• The emphasis on the Congolese BIFs’ grades and mineral resources underlines their economic attractiveness.

• The lithological associations, as well as the very high (Eu/Eu*)SN ratios> 1.8 shown by the BIFs, suggest that they are related to Algoma-type BIFs.

• The Congolese BIFs were formed in extensional back-arc basins with redox heterogeneity, varying from suboxic to anoxic and from oxic to anoxic conditions.

• They were formed through hydrothermal vents in the seawater, with relatively low proportions of detrital inputs derived from igneous sources through continental weathering.

Iron ranks as one of the predominant elements present in the Earth's crust, with a Clarke value of 5.76 wt% Fe (Clarke and Washington, 1924). However, due to its propensity to readily rust, it is rarely encountered in its native state. Instead, it is predominantly found as an ore, often in combination with other elements such as oxygen or sulfur (Kamble et al., 2013). It is mostly extracted from deposits, which can be classified into four major groups according to their origin: iron deposits of a magmatic nature, iron deposits associated to geological structures, iron deposits originating from sedimentary processes and metamorphism-related iron deposits (Dill, 2010). The last group is that of Banded Iron Formations (BIFs) (Dill, 2010). These rocks, whose formation took place during the Precambrian period with a peak deposition ca. 2.5 Ga (Klein, 2005; Bekker et al., 2010) provide more than 90% of the world’s mined iron ore (Isley, 1995). The traditional classification of BIFs, based on the tectonic environments and the associated rocks, distinguishes Algoma, Superior and Rapitan types (Hagemann et al., 2016). While their genesis and evolution are still subjects of controversy and remain not entirely comprehended, according to Sośnicka et al., (2015), scientific research on BIFs has experienced significant growth on a worldwide scale in recent times.

In Congo, traces of ancient mining activities related to iron before French colonization have been attested in Zanaga (The Chaillu Massif). Local populations have long sought iron for their own use by digging wells and subsequently digging deeper wells (Meloux et al., 1983). During the colonial period, preliminary exploration in the northwestern region of Congo in 1938 led to the discovery of iron. Initially, only a small-scale study was conducted on this iron ore, but larger investigations were carried out later (Meloux et al., 1983). In modern times, numerous documents, including published scientific literatures and unpublished reports, attest to the existence of several iron deposits and occurrences in Congo. The abundance of iron ore is attributed to the presence of an Archean craton in Central Africa, known as the Congo craton. This craton plays a vital role in mining, making the West-Central Africa region an important metalliferous province with significant deposits located in Cameroon, Congo and Gabon (De Waele et al., 2015; Ganno et al., 2015, 2017; Ilouga et al., 2017; Nkoumbou et al., 2017; Kondja et al., 2017; Soh Tamehe et al., 2018, 2019; Ndime et al., 2019; Teutsong et al., 2021; Gourcerol et al., 2022) .

The aim of this paper is to provide an overview on BIFs in the Republic of Congo by conducting a comprehensive census of all known iron deposits and occurrences. This includes their locations, geological contexts, grades, country rock associations, economic resources, geochemistry and depositional settings. Considering the limited literature available on Congolese BIFs, this review presents a remarkable compilation of data from both scientific and gray literature sources (annual, project and technical reports). These sources provide valuable information regarding tonnages, grades, spatio-temporal distribution, resources, occurrences, geochemistry and depositional conditions.

The West-Central Africa region is currently one of the richest areas in the world in terms of iron ore (Fig. 1). This natural advantage, with promising economic benefits, is attributed to its geological landscape, primarily characterized by the Ntem-Chaillu block, which forms the northwestern part of the Congo craton (Fig. 2) (Caen-Vachette et al., 1988; Feybesse et al., 1998). This extensive region spans across multiple countries, including Cameroon, Congo and Gabon, and hosts numerous world-class iron ore deposits. In Cameroon, some of these deposits include Kelle Bidjoka, Kouambo, Kpwa-Atog Boga, Mamelles, Ngovayang, Sanaga, Zambi, Bikoula, Elom, Mbalam, Meyomessi, Bibole, Anyouzok and Nkout, all located in the Ntem Complex (Nforba et al., 2011; Ilouga et al., 2013, 2017; Ganno et al., 2015, 2017; Ndong Bidzang et al., 2016; Teutsong et al., 2017, 2021; Soh Tamehe et al., 2018, 2019, 2021; Ndime et al., 2018, 2019; Moudioh et al., 2020; Nzepang Tankwa et al., 2021; Djoukouo Soh et al., 2021; Swiffa Fajong et al., 2022).

Figure 1. Map showing the distribution of iron deposits and occurrences of West-Central Africa (source: De Waele et al., 2017; Soh Tamehe et al., 2019)

Figure 2. Simplified geological map of West-Central Africa showing geological domains (source: Thiéblemont et al., 2018)

