Depositional Environment and Genesis of the Nabeba Banded Iron Formation (BIF) in the Ivindo Basement Complex, Republic of the Congo: Perspective from Whole-Rock and Magnetite Geochemistry

: The Nabeba high-grade iron deposit (Republic of the Congo) is hosted by banded iron formation (BIF) in the Ivindo Basement Complex, which lies in the northwestern part of the Congo Craton. The Nabeba BIF is intercalated with chlorite-sericite-quartz schist and comprises two facies (oxide and a carbonate-oxide). In this study, whole-rock and LA-ICP-MS magnetite geochemistry of the BIF was reported. Magnetite samples from both BIF facies had fairly similar trace element compositions except for the rare earth element plus yttrium (REE + Y) distribution patterns. The high V, Ni, Cr, and Mg contents of the magnetite in the Nabeba BIF could be ascribed to the involvement of external medium-high temperature hydrothermal ﬂuids during their deposition in relatively reduced environment. The Post-Archean Australian Shale (PAAS)-normalized REY patterns of the Nabeba BIF magnetite were characterized by LREE depletion coupled with varying La and positive Eu anomalies. Processing of the information gathered from the geochemical signatures of magnetite and the whole-rock BIF suggested that the Nabeba BIF was formed by the mixing of predominantly anoxic seawater (99.9%) with 0.1% of high-temperature (>250 ◦ C) hydrothermal vent ﬂuids, similar to the formation mechanism of many Archean Algoma-type BIFs reported elsewhere in the world. Y.X., and Y.Q.; investigation, C.G.E., Y.X., K.A.-A., B.G., and Y.Q.; resources, Y.X.; writing—original draft preparation: C.G.E.; writing—review and editing, C.G.E., Y.X., K.A.-A., B.G., and Y.Q.; funding acquisition: Y.X. All authors have version the manuscript.

The northwestern margin of the Congo Craton extends from Cameroon southward through northern Gabon to the northwestern Republic of the Congo (R.C.), where various BIF-hosted iron deposits were discovered within the Precambrian terranes and the narrow greenstone belts around them [11][12][13][14][15]. The BIFs in Central Africa and the Congo are much less studied than those in Cameroon. In the northwestern Congo, recent exploration by the Core Mining Ltd., Equatorial Resources, and Congo Iron SA discovered several high-grade iron deposits, including Avima, Badondo, Nabeba, and Cabosse [16] (Figure 1). Very little The Precambrian Nabeba BIF is associated with mafic and felsic volcanic rocks ( Figure 2a). Their lithological associations are similar to those of the well-studied Algomatype Musselwhite and Meliadine BIFs. Recent studies on Algoma-type BIFs demonstrated that their depositional settings [2,[20][21][22][23][24] involve submarine hydrothermal activity and a seawater component with varying detrital/volcaniclastic input. Despite being influenced by post-depositional processes, such as regional greenschist-/amphibolite-facies metamorphism, mineral geochemistry from Si-rich and Fe-rich layers can be used to constrain petrogenesis and depositional environment [2,[25][26][27][28][29][30][31][32][33], especially with the recent advances in the laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analytical technology [28].
This study focuses on the BIF that hosts the Nabeba iron deposit-the deposit with the highest iron ore potential amongst the other BIFs in the Invido Basement Complex (IBC) of the Republic of Congo ( Figure 1)-but its host BIF depositional environment and genesis are not well-studied. In this paper, we reported BIF petrographic features and whole-rock geochemical and LA-ICP-MS magnetite trace element compositions. Based on comparisons with well-studied BIFs around the world, we discussed the controlling factors on the magnetite trace element characteristics and the implications that all of these data have for the BIF depositional environment and petrogenesis at Nabeba.

