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

Banded iron formation (BIF)-related country rocks at the northern limit of the Mbalam iron ore district constitute part of the Archean to Proterozoic greenstone belt of the Congo Craton (CC). In this study the petrology of BIFs and amphibolites from the lower part of the Njweng ridge is presented in a bid to decipher the source of the sediments and their inherent iron enrichment. The BIFs vary from oxide to silicate facies categories, composed mainly of magnetite bands alternating with quartz bands and their mineralogy is dominated by magnetite, hematite and quartz. The amphibolites contain quartz, magnetite, amphibole, biotite, garnet with a distinct schistocity. The Fe 2 O 3 and SiO 2 content of the BIFs reach highs of 54.55 wt% and 57.83 wt%, respectively. The average Fe 2 O 3 content of 46.7 wt% coupled with SiO 2 and Al 2 O 3 as deleterious elements indicate a relatively low-grade iron ore. Gold content reaches a maximum of 1.1 ppb. Low Al 2 O 3 , TiO 2 and High Field Strength Elements (HFSE) indicate BIF derivation from detritus-free chemical sediments. Their Fe/Ti, Fe/Al and Si/Al ratios are typical of marine BIF systems with hydrothermal overprint. On Post Archaean Australian Shale (PAAS)-normalized Rare Earth Elements-Yttrium (REE-Y) patterns the BIFs present a slightly negative Ce anomaly and positive Eu and Y anomalies; characteristics typical of modern seawater. The amphibolites are enriched in Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE) and show peaks of Y, Nb, La Ta and their Au content attains a maximum of 1.7 ppm. Gold contents in the BIFs and amphibolites indicate the BIF-related country rocks as a potential primary source of the alluvial gold mined in the Mbalam district. une légère anomalie négative de Ce couplée à une anomalie positive de Eu et Y, caractéristique d’un environnement marin récent. Les amphibolites sont enrichies en tarres rares légères par rapport aux tarres rares lourdes et montrent des pics de Y, Nb, La et Ta, Leurs teneurs en Au atteignent un maximum de 1,7 ppm. Les teneurs en Or dans les BIFs et les amphibolites indiquent que les formations de fer rubanées (BIFs) en place dans le district de Mbalam sont une source potentielle d’or.


INTRODUCTION
Banded iron formations (BIFs) are rock units with distinct alternations of Fe-rich and chert-rich bands (Kimberley, 1989). They typically vary in age from Archean (3.8 Ga) to Paleoproterozoic (2.5 Ga) and are thought to have formed from seawater as chemical precipitates. The total Fe content of BIFs falls between 20 wt% and 40 wt% and silica content ranges from 34 to 56 wt% (Klein, 2005). BIFs generally occur in greenstone belts and other super crustal volcanic and/or sedimentary sequences of deep marine environments and they range from oxide-, carbonate-, silicate-to sulfide-rich varieties. Precambrian BIFs represent an important source of mineable iron (Teutsong et al., 2017). According to Suh et al. (2008) the high demand for steel worldwide is partly driven by an enormous economic growth currently underway in India and China. Consequently, the search for new iron deposits is experiencing an upsurge in Cameroon where Archean to Early Proterozoic BIF deposits are limited to the south and southeastern part of the country (Suh et al., 2008;. 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 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).
The last two decades have witnessed a boom in alluvial gold mining in the iron ore-bearing terrains of southern Cameroon. Understanding the nature of gold mineralization in this greenstone metamorphic belt has been the focus of some recent studies (Fuanya et al., 2019;Omang et al., 2015;Dongmo et al., 2019;Nono et al., 2021), yet the primary source of gold mineralization is unknown. Previous studies are in favour of an ultramafic host for the primary mineralization (Omang et al., 2015;Dongmo et al., 2019;Fuanya et al., 2019). Studies of gold deposits associated with BIFs in the Archean cratons have shown that gold is associated with localized sulfide facies zones within regionally extent oxide-facies units (Biczok et al., 2012). This study is aimed at characterizing the Feenriched rocks of part of the Mbalam district through detailed petrography and geochemical analysis of BIFs and associated host rocks. This data set should provide insights into their genesis and serve as a gate way to determine the primary gold host rock and thus enhance exploration in the area.

