Depositional age and tectonic environment of the Gouap banded iron formations from the Nyong group, SW Cameroon: Insights from isotopic, geochemical and geochronological studies of drillcore samples

Abstract

The discovery of the Gouap banded iron formations (BIFs)-hosted iron mineralization in the northwestern of the Nyong Group (Ntem Complex) in southwestern Cameroon provides unique insights into the geology of this region. In this contribution, we firstly report detailed study of geochemistry, isotopic and geochronology of well preserved samples of the Gouap BIFs collected from diamond drillcores. The Gouap BIFs consist mainly of amphibole BIFs and amphibole–pyrite BIFs characterized by dominant Fe2O3 ​+ ​SiO2 contents and variable contents of CaO, MgO and SO3, consistent with the presence of amphibole, chlorite, epidote and pyrite, formed during amphibolite facies metamorphism and overprinted hydrothermal event. The amphibole–pyrite BIFs are typically enriched in trace and rare earth elements (REE) compared to the amphibole BIFs, suggesting the influence of detrital materials as well as secondary hydrothermal alteration. The Post Archean Australian Shale (PAAS)-normalized REE–Y profiles of the Gouap BIFs display positive La, Eu anomalies, weak negative Ce anomalies, indicating a mixture of low-temperature hydrothermal fluids and relatively oxic conditions probably under relative shallow seawater.We present here the first isotopic data of BIFs within the Ntem Complex. The δ30SiNBS28 values of the quartz from the Gouap BIFs vary from −1.5‰ to −0.3‰ and from −0.8‰ to −0.9‰ for the amphibole BIFs and amphibole–pyrite BIFs, respectively. The quartz has δ18OV-SMOW values of 6.8‰–9.5‰ (amphibole BIFs) and 9.2‰–10.6‰ (amphibole–pyrite BIFs). The magnetite from the Gouap BIFs shows δ18O values ranging from −3.5‰ to −1.8‰ and from −3‰ to −1.7‰ for the amphibole BIFs and amphibole–pyrite BIFs, respectively. Moreover, the pyrite grains in the amphibole–pyrite BIFs display δ34S values of 1.1‰–1.8‰. All isotopic data of the Gouap BIFs confirm that they might have precipitated from low-temperature hydrothermal fluids with detrital input distant from the volcanic activity. According to their geochemical and isotopic characteristics, we propose that the Gouap BIFs belong to the Superior type.In situ U–Pb zircon dating of BIFs was conducted to assess the BIF depositional age based on strong evidence of zircon in thin section. The Gouap BIFs were probably deposited at 2422 ​± ​50 ​Ma in a region where sediments extended from continental shelf to deep-water environments along craton margins like the Cauê Formation of the Minas Supergroup, Brazil. The studied BIFs have experienced regional hydrothermal activity and metamorphism at 2089 ​± ​8.3 ​Ma during the Eburnean–Transamazonian orogeny. These findings suggest a physical continuity between the protocratonic masses of both São Francisco and Congo continents in the Rhyacian Period.