Abstract

The Endengue iron formations (IFs) belong to the Archaen Ntem greenstones belt at the NW edge of Congo craton and comprise magnetite-bearing gneiss associated with biotite-pyroxene gneiss and intruded by biotite granite and charnockite. The studied IFs are fine-grained; weak foliated rocks, composed of quartz-pyroxene-magnetite-plagioclase mineral assemblages, suggesting high-grade metamorphism. Whole-rock geochemical composition reveals that iron and silica are the main chemical components of the studied IFs with an average TFe2O3 + SiO2 of 93.06 wt-%, suggesting the purity of the chemical precipitation. In addition, the high average Si/Al (38.69); Fe/Ti (766.83) and Fe/Al (56.36) ratios and the Fe/Ti vs Al/(Al+Fe+Mn) plot suggest that the major components (> 80%) of the Endengue IFs are predominantly hydrothermal in origin. However, the slightly high Al2O3+TiO2 content (average of 2.14 wt-%) suggested a detrital input during their deposition. The studied IF samples with an average Total Fe content of 39.40 wt-%, low gangue (34.68 wt-% SiO2 and 1.05 wt-% Al2O3) and deleterious (P2O5: 0.07 wt-%) elements contents, correspond to accepted commercial low grade siliceous ore by global standards. The Edengue IFs shared chemical similarities with other Precambrian IFs worldwide.

1. Introduction

Iron formations are iron-rich chemically precipitate rocks, typically thin-bedded and/or finely laminated, commonly but not necessarily containing layers of chert with at least 15% iron ¹. They are usually found on Precambrian area generally associated to greenstone belt ². The banded iron formations (BIFs) with alternating Fe-rich and Si-rich bands which are the principal sources of iron ore in global steel industries with 90% of iron ore exploitation ³.

In Cameroon, BIFs occur within Archean to early Paleoproterozoic Ntem complex at the NW of Congo craton. This complex hosts many BIF-hosted iron ore deposits such as Mballam ^{4, 5}, Nkout ^{6, 7}, Bikoula ⁸, Ngovayang ⁹, Kouambo ¹⁰ and Gouap ¹¹ deposits (Figure 1a). Various iron-formation sequences, particularly those of Archean in age, are the metamorphic products of the iron minerals ¹². Previous studies have revealed that the Ntem Complex BIFs have been metamorphosed to various grade, ranging from greenschist to granulite facies metamorphism ^{6, 10, 11}. As result, most BIFs appear as magnetite quartzite and where the metamorphic grade was high (upper amphibolite to granutlite facies), as magnetite gneiss. In the Endengue area, metamorphosed iron formations have been recently discovered and no data are yet available on these formations. In this paper, we report the first detailed petrographical and whole rock geochemical data of the Endengue magnetite gneiss with the aim to discuss the origin and the iron ore potential of these rocks.

2. Geological Setting

The Endengue area lies within the Archean Ntem unit which belongs to the Ntem Complex greenstones belt at the northwestern edge of the Congo Craton in southern Cameroon (Figure 1b). The Congo craton is a vast Archean (> 2.5 Ga) unit which includes the Bangweulu block in Zambia; the Tanzanian craton, the Kasaї Angola and Congo Democratic Republic (CDR) block, the Tete block; the Chaillu craton in Gabon and Congo, the Bomu-Kibalien block extending in Congo, CAR (Central Africa Republic) and CDR and the Ntem complex in Cameroon ^{14, 15}. The Ntem complex is characterized by two orogenic cycles. The first cycle corresponds to Liberian orogeny and is marked by the formation of greenstone belt, followed by the intrusion of TTG (tonalite, trondhjemite, granodiorite) between 2900 and 2800 Ma. This cycle is ended by the anatectic potassic granitoid intrusion between 2600 and 2500 Ma ^{16, 17}. The second cycle comprises metasedimentary and metaigneous rocks form during the Eburnean high-grade tectono-metamorphic event. This event is marked by the intrusion of alkali syenite at ca 2300 Ma follow with the emplacement of doleritic dykes at 2100 Ma and the amphibolite to granulite facies metamorphism at 2050 ^{18, 19, 20, 21}. Zircon U-Pb ages and Hf-O isotopes data from the Ntem unit reveals that the charnockites crystallized at ca. 2.92 Ga while the trondhjemites and associated amphibolite protoliths crystallized synchronously at around 2.87-2.86 Ga ²². Recent works in this unit have reported the existence of well-preserved eclogites and serpentinized peridotites interpreted as related to supra subduction zone setting ^{23, 24, 25}.