In recent years, iron mineralization in Cameroon has been the subject of extensive research aimed at constraining its genesis. These studies have involved petrographic descriptions and geochemical analyzes of BIFs and associated rocks, as well as structural, geochronological, mineralogical and lithostratigraphic investigations (Ilouga et al., 2013; Ganno et al., 2015, 2016, 2017, 2018; Teutsong et al., 2017; Soh Tamehe et al., 2018, 2019, 2021; Ndime et al., 2018; Moudioh et al., 2020; Nzepang Tankwa et al., 2021; Djoukouo Soh et al., 2021; Swiffa Fajong et al., 2022). BIFs in Cameroon are predominantly of oxide-facies, characterized by alternating layers of hematite and/or magnetite-rich layers with quartz-rich layers (Ganno et al., 2015; Soh Tamehe et al., 2021). These BIFs are found hosted within a diverse range of rock types, encompassing metasedimentary rocks (gneisses, quartzites and schists), greenstones (amphibolites, epidotites and serpentinites) and intrusive rocks (syenites), as noted by Soh Tamehe et al., 2019. The hematite-rich deposits are located in Mbalam and Nkout, while the magnetite-rich deposits are found in various sites such as Elom, Zambi, Kouambo, Bikoula, Kpwa–Atog Boga and Meyomessi (Fig. 1) (Soh Tamehe et al., 2019). The Ntem Complex BIFs were deposited on a passive margin, similar to that of Superior-type (Ganno et al., 2016, 2017; Soh Tamehe et al., 2018), except for the Bikoula and Kelle Bidjoka BIFs, which are closer to the Algoma-type (Teutsong et al., 2017; Nzepang Tankwa et al., 2021). According to Nzepang Tankwa et al., (2021), the Ntem Complex BIFs were formed in a submarine volcanic arc setting from Mesoarchean (ca. 2883 Ma) to early Paleoproterozoic (ca. 2423 Ma) and experienced high-grade granulite facies metamorphism during the Eburnean/Transamazonian orogeny (ca. 2050 Ma). In Gabon, the Bélinga iron ore deposit is classified into four major groups: blue and yellow ores, hematitic phyllites, enriched itabirites and canga ores (Kondja et al., 2017). These deposits are associated with various rock types, including meta-ultrabasites, gneisses, quartzites, mica schists, as well as greenschists and amphibolites (Kondja et al., 2021). These iron ores display a paragenesis of minerals formed under high-pressure metamorphic conditions, ranging from greenschist to granulite-BIF facies (Kondja et al., 2021). Other Gabonese BIFs include the Minkebe, Boka-Boka, Bélinga-Sud, Batouala, Mebaga, Mekambo-Est, Kango, Kango-North, Ngama, Baniaka, Koumbi-Magnima, Lobi-Lobi, Méla, Minvoul and Tchibanga deposits (Thiéblemont et al., 2009; Frost-Killian et al., 2016). In the Tchibanga BIFs, mineralization primarily consists of fine-grained hematite and goethite, with an average grade of 45.2 % Fe (Frost-Killian et al., 2016).

3.1. Regional Geological Setting of West-Central Africa

Similar to many other Archean cratons across the world, the Congo craton is essentially made up of a granito-gneissic basement (TTG suites) and greenstone belts (Thiéblemont et al., 2018). The West-Central Africa craton is composed of three main blocks: the Ntem-Chaillu block in the western region, the NE-Congo block in the northeastern area and the Kasai block in the southern part (Fig. 2). Surrounding the craton are Proterozoic orogenic belts, including the Western Central Africa belt (Eburnean), the Usagaran belt, the Ubendian belt, the Ruwenzori-Toro belt, Kibarian belt, the Lufilian belt and the W-Congo belt, all associated with continental extension (Fig. 2) (Thiéblemont et al., 2018). In this overview, we focus on the Ntem-Chaillu block, which is characterized by rock exposures in the Chaillu Massif (southwestern Congo) and the Ivindo Basement Complex (northwestern Congo) (Figs. 3 and 4). The Ntem-Chaillu block is also well developed in Cameroon and Gabon (Figs. 1 and 2) (Toteu et al., 1987; Ledru et al., 1989; Nédélec et al., 1990). Its geological evolution includes a Mesoarchean granite-greenstone-BIF and high-grade terrains dating between ca. 3.2 and 2.9 Ga, as well as subsequent Neoarchean granitoid intrusions associated with a tectono-metamorphism event (ca. 2.8-2.7 Ga). These are followed by Late Archean granite activity (Feybesse et al., 1998; de Wit and Linol, 2015).

Figure 3. Simplified geological map of Congo (source: Desthieux et al., 1993)

Figure 4. Map of Iron ore location in the Republic of the Congo (source: Waratah Gold Ltd, 2010)

3.2. Geological Setting of the Republic of Congo

The geological formations of Congo mainly belong to the Precambrian and Mesozoic to Cenozoic ages (Desthieux et al., 1993). These structural units, from the southwest to the far north, include the Coastal Basin, the Mayombe Chain, the Niari Basin, the Chaillu Massif, the Batéké Plateaus, the Congolese Basin, the Ivindo Basement Complex and the Sembé-Ouesso Basin (Fig. 3) (Desthieux et al., 1993).

The Chaillu Massif and the Ivindo Basement Complex are the oldest terrains in the country and form part of the Congo craton (Figs. 2 and 3). These terrains consist of a granito-gneissic basement and a supracrustal member, typical components of Archean domains (Thiéblemont et al., 2018). The supracrustal member, known as the greenstone belt, comprises a range of volcano-sedimentary rocks, including ultrabasic and basic rocks (such as picrites, amphibolites and pyroxenites), metasediments (mica schists, chlorite schists and chlorite-sericite-quartz schists) and BIFs (Meloux et al., 1983; Kessi, 1992). The granito-gneissic basement can be subdivided into two sets (Thiéblemont et al., 2009, 2018): · A granitic basement consisting of grey granitoids of tonalitic and granodioritic composition and pink granites mainly of monzogranitic nature; · A gneisso-migmatitic unit cut by mesocratic to leucocratic granites. Geochronological results (²⁰⁷Pb/²⁰⁶Pb) on the mono-zircons of the Chaillu Massif indicate an Archean age (2.8-2.5 Ga) (Kessi, 1992). In terms of structural features, the Chaillu Massif exhibits four major fault directions oriented N-S, NW-SE, NE-SW and E-W, associated with two deformation phases, while the Ivindo Basement Complex shows fault directions oriented subequatorial, N-S, NNE-SSW and NW-SE, caused by three distinct deformation episodes (Desthieux et al., 1993; Loemba et al., 2022; Mavoungou et al., 2023). BIFs in Congo are essentially restricted to these two structural units (Martini et al., 1995).

