Bulk geochemistry, Rb–Sr, Sm–Nd, and stable O–H isotope systematics of the Metzimevin high-grade iron ore deposit, Mbalam iron ore district, southern Cameroon

Abstract

Bulk geochemistry, Sr, Nd, and O–H isotope systematics are reported for the first time on banded iron formation (BIF)-hosted high-grade iron ore at the north-western segment of Congo Craton (CC). Located in Mbalam iron ore district, Southern Cameroon, Metzimevin iron ore deposit is a hematite-magnetite BIF system, dominated by SiO₂ + Fe₂O₃ (97.1 to 99.84 wt%), with low concentrations of clastic elements e.g., Al₂O₃, TiO₂, and HFSE, depicting a nearly pure chemical precipitate. The REE + Y signature of the iron deposit displays strong positive Eu anomaly, strong negative Ce anomaly, and chondritic to superchondritic Y/Ho ratios, suggestive of formation by mixed seawater-high temperature hydrothermal fluids in oxidising environment. The ⁸⁷Sr/⁸⁶Sr ratios of the BIF are higher than the maximum ⁸⁷Sr/⁸⁶Sr evolution curves for all Archean reservoirs (bulk silicate earth, Archean crust and Archean seawater), indicating involvement of continentally-derived components during BIF formation and alteration. The Ɛ_Nd(t) (+ 2.26 to + 3.77) and Nd model age indicate that chemical constituents for the BIF were derived from undifferentiated crustal source, between 3.002 and 2.88 Ga. The variable and diverse O and H isotope data (− 1.9‰ to 17.3‰ and − 57‰ to 136‰ respectively) indicate that the Metzimevin iron ore formed initially from magmatic plumes and later enriched by magmatic-metamorphic-modified meteoric fluids. Mass balance calculations indicate mineralisation by combined leaching and precipitation, with an average iron enrichment factor of > 2.67 and SiO₂ depletion factor of > 0.99. This is associated with an overall volume reduction of 28.27%, reflecting net leaching and volume collapse of the BIF protholith.