3. Sampling and Analytical Methods

For this study, twelve fresh representative samples of magnetite gneiss were selected for both petrographical and geochemical studies. Sampling location sites are shown in Figure 1b. Polished thin sections were prepared at the Geotech Lab, Vancouver (Canada) using conventional techniques. Textural and mineralogical studies of these samples were carried out using optical microscopy at the University of Yaoundé I, Cameroon. Whole rock geochemical analysis was performed using the pulp by Inductively Coupled Plasma-Atomic Emission (ICP-AES) for major elements at ALS Minerals Global Group, Vancouver (Canada). The samples were pulverized to obtain a homogeneous sample out of which 50-60 g was obtained for the analyses, 0.2 g of rock powder was fused with 1.5 g LiBO2 and then dissolved in 100 mm 35% HNO₃. Analytical uncertainties vary from 0.1% to 0.04% for major elements. Loss on ignition (LOI) was determined by weight difference after ignition at 1000°C. Various standards were used and data quality assurance was verified by running these standards between samples as unknowns.

4. Results

The Endengue IFs are associated to biotite-pyroxene gneiss, biotite granite and charnockite (Figure 1b). They crop as flagstone (Figure 2a), dark gray to yellowish in color when and display finely alternation of light quartzo-feldspathic bands and dark iron-rich bands (Figure 2b). Under transmitted light, the magnetite gneiss displays a heterogranular granoblastic microstructure composed of quartz (33-40%), magnetite (30-38%), pyroxene (10-15%), amphibole (6-15%), plagioclase (5-12%) and secondary goethite (2-5%) (Figure 2c). Quartz is the most abundant mineral of the rock. It appears as subhedral to euhedral crystals forming silica-rich layers. Some quartz crystals are as inclusions in magnetite. Magnetite occurs as euhedral to anhedral crystals, 0.2 to 1.7 mm in size, disseminated within the rock, and sometimes associated with pyroxene and goethite (Figure 2d). Hydratation to goethite is common in the weathered samples (Figure 2e). Orthopyroxene appears as subhedral to euhedral crystals, < 1 mm in size, associated with plagioclase and magnetite (Figure 2c, d and f). Some pyroxene crystals show conspicuous transformation to magnetite. Amphibole crystals green in color, euhedral in shape. They are secondary minerals and represent the alteration products of pyroxene. Apatite and allanite are accessory mineral phases.

Biotite-pyroxene gneiss are gray-dark in color and crop as flagstones (Figure 3a). Under the microscope, the rock is composed of quartz, potassic feldspar, biotite, plagioclase, pyroxene and opaque minerals and displays a granoblastic microstructure (Figure 3b). Quartz (25-30%) is the most abundant mineral and appears as subhedral to euhedral crystals with variable sizes, ranging between 0.5 and 1 mm. Potassic feldspar (15-20%) crystals are euhedral in shape with grain size up to 1.8 mm and host muscovite flakes inclusions. Some crystals show almond-shape suggesting the mylonitisation effects. Biotite (15-20%) appears as small flakes (< 0.5 mm) frequently found in association with pyroxene and opaque minerals (Figure 3b). Pyroxene (5%) occurs as prisms of variable sizes (0.5 to 1.5 mm), often associated with quartz and opaque crystals (Figure 3b).

Biotite granites are medium to coarse-grained (Figure 3c) and display porphyritic microstructure compose of quartz (35-45%), potassic feldspar (30-40%), biotite (5-10%) and magnetite (<5%) (Figure 3d). Quartz appears as anhedral fine crystals, sometimes encountered as inclusion in K-feldspar crystals. The later occurs as subhedral to euhedral crystals with average grain size of 1.2 mm. Biotite exhibits distinct small flakes, subhedral in shape (Figure 3d). Magnetite minerals are present as anhedral crystals, < 1 mm size associated with biotite (Figure 3d).

