Petrology and geochemistry of metamorphosed rocks associated with iron formations of the Toko‐Nlokeng iron deposit, (Southern Cameroon): Implications for geodynamic evolution and mineralization

Abstract

Detailed petrographic, lithostratigraphic, and geochemical data of metamorphosed orthogneisses and mafic‐ultramafic metavolcanic rocks intruded into the greenstone belt of the Toko‐Nlokeng iron deposit have enabled the reconstruction of the tectonic and geodynamic setting and crustal evolution of the Nyong Complex. Samples were collected from the drill holes TNF11_01 and TNF11_02 from 17.16 m to 335.85 m depth. The lithostratigraphy supported by field and petrographic observations outlines two main lithologies: Iron formations (IFs) and metamorphosed host rocks. The IFs (granular iron formations (GIF) and banded iron formations (BIF)) are intercalated with host rocks consisting of orthogneisses and mafic‐ultramafic rocks. New and published geochemical data of metamorphosed associated IFs from Anyouzok (TNF08 prospect) of the Toko‐Nlokeng iron deposit, Mewengo iron deposit, Bipindi, and Kribi metavolcanic rocks in the Nyong Complex, suggest that these rocks were formed by material from intrusive and extrusive magmatic episodes with mid‐ocean ridge basalts (MORB) contaminated by either subduction or crustal components. CaO/Al2O3 and (Gd/Yb)N ratios >1 and positive Nb anomalies in ultramafic rocks indicate a mantle plume source contaminated by metasomatized subduction mantle lithosphere. These data also show that the mafic‐ultramafic metavolcanic rocks derive from magma of basaltic and basaltic andesite compositions, with a tholeiitic to calc‐alkaline tendency characteristic of the upper mantle. These data reveal that gneisses derive from granite and diorite subalkaline, peraluminous, and ferroan to magnesian compatible with Cordilleran magmas and island arcs with polygenetic crustal signatures. All samples of mafic granulites, garnet‐amphibolite, metabasites, and ultramafic granulites in the Nyong Complex reveal no residual garnet and predominantly show ca. 4% partial melting of an amphibole‐spinel‐peridotite source in the Dy/Yb versus La/Yb. Otherwise, hornblendites of Toko‐Nlokeng display ca. 5% partial melting of an amphibole‐garnet‐peridotite source, average (Gd/Yb)CN ratio of the hornblendites is (Gd/Yb)CN = 7.56 > 2. This result suggests these hornblendites do not have the same magmatic source as other mafic‐ultramafic rocks of the Nyong Complex. Mafic‐ultramafic host rock protoliths are classified as E‐MORB, P‐MORB, and G‐MORB (hornblendites) compositional types and arc, back‐arc ‘B’ in subduction unrelated, rifted margin setting with minor crustal contamination. The tholeiitic to calc‐alkaline and peraluminous affinity of these rocks indicate a mature arc and thickened crust during the Eburnean Trans‐Amazonian orogenic belts of the Congo Craton. This study shows that the host rocks were emplaced in a convergent tectonic setting and affected by a syn‐collisional episode where melts were derived from the partial melting of thick basaltic crust into amphibolite–eclogite facies in the subduction zone. The geochemical signatures of the mafic‐ultramafic rocks support the tectonic accretionary of the Palaeoproterozoic plume arc in Nyong Complex. Furthermore, the resulting hydrothermal alteration process was accompanied by an increase in Fe; particularly under conditions dominated by seawater, the origin and source of iron and silica in the Toko‐Nlokeng deposit.