Regional Geology
Due to the inaccessibility of the region to conduct regionally extensive geological works and the lack of extensive outcrops, the regional geology reported in this paper was based on the few outcrops that we encountered at the company site and environs, the company records on exposures during its geological exploration expeditions, and the scant literature available on the Nabeba area (e.g., the publication of Meloux et al. [34]). In NW Congo, a granitoid massif, which is known as the Chaillu Massif [35] and an equivalent of the IBC, contains two generations of granitoids with mainly N-S foliation. Greenstone belts (including BIF) and schist are found in several locations within the Chaillu Massif, including the Mayoko and the Zanaga Regions in southern Congo [36] and at Mt. Nabeba. To the east of Mt. Nabeba, the margins of the Chaillu Massif are overlain by low-grade metamorphic rocks of the Proterozoic Sembé-Ouesso Group (i.e., a sedimentary sequence of sandstone-shale and diamictite [19,34,35,37] (Figure 1). Cenozoic sediments of the Congo Basin cover much of the area, including the surroundings of Mt. Nabeba.
The Precambrian Nabeba iron deposit in northwestern Congo ( Figure 1) is geologically located in the Archean IBC. The IBC comprises mainly two types of rock formation, namely supracrustal and crystalline rocks (Figure 1; [19,34,35,37]). The supracrustal rocks are mainly volcano-sedimentary in origin and comprise metamorphosed, EW-trending greenstone belts (commonly chlorite-sericite-quartz schists and amphibolite) and banded iron formations with mafic and/or ultramafic intrusive rocks [34]. The Archean greenstone belts narrowly frame the crystalline rocks along regional NE-trending structural corridors. The crystalline rocks generally comprise, as elsewhere in the Congo Craton, cataclasized granites, orthogneiss, and migmatites. These rocks constitute the greater part of the basement (80% to 90%). However, the most widespread lithological assemblages are calc-alkaline granites, biotite, green hornblende granodiorites, and, more locally, quartz meta-diorites or metagabbros [34].
In the Souanké area, which is at the northeastern margin of the IBC, the greenstone belts and the amphibolite associated with banded iron formation ( Figure 2a) are thought to be the remnants of greenstone belts, including those at Nabeba [34]. The good exposures in the Souanké area reveal intrusions of Archean TTG suites and potassic granite in the amphibolite, suggesting that the Amphibolite and the BIF are at least of Precambrian age [38,39].

Deposit Geology
The principal rock types that represent the volcano-sedimentary units in the structurally complex Nabeba project area are BIF and chlorite-sericite-quartz schists [40]. The Nabeba rocks are metamorphosed to the amphibolite facies and strongly deformed [16].
Structural geological studies revealed at least two major deformation events (D 1 and D 2 ) [16]. D 1 consists of the NE-trending S 1 foliation and the F 1 north-plunging isoclinal fold, while D 2 formed regional N-S-trending folds (F 2 ) and extensive E-W-trending S 2 shearing [16]. The Nabeba BIF was suggested to be structurally dismembered and fanshaped, which was confined by NE-trending thrust/reverse faults. These faults converged into an E-W trending regional shear zone north of Nabeba [16].
The Nabeba BIF consists of alternating thin iron oxide-rich and cherty bands with minor iron carbonate minerals, such as siderite and ferroan-magnesite. The iron oxide minerals comprise hematite, magnetite, and martite. The BIF is crosscut by quartz veins of varying thicknesses [16].