Charnockites occur as massive blocks, pink in color and medium to coarse-grained (Figure 3e). The main minerals of this rock are quartz (25-30%), orthopyroxene (15-20%), potassium feldspar (15-20%), plagioclase (5-10%), biotite (3-8%) and magnetite (2%). Quartz occurs as anhedral crystals, 0.3 to 1mm in size, commonly associated to feldspar (Figure 3f). Orthopyroxene appear as large (0.3 to 1.2 mm) prismatic crystals associated with plagioclase and biotite (Figure 3f). Potassium feldspar is present as subhedral and euhedral crystals to up 1 mm in size, sometimes associated to plagioclase and quartz (Figure 3f). Biotite appears as very small crystals (< 0.5 mm in size), associated to the magnetite and orthopyroxene (Figure 3f).

Pearson’s inter-elements correlation matrix is presented in Table 2. The strong negative correlation (r= -0.88) between SiO₂ and Fe₂O₃ indicate a reverse distribution of silica and magnetite in the rock. We can notice a weak positive correlation (r= 0.54) between Al₂O₃ and CaO. Indeed, the Al₂O₃ and Fe₂O₃ displays weak negative correlation (r= -0.40), while Al₂O₃ with SiO₂, TiO₂, K₂O, MnO and Cr₂O₃ show weak to strong positive correlation (Table 2). The Endengue IFs show low concentrations of CaO and MgO, ranging from 0.01 to 0.52 wt-% (average of 0.16 wt-%) and 0.04 to 2.58 wt-% (average of 0.77 wt-%) respectively. The strong positive correlation between CaO and MgO (r= 0.80), indicate the mixing of MgO in carbonate ²⁸. The K₂O and Na₂O contents are strongly depleted in all samples (average =0.07 wt-% and 0.06 wt-%, respectively). MnO and P₂O₅ concentrations (0.04 to 0.14 wt-% average= 0.10 wt-%; 0.02 to 0.012 wt-%, average=0.07 wt-%, respectively) are generally very low in all samples. MnO and Fe₂O₃ show very weak positive correlation (r= 0.01). The LOI content range from -0.45 to 2.46 wt-%.

5. Discussion

The Endengue IFs are located within the high grade terrain of the Archean Ntem unit. Their mineral assemblages of orthopyroxene, amphibole and biotite contain key minerals that indicated high grade metamorphic conditions. The presence of apatite as accessory mineral in the investigated rocks, suggests metasomatic alteration. Assuming limited vertical phosphate remobilization during diagenesis, it is likely that the apatite precipitated either as two separate phases, preferentially with primary ferric hydroxides (now goethite) ³¹ or it was associated with the organic remains of planktonic bacteria that settled out to the seafloor ³². The destabilization of magnetite to goethite in the IFs is due to the precipitation of hydrothermal fluids process ³³. In addition, the presence of magnetite can also be explained by conversion of pyroxene to magnetite in the presence of hydrothermal fluid through diagenetic process and/or the conversion of goethite to magnetite. Some iron oxide laminar are derived from abundant hematite and/or goethite particles that were originally encapsulated in chert layers and subsequently liberated by removal of quartz during post-depositional deformation through dissolution and precipitation ³⁴. The alteration of orthopyroxene to magnetite observed in the Edengue IFs support the metamorphic and hydrothermal input. In addition, the studied IFs show conspicuous silicification which is attributed to silica overprint from hydrothermal fluids linked to late granitoid emplacement in the study area. Similar metasomatic signatures were observed in the Ngovayang IFs in the northwestern part of the Ntem Complex ⁹.

