Tectonic setting of the Bipindi greenstone belt, northwest Congo craton, Cameroon: Implications on BIF deposition

Highlights

• The Bipindi greenstone belt lies to the NW edge of the Congo craton in Cameroon.

• Metavolcano-sedimentary rocks are associated with the Bipindi BIFs.

• Metavolcanics derived from tholeiitic to transitional and calc-alkaline basalts.

• Associated metasediments derived from mafic and intermediate igneous rocks.

• Deposition in a back-arc environment is suggested for the Bipindi BIFs.

Abstract

The banded iron formations (BIFs) in the Bipindi greenstone belt (GB) are hosted within metavolcano-sedimentary sequence of the Nyong Group at the northwestern edge of the Congo Craton (Ntem Complex) in southern Cameroon. New data on the BIFs and its associated rocks allow us to discuss the tectonic setting of this greenstone belt in a regional perspective. Metavolcanic rocks consist of epidosites, mafic granulites, and garnet-pyroxene gneisses, whereas the metasedimentary rocks comprise chlorite-epidote schists. These rocks have been metamorphosed under greenschist-to granulite-facies conditions. The geochemical features of the metavolcanic rocks such as the trace element ratios and rare earth elements (REE) patterns suggest that their protolith corresponded to tholeiitic to transitional and calc-alkaline basalts. The associated metasedimentary rocks are characterized by low La/Sc and Th/Sc discrimination ratios, and REE abundances, indicating that they were derived from mafic and intermediate igneous rocks. Similar protoliths were recently described for metamorphic rocks interbedded with BIFs at the Nyong Group, which were formed in a shallow basin near the continent. The Bipindi BIFs are further characterized by both weak (1.31–1.34) and slightly high (2.00–2.51) positive (Eu/Eu)_SN ratio, that are typical of Superior- and Algoma-type BIFs respectively. However, the geochemical features of the host rocks (e.g., REE patterns, Th/Nb, La/Yb, and La/Sc ratios) suggest that the Bipindi BIFs were deposited at an extensional geodynamic setting likely related to a back-arc, similar to the depositional environment for many Algoma-type BIFs. Archean-Paleoproterozoic arc setting is widely distributed across the Congo Caton and the West African Craton, suggesting integration of the tectonic evolution of the Bipindi GB into a continental-scale paleogeography.

Introduction

Iron ore deposits are widely distributed in most of the cratonic areas in the world, where they are commonly represented by iron formations (IFs). Based on their textural features, IFs can be divided into banded iron formations (BIFs) and granular iron formations (GIFs) (Trendall, 2002). Both IFs have been extensively investigated owing to their economic and scientific importance. The BIFs are chemical sedimentary rocks that precipitated throughout the Archean to Paleoproterozoic with a peak at 2.7–2.5 Ga with fewer examples younger than ~1.85 Ga (Bekker et al., 2010). These rocks are characterized by alternating silica-rich and iron-rich layers with variable thickness ranging from micro- (submillimeter- and millimeter-thick), meso- (centimeter-thick) to macro-scale (meter-thick) layering (Trendall and Blockley, 1970; Trendall, 2002; Konhausser et al., 2017). Most BIFs have been deformed and metamorphosed to various grades making it a difficult task to reconstruct their depositional settings (Klein, 2005). However, the study of rocks interbedded with BIFs has been widely applied to constrain their depositional environment as well as their geotectonic settings (e.g., Cen et al., 2012; Angerer et al., 2013; Wang et al., 2014; Silveira et al., 2015; Yang et al., 2016; Ghosh and Baidya, 2017; Soh Tamehe et al., 2018; Tong et al., 2019).

The Congo Craton (CC) is represented in southern Cameroon by the Ntem Complex, which encompasses the Neoarchean-Paleoproterozoic Nyong Group to the west and the Archean Ntem Group to the east (Fig. 1a; Maurizot et al., 1986; Lerouge et al., 2006; Chombong et al., 2017; Soh Tamehe et al., submitted). The Ntem Complex has been the topic of recent geological investigations regarding their BIF-hosted iron deposits (Fig. 1a; Suh et al., 2008, 2009; Anderson et al., 2014; Ganno et al., 2015, 2016, 2017, 2018; Ilouga et al., 2013, 2017; Teutsong et al., 2017; Ndime Ekah et al., 2018, 2019; Soh Tamehe et al., 2018, 2019). Although the BIFs are less abundant in the Ntem Complex, they are major components of the greenstone belts (GB) in this region (e.g., the Bipindi GB, the Bikoula-Meyomessi GB; Fig. 1a), representing important targets for iron mineral exploration in Cameroon (Suh et al., 2008; Ganno et al., 2016, 2017; and references therein). For example, the Bipindi GB has been intensively explored and several iron ore prospects (e.g., Kouambo, Gouap, Kpwa-Atog Boga, and Zambi) were discovered (Ganno et al., 2016, 2017; Soh et al., 2019; http://mississaugamining.com). In contrast, fewer studies have focused on the rocks associated with the Ntem Complex BIFs (e.g., Chombong and Suh, 2013; Chombong et al., 2013; Ganno et al., 2017; Teutsong et al., 2017; Ndime Ekah et al., 2018; Soh Tamehe et al., 2018, 2019). Due to poor exposures influenced by thick soil and heavy vegetation, the relationship between these BIFs and their surrounding rocks is not well documented. Moreover, the geotectonic setting in which the Ntem Complex BIFs formed is poorly constrained and little is known regarding the geochemistry of both BIFs and their surrounding rocks.

In this regard, this work presents detailed petrographic and geochemical features of the BIFs and associated rocks in the Bipindi greenstone belt, which is part of the Nyong Group (Fig. 1b). This dataset enables discussions towards the geotectonic setting of the Bipindi GB, on which these rocks have formed. The combination of our new geochemical data and those recently published (Ganno et al., 2015, 2017; Chombong et al., 2017; Loose and Schenk, 2018; Soh Tamehe et al., 2018; Bouyo Houketchang et al., 2019; Fuanya et al., 2019a; Nga Essomba et al., 2020) enables interpretations on the geodynamic setting of the northwest edge of the CC during the Archean-Paleaoproterozoic. Considering that other GB have been recently documented within the CC at the Democratic Republic of Congo (D.R.C) (Bird, 2016; De Putter et al., 2018; François et al., 2018) and Western African Craton (WAC) in Mauritania (Taylor et al., 2016), the present paper provide key information in order to build an integrated regional model for the CC and its surroundings.