The Mayombe Chain is the Congolese component of the Araçuaí-West Congo system, which was sculpted during the Brazilian/Pan-African orogeny of the late Paleoproterozoic and Neoproterozoic (Alkmim et al., 2006). It is composed of granitic intrusions, amphibole dykes, quartzites, gneisses, schists, conglomerates with dykes and mafic veins of Paleoproterozoic age and ignimbrite, tuff, agglomerate, magmatic intrusions, sandstones, pyritic quartzites, greenish schists, argillites and carbonate formations of Neoproterozoic age (Callec et al., 2015; Fullgraf et al., 2015; Le Bayon et al., 2015; Bouenitela, 2019).

The Niari and Sembé-Ouesso Basins are of Neoproterozoic age. The Niari Basin is the foreland of the Mayombe Chain. Both basins are composed of diamictites, schisto-limestones, conglomerates, sandstones, schisto-quartzites, quartzite sandstones, black pelites, ampelites and carbonate rocks (Dadet, 1969; Meloux et al., 1983; Desthieux et al., 1993; le Bayon et al., 2015).

The Coastal Basin, the Congolese Basin and the Batéké Plateaus constitute the Phanerozoic cover. They are primarily composed of salt series, sandstones, marls, carbonates, clays, sands and argillites (Dadet, 1969; Meloux et al., 1983). These formations are the most widespread and cover 70% of the Congolese territory.

BIFs are located in the northwestern and southwestern parts of Congo, within the greenstone belts of the Chaillu Massif and the Ivindo Complex Basement (Figs. 1 and 4). These deposits consist of magnetite-hematite itabirite (Martini et al., 1995). The iron deposits and occurrences in Congo are listed below: Mayoko-Lékoumou, Mayoko-Moussondji, Zanaga, Bikélélé, Avima, Badondo, Nabeba, Letioukbala, Youkou, Okanabora, Elogo, Odia, Obélé, Kékélé (Figs. 1 and 4) (Gourcerol et al., 2022). Table 1 summarizes the geological and economic characteristics of the aforementioned deposits and occurrences.

Table 1

Geological and economic parameters of Iron deposits and occurrences in Congo.

Deposits/Occurrences	Host rock/Surrounding outcrops	Resources/Grades	Types	References
Mayoko-Lékoumou	Biotite gneiss, amphibolite, talc schist and greenish actinolite-chlorite schist	795Mt @ 36%Fe	Algoma	(Meloux et al., 1983; Exxaro, 2015; Gourcerol et al., 2022)
Mayoko-Moussondji	Amphibolite, granitoid and pegmatite	917Mt @ 31.4%Fe	Algoma	(Equatorial Resources Ltd, 2013, 2014; De Waele et al., 2017)
Zanaga	Amphibolite, ultramafic rock	6.8 Gt @ 30 to 35%Fe	Algoma	(Meloux et al., 1983; De Waele et al., 2017; Gourcerol et al., 2022)
Bikélélé	Amphibolite, quartzite, ultramafic rock, and greenish actinolite-chlorite	-	Algoma	(Samba, 2013; Gourcerol et al., 2022)
Avima	Schist, amphibolite	580 Mt @ 60%Fe	Algoma	(De Waele et al., 2017; Gourcerol et al., 2022)
Badondo	Schist, amphibolite	370 and 620 Mt @ 58 to 67%Fe	Algoma	(Equatorial Resources Ltd, 2015a, 2015b, 2016; Barry, 2018; Gourcerol et al., 2022)
Nabeba	Amphibolite, greenish actinolite- chlorite schist	1.714 Mt @ 34.1%Fe	Algoma	(Sundance Resources Ltd, 2020; Gatsé et al., 2021; Gourcerol et al., 2022)
Letioukbala	Amphibolite, greenish actinolite-chlorite schist	-	Algoma	(Gourcerol et al., 2022)
Youkou	Amphibolite, greenish actinolite-chlorite schist	1 to 2 Bt @ 60%Fe	Algoma	(Waratah Gold Ltd, 2011; Gourcerol et al., 2022)
Elogo	ultramafic rock, amphibolite, talc schist, greenish actinolite-chlorite schist.	-	Algoma	(Gourcerol et al., 2022; Makamba et al., 2023)
Okanabora	amphibolite, mesocratic gneiss, greenish actinolite-chlorite schist	-	Algoma	(Waratah Gold Ltd, 2011; Gourcerol et al., 2022)
Obélé, Odia and Kékélé	Amphibolite, ultramafic rocks and greenish actinolite-chlorite schist	-	Algoma	(Gourcerol et al., 2022)

4.1. Mayoko-Lékoumou

This deposit is located in the Chaillu Massif and consists of ferruginous quartzites associated with amphiboles, biotite gneisses, talc schists and greenish actinolite-chlorite schists (Meloux et al., 1983; Gourcerol et al., 2022). Four types of iron mineralization are encountered: 1) fresh ferruginous quartzites (fresh BIFs), which constitute the primary mineralization; 2) weathered ferruginous quartzites also known as enriched BIFs; 3) supergene hematite; and 4) ferruginous cuirass (Exxaro Resources Ltd, 2015). The deposit encompasses a vast tenement covering 1,000 km² with resources of 795 Mt and a grade of 36% Fe (Exxaro, 2015). Production from this deposit was launched in 2019 by Sapro Mayoko SA.

4.2. Mayoko-Moussondji

It is located in the Chaillu Massif, near the previous deposit. Mineralization is associated with amphibolites, granitoids and pegmatites (Equatorial Resources Ltd, 2013; De Waele et al., 2017). The BIFs are overlain by friable hematite, which in turn is covered by colluvial hematite that extends laterally from the BIFs (Equatorial Resources Ltd, 2013). The total estimated iron resources reached a peak of 917 Mt at 31.4% Fe (Equatorial Resources Ltd, 2013, 2014).

4.3. Zanaga

The Zanaga ferruginous quartzites are located in the Chaillu Massif. This deposit is one of the largest in Congo, and the ferruginous quartzites are found associated with amphibolites and ultrabasic rocks (Dadet, 1969; Meloux et al., 1983; Gourcerol et al., 2022). The ferruginous quartzites cover an area of approximately 25 km² (Meloux et al., 1983). This deposit contains enormous primary BIF reserves of 2.5 Gt at 34% Fe, as well as mineral resources of 6.8 Gt at 30% to 35% Fe (Meloux et al., 1983; De Waele et al., 2017).