Iron formations can be resulted from interaction of shallow seawater, hydrothermal alteration of ocean crust, weathering of continental landmass, mixture of seawater hydrothermal fluids and weathering of continental crustal material ^{35, 36, 37}. Due to this complexity, different approaches mainly based on geochemistry, were suggested to investigate the origin of chemical components in IFs. The chemical statistics of the Endengue IFs shows high contents of total Fe₂O₃ + SiO₂ (average of 93.06 wt-%), similar to the TFe₂O₃ +SiO₂ values of the iron formations in the Ntem Complex ^{6, 9, 11, 28, 38, 39}, suggesting the purity of the chemical precipitation. Otherwise Al₂O₃ and TiO₂ are used as proxies to quantify clastic input, that should increase the Al₂O₃+TiO₂ concentration up to 2 wt-% in BIFs since they are immobile and cannot be introduced in solution during hydrothermal, diagenetic and weathering processes ^{27, 40}. The average Al₂O₃+TiO₂ content of 2.14 wt-% suggested a significant detrital input during deposition of the Endengue IFs. This clastic involvement is supported by the strong positive correlation between Al₂O₃ and TiO₂ (r= 0.93). Moreover, the strong positive correlation between Al₂O₃ with K₂O, Na₂O, MgO and CaO (Table 2) indicates that terrigenous clastic materials were involved during the deposition of the Endengue IFs which can be present in detrital particles ^{41, 42}. These results are comparable to those obtained in the Bikoula BIFs from the Ntem unit ⁸ and in the Ngovayang magnetite gneiss in the Nyong unit ⁹. However, Ganno et al. ²⁸ have suggested that the Elom BIFs, situated some 10 Km NE of the Endengue area, have recorded trivial detrital input.

According to their resistance and insoluble character, Al³⁺ and Ti⁴⁺ cations are used to identify the input in hydrothermal fluid and seawater during sedimentation ^{43, 44, 45}. Indeed, Si/Al, Fe/Al, and Fe/Ti ratios are high in hydrothermal fluids ^{11, 38}. Therefore, the average Si/Al (38.69); Fe/Ti (766.83); Fe/Al (56.36) ratios of the Endengue IFs are comparable to other BIFs in the Ntem Complex, and indicate the input of hydrothermal derived component during sediments deposition. Wonder et al. ⁴⁶ have proposed the use of Al₂O₃-SiO₂ diagram to discriminate the origin of BIFs. On this diagram, all the Endengue IFs samples (except LBR65) plot in hydrothermal origin field. The high detrital contaminated sample LBR65 falls on the boundary between the hydrothermal and hydrogenous fields (Figure 5). This result is supported by the Fe-Mn-Al ternary diagram ⁴⁸, that shown predominant hydrothermal origin of the Endengue IFs (Figure 6) similar to the Mesoarchean Bikoula BIFs ⁸. Since Fe₂O₃ and MnO are rich in components of hydrothermal fluids, many authors (eg. ^{11, 44, 49}) have proposed that the MnO/Fe₂O₃ ratio in BIFs characterized hydrothermal origin. The MnO/Fe₂O₃ ratio of the Endengue IFs is extremely low (0.07-2.5*10⁻³; average= 1.8*10⁻³), when comparable to the Kpwa-Atog Boga BIFs within the Paleoproterozoic Nyong unit ¹¹, suggesting a predominant hydrothermal origin.

To evaluate the degree of dilution of hydrothermal input by clastic or volcanic detritus, Barrett ⁵⁰ proposed a plot of Fe/Ti versus Al/(Al+Fe+Mn). The studied IF samples plot closer the hydrothermal Red Sea deposits and East Pacific Rise deposits, very far from the field of modern pelagic terrigenous sediments (Figure 7). From the above discussion, it can be suggested that the major components (> 80%) of the Endengue IFs are predominantly hydrothermal in origin.

An ore is a natural occurrence of rock or sediment that contains sufficient minerals with economically important elements, typically metals ^{51, 52}. The geological literature comprises several schemes for classifying ore minerals. Clout and Manuel ⁵³ classified iron ore in “ore group” related with the dominant mineral type, hardness, and texture. Moreover, there is no specific classification of iron ores based on their grade in fact that the price of iron is not standard. Based on the Fe₂O₃ concentration, exploited iron ores can be classified into three basic classes: (i) high-grade iron ores with a TFe₂O₃ content above 65 wt-%; (ii) medium-grade ores with Fe₂O₃ concentration fluctuated between 52 and 65 wt-%; and (iii) low-grade ores with Fe₂O₃ values 52 wt-% ⁵⁴.

