Host rock geochemistry, texture and chemical composition of magnetite in iron ore in the Neoarchaean Nyong unit in southern Cameroon

Abstract

A combination of petrography, whole-rock geochemistry, geochronology and compositional variation in magnetite is employed in this study to elucidate the nature and origin of enigmatic magnetite mineralisation hosted within gneissic rocks in the Nyong Unit in southern Cameroon. The mineralisation occurs as magnetite-bearing calc-silicate gneisses. The host rock mineral assemblage comprises quartz-plagioclase-biotite-amphibole-chloriteclinopyroxene-garnet that provides evidence of medium-grade metamorphism and retrograde alteration. Textural and chemical analyses of the magnetite grains show variable textural and trace element chemical characteristics attributed to metamorphic-hydrothermal overprint and weathering. Magnetite occurs as disseminations and stringers commonly intergrown with amphiboles. It is also observed to show vermiforms wrapped around quartz and clinopyroxenes within a biotite-chlorite-plagioclase groundmass. Massive-granoblastic magnetite is rare and mainly observed within vein-like domains. On backscattered secondary electron images the magnetite grains are anhedral, with minor spinel exsolution lamellae. Electron microprobe analysis on magnetite suggests both a hydrothermal skarn and banded iron formation (BIF) affinity. The lack of negative Ce anomalies excludes a Proterozoic BIF setting, but it is in agreement with Archaean BIF. Sensitive high resolution ion microprobe U-Pb isotope data on zircon in the magnetite gneiss define an identical Wetherill concordia and Tera-Wasserburg Neoarchaean age of 2699 ± 7 Ma (1σ; MSWD (mean square weighted deviation) = 1.3; n = 13), and Pan-African disturbance at about 500 ± 200 Ma. The Neoarchaean age is in accordance with the known onset of BIF deposition at the northern edge of the Congo Craton and therefore constrains the maximum age of formation of the Nyong magnetite gneisses.

The petrostructural characteristics and 207Pb/206Pb zircon data from the Ngomedzap-Akongo area (Nyong complex, SW-Cameroon)

“…Our new NyC Pb-Pb zircon ages on metagranodiorite and metasyenites that range between 2816-2764 Ma are consistent with those obtained from Edea gneiss with ages between 3174-2519 Ma (Pb/Pb zircon, Lerouge et al 2006). They are older to Nyong magnetite bearing quartzite of 2776 ± 34 Ma (SHRIMP U-Pb zircon, Lerouge et al 2006) and 2699 ± 7 Ma (SHRIMP U-Pb zircon; Chombong et al 2017). They are reliable to 3245-2526 Ma in Ndjole sandstone in the Paleoproterozoic Franceville and Ogoué complexes in Gabon (Pb/Pb zircon, Feybesse et al 1998).…”

New U–Pb zircon ages of Nyong Complex meta‐plutonites: Implications for the Eburnean/Trans‐Amazonian Orogeny in southwestern Cameroon (Central Africa)

“…For example, neodymium‐isotope data with T DM ages of ∼3,000 Ma corroborate the Archean origin of most protoliths (Pénaye et al, 2004). Petrographically, the Nyong Complex rocks comprise four main groups: (a) Meta‐volcanic sedimentary rocks, probably remnants of greenstone belts, with orthopyroxene‐bearing gneiss, garnet‐rich amphibole‐pyroxenite and related gneiss, banded iron formation, and mafic–ultramafic meta‐volcanic rock; (b) migmatitic grey gneiss of tonalite–trondhjemite–granodiorite (TTG) composition; (c) syn‐ to late‐tectonic (in respect to the Eburnean/Trans‐Amazonian Orogeny) charnockite, (grano)dioritic augengneiss, granite, and syenite; (d) post‐tectonic meta‐dolerite (e.g., Chombong et al, 2017; Maurizot, Abessolo, Feybesse, Johan, & Lecompte, 1986; Owona et al, 2012; Pénaye et al, 2004). Structurally, the Nyong Complex is dominated by a NNE strike and shows a polyphase tectonic evolution.…”

Petrology of banded iron formation-related country rocks at the northern limit of the Mbalam iron ore district, southeastern Cameroon

“…According to the Sm/Yb-Eu/Sm and Y/Ho-Eu/Sm diagrams extremely small amounts of submarine high-T hydrothermal fluids (<0.1%) were enough to cause positive Eu anomalies in the Njweng BIF. Therefore, as observed in BIFs from Dales Gorge; Agus Claras, Elom, Witwatersand, Krivoy Rog, Bikoula, Mbalam, and Nkout west (Spier et al, 2007;Ilouga et al, 2013;Viehmann et al, 2015a,b;Ganno et al, 2015Ganno et al, , 2017Teutsong et al, 2017;Chombong et al, 2017;Ndime et al, 2019), it can be deduced that relatively small inputs of high-T hydrothermal fluids may have been sufficient to account for REE-Y distribution in the BIF. All the samples in this study show a positive values for Eu/Eu* (2.65-2.71), which provide an indication of the relative influence of lowtemperature hydrothermal fluids and thus distal BIFs deposition relative to fluid vents (Ganno et al, 2017).…”

“…All the samples in this study show a positive values for Eu/Eu* (2.65-2.71), which provide an indication of the relative influence of lowtemperature hydrothermal fluids and thus distal BIFs deposition relative to fluid vents (Ganno et al, 2017). (Spier et al, 2007;Ilouga et al, 2013;Viehmann et al, 2015a,b;Ganno et al, 2015a;Teutsong et al, 2017;Chombong et al, 2017;Ndime et al, 2019). It is worth noting that high contents of Zr and Nd is significant of detrital input.…”

“…This part of the country is at the northern edge of the Congo Craton (CC) where a new iron ore exploration belt has opened (Fig. 1) and has been the subject of recent publications (Ilouga et al, 2013;Teutsong et al, 2017;Ganno et al, 2015Ganno et al, , 2017Chombong et al, 2017Ndime et al, 2018Ndime et al, , 2019Ndema et al, 2020;Bonda et al, 2022).…”