4.4. Bikélélé

Located in the center of the Chaillu Massif, the Bikélélé BIFs are composed of silicate-rich chert layers of centimeter to millimeter thickness, alternating with oxide-rich bands bands (Gourcerol et al., 2022). The associated rock formations consist mainly of amphibolites, quartzites, ultramafic rocks, greenschists and gneisses (Samba, 2013; Gourcerol et al., 2022).

4.5. Avima

This deposit is located in the Ivindo Basement Complex and encompasses a mineralized ridge known as Mount Avima, which includes Avima West, Avima Center and Avima East (Meloux et al., 1983). In this deposit, mineralization occurs in various styles from west to east: platelet ores, hydrated ores and hard ores (Meloux et al., 1983). The deposit's estimated resources amount to 580 Mt with a grade of 60% Fe (De Waele et al., 2017). Mineralization is closely associated with schists and amphibolites (Gourcerol et al., 2022).

4.6. Badondo

The Badondo deposit is situated on Mount Badondo, which belongs to the Ivindo Basement Complex. Mount Badondo is a long ridge of ferruginous quartzite arranged in a circular arc (Meloux et al., 1983). Mineralization consists mainly of magnetite BIF, hematitized BIF, massive hematite and lateritized scree deposits (Fig. 5) (Cunningham and De Waele, 2012). The estimated global exploration target for the Badondo iron ore is between 370 and 620 Mt, with a grade of 58% to 70.2 % Fe (Cunningham and De Waele, 2012; , 2016; Barry, 2018). Like the Avima deposit, Badondo is associated with schists and amphibolites (Gourcerol et al., 2022).

Figure 5. Geological sketch map of the Badondo Iron deposit (source: Cunningham and De Waele, 2012)

4.7. Nabeba

This deposit is located in the Ivindo Basement Complex. Mineralization found in this area consists mainly of scree or exposed hydrated platelet formations, which can be weathered or unaltered itabirites (Meloux et al., 1983). The Nabeba BIFs exhibit two distinct facies: oxide and carbonate-oxide (Gatsé et al., 2021). The paragenesis of this deposit includes minerals such as magnetite, hematite, quartz, siderite and magnesite (Gatsé et al., 2021). In terms of rock types encountered in the Nabeba drill holes, itabirite and weathered white mica-chlorite schists have been found to be dominant (Longley et al., 2013). The inferred resources of this deposit amount to 1.714 Mt, with a grade of 34.1% Fe (Sundance Resources Ltd, 2020). The host rocks associated with the Nabeba BIFs are amphibolites and greenish actinolite-chlorite schists (Fig. 6) (Gatsé et al., 2021; Gourcerol et al., 2022).

Figure 6. Geological sketch map of the Nabeba Iron deposit (source: Gatsé et al., 2021; Longley et al., 2013)

4.8. Letioukbala

This mineralization is located in the Ivindo Basement Complex and was initially documented during a reconnaissance study on iron in northern Congo conducted by the French geological survey Bureau de Recherches Géologiques et Minières (BRGM) between 1965 and 1966 (Henry et al., 1965; Wissink, 1966). The area has limited occurrences and scree of valuable minerals. Mineralization in this zone consists of low-iron quartzites and itabirites (Meloux et al., 1983). The Letioukabala BIFs belong to the oxide facies and are associated with amphibolites and greenschists (Gourcerol et al., 2022).

4.9. Youkou

The Youkou deposit is situated in the Kellé-Mbomo region, part of the Ivindo Basement Complex, and shows similarities with the Mekambo-Est deposit in Gabon. Mineralization at Youkou primarily consists of hematite- and magnetite-rich itabirite (Waratah Gold Ltd, 2010). The initial exploration target for Youkou iron mineralization ranges from 1 to 2 Bt, with a grade of 60% Fe (Waratah Gold Ltd, 2011a). Surrounding the mineralization are found talcs, greenschists and amphibolites (Gourcerol et al., 2022).

4.10. Elogo

This deposit is situated within the Ivindo Basement Complex, in close proximity to the Nabeba iron deposit. It primarily consists of oxide-facies BIFs (Moutou, 2021; Gourcerol et al., 2022). They are associated with ultramafic rocks, amphibolites, talc schists and greenschists (Gourcerol et al., 2022; Makamba et al., 2023).

4.11. Okanabora

The Okanabora deposit is also situated in the Kellé-Mbomo area, close to the Youkou deposit. It has been identified as having the potential for high-grade itabirite iron ore (Waratah Gold Ltd, 2011b). The Okanabora BIFs consist of magnetite and hematite iron rich. The estimated resources range from 1 to 2 Bt at 60% Fe (Waratah Gold Ltd, 2011a). These BIFs are associated with various rock types such as amphibolites, gneisses and greenschists (Gourcerol et al., 2022).

4.12. Obélé, Odia and Kékélé

These mineralizations are located in the Kellé-Mbomo area. The Obélé, Odia and Kékélé iron mineralizations belong to the oxide-facies BIFs and are associated with amphibolites, ultrabasic rocks and greenschists (Gourcerol et al., 2022).