The petrography studies of the Endengue magnetite gneiss have shown that they are well foliated and coarse-grained; hard; slightly weathered; dominated by quartz, magnetite and goethite. The major ore mineral (magnetite) is coarse grained and has a granoblastic heterogrnular texture. The geochemical statistic of the studied iron samples reveals the high content of Fe and Si which represent more than 96 wt-% of the average composition. Furthermore, the TFe₃ concentration which range from 35.98 to 43.96 wt-% (average of 39.40 wt-%) corresponds the low-grade iron ores by global standards ⁵⁴. Guider ⁵⁵ and Dobbins and Burnet ⁵⁶ proposed the tolerable concentration of deleterious elements (P and S) in the commercial iron ore should be lower than 0.07wt-% P and 0.1wt-% S respectively. The studied IF samples have low gangue (34.68 wt-% SiO₂ and 1.055 wt-% Al₂O₃) and deleterious (P: 0.07 wt-%) elements concentration.

To determine the origin of ore type, Angerer et al. ⁵⁷ have suggested the use of the whole-rock geochemical data of the iron ore and proposed the SiO₂/Fe₂O₃ and (MgO + CaO + MnO)/Fe₂O₃ discrimination diagram. Consequently, the higher (Mg + Ca + Mn)/Fe and lower Si/Fe ratios reveal carbonate-silicate metasomatism and the decrease in the Si/Fe and (Mg + Ca + Mn)/Fe ratios of the high-grade magnetite-hematite-goethite ores in all high-grade ore deposits worldwide. Moreover, this diagram highlights the ratio between primarily dominant silica and additional elements through carbonate-silicate alteration (mainly Mg, Ca, Mn) and hence differentiates unaltered siliceous BIF from BIF that was likely hypogene-altered and associated hypogene ore ⁵⁷. In this diagram most of the investigated samples plots in the field of medium-grade siliceous ore and four samples (LBR59, LBR 63, LBR 65 and LBR 69) fall in the low-grade siliceous BIFs field (Figure 7). This suggests that the Edengue IFs can be classified as low grade siliceous ore deriving from the BIF protore.

A comparison of the major element composition of Endengue IFs with Ngovayang IFs and BIFs from other parts of the Ntem Complex has revealed a nearer chemical similarity with other Precambrian BIFs worldwide (Table 3). The average concentration of Na₂O, K₂O, TiO₂ and P₂O₅ of the Endengue IF samples show close pattern with the Elom BIFs situated some 10 Km North of the studied area (Figure 9). The Al₂O₃, K₂O, Na₂O, CaO, MgO and P₂O₅ contents are depleted in Endengue IFs but high than the Elom BIFs. MgO and CaO are depleted in Endengue iron deposit than the Ngovayang IFs within the Nyong unit. The Al₂O₃ content is high in the Um Anab (Egypt) IFs, Bikoula BIFs and Ngovayang IFs than Endengue magnetite gneiss but lower in the Jerome (USA) IFs and Nkout BIFs. Additionally, the studied IFs display high Fe₂O₃ content when compare with the global Superior BIFs and Algoma BIFs (Figure 9).

6. Conclusion

The petrographical and geochemical analysis of the Endengue IFs has directed to the following conclusions:

(1) The IF samples are hard, finely banded and composed of magnetite, pyroxene, quartz and goethite. Goethite is the secondary mineral, resulting in the hydration of magnetite due to supergene alteration.

(2) Whole-rock composition of the studied IFs reveals that iron and silica are the main constituents (average Fe₂O₃ + SiO₂ of 93.06 wt-%), suggesting the purity of the chemical precipitation.

(3) Their high Si/Al, Fe/Al, Fe/Ti, and low Fe₂O₃/MnO ratios are consistent with a predominantly hydrothermal origin.

(4) The average TFe content of 39.40 wt-%, low gangue (34.68 wt-% SiO₂ and 1.05 wt-% Al₂O₃) and deleterious (P₂O₅: 0.07 wt-%) elements contents correspond to accepted commercial low grade siliceous ore by global standards

(5) The Edengue IFs shared chemical similarities with other Precambrian IFs worldwide.

Acknowledgements

This study is part of the PhD work of the senior author at the University of Dschang. We gratefully acknowledge the mineral exploration company Divine Mining Ltd for logistic support during the field trip. This is a contribution to the IGCP 646 project.

References