Abstract

The Sanaga magnetite bearing iron ore deposit is hosted in the eburnean Nyong complex which constitutes the northwestern edge of the Congo Craton. It is compose predominantly of magnetite bearing quartzite and magnetite-biotite gneisses related to charnockites and amphibole orthogneisses. In this study we use the composition of the magnetite bearing ore to determine their origin and ore formation process. A deposit model is also proposed for a better understanding of the emplacement of the iron ore. EMPA analysis on magnetite reveal variable amount of V, Ti, Al, and Mn. Most of the samples present Ti contents > 0.1%, this indicates a hydrothermal overprint. Although the texture and chemical composition of the magnetite bearing rocks neither represents typical skarn nor BIFs, on Ca + Al + Mn vs Ti + V and Ni + Cr vs Ti + V discrimination diagrams the magnetite reveals a double affinity for skarn and BIF. Elevated contents of Al, Mn and Mg in the magnetite signify crustal contamination while BIF signatures are related to hydrothermal activities. The variable content of V and Ti/V ratio suggests a mixture of reducing and oxidizing environments. On the Al + Mn vs Ti + V binary diagram the magnetite bearing ore reveal hydrothermal temperatures that vary between 200-300°C and 300-500°C. This suggests their precipitation from hydrothermal fluid with medium to high temperature and slight enrichment in Al and Ti. Integrating the data obtained from studies such as regional geology, ore geology and mineral microchemistry, we suggest that the Sanaga magnetite bearing iron ore deposit is similar to the Lake Superior iron ore type and was formed from transgression-regression in back arc basin or continental margin.

1. Introduction

Magnetite is a mineral that has been widely used as a petrogenetic indicator ¹, it is the most abundant oxide mineral in the earth’s crust. It is generally most resistant to alteration and transport than other mineral phases with which it coexists ². Moreover, it’s an index mineral with a wide range of applications in geophysical studies, igneous petrology and mineral exploration ^{1, 3}. Magnetite have AB₂O₄ as stoichiometric formula, with an average content of trace elements such as Al, Ti, V, Si, Ca, Mn and Mg ⁴ which can be used to discriminate iron ore types and ore forming processes. The substitution of cations in the different A and B sites probably takes place with low oxygen fugacity; thus Mn, Mg, Zn and Ni can substitute Fe²⁺ while Fe³⁺ can be replaced by Al, V and Cr ⁵. Magnetite is an accessory mineral generally found in igneous, metamorphic and sedimentary rocks. It is form under variables conditions such as high temperature crystallization in igneous rocks with silicates, sulphur and carbonates magmas or low temperature with hydrothermal fluids. This mineral occurs frequently in hydrothermal magnetite, Fe-Ti-(V) igneous and kiruna deposits. It also occurs commonly in skarn and porphyry Cu deposits. In mineral exploration, one of the toughest challenges is to detect geochemical signatures of proximal and distal deposits of the main mineralization.

In Cameroon several studies have targeted iron ore formations in the southern part of the country. These studies focused on banded iron formation [6-21], magnetite gneisses ^{22, 23, 24} and magnetite-martite bearing quartzite ²⁵ with the aim of determining their origin, age and depositional setting. Iron ore in the Sanaga prospect has been evaluated at 10 million tons (see www.africanmineral.com). Besides ²³ that evaluated the chemical composition of magnetite from the Nyong complex, studies which focus on determining the chemical composition of magnetite in Cameroon is rare.

In this study we provide new insight of the chemical composition of magnetite bearing iron ore in the Nyong complex. We present variable textures of magnetite bearing iron ore and their microchemical signatures with the aim to understand their origin and ore-forming processes. A deposit model is also provided for a better understanding of the emplacement of the iron ore.

2. Geological Background

The Sanaga magnetite bearing iron ore deposit lies within the Paleoproterozoic Nyong complex, which constitute the northwestern edge of the Archaean Congo Craton in southern Cameroon (Figure 1). The Nyong complex (Figure 1) forms part of the Precambrian geology of Cameroon; it constitutes the main Neoarchaean to Paleoproterozoic complex in Cameroon ^{23, 26, 27}. It’s a wide band with a NNE-SSW orientation and is composed mainly of migmatitic gneisses ^{28, 31}. It is a well preserved rock unit of the West Central African Belt (WCAB), situated on the Eburnean layer of the Congo Craton.

Lithologically, the Nyong complex is composed of charnockites, gneisses with composition of TTG, mafic-ultramafic rocks are expressed by pyroxenites and amphibolites; meta quartzites, Banded Iron Formation, granodiorite, syenites and augen diorites. Genetically, the Nyong complex is attributed to a proximal domain characterized by the remobilization of the Archaean adjacent cratonic crust ³⁰. According to ²⁷, the Nyong complex is characterized by a flat S1/S2 foliation, locally open folds and stretching lineation, all associated with N-S sinistral strike slip faults in the western edge ^{23, 28, 31, 32}. According to ²⁶ the Nyong complex has experienced poly-phase deformation with a granulite-amphibolite facies metamorphism attained in the eburnean.