REGIONAL AND DISTRICT GEOLOGICAL ASPECTS
The Mbalam iron ore prospect constitutes a segment of the Ntem complex (Fig. 2,  which is part of the CC composed of Archean rocks (2.8 and 2.9 Ga) that appear to have been partially reworked during the Paleoproterozoic Trans Amazonian cycle (Shang et al., 2004Tchameni et al., 2001;Van Schmus et al., 2008;, Li et al., 2016. The magmatic rocks here include charnockitic and Fig. 1: New iron ore exploration belt in the northern part of the Congo Craton with iron ore deposits in Cameroon, Congo and Gabon TTG (Tonalite-Trondjemite-Granodiorite) suites (Pouclet et al., 2007) and the granitoids are mainly I-type (Shang et al., 2004). The banded series is represented by highly deformed granulitic gneisses (Takam et al., 2009) while the greenstone rocks occur as disrupted belts and as xenoliths in the intrusive series (Shang et al., 2004. These supracrustal rocks consist of BIFs, metagraywackes, sillimanite-bearing paragneisses, amphibolites and pyroxenites affected by granulite-facies metamorphism with estimated peak temperatures of 750 ± 50 °C at 5-6 kbar (Tchameni et al., 2001). Rock units of economic importance in the Ntem complex include high-grade iron ore and itabirites (Suh et al., 2008;Ilouga et al., 2013;Ndime et al., 2019) with less prominent ultrabasic rocks containing elevated Ni, Cr and Co (Milesi et al., 2006). Polyphase deformation in the Ntem complex is revealed by the prominent E-W-trending foliation truncated by late NE-SW shear zones adorned with late felsic plutons. The Njweng prospect is a ridge trending N-S with a surface area of ~ 80 km 2 . It is 7 km long and constitutes one of the four ridges of the Mbalam iron ore district (Fig. 3). The BIF is hundreds of meters thick (more than 600 m in depth), with maiden resources of approximately 3.5 Mt of iron at Mbarga the main ridge of the district (www.sundanceresources.com.au). The prospect is composed predominantly of metamorphosed chemical sediments . Associated supracrustal rocks consist of matagranitoids and amphibolites. The BIF is intercalated with metasandstone and metasiltstone with variable ages from 1000-3000 Ma . Structurally the first deformation event in the area involved regional E-W shortening. This resulted in a dominant isoclinal recumbent F1 fold with the early Nverging, overprinted by a late W-verging open F2 fold expressed in the itabirite. The itabirite bands have N-S strike with a near vertical dip (average 75 o ), and a dip direction that varies between E and W.

MATERIAL AND METHODS
During field mapping for this study of the southern end of the Njweng ridge previously documented by Ilouga et al. (2013), rock samples were collected preferably in the vicinity of alluvial gold workings as the desire was to pinpoint potential gold source rocks. Eight representative BIF and amphibolites samples were subsequently selected for petrography and geochemical analysis. Samples collected were grouped into facies based on their mineralogy, texture and field distribution and subsequently thin sections prepared from them were studied under a petrographic microscope. Whole rock geochemical analysis was achieved at Acme Analytical Laboratories Ltd., Vancouver, Canada using a combination of inductively coupled plasma emission spectrometry (ICP-ES) and mass spectrometry (ICP-MS). A 0.2 g of pulverized sample was fused using lithium metaborate/tetraborate and digested using dilute nitric acid. Loss on ignition was determined by weight difference after ignition at 1000°C. In addition, a separate 0.5 g split was digested in aqua regia and analyzed by ICP-MS for precious and base metals. The major elements were analyzed by ICP-ES while total C and S was determined by Leco. Trace, REE as well as the precious and base metals were analyzed by ICP-MS. In order to ensure data quality and to calibrate the equipment for optimal precision, a replicate, standard (STD SO-18) and blank was run after every sample analyzed. Accuracy for major, trace and rare earth element (REE) analyses was within 5% error margin. The detection limits for the major elements vary between 0.002 and 0.01 wt%, 0.05 to 1 ppm for trace and REE elements and 0.01 to 1 ppm for precious and base metals. The Mbalam iron ore district is part of the Ntem complex in the northern limit of the Congo craton.  The oxide facies BIF is marked by distinct alternation of the iron-rich bands that vary in thickness from 1 to 3 mm and silica-rich bands with thicknesses that range from 2 to 5 mm ( Fig.  4a, b). In some cases, the bands are folded (Fig.  4) and granular to massive. The iron-rich band is composed of iron oxides (magnetite with minor hematite). Magnetite crystals occur as large subhedral to anhedral grains dominantly showing a granoblastic texture with inclusions of quartz ( Fig. 5). Some magnetite grains are oxidized to hematite around their rims (Fig. 5). The silicarich bands are mainly composed of quartz with disseminations of magnetite crystals. Quartz is medium to coarse-grained, subhedral to anhedral in shape and exhibits undulose extinction. The silicate facies BIFs is essentially composed of quartz that is sparingly stained by hematite (Fig. 4). The silicate facies BIF samples show alternating silica-and Fe-rich bands some of which are weakly to strongly folded (Fig. 4). While the silica-rich bands vary in thickness from 3 to 9 mm, the iron-rich bands reveal thicknesses that range from 1 to 4 mm. The main iron oxide in the silicate facies is magnetite with a vesicular texture ( Fig. 5).

