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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 ƐNd(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. [ABSTRACT FROM AUTHOR]

The Sanaga iron ore prospect within the Palaeoproterozoic Nyong Series in Cameroon is a magnetite‐dominated iron ore deposit hosted within granodioritic gneisses with an estimated resource of 82.9 Mt @ 32.1% Fe. The main ore mineral being magnetite, provides an opportunity to study the magnetite texture and iron‐oxygen isotope composition of the mineralization, which has heretofore not been reported. These offer insights into the nature and condition of formation of the mineralization thereby paving the way for better understanding the genesis and evolution of the mineralization as well as classifying it. The mineralization is characterized by Type I and Type II magnetite varieties based on their textural features. Type I magnetite is the dominant magnetite and is characterized by ilmenite oxy‐exsolution features with triple junction contacts, while Type II magnetite lacks these features. The presence of ilmenite oxy‐exsolution features in the Type I magnetite indicates a primary magmatic formation of the magnetite followed by re‐equilibration of primary Ti‐rich magnetite in the presence of a high‐temperature fluid. Oxygen isotope (δ18O) values for the magnetite range from +2.89% to 9.30‰ indicating precipitation from evolved ore‐forming fluids through reaction with country rocks. The iron isotope (δ56Fe) of the magnetite shows little variation, with positive values ranging from +0.49–0.66‰. These values plot within the magmatic and magmatic‐hydrothermal magnetite fields when compared to the published global range of δ56Fe values for different deposit types. A similar result is obtained by plotting the coupled Fe–O isotope pair, further buttressing the magmatic‐hydrothermal origin of the mineralization. Based on these data, the Sanaga iron mineralization is best explained as a magmatic‐hydrothermal deposit. [ABSTRACT FROM AUTHOR]

The Sanaga iron ore prospect is a recent discovery in the Nyong Series with a resource estimated at 82.9 Mt at 32.1% Fe and whose origin remains debatable. The mineralization occurs as NE-SW oriented discontinuous lenticular bodies of magnetite-bearing pyroxene-gneisses (MPG) hosted by ortho-derived gneisses. Rare amphibolites are observed. The MPG mineral assemblage consists of quartz-magnetite-orthopyroxene-garnet-tremolite/actinolite exhibiting a granoblastic texture, which is characteristic of granulite facies metamorphism. The granodioritic gneisses show compositional features of the tonalite-trondhjemite-granodiorite association. Their trace and REE element geochemistry indicate their protolith melt resulted from the partial melting of a subducted oceanic slab, with interaction with the overlying mantle wedge during ascent. The amphibolites show enrichment in LILE with negative Ta–Nb and Zr–Hf indicating arc-related magmas generated by partial melting of a sub-continental lithospheric mantle source with metasomatism by subduction-related fluids. The MPG exhibits oxidation-exsolution features characterized by ilmenite lamellae, with hematite fracture-filling in magnetite, and lacks features characteristic of typical BIF such as LREE depletion relative to HREE, positive Eu, La, and Y anomalies. Based on the results of this study, we interpret the Sanaga MPG as a possible skarn-type mineralization formed by the metamorphism/metasomatism of a possible BIF protolith. The results of this study compare with similar magnetite-rich mineralization in the São Francisco craton in northeastern Brazil and enhance the correlation of pre-drift reconstructions of the São Francisco–Congo Cratons. [ABSTRACT FROM AUTHOR]

In Cameroon, most of the iron formation occurrences reported are found within the Nyong and Ntem Complexes. The Anyouzok iron deposit is located in the Nyong Complex greenstone belts, which represent the NW margin of this Congo craton. The main lithological units comprise the iron formations (IFs) unit, consisting of banded IFs (BIFs) and sheared BIFs (SBIFs), and the associated metavolcanic rocks unit consisting of mafic granulite, garnet amphibolite, and biotite gneiss. Within the Anyouzok area, BIFs are rare, while SBIFs are ubiquitous. This study reports the petrography, mineralogy, and whole rock geochemistry of IFs and interbedded metavolcanic rocks of the Anyouzok iron deposit. The abundance of cavities, higher Fe contents (49.60–55.20 wt%), and strong Eu anomalies (Eu/Eu* = 2.14–3.17) within the SBIFs compared to the BIFs suggest that SBIFs were upgraded through post-depositional hydrothermal alteration activities. REE signatures indicate the contribution of both seawater and hydrothermal fluids during BIFs precipitation. Mafic granulite and garnet amphibolite protoliths were derived from the partial melting of a metasomatized spinel lherzolite depleted mantle source. The overall compositional variations of the Anyouzok IFs and interbedded metavolcanic rocks endorse an Algoma-type formation deposited in the back-arc basin under suboxic to anoxic conditions. [ABSTRACT FROM AUTHOR]

Abstract: Gold nuggets from specular hematite-rich, sulphide-free, auriferous veins (jacutinga) at Gongo Soco, an iron ore deposit in the Quadrilátero Ferrífero of Minas Gerais, Brazil, have inclusions of palladium arseno-antimonides (isomertieite and mertieite-II) and hematite. Both isomertieite and mertieite-II are commonly fractured and their fragments are displaced in relation to each other without any optically recognisable brittle failure of the host gold. More rarely, isomertieite has sub-parallel S-shaped intragranular microfractures terminating in angular edges. Hematite shows microfracturing and displacement, as well as microgouge formation and slumping. It occasionally exhibits deformation twins. Microcracking also occurs in gold itself. Goethite seals microcracks in gold with or without angular fragments of goethite. Crack sealing in nugget gold by Hg–Pd-enriched gold–hematite–kaolinite infill indicates recurrent fracturing and mineralisation. Although the microstructures are enigmatic, two hypotheses are suggested for their formation: (i) fluid-pressure increase and its sudden (seismogenic?) release could fracture erstwhile ductile gold; (ii) the ductile gold transferred tectonic stresses to the harder mineral inclusions, which deformed in a brittle manner. [Copyright &y& Elsevier]

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 f O 2. Geochemical analyses show that the supergene and specularite ores have higher Fe 2 O 3 (88.27 to ~100 wt%) and lower SiO 2 (<0.01 to 0.18 wt%) contents than the itabirites (31.95 wt% Fe 2 O 3 , 67.16 wt% SiO 2). 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 Al 2 O 3 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. • Itabirite at Mbarga is linked with supergene and specularite ores. • Magnetite microchemistry suggests formation under low- T hydrothermal conditions. • The itabirite's isotopic signature suggests mixed magmatic and metamorphic fluid sources. • Itabirite enrichment was aided by isotopically light-evolved meteoric water. • A polygenic-supergene-hydrothermal model is proposed for the iron ores formation. [ABSTRACT FROM AUTHOR]