Several studies have been carried out on BIFs in Congo, providing valuable information on their geochemical characteristics, which were previously unknown. To gain insights into these characteristics, carefully selected samples from the aforementioned deposits and occurrences were analyzed using geochemical techniques. The aim was to identify and characterize the major, trace and rare earth elements present in BIFs (Gatsé et al., 2021; Gourcerol et al., 2022). In this section, the average compositions of a few deposits mentioned earlier, along with globally recognized deposit types such as Algoma, Rapitan and Superior have been reported (Tables 2 and 3). Such method has been already used to review the geochemical features of BIFs in China and India (Moon et al., 2017a; Mukhopadhyay, 2020). The chosen samples from the Congolese BIFs encompass various facies, including oxide and carbonate-oxide-facies BIFs. The samples primarily originate from deposits such as Avima, Bikélélé, Létioukbala, Nabeba, Odia, Youkou, Elogo, Okanabora and Zanaga (Gatsé et al., 2021; Gourcerol et al., 2022). To facilitate comparison and analysis, the data were normalized with respect to PAAS (Post-Archean Australian Shale) values (Taylor and McLennan, 1985). The PAAS-normalized diagram is one of the reference compositions used to illustrate the degree to which the composition of a sediment differs from the average composition of the continental crust (Rollinson and Pease, 2021). Normalization using PAAS-values also offers an opportunity to evaluate relative contributions between hydrothermal and continental sources (Mukhopadhyay, 2020).

Table 2

Average compositions of major oxides of some selected Congolese BIFs and worldwide deposit types.

Occurrences	Avima1 (n=6)	Letioukbala1 (n=5)	Elogo1 (n=2)	Youkou1 (n=2)	Okanabora1 (n=2)	Odia1 (n=3)	Bikélélé1 (n=3)	Zanaga1 (n=8)	Nabeba2 (n=10)	Algoma3	Superior3	Rapitan4
Elements (wt%)
SiO2	43.5	46.98	48.6	58.95	51.4	43.07	44.63	41.75	47.48	48.9	47.1	44.3
TiO2	0.02	0.01	0.005	0.01	0.01	0.01	0.02	0.02	0.007	0.12	0.04	0.27
Al2O3	0.5	0.19	0.12	0.18	0.3	0.29	0.89	0.63	0.15	3.7	1.5	3.18
Fe2O3	48.82	50.82	48.65	37.9	45.75	55.6	50.7	52.31	46.68	38.68	40.31	44.3
MnO	0.07	0.02	0.02	0.07	0.04	0.02	0.11	0.08	0.12	0.19	0.49	0.23
MgO	1	0.01	0.04	0.05	0.09	0.02	1.3	0.8	0.4	2	1.93	1.24
CaO	0.79	0.02	0.02	0.02	0.04	0.03	1.58	1.35	0.04	1.87	2.24	1.79
Na2O	0.02	0.007	0.01	0.01	0.008	0.01	0.05	0.07	0.005	0.43	0.13	0.28
K2O	0.07	0.01	0.008	0.01	0.01	0.03	0.03	0.07	0.009	0.62	0.2	0.45
P2O5	0.98	0.11	0.09	0.14	0.04	0.1	0.15	0.08	0.04	0.23	0.08	0.35

¹ Gourcerol et al., 2022, ² Gatsé et al., 2021, ³ Gross and McLeod, 1980, ⁴ Yeo, 1986.

Table 3

Average compositions of trace and rare earth elements of some selected Congolese BIFs and worldwide deposit types.

Occurrences	Avima1 (n=6)	Letioukbala1 (n=5)	Elogo1 (n=2)	Youkou1 (n=2)	Okanabora1 (n=2)	Odia1 (n=3)	Bikélélé1 (n=3)	Zanaga1 (n=8)	Nabeba2 (n=10)	Algoma3	Superior4	Rapitan5
Elements (ppm)
Sc	0.92	0.4	0.5	0.75	0.5	0.67	1	1.94	0.43	8	18	2.84
V	11	13	7	19.5	11	60.67	9	8.63	4.2	109	42	-
Cr	20	34	15	35	35	23.33	16.67	13.75	29.6	118	112	25
Co	2.17	2.16	0.5	4	4.5	5.6	2.17	1.69	2.14	41	28	2.06
Ni	2.33	12.9	4.5	14	18.75	5.5	1.17	1.13	7.19	103	37	10.33
Cu	9	12.2	8.5	23	37.5	15.33	7	12.88	1.51	149	14	59
Zn	11.67	17.2	59.5	43	15	17	23.33	14.13	34.6	330	40	44
Rb	4.63	0.52	0.45	0.55	0.5	0.87	2.23	6.99	0.74	-	-	16.67
Sr	3.58	4.92	0.5	1.6	1.9	8.17	8.77	9.48	3.45	116	37	76.33
Y	5.88	5.46	5.1	5.25	3.6	2.53	4.03	4.5	4.66	54	47	0.22
Zr	11.83	3.8	4.5	5.5	6	7.67	13	9.25	2.29	98	81	10
Nb	0.58	21	0.2	0.25	0.35	0.57	1.3	0.59	0.26	-	-	6
Ba	13.37	17.2	2.9	7.45	12.25	10.13	5.07	15.45	25.07	-	-	127
Hf	0.28	0.16	0.1	0.1	0.15	0.23	0.4	0.25	0.12	-	-	0.32
Th	0.26	0.32	0.03	0.08	0.35	0.46	0.5	0.27	0.06	-	-	0.31
U	0.52	0.5	0.03	0.57	0.09	0.42	0.66	0.17	0.03	-	-	0.1
La	2.5	2.74	2.75	2.35	3.7	1.43	2.87	2.64	1.63	1.5	12.59	5.98
Ce	4.38	4.36	2.75	3.35	4.35	2.63	6.43	3.33	3.64	2.84	17.06	14.15
Pr	0.54	0.51	0.54	0.52	0.8	0.3	0.65	0.5	0.4	0.37	3.12	1.8
Nd	2.4	2.04	2.1	2.4	3.05	1.27	2.7	2.1	1.73	1.52	12.26	7.67
Sm	0.54	0.53	0.53	0.47	0.55	0.26	0.55	0.4	0.43	0.34	2.44	1.65
Eu	0.31	0.24	0.28	0.23	0.19	0.14	0.21	0.22	0.23	0.31	0.82	0.41
Gd	0.67	0.54	0.48	0.55	0.55	0.29	0.58	0.49	0.59	0.5	2.5	1.81
Tb	0.11	0.1	0.1	0.09	0.07	0.06	0.09	0.08	0.1	0.08	0.39	0.27
Dy	0.71	0.71	0.59	0.59	0.54	0.34	0.47	0.48	0.6	0.53	2.46	1.72
Ho	0.16	0.16	0.14	0.15	0.11	0.08	0.12	0.11	0.13	0.13	0.52	0.34
Er	0.53	0.48	0.44	0.51	0.35	0.28	0.35	0.38	0.4	0.44	1.51	0.99
Tm	0.1	0.08	0.08	0.1	0.05	0.05	0.08	0.08	0.06	0.07	0.21	0.14
Yb	0.51	0.44	0.49	0.52	0.37	0.27	0.3	0.33	0.4	0.45	1.35	0.85
Lu	0.09	0.07	0.08	0.09	0.05	0.04	0.07	0.06	0.07	0.07	0.2	0.12
ΣREE	13.55	13	11.32	11.89	14.71	7.44	15.47	11.18	10.39	0.65	57.43	37.89
ΣREE+Y	19.43	18.46	16.45	17.17	18.33	9.97	4.03	4.5	15.08	54.65	104.43	38.11
(La/Yb)SN	0.28	0.36	0.33	0.26	0.58	0.31	0.54	0.46	0.23	0.2	0.54	0.4
(Ce/Ce*)SN	0.89	0.85	0.65	0.72	1.03	0.91	1.52	0.7	1.05	0.88	0.63	0.99
(Pr/Pr*)SN	0.97	0.99	1.2	1.03	1.03	0.98	0.84	1.07	0.92	1.04	1.23	1.01
Y/Ho	36.75	34.13	36.43	35	32.73	31.63	33.58	40.91	35.85	415.38	90.38	0.65
(Eu/Eu*)SN	2.26	1.9	2.48	2.05	2.03	1.97	1.81	2.57	2.03	3.49	1.53	1.12
(La/La*)SN	1.6	1.57	1.29	1.82	2.26	1.42	1.7	2.04	1.43	1.09	0.98	0.98
(Gd/Gd*)SN	1.2	1.05	0.93	1.17	1.15	0.95	1.19	1.28	1.2	1.27	1.19	1.25
(Y/Y*)SN	1.1	1.01	1.1	1.12	0.94	0.97	1.05	1.19	1.03	12.74	2.57	0.02