Amphibolite
The amphibolites are dark green, foliated and medium-to coarse-grained (Fig. 6). Two varieties have been distinguished based on their mineralogy: garnet-bearing and non-garnet types. The garnet-bearing amphibolite has an equigranular to granoblastic texture ( Fig. 6) with platy and elongated aligned amphibole grains ( Fig. 7) coupled with garnet, biotite, hornblende and quartz. Garnet porphyroblasts (0.23 -1 mm) are subhedral and show characteristic irregular fractures (Fig. 7a). Hornblende is subhedral and needle-like to elongate in shape ( Fig. 7a, b). Two quartz types have been identified herein referred to as qtz1 and qtz2. While qtz1 is fine-grained and makes up the groundmass, qtz2 is coarse-grained and occurs both in veinlets and in the groundmass of the rock (Fig. 7a-c). Quartz shows a characteristic ribbon texture and wavy extinction. The non-garnet amphibolite variety is granoblastic to heteroblastic and occurs in direct contact with the BIFs. Minerals identified include amphiboles (notably hornblende), quartz and magnetite ( Fig.  7d-f). The schistosity here is defined by the preferred alignment of the amphiboles (Fig. 7d-f) which are poikiloblastic with quartz inclusions (Fig. 7d). The amphiboles are altered to chlorite to a limited extent. Quartz occurs as ribbons and shows characteristic undulose extinction. Magnetite occurs as disseminations along the cleavages of the amphiboles.     Their fine lamination (microbanding) is interpreted to represent chemical valves or annual layers of sedimentation (Klein, 2005). Magnetite is the main iron oxide with hematite as a product of alteration. This is similar to magnetite reported by Suh et al. (2009) in the Metzimevin and Mbalam iron ore prospects. In the Aguas Claras and Matuca BIF deposits of the Quadrilatero Ferrifero in Brazil the principal iron oxide is heamatite (Spier et al., 2008). This is in contrast to other Archean and Early Proterozoic BIFs with essentially magnetite as the main Fe-oxide mineral (Carlut et al., 2015).
Magnetite is coarse-grained and granular to massive in texture with characteristic voids and inter-granular triple junctions. While the granular texture can be attributed to deposition in a shallow marine environment, the vuggy and massive texture represents leaching of gangue minerals by hypogene processes (Angerer and Hagemann, 2010;Duuring and Hagemann, 2013). Characteristic subhedral quartz inclusions in the magnetite are sug gestive of metamorphic recrystallization Ndime et al., 2019). The shape of the magnetite and quartz grains is a product of post-depositional recrystallization.
The amphibolites vary from granoblastic to heteroblastic with preferred mineral orientation, indicative of mineral recrystallization.  Fig. 8b). Globally, the chemical compositions of metamorphosed BIFs are commonly used to decipher the effects of metamorphism. LILEs such as Ba, K, Rb, Th, and U are mobile during diagenetic and metamorphic processes, thus, are good indicators to detect the effects of metamorphism in the BIF (Ndime et al., 2019). The LILE in the samples from the Njweng prospect are generally very low. This therefore supports their mobility during the process of diagenesis and metamorphism. This is different from the Nkout west BIF that shows a stable behavior in their LILE suggesting that the metamorphosed BIF kept their original chemical compositions during post-depositional processes (Ndime et al., 2019).