The Sanaga prospect (Figure 2) is composed predominantly of charnockitic gneisses, amphibole bearing orthogneisses, magnetite-biotite gneisses that dips 10° to the NE and magnetite-martite bearing quartzite situated at the southern slope of the Mangombe hill, in the center of the studied area. The magnetite-martite bearing quartzite is oriented NE-SW evidence of an eburnean fingerprint. It is constituted of faults in the eastern and western parts that form a corridor between which exist a horst called Mangombè hill.

The magnetite-martite bearing quartzite is the main iron ore formation in this area. It is subdivided into two facies: banded and massive ²⁵. Both facies presents a succession of discontinuous quartz rich and magnetite-martite rich bands. Magnetite + quartz + pyroxene + martite ± amphibole ± biotite ± apatite is the main mineral assemblage. Magnetite varies from irregular crystals in the magnetite rich band to crystals associated with pyroxene in bands rich in quartz and isolated magnetite grain ²⁵. Their whole rock geochemistry shows that the magnetite-martite bearing quartzite has a sedimentary origin. According to ²⁵ the magnetite- martite bearing quartzite may have undergone a significant input from hydrothermal sources with fingerprints of the clastics materials during their deposition.

3. Sampling and Analytical Techniques

In this study, a total of eleven (11) representative samples of magnetite bearing ore were used for mineralogical and microchemical analysis at the petrology and mineralogy laboratory of the Department of Geosciences, University of Padova, Italy. Samples used for polished thin sections were cut using a rock cutter, to rectangular cubes (4×2.5×1 cm) and placed on a glass slide using araldite gum. It was then polished down to 0.3 mm. The mineralogical composition of the samples was determined using a petrographic microscope in transmitted and reflected light.

Their microchemical signature was realized with a CAMECA SX-100 Electron Probe Micro-Analyzer using a 1 µm spot size with acceleration voltage of 15 kV, and a beam current of 20 nA. Magnetite crystals were targeted to determine major and trace elements such as Mg, Al, Si, Ca, Ti, V, Cr, Mn, Fe, Co, Ni and Zn. The detection limit for major elements was set at 0.01% and 0.001 ppm for trace elements. Standards used include Si4+, Mg²⁺, Ti⁴⁺, Fe²⁺, Al³⁺, Mn²⁺, Zn²⁺, Ni²⁺, Cr²⁺, O²⁺, V³⁺, Ca²⁺,Co²⁺, for O, MgO new for Mg, Al₂O₃-20 for Al, Woll-20 for Si, Diopside-20 for Ca, MnTiO₃-20 for Ti, Vanadium for V, Cr₂O₃-20 for Cr, MnTiO₃-20 for Mn, Fe₂O₃-20 for Fe, Co for Co, NiO-20 for Ni and Blende (ZnS) for Zn.

4. Analytical Results

Magnetite in the Sanaga iron ore prospect is hosted by magnetite martite bearing quartzite and magnetite-biotite gneiss. The petrography and mineralogical composition of the magnetite martite bearing quartzite is presented in ²⁵. Two facies have been determined: the massive and the banded facies.

The magnetite-biotite gneisses show a characteristic augen texture with tiny bands of magnetite (Figure 3a). The gneisses present mylonitic shistosity with preferential oriented sigmoidal feldspar. It is mainly composed of microcline, biotite, quartz, pyroxene, plagioclase, aegerine-augite and magnetite (Figure 3b). Quartz, microcline and plagioclase show euhedral crystals and occur as disseminations. Plagioclase reveals a characteristic twin while magnetite occurs disseminated or as inclusions in biotite and aegerine-augite. Biotite also defines the rims of plagioclase and K-feldspars.

The microchemical composition of magnetite from the magnetite-martite bearing quartzite is summarized in Table 1. The magnetite show Fe₂O₃^T contents between 92 and 94 wt.%. Besides Fe₂O₃^T it is enriched in TiO₂, Al₂O₃, MgO and MnO. However the massive facie shows higher concentrations of TiO₂, MgO and MnO compared to the banded facie (Table 1). The concentration of others major oxides are relatively low. Cr₂O₃ show values that range from 0 to 0.04 wt.%; V₂O₃ varies between 0 and 0.083 wt.%. The concentrations of CoO and NiO are generally less than 0.017 wt.%. The samples show low concentrations of trace elements such as Co, Ni, Zn, Cr, V, Ti and Al. On binary plots the massive and banded facies of the magnetite bearing ore shows a positive correlation between Ti and Al; Al and Si (Figure 4).