¹Gourcerol et al., 2022, ²Gatsé et al., 2021, ³Gross and McLeod, 1980; Moon et al., 2017b, ⁴Gross and McLeod, 1980; Soh Tamehe et al., 2018, ⁵Klein and Beukes, 1993; Halverson et al., 2011.

5.1. Major elements

Table 2 presents data on the average composition of major elements in the BIFs. In most deposits, Fe₂O₃ is the predominant oxide, with contents ranging from 37.9 wt% to 55.6 wt%. However, in the Nabeba, Okanabora and Youkou BIFs, the average composition is dominated by SiO₂, with contents ranging from 47.48 wt% to 58.95 wt%. Other oxides have significantly lower contents than Fe₂O₃ and SiO₂. Furthermore, the Congolese BIFs exhibit much lower Al₂O₃ content compared to the Algoma, Rapitan, and Superior-type BIFs.

The PAAS-normalized diagram reveals that the majority of the Congolese BIFs are enriched in SiO₂, Fe₂O₃, CaO and P₂O₅, but depleted in TiO₂, Al₂O₃, MgO, Na₂O and K₂O. These BIFs share similarities in oxide composition with the Algoma, Rapitan and Superior BIFs (Fig. 7).

Figure 7. PAAS-normalized spider diagram for major oxides of the Congolese BIFs and world-known deposits (PAAS values after Taylor and McLennan, 1985)

5.2. Trace and REEs

Table 3 presents data on the average composition of trace and rare earth elements (REEs) in Congolese and worldwide BIFs. When normalized with respect to the PAAS values (Fig. 8), most BIF samples exhibit enrichment in elements such as Cr, Cu, Y, Nb, Hf, U, and depletion in Sc, Co, Ni, Rb, Zr, Ba and Th. The Rapitan-type BIFs show pattern relatively similar to Congolese BIFs, but display a negative Y anomaly comparatively to other BIFs. The Létioukbala BIFs display a high concentration of Nb, averaging 21 ppm. The Odia BIFs show enrichment in V, while the Nabeba BIFs exhibit depletion in Cu. The average values of Hf range from 0.04 to 0.42 ppm, Th from 0.06 to 0.50 ppm, Sc from 0.43 to 5.28 ppm and Zr from 2.26 to 14.67 ppm.

Figure 8. PAAS-normalized spider diagram for trace elements of the Congolese BIFs (PAAS values after Taylor and McLennan, 1985)

ΣREE data are shown in the PAAS-normalized plot (Fig. 9). The patterns show slight light rare-earth elements (LREEs) compared to heavy rare-earth elements (HREEs) [(La/Yb)_SN = 0.23-0.58], as well as positive La anomalies [(La/La*)_SN = 1.29-2.26], negative to positive Ce anomalies [(Ce/Ce*)_SN = 0.65–1.52], and high positive Eu anomalies [(Eu/Eu*)_SN = 1.81-2.57]. The average values of ΣREE vary from 7.44 ppm to 15.47 ppm for all the selected Congolese BIFs, with Y/Ho ratios ranging from 31.63 to 40.91. Compared to known BIF-types, the Congolese BIFs show similar patters to the Algoma-type BIFs (Fig. 9).

Figure 9. PAAS-normalized spider diagram for REEs of the Congolese BIFs (PAAS values after Taylor and McLennan, 1985)

Based on the assessment of major, trace and REEs, we will discuss the origin and the redox state of the Congolese BIFs, the incidence of syn-and-post depositional processes, as well as their tectonic setting and depositional environment.