The influence of detrital aluminosilicates on the trace element budget of the BIFs
Precambrian BIFs may serve as proxies that provide information on the chemical composition and the physio-chemical conditions in ambient seawater (Viehmann et al., 2015a,b) at the time of their deposition. The close similarity between the REY distribution in modern seawater and Precambrian marine chemical sediments and, hence, the respective ambient seawater (except for redox sensitive Ce and Eu) provides a robust tool to verify a marine origin of a sequence of chemical sediments (e.g. Bau and Dulski, 1996). Previous studies concluded that BIFs with REY SN patterns sub-parallel to those of modern seawater directly reflect the REEY composition of ancient ambient seawater and can, therefore, be used as prime archives for Precambrian seawater (e.g. Bau and Dulski, 1996;Viehmann et al., 2015a,b). However, only pure chemical sediments are reliable archives of Precambrian seawater and syn-and post depositional processes such as diagenesis, metamorphic fluid rock alteration and detrital aluminosilicate contributions may alter the pristine seawater signature (e.g. Bau, 1993 Fig. 11). The BIFs also show positive La SN anomalies (Fig.9b). According to Bau and Dulski, (1996) significant La in the BIFs mask the Ce anomaly thus the samples only show a slightly negative Ce anomaly (Fig. 9a). The negative Ce anomaly indicated by the BIFs shows their deposition in an oxidizing environment (Bau and Dulski, 1996). Positive Eu SN anomaly indicates that ambient seawater carried REY that had entered the ocean by moderate-hightemperature hydrothermal fluids (200-300 o C, Fig. 8b). These ocean waters are considered as the source of the iron as well as of SiO 2 in the BIFs (Bau et al., 1993). The strong positive Eu anomalies in the BlFs may only result from an input of Eu-enriched hydrothermal fluids into the water column (Alibert et al., 1993).

Source of chemical components in the BIFs
The source of iron and silica, main components of BIFs, was firstly suggested by Cloud (1973) as derived from seawater. More recently, Klein (2005) proposed a deep ocean derived source for Fe and Si. However, not all BIFs are made up of pure chemical sediments, but some are commonly contaminated by clastic components (Bau and Dulski, 1996;Basta et al., 2011). Thus the composition of Archean chemical sediments reflects input of solutes from both crustal weathering and hydrothermal emissions. Detrital inputs can also occur in shallow water facies iron formation and such clastic particles tend to obscure the seawater signal. The Al 2 O 3 , Na 2 O, K 2 O and TiO 2 contents are mainly hosted in silicates and are usually used as contamination input tracers. Such impurities in terrigenous sediments are carried by rivers or winds from weathered pre-existing rock, or deposited by volcanic activities or pelagic sediments (Lascelles, 2007). A strong positive correlation between Al 2 O 3 and TiO 2 has been reported for BIF worldwide (Basta et al., 2011;Yang et al., 2015) and is attributed to clastic input. Detrital component in BIF can also be identified through elevated concentrations of HFSE (e.g. Zr, Hf, Ta, and Th), "REE (Bau, 1993) as well as co-variation between HFSE and REE ratios (La/La*, Y/Ho, Pr/Yb, Ce/Ce* (Bau, 1993). Pure chemical sediments are enriched in Mn and Fe but addition of detrital or volcanic material causes their dilution and enrichment of Ti, Al and Zr (Bonatti, 1979 Klein, 2005). This is different from BIF reported from the Dales Gorge, Witwatersrand; Elom, Bikoula, Kouambo and Nkout west prospects which records input of detrital material (Pecoits et al., 2009;Viehmann et al., 2015;Ganno et al., 2015Ganno et al., , 2017Teutsong et al., 2017;Ndime et al., 2019).
Furthermore, the BIF investigated in this study shows Fe/Ti ratios that range from 26.85 to 38.15, Fe/Al ratios between 1345.5 and 1907.5 and Si/ Al ratios that vary from 1351.5 to 984. This is similar to sediments with predominantly hydrothermal components (Barrett, 1981). On the Si vs Al binary diagram of Choi and Hariya 1992, the BIF samples show hydrothermal signatures (Fig. 8a). Pure hydrothermal deposit contains little Al and high Al/Ti ratio (Marchig et al., 1982). This is confirmed on the Al-Fe-Mn ternary diagram in which the BIFs plot in the hydrothermal field ( Fig. 13) indicating that Si and Fe were likely derived from hydrothermal sources. On the two component diagrams of Alexander et al. (2008) the BIF from Njweng plot in the <0.1 high temperature hydrothermal fluid field (Fig. 14a,  b). 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).   (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.   (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).