Sampling and Analytical Methods
Thirty-three representative BIF samples were collected from drill cores ( Figure 3) and outcrops (Figure 4a,b) for petrographic observations, from which ten samples were selected for whole-rock geochemical analysis and five for in situ LA-ICP-MS magnetite trace element analyses (Table 1).
These samples contain iron oxide bands with dominantly anhedral magnetite grains and rare hematite. Polished thin sections were prepared and examined using both transmitted and reflected light microscopy along with scanning electron microscopy (SEM) in order to select suitable geochemical analysis site with minimal intergranular matrix or mineral (e.g., quartz, siderite, monazite) inclusions.
Whole-rock major and trace (including rare earth element (REE)) analyses were performed at the ALS Laboratory (Guangzhou, China). The samples were crushed and powdered in an agate mill. Major elements were analyzed with X-ray fluorescence (XRF) Minerals 2021, 11, 579 5 of 24 spectroscopy after the powders were fused with lithium borate into glass disks. The analytical uncertainties varied from 1% to 0.04%.   Trace element compositions were analyzed with solution ICP-MS, with approximately 2% to 5% analytical uncertainties. The sample powder was mixed with distilled HF and HNO 3 acids in Teflon screw-up capsules at 140 • C and then dried and digested with HNO 3 acid at 190 • C for 48 h in an oven. The solution was dried again and digested with HNO 3 acid at 150 • C for 12 h. A solution with 800 ng/g rhodium was added into the dissolved samples as an internal standard. The precision for trace elements and REEs varies between 0.1% and 0.5%. For REE-Y, the detection limits were 0.01 ppm. Data quality was monitored by analyzing various standards between unknown samples.
LA-ICP-MS trace element analysis was performed on selected magnetite grains with an Agilent 7900 quadrupole ICP-MS (Agilent Technologies, Santa Clara, CA, USA) attached to a Photon Machines Analyte (Photon-Machines INC., Redmond, WA, USA) HE 193 nm Excimer laser ablation system equipped with a SQUID signal smoothing (Applied Spectra, INC., West Sacramento, CA, USA) device. The analysis was performed at the In-situ Mineral Geochemistry Laboratory, Ore Deposit and Exploration Centre (ODEC), Hefei University of Technology (Anhui Province, China). Sample ablation was done using He as a carrier gas, which mixed with the Ar make-up gas before entering in the ICP. The 30 µm laser spot and the 8 Hz frequency were maintained throughout the analysis. Analysis of all grains was done using laser energy at 3 J/cm 2 . Each analysis comprised a gas blank (laser-off) background measurement of 20 s, followed by a 40 s laser-on sample signal measurement. Calibration was done using multiple reference materials (BCR-2G, SRM 612, and SRM 610), and 57 Fe was used as an internal standard [41]. The ICPMSDataCal program was used for data processing, following the method described by Liu et al. [42]. This software is generally used for the integrated selection of contextual and for signal analysis, quantitative calibration, and correction of time drift. Fifty-four isotopes were measured. Forty-seven sample analysis spots were bracketed by analysis of the standard (e.g., BCR-2G, SRM610, and SSRM612). In the study, the detection limits were significantly lower than the rare earth element (REE) contents of our magnetite samples (Tables S1 and S2).
Siderite was often associated with ferroan-magnesite, and both minerals appeared as massive aggregates in the continuous iron carbonate-rich bands (Figure 6a,b). Siderite was often associated with magnetite or quartz (Figure 6c). Anhedral ferroan-magnesite occurred as micro-laminae in a siderite groundmass and appeared as an alteration product of siderite.

Major Elements
Major element analyses of the representative Nabeba BIF were performed on oxideand carbonate-oxide-facies-BIF samples (six from outcrop and four from drill core) ( Table 1). The results showed that the BIF contained mainly Fe 2 O 3 and SiO 2 , with a weak positive correlation between them (r = 0.17 and 0.27 for oxide-facies and carbonate-oxide-facies-BIF, respectively; Tables 2-4).

Rare Earth Elements
The Nabeba BIF had generally low total REE contents (ΣREE = 8.32-12.11 ppm), with an average of 10.06 ppm for the oxide facies-BIF and 9.81 ppm for the carbonate-oxide facies-BIF samples ( Table 5). The low ΣREE content feature was also reported in many other Archean BIFs [46,50,51]. The PAAS-normalized REE+Y (REY) patterns for the studied BIF were characterized by greater depletions in LREE/HREE ((La/Yb) SN   Determining Ce anomalies is often complicated due to the anomalous behavior of La. Bau and Dulski [3] suggested the use of Ce/Ce* vs. Pr/Pr* diagram to determine La and Ce anomalies in BIFs. The Ce/Ce* and the Pr/Pr* ratios were calculated as Ce SN /(0.5 La SN + 0.5 Pr SN ) and as Pr SN /(0.5 Ce SN + 0.5 Nd SN ), respectively. The Ce/Ce* vs. Pr/Pr* plot indicated that the majority of our samples had no negative Ce anomalies and had very small to negligible positive La and Ce anomalies ( Figure 10).   (Table S1), except for SiO 2 (0.07-9.40 wt.%), which was attributed to the presence of quartz near the analysis spot. Contents of other major element oxides (e.g., MgO and MnO) were very low (<0.01 wt.%), while those of most trace elements were either close to the detection limit or showed insignificant concentration change, although Ni (6.37-80.00 ppm), Co (1.06-12.70 ppm), Zn (7.34-77.80 ppm), and Y (0.17-50.80 ppm) displayed high values (Table S1) (Figure 11b,c).
The magnetite REY contents of the Nabeba oxide facies-BIF were generally above the detection limits (Table S1)