6.1. Syn-and-post depositional contamination

The deposition of continental input has influenced the occurrence of BIFs worldwide, and this influence is often reflected in their chemical compositions. Certain elements, including Al₂O₃, TiO₂, Zr, Hf, Rb, Y, Cr, Ni, Co, V and Sc, play a crucial role in identifying the impact or effect of continental contamination. As a result, their contribution significantly modifies the chemical composition of BIFs during their precipitation. These elements mostly originate from the weathering of crustal felsic rocks or mafic sources (Kato et al., 1996; Basta et al., 2011). Most of the Congolese BIFs exhibit low contents of major oxides and trace elements such as Al₂O₃, TiO₂ (<1 wt%), Zr, Hf, Th, Sc (<20 ppm) and elevated Y/Ho ratios (>30), indicating minimal detrital input during their precipitation (Bau, 1993; Bolhar et al., 2004, 2005).

In summary, the chemical compositions of BIFs are influenced by continental input, with certain elements being highly mobile during geological processes. The Congolese BIFs show low contents of major oxides and trace elements, indicating limited detrital input during their formation.

Gourcerol et al., 2022 used the La-Th-Sc diagram to assess the detrital components in some Congolese BIF samples and identified three sources of detrital contamination: felsic, intermediate and basic sources. Considering that trace elements like Zr, Hf, Rb and Sr originate from crustal felsic rocks, while Cr, Ni, Co, V and Sc are derived from mafic rocks (Rao and Naqvi, 1995), a covariance relationship between certain elements can help constrain the origin of contamination (Hou et al., 2019). Despite the lack of correlation between Zr and Y (Fig. 10d), significative positive correlations are observed between Zr and TiO₂, Al₂O₃, Hf (Figs. 10a-c), implying that the contamination originates from felsic rocks. However, the absence of correlations between MgO and Cr, Ni suggests that contributions from mafic sources were insignificant (Figs. 11a-b). These observations indicate that the Congolese BIFs have mostly undergone contamination from felsic sources.

Figure 10. Covariance correlations between (a) Zr vs TiO₂; (b) Zr vs Al₂O₃; (c) Zr vs Hf; (d) Zr vs Y

Figure 11. Covariance correlations between (a) Ni vs MgO; (b) Cr vs MgO

The Fe/Ti vs Al/(Al + Fe + Mn) binary diagram, proposed by Boström, 1973, is a useful tool to distinguish between terrigenous sediment and hydrothermal deposits. The diagram (Fig. 12) shows that the majority of iron ore is found close to the hydrothermal field and far from the pelagic and terrigenous sediments field. This indicates that over 80% of the Congolese BIFs are of hydrothermal origin.

Figure 12. Discrimination diagram of Fe/Ti vs. Al/(Al + Fe + Mn) ratios (Boström, 1973)

6.2. Origin of the Congolese BIFs and Redox State

The formation of the Congolese BIFs likely began with the chemical precipitation of elements (Si and Fe) within oceanic basins. Some Congolese BIFs exhibit high contents in Cr, Ni and Cu suggesting that iron (Fe) and silicon (Si) have been leached through hydrothermal processes associated with submarine volcanism. This assumption of chemical precipitation is also confirmed by the SiO₂ vs. Al₂O₃ discrimination diagram ( Fig. 13) (Wonder et al., 1988), which shows the importance of hydrothermal activities during the deposition of the Congolese BIFs. The relative low proportion of detritus elements were derived from igneous sources. Weathering, controlled by favorable climatic and environmental conditions, may have played an important role in the leaching of these elements from the continent.

Figure 13. SiO₂ vs. Al₂O₃ discrimination diagram (Wonder et al., 1988)

Chemical sediments, like BIFs, show chondritic and super-chondritic Y/Ho ratios, respectively (~28) and (~44), LREEs depletion compared to HREEs, a negative Ce anomaly and positive La and Eu anomalies, signatures of both seawater and hydrothermal fluids (Bau and Dulski, 1999; Bolhar et al., 2004; Alexander et al., 2008). Most Congolese BIFs have similar characteristics, with the Y/Ho ratios ranging from 31.63 to 40.91, a negative Ce anomaly and positive La and Eu anomalies, with the exception of the Bikélélé and Nabeba BIFs, which have positive Ce and negative La anomalies (Fig. 9; Table 3). These features suggest the influence of seawater and hydrothermal fluid components in the BIF precipitation. Eu/Sm vs. Sm/Yb and Eu/Sm vs. Y/Ho diagrams were proposed to assess the amount of seawater and hydrothermal components in the mixing solution (Alexander et al., 2008). Figs. 14a,b show that the Congolese BIFs were precipitated from a mixture solution of seawater and little proportions of high-temperature hydrothermal fluids (0.1%). Therefore, the formation of the Congolese BIFs can be attributed to the chemical precipitation of Si and Fe from seawater with the influence of high-temperature hydrothermal fluids and relatively low proportions of felsic detrital inputs. The formed BIFs underwent metamorphism under lower greenschist to amphibolite facies, leading to their recrystallization as magnetite-hematite Itabirites.

Figure 14. Two-component conservation mixing lines of (a) Eu/Sm vs. Sm/Yb and (b) Eu/Sm vs. Y/Ho ratios for the Congolese BIFs (Alexander et al., 2008)