Ore potential of the Njweng BIFs and associated country rocks
The Njweng prospect forms part of the Precambrain iron ore belt of Cameroon with the three main iron ore deposits namely Mbalam, Nkout and the Mamelles (Suh et al., 2008;Ganno et al., 2015). Iron ore deposits within Precambrian BIFs are the most profitable sources of iron making them very attractive exploration targets (Clout, 2006;Duuring et al., 2012). According to Clout, (2006), iron ore can be classified into three groups; (1) Primary unenriched Fe (25-45 wt %), (2) Martite-goethite ore with Fe of 60-63 wt% formed by supergene processes.
(3) High grade hematite ore (60 to 68 wt %) formed by hypogene process and subsequently supergene enrichment. According to Klein (2005), the average bulk chemistry of low-grade BIFs from the Archean and Paleoproterozoic are always found to be similar, with Fe 2 O 3 ranging from about 20 to 40 wt%. The average concentration of Fe 2 O 3 in the BIFs is 46.47 wt% and 30.13 wt% in the amphibolite (Table 1) coupled with relatively high contents of gangue (average of 50 wt %, SiO 2 + Al 2 O 3 ). Deleterious elements such as Al 2 O 3 , P 2 O 5 , and TiO 2 , are generally low (Table 1). Phosphorous and sulfur represent deleterious elements in the steel making process and are specific targets during iron ore beneficiation (Guider, 1981). The acceptable contents of phosphorous and sulfur in commercial ores should be lower than 0.07% P and 0.1% S, respectively (Guider, 1981). The Njweng BIF shows S contents that are generally below 0.02 wt% which is well below the acceptable average. The phosphorus contents are slightly high (average 0.22%), but within the acceptable levels for commercial ores. Although this can be regarded to as suitable for exploration, the beneficiation process may be problematic (Anderson et al., 2014). Thus the Njweng BIF samples in this study is a magnetite-dominated low grade iron ore, similar to other BIF-hosted iron deposits in Cameroon (e.g. Kouambo iron ore deposit (Ganno et al., 2017), Nkout iron deposit (Suh et al., 2009); Zambi iron deposit (Ganno et al., 2015) and worldwide (e.g. Wittenoom and Mt. Whaleback, Australia (Webb et al., 2006). Major BIF deposits around the world have recorded gold contents as high as 5 ppm and reserves up to 44.6 Mt (Pereira et al., 2007;Morales et al., 2016;Vitorino et al., 2020). In the Njweng prospect the BIFs and amphibolites show gold contents that reach a maximum of 1.1 and 1.7 ppb, respectively. Gold indices identified in the BIFs and associated rocks are lower than that of world class deposits. Although their contents are low they may serve as potential source of primary gold in the Njweng area and confirm that alluvial gold widely won in the area is derived from the weathering of these rocks.

CONCLUSIONS
The following conclusions can be drawn from this studyThe Njweng prospect BIFs occur in association with amphibolites. The BIFs vary from oxide to silicate facies and are composed predominantly of recrystallized quartz associated with magnetite and hematite which are the major ore minerals. The magnetite shows a vuggy texture indicative of leaching of gangue minerals by hypogene fluids 1. The amphibolites bear testament to high grade metamorphic overprint in the area with the development of a granulite facies mineral assemblage 2.
The Njweng BIF is predominantly composed of Fe 2 O 3 and SiO 2 and their geochemical features indicate pure chemical sediments with no detritus input. Low LILE contents indicate their mobility during the process of diagenesis and metamorphism.

4.
The positive Eu anomaly indicates that metals (Fe and Si) in the Njweng BIF were derived from a high temperature hydrothermal fluid. Negative Ce anomaly implies the anoxic nature of the seawater from which this BIF precipitated.

5.
Gold indices identified in the BIFs and associated rocks points to these lithologies as primary gold sources for the alluvial gold workings in the area.