The composition of magnetite from the magnetite-biotite gneiss is presented in Table 2. The concentration of Fe₂O₃^T in the magnetite varies between 91.26 and 94.45 wt%. Concentrations of SiO₂ (0.01-0.07 wt.%); Al₂O₃ (0-0.09 wt.%); V₂O₃ (0-0.08 wt.%); Cr₂O₃ (0-0.03 wt.%) and NiO (0-0.01 wt.%) are low while the content of CaO in the samples reach a maximum of 0.03wt%. On binary plots (Figure 4) the magnetite-biotite gneiss shows a positive correlation between Ti and Al as well as Al and Si (Figure 4). On continental crust normalized trace element diagrams after ³⁵, the magnetite presents similar spider patterns, with depletion in Si, Al, Ca, and enrichment in Mn, Mg, Ti, Zn, V, Ni and Cr (Figure 5). They are also enriched in Mn, Mg, Ti, Ni and Cr compared to the continental crust.

5. Discussion

Numerous studies have focused on the chemical composition of magnetite ore around the world ^{1, 3, 4, 35, 36, 37}. Trace element behavior in magnetite has been used as a tool for petrogenetic studies and the determination of provenance ³⁶. On the Ti vs Ni/Cr diagram after ³⁷ and/or ³⁸, magnetite can be classified as hydrothermal or magmatic in origin. If the magnetite has a hydrothermal affinity the Ni/(Cr + Mn) vs Ti + V or (Ca + Al + Mn) vs Ti + V diagrams after ³ can be used to differentiate its signature. It has been modified, by ¹ to differentiate magnetite into IOCG, Skarn, BIF, Fe-Ti, Kiruna and Porphyry types. If the magnetite shows a magmatic affinity, then the MgO/(MgO + Al₂O₃) diagram after ³⁹ can be used to differentiate the rock into felsic, mafic and intermediate domain.

The studied magnetite ore have a chemical composition characterized by enrichment in Si, Al, Fe, Mn, Mg, V, Ca and Ti. According to ³ and ¹ Ti and Cr are incompatible elements in magnetite during hydrothermal alteration. Magnetite with hydrothermal and BIF affinities have Ti< 2%; moreover, magnetite with typical BIF affinity have Ti < 0.1%. The magnetite in this study show Ti > 0.1%, this suggests a hydrothermal signature. Furthermore, on the Ti versus Al discrimination diagram of ³⁸ magnetite shows a hydrothermal (Figure 6) signature. However, discrimination diagrams after ³ reveals both skarn and BIF affinities (Figure 7) for the magnetite. This can be related to the behavior of Al in hydrothermal process. In this study, the massive facies is Al-rich compared to the banded facies; so Al, Si and Ca are incompatible elements in magnetite. Hydrothermal skarn is enriched in Mg, Al, Mn, Co, Ni and Zn ¹. We have shown that magnetite in the study area is enriched in Mg, Mn and Al with minor amounts of Cr, Co, Ni and Zn. This suggest that magnetite from the Sanage prospect is not skarn in origin. The high content of Al, Mn, and Mg in the magnetite suggests crustal contamination.

Although the magnetite show BIF signatures their texture and chemistry differs from that of typical BIF. BIFs are characterized by regular alternating and continuous bands, a negative Ce and positive Eu anomalies. According to ²⁵ the magnetite- martite bearing quartzite shows irregular discontinuous alternating bands. They lack a positive Eu and negative Ce anomalies. This therefore excludes the BIF setting ²³. BIF signature presented by the magnetite can be attributed to hydrothermal activity at temperatures < 500°C.

Hydrothermal magnetite has several factors that controls their formation such as: temperature, pressure, fluids composition, oxygen fugacity, host rock composition or coexists minerals ^{1, 40}. In natural fluids, V can be present as V³⁺, V⁴⁺ and V⁵⁺. Oxygen fugacity (fO2) may control the V content in magnetite. Moreover, only V³⁺ can be highly partitioned into magnetite. ¹ indicate that V can be enriched in magnetite formed from reducing fluids. Magnetite formed in reducing fluids has lower Ti/V ratios relative to those in oxidizing fluids ⁴⁰. The magnetite bearing ore from the Sanaga prospect show high concentrations of V in the magnetite-biotite gneiss compared to the magnetite-martite bearing quartzite. Ti/V ratios for biotite gneisses are low (range between 0.0069-1.33), only one sample shows a high ratio (12.28). Thus the magnetite from the magnetite-biotite gneisses was formed under reducing conditions. Ti/V ratios calculated for magnetite-martite bearing quartzite are variables in the massive facie. Ratios range between 0.38 and 9.06 for the massive facie while the banded facie reveals ratios that vary from 0.06 to 2.17. This suggests that the magnetite-martite bearing quartzite was formed in an environment with both reducing and oxidizing conditions.