Ce anomaly is often used as a proxy to constrain the redox state of the paleo-ocean at the time of BIF deposition. Oxygenated modern seawaters usually display strong negative Ce anomaly, while sub-oxic and anoxic seawaters show lack of negative Ce anomaly. In the former case, this results from the oxidation of Ce³⁺ to Ce⁴⁺ and the integration of Ce into Fe-Mn oxyhydroxides, whereas in the latter case, it is due to the reductive dissolution of Fe-Mn particles (Byrne and Sholkovitz, 1996; Bau and Koschinsky, 2009; Bekker et al., 2010). The Pr/Pr* vs Ce/Ce* diagram of Bau and Dulski (1996) helps to identify true La and Ce anomalies. Fig. 15 illustrates the redox heterogeneity existing within the basins during the deposition of the Congolese BIFs. Three Avima BIF samples show positive La anomaly and no Ce anomaly, implying that they were deposited in a suboxic to anoxic environment. Similarly, most of the Letioukbala BIF samples also show positive La anomaly and no Ce anomalies, with the exception of one sample that shows true negative Ce anomaly. This indicates that the Letioukbala BIFs were deposited in a suboxic to anoxic environment. Interestingly, the Youkou, Elogo and Zanaga BIFs show similar plots, with some samples (one sample from the Youkou and Elogo BIFs and two samples from the Zanaga BIFs) showing positive La anomaly and no Ce anomaly, while others show true negative Ce anomaly (one sample from the Youkou and Elogo BIFs and three samples from the Zanaga BIFs), indicating deposition in an oxic to anoxic environment. The Odia BIFs have one sample plotting in positive Ce anomaly, while two samples yield at the boundary between no Ce anomaly and negative Ce anomaly, suggesting a combination of oxic and anoxic conditions in the basin. Samples The Okanabora BIF samples plot in positive Ce anomaly (one sample) and in negative Ce anomaly (one sample), implying precipitation in both oxic and anoxic environment. Similarly, the Bikélélé BIFs show samples plotting in positive Ce anomaly (one sample), no Ce anomaly (one sample) and in negative Ce anomaly (one sample), suggesting an environment fluctuating between oxic and anoxic conditions. The Nabeba BIFs have three samples that show positive Ce anomaly, while four samples display no Ce anomaly, suggesting that they were deposited under suboxic to anoxic conditions.

Figure 15. PAAS-normalized Ce/Ce* vs. Pr/Pr* diagram of the Congolese BIFs (Bau and Dusky, 1996)

6.3. Tectonic setting and depositional environment

BIFs are abundant in the Congo Craton, which appears to be an important metallogenic province in Central Africa. It is suggested that the Congo craton was formed by the amalgamation of various cratonic pieces through multiple geological events: collisions, metamorphism, magmatism, rifting and welded around 2.1 Ga (Fernandez-Alonso et al., 2011). The Congolese BIFs occur within the greenstone belts of the Chaillu Massif and Ivindo Basement Complex, which exhibit similar rock associations. They consist mainly of amphibolite, greenschist and mafic-ultramafic rocks (Gatsé et al., 2021; Gourcerol et al., 2022). These characteristics, together with their higher (Eu/Eu*)_SN ratios > 1.8 (Table 3), suggest that they are related to Algoma-type BIFs (Gross, 1980; Huston and Logan, 2004). U-Pb zircon dating applied to the Congolese greenstone belts yielded ages ranging from 3080 Ma to 2845 Ma. The age of 2845 Ma represents the upper limit of the depositional age of the greenstone belts, occurring after 2845 Ma but before 2750 Ma, which is the age of the Neoarchean tectono-metamorphism event (Blein et al., 2017; Chevillard et al., 2017; Fullgraf et al., 2017). BIF deposition likely occurred within that timeframe.

To date in Congo, only the greenstone belt associated with the Elogo BIFs has been studied to constrain the geodynamic setting (Makamba et al., 2023). On the basis of their geochemical features, it has been suggested that the tholeiitic basalts of the Elogo greenstone belt were formed in a back-arc basin subject to an extensive regime, while the calc-alkaline andesitic basalts and dacites indicate a volcanic arc setting, related to a subduction regime (Makamba et al., 2023). Such a model implies that BIFs were deposited in an extensional back-arc basin, which also recorded hydrothermal vent fluids and was later affected by a compressive orogenic event around 2750 Ma. Given the similarity of geological sequences across the Congolese greenstone belts, we think that other BIFs were also deposited in the same tectonic setting. Similar cases have been highlighted in Cameroon (Ganno et al., 2017; Soh Tamehe et al., 2021; Swiffa Fajong et al., 2022; Evina Aboula et al., 2023) and Gabon (Thiéblemont et al., 2009).

Iron mineralization in Congo is distributed across two distinctive metallogenic provinces: the Chaillu Massif and the Ivindo Basement Complex. These two mineralized provinces collectively compose the northwestern part of the Congo craton. The substantial number of identified iron ore deposits and occurrences highlights significant economic attractiveness due to their notable grades and mineral resources. BIFs primarily exhibit oxide facies dominated by hematite and magnetite, but carbonate facies can also be found notably at Nabeba. The geochemical composition of these deposits indicates relatively low contents of most major oxides, except SiO₂ and Fe₂O₃. An assessment of detrital influence has identified felsic rocks as the principal source materials. The formation of the Congolese BIFs can be attributed to the chemical precipitation of Si and Fe from seawater with the influence of high-temperature hydrothermal fluids and relatively low proportions of felsic detrital inputs from the weathering of the continent. The lithological sequences across the Congolese BIFs, as well as their geochemical characteristics suggest that they are related to Algoma-type BIFs and were formed in extensional back-arc basins, under redox heterogeneity varying from of are related to a back-arc setting. The Congolese BIFs were formed in basins with redox heterogeneity, which vary from suboxic to anoxic (Avima, Letioukbala and Nabeba) and from oxic to anoxic (Youkou, Elogo, Zanaga, Odia, Okanabora and Bikélélé) conditions.

The authors express their gratitude to Mr. Urbain Fiacre OPO, the Acting Director-General of Mines, for providing valuable reports that were used in preparing this manuscript. The first author would also like to extend his gratitude to Dr. Vicky Bouenitela and Dr. Chesther Gatsé whose precious comments helped us improve the manuscript. Furthermore, we also thank the anonymous reviewers and the editor for their thorough reviews and insightful comments, which greatly enhanced the quality of the manuscript. This paper is part of the first author's PhD thesis at the Pan African University Institute of Life and Earth Sciences (including Health and Agriculture).

The authors reported no potential conflicts of interest regarding the publication of this paper.

The first author’s PhD thesis is being funded by the African Union Commission via the Pan African University Institute of Life and Earth Sciences (including Health and Agriculture).