Magnetite in Carbonate-Oxide Facies-BIF
Apart from the high major iron oxide content, other major element oxides (e.g., MgO, MnO and TiO 2 ) had concentrations generally below 2 wt.% (Table S2).
Compared to the magnetite samples from the oxide facies BIF, REY contents of most magnetite samples from the carbonate-oxide facies BIF were below the detection limit, which might have been due to poor signals and/or alterations. This explains the very few data points used for the carbonate-oxide facies BIF. The data were not used in the bivariate plots to test for any correlations between analysed elements. Correlations based on bivariate plots for the Nabeba BIF were done using the whole rock geochemical data ( Figure 8) and the magnetite LA-ICP-MS data for oxide-facies and carbonate-oxide-facies BIF ( Figure 11). The magnetite LA-ICP-MS data for carbonate-oxide facies BIF were used cautiously, with emphasis on (1) the nature of the REY patterns when compared with other well-studied BIFs and the Nabeba oxide facies BIF in this study, and (2) the absolute values (i.e., amounts) of the elemental concentrations and whether they were higher or lower than the concentrations in the oxide facies BIF'.

Primary Geochemical Signature of the Nabeba BIF Magnetite
The primary iron minerals (e.g., Fe-oxyhydroxide) in most Archean BIFs were modified by later diagenesis and regional metamorphism [52]. These assertions in which magnetite in BIF were formed by diagenetic and/or metamorphic processes are illustrated by the following three equations: (1) 2Fe(OH) 3  Recent studies [3,29,55] showed that the separation of Y and REE in the ambient seawater column represents depletions in LREEs relative to HREEs, in association with Y and La positive anomalies, which is explained by the preferential removal of these elements in organic matter, Fe oxyhydroxides, and clay minerals. Magnetite is the most abundant iron oxide mineral in both facies of the Nabeba BIF. In order to use the magnetite as a proxy for understanding the environment of deposition, it is essential to establish whether the Nabeba BIF magnetite retained the REY signature of the precursor minerals, notably Fe oxyhydroxides [1,4,[56][57][58][59][60].
In our study, the magnetite REY patterns in the two-facies BIF samples showed LREE/HREE and MREE/HREE depletions, negative to slightly positive La anomalies, slightly negative to positive Y anomalies and positive Eu anomalies, negligible to weak positive Ce and Gd anomalies, and super-chondritic Y/Ho ratios (Table S1). The Nabeba BIF magnetite showed similar REY characteristics as those in other well-studied Archean BIF magnetites from Isua, North Caribou, Temagami [61], Badampahar [62], and Musselwhite and Meliadine BIFs [30], regardless of their diagenetic, later metamorphic, and hydrothermal histories. The similarity with magnetite data of the other well-studied BIFs indicates that the Nabeba BIF magnetite possesses the primary geochemical signatures that would be useful in determining the depositional environment and the genesis of the Nabeba BIF. The magnetite trace element contents of the Nabeba BIF were generally similar to those of other BIFs (e.g., Meliadine and Musselwhite), except for the high Y-Ni contents in the former (Figure 13a,b). Most of our samples were plotted in the extended BIF field of Gourcerol et al. [30] and overlapped with the extended field of Sun et al. [32] (Figure 14).
Since BIFs are formed by chemical precipitation, any detrital input would produce high contents of Al 2 O 3 and TiO 2 and correlations between HFSE contents and REE ratios (e.g., La/La*, Y/Ho, Pr/Yb, and Ce/Ce*) [66][67][68]. As we mentioned previously, the three samples of carbonate-oxide facies BIF were not good for correlation such as in Figure 15, but we used their element ratios to discuss other aspects of the BIF characterisictics. Thus, the variations and the negative correlations of low Al 2 O 3 concentrations with SiO 2 and the weak ΣREE vs. Al 2 O 3 correlations in the oxide-facies BIF samples (Tables 3 and 4; Figure 15a) suggested insignificant detrital input in the Nabeba BIF formation [5,67,69,70]. However, the strong positive ΣREE vs. P 2 O 5 correlation in oxide-facies indicated that P 2 O 5 controlled REY contents during the BIF precipitation ( Figure 15). Slight negative Zr vs. Y/Ho correlation (r = −0.11; Figure 15c) of the oxide-facies BIF further argued against detrital (terrigenous) contamination during the BIF chemical precipitation [71].