Temperature may have an influence on few elementals contents in magnetite, such as Mg, V, Ti and Al. Incorporation of these elements in magnetite is evident in magmatic systems with high temperatures, but they are immobile in low-temperature hydrothermal fluids ⁴¹.

High contents of Al, V, Ti and Mn in magnetite suggest that magnetite samples are related to volcanic rocks ⁴⁰. Magnetite from the magnetite-martite bearing quartzite and biotite gneiss show variable content of Al, V, Ti and Mn in the (Al + Mn) vs (Ti + V) diagram that discriminates the formation temperatures of magnetite ¹. Most of the samples from the magnetite-martite bearing quartzite and biotite gneiss plot in the 200-300°C field while few sample plot in the 300-500°C field (Figure 8). This suggests that magnetite in the Sanaga prospect was precipitated from medium to high temperature fluids slightly enriched in Al and Ti. The slight enrichment in Al + Mn and Ti + V also suggests that the magnetite-martite bearing quartzite is not from a volcanic origin. According to ¹, coexisting silicates and sulfides phases have an important compositional control in hydrothermal magnetite. Silica and sulfide minerals preferentially incorporate chalcophile and lithophile elements respectively ^{1, 40}. The Sanaga magnetite iron ore is particularly composed of magnetite, without sulfides and variable amount of silicates (quartz and pyroxene). The studied magnetite indicates low concentrations of Ti, V and Cr thus the presence of the silicate phase did not affect their concentration in the hydrothermal magnetite. This situation may explain that minor amount of silicates phases in the magnetite-martite bearing quartzite did not affect the composition of magnetite during hydrothermal process.

Recently, ²⁵ and ⁴² showed that the Sanaga iron ore are formations with sedimentary origin. They are different from BIF by lack of Ce anomaly and textural evidence. Therefore, they are related to rocks of the Nyong complex which represent the NW edge of the Congo Craton. Compared to the Algoma type, the magnetite-martite bearing quartzite have low amount of transition metals such as Ni (mean 24.8 ppm), Co (mean 4.3 ppm) and V (mean 20.5 ppm) (see ²⁵). This suggests that the Sanaga iron ore are not volcanic in origin. These values are similar to those of Lake superior iron formation reported by ⁴³ (32 ppm; 27 ppm; 30 ppm respectively). Moreover the Sanaga iron ore is situated at the edge of the Congo Craton. We can therefore suggest that the mode of formation of the Sanaga iron ore deposit is similar to that of the Lake superior type. However, ⁴⁴ reported that rocks formed in a rift context or residual sea is enriched in LREE because these environments are close to the continent. The magnetite-martite bearing quartzite presents enrichment in LREE (see ²⁵) with signatures of crustal contamination. Moreover all discriminating diagrams show that these magnetite-martite bearing quartzites are formed by hydrothermal activity. Thus we suggest that the Sanaga iron ore was deposited in continental margin or back arc basin like the Lake superior iron formations by transgression-regression phenomenon with a combination of three process: (1) hydrothermal solutions and fluvial activity from which Fe, Si and REE precipitated; (2) terrigenous sedimentation; (3) oxidation, producer of iron formation (Figure 9).

6. Conclusions

The main conclusions from this work are as follows:

1) The magnetite martite bearing quartzite and the magnetite-biotite gneiss are the main sources of magnetite in the Sanaga prospect.

2) Magnetite from the study area show a hydrothermal signature on discrimination diagrams. This is supported by Ti contents > 0.1 %.

3) Although the magnetite show signatures of skarn and BIF on discrimination diagrams, they lack characteristic signatures of skarn. High contents of Al, Mn and Mg in the magnetite can be attributed to crustal contamination. The magnetite martite bearing quartzite lacks characteristic textures and chemical signatures of BIF. Their BIF signature can be related to hydrothermal activities at temperatures < 500°C.

4) The behavior of V and Ti/V ratio calculated suggest a mixture of reducing and oxidizing environment for the magnetite. Furthermore, magnetite grains on Al + Mn vs Ti + V diagrams show moderate to high temperatures with a slight enrichment in Al and Ti.

5) Sanaga iron deposit have a minor amount of transition metals such as Ni, Co and V suggesting that they are not related to volcanic rocks but similar to Lake Superior iron formation type. Ore deposition took place in continental passive margin or back arc basin by transgression-regression.

Acknowledgements

We are grateful to Prof. Paolo Nimis and Prof Claudio Mazzoli of the Department of Geosciences, University of Padova, Italy for their availability and assistance in the acquisition of the EMPA data. We are also gratefully to anonymous reviewer and the Editor Chief whose comments improved on the quality of this work.

References