Cameroon, Congo Craton

Abstract

The Mbarga itabirite deposit in the Mbalam iron district on the northwest edge of the Congo Craton (CC) hosts two main types of iron ore enrichments: supergene and specularite ores. This study presents mineralogical, geochemical, and isotopic datasets on these ores to determine their genesis.

Ore microscopic studies indicate that the itabirites are of the oxide facies type, with magnetite showing partial to extensive alteration to hematite-martite. The supergene ores consist of hematite + martite + goethite ± gibbsite ± magnetite ± quartz, while the specularite ores are mainly composed of hematite + martite ± quartz. Magnetite microchemistry suggests formation under low-T hydrothermal conditions (~200–300 °C) with high fO₂. Geochemical analyses show that the supergene and specularite ores have higher Fe₂O₃ (88.27 to ~100 wt%) and lower SiO₂ (<0.01 to 0.18 wt%) contents than the itabirites (31.95 wt% Fe₂O₃, 67.16 wt% SiO₂). The enrichment of Fe in the supergene ores is attributed to the depletion of major oxides and trace elements due to weathering and supergene enrichment, while the high Fe content in the specularite ores stems from the precipitation of iron-rich, but trace- and rare earth elements (REE)-deficient hydrothermal fluids. The slightly higher Al₂O₃ content and positive Ce anomalies in the supergene ores suggest the retention of Al-bearing minerals (gibbsite) and reveal highly oxidative conditions during martitization. Stable isotope analyses reveal that the supergene and specularite ores have δ¹⁸O values of −2.5 to −0.3 ‰ and − 2.0 to −3.4 ‰, and δ²H values of −75 to −123 ‰ and − 70 to −119 ‰, respectively, suggesting the involvement of isotopically light-evolved meteoric water in their formation. In contrast, the itabirites exhibit heavier δ¹⁸O (8.5 to 10.2 ‰) and δ²H (−85 to −91 ‰) values, suggesting formation from mixed magmatic and metamorphic fluid sources. A “polygenic-supergene-hydrothermal” model is suggested for the formation of the Mbarga itabirite-hosted iron ores.