Fluid Mixing and T-f O 2 Conditions of Nabeba BIF Precipitation
The Nabeba BIF magnetite and the whole-rock samples had weak positive Ce anomalies (except for two samples of CG-NB3 and CG-NB6) ( Figure 10). Their REY distribution patterns showed LREE depletions, a mild Ce incongruity, and marked positive anomalies for Eu and weakly positive to negligible Y ( Figure 16). This indicated that the Nabeba BIF formation was likely influenced by reduced bottom water [77] and the chemical precipitates were derived from a mixture of anoxic seawater and medium-to-high-temperature hydrothermal fluids. Similar geochemical trends were found in the magnetite from many well-studied Archean BIFs, such as Musselwhite and Meliadine [30], Badampahar [62], and Middle Back Ranges [33]. This implied that, despite the overprinting diagenetic, metamorphic, and hydrothermal processes, the Nabeba magnetite still retained the geochemical signature of the fluid mixing that formed the original Nabeba chemical precipitates on the paleo-seafloor.
To determine the proportion of fluid-mixing components (i.e., hydrothermal vent fluid and seawater) for the Nabeba BIF, variations of Sm/Yb and Eu/Sm ratios [78] were used (Figure 17), which yielded~0.1% for the high-temperature hydrothermal vent fluid input [2,30]. Strong positive Eu anomalies were associated with high-temperature hydrothermal fluids, medium-to-weak Eu anomalies with low (or medium)-temperature hydrothermal fluids, and no Eu anomaly with seawater ( Figure 16; [3,23,71]). Thus, the weak Eu anomalies exhibited by both facies of the Nabeba BIF ( Figure 16) was consistent with the fluid mixing that dominantly comprised (~99.9%) ambient seawater with minor (~0.1%) high-temperature vent fluids.
The presence of W, As, Pb, Mo, and Sn in the precipitates from seawater was influenced by hydrothermal fluids associated with felsic rocks. In contrast, hydrothermal fluids related to mafic-ultramafic rocks would produce elevated Ti, V, Ni, Cr, and Mg contents [28,79], which was the case for the Nabeba BIF (Figures 2 and 11) [28,79]. Titanium and V contents in magnetite are sensitive indicators of redox conditions and mafic material input [80,81]. The magnetite precipitated from reduced fluids had lower Ti/V ratios than those of the magnetite precipitated from oxidized fluids [32,[80][81][82]. Overall, the Ti/V ratios from both facies of the Nabeba BIF were considered as low. Therefore, the magnetite trace element compositions of these two facies indicated medium-high temperature and relatively reducing conditions and were likely related to mafic-ultramafic hydrothermal input during the BIF deposition on the paleo-seafloor. Figure 17. Sm/Yb vs. Eu/Sm plot (modified after Alexander et al. [78]) for the Nabeba BIF and other BIFs shown in Figure 16. Average compositions of high-temperature (>350 • C) hydrothermal fluids [76], low-temperature hydrothermal fluids [83], and Pacific seawater [84] are shown for comparison.
However, when comparing both facies of the Nabeba BIF, magnetite from the carbonateoxide facies-BIF had higher V content than its oxide facies-BIF counterpart (Figure 11g), yielding lower magnetite Ti/V ratio for the carbonate-oxide facies (3.72−16.02 ppm, average 10.95 ppm) than that for the oxide facies (18.51−136.69 ppm, average 60.63 ppm). This suggested that the depositional environment was more reducing for the carbonate-oxide facies than for the oxide facies [28,65].

1.
The Nabeba BIF comprised two facies (oxide and carbonate-oxide) and displays alternating iron-rich and quartz-rich layers. The BIF consisted of magnetite, hematite, and quartz, together with minor siderite and magnesite. Geochemical analysis suggested that the Nabeba BIF had mainly Fe and Si, with minor CO 3 and trace detrital material.

2.
Magnetite from both BIF facies had a wide range of trace element contents (e.g., Mn, Ti, Ni, Mg, Cr, V, and Zn), which suggested input of medium-high-temperature hydrothermal vent fluids. Redox and temperature conditions likely controlled the magnetite chemical compositions.

3.
Major and trace (including REE) element compositions suggested that Fe and Si were sourced from anoxic seawater mixed with medium-high-temperature (>250 • C) hydrothermal fluids. The Nabeba BIF was likely deposited in a reducing marine environment.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/min11060579/s1, Table S1: Chemical composition of magnetite from the oxide facies BIF of the Nabeba deposit major element (wt.%) trace elements (ppm), Table S2: Chemical composition of magnetite from the carbonate facies BIF of the Nabeba deposit major element (wt.%) trace elements (ppm).