| Citation: | Landry Soh Tamehe, Huan Li, Sylvestre Ganno, Zuxing Chen, Yanick Brice Lemdjou, Safiyanu Muhammad Elatikpo. Insight into the Origin of Iron Ore Based on Elemental Contents of Magnetite and Whole-Rock Geochemistry: A Case of the Bipindi Banded Iron Formations, Nyong Complex, SW Cameroon. Journal of Earth Science, 2024, 35(1): 16-28. doi: 10.1007/s12583-022-1622-4
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The Bipindi iron ore district is located in the central section of the Nyong Complex at the northwestern margin of the Congo Craton in Southwest Cameroon. This iron district contains numerous iron mineralization hosted by the Mewongo, Bibole, Kouambo, and Zambi banded iron formations (BIFs). These BIFs contain magnetite as the main iron ore mineral associated with pyrite, and gangue minerals are quartz with minor chlorite and amphibole. The origin of iron ore from these BIFs was investigated using a combination of
Archean to Proterozoic terranes commonly host iron ore deposits that occurred in banded iron formations (BIFs). The BIFs are widely defined as chemical sedimentary rocks characterized by alternating thin to thick layers of iron oxide-rich and silica-rich minerals (Trendall, 2002). These rocks have received much attention during the last decades since they provide an excellent opportunity to decipher the Earth environmental history, the chemistry of ancient seawater and continents as well as representing the main source for iron ore worldwide (cf. Hagemann et al., 2016; Bekker et al., 2014). Nevertheless, the origin of BIF-hosted iron ore continues to be greatly debated due to the effects of post-depositional processes (e.g., hydrothermal alteration, metamorphism, and weathering) that most BIFs have experienced (cf. Hagemann et al., 2016). These processes have commonly obliterated the primary features of the BIFs and influenced their chemical compositions. Some elements such as high field strength elements (HFSE) and rare earth elements (REE) remain immobile (e.g., Bolhar et al., 2004; Bau and Dulski, 1996). These elements are used widely to unravel the genesis of BIF-hosted iron ore deposits (e.g., Wang et al., 2015; Spier et al., 2008).
The BIF-hosted high-grade iron ore mainly comprises martite and hematite as the main ore minerals, which are usually attributed to hypogene process and supergene hydrothermal enrichment of BIFs (e.g., Hagemann et al., 2016; Hensler et al., 2015). Magnetite is a common ore mineral in BIF-hosted low-grade iron ore (e.g., Hagemann et al., 2016; Li et al., 2014). Since magnetite can include considerable major and trace elements in its crystalline structure, the chemical composition of this mineral can provide valuable information about the physiochemical conditions of iron ore-forming processes. Compared to the traditional electron microprobe analyses (EPMA), the improvement of in-situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) technique provides a larger number of trace elements for magnetite with detection limits of low to sub ppm (e.g., Sun et al., 2017; Dare et al., 2014; Nadoll et al., 2014). The chemical composition of magnetite by LA-ICP-MS has thus enhanced our understanding of iron ore-forming processes and controlling factors for magnetite formation (e.g., Sun et al., 2020; Zhou et al., 2017).
Recent efforts in mineral exploration have led to the discovery of numerous BIF-hosted iron ore occurrences and deposits in southern Cameroon (Fig. 1; Swiffa Fajong et al., 2022; Soh Tamehe et al., 2019; Ndime et al., 2018; Teutsong et al., 2017; Ganno et al., 2016). These BIFs are mainly distributed in greenstone belts of the Nyong and Ntem complexes, which represent the northern edge of Congo Craton (Fig. 1a). The Nyong and Ntem complexes BIFs have been the focus of research projects in recent years. In light of the geochemical and geochronological studies, the tectonic setting and depositional age of these BIFs were investigated (Deassou Sezine et al., 2022; Djoukouo Soh et al., 2021; Nzepang Tankwa et al., 2021; Soh Tamehe et al., 2021; Ndime et al., 2019). The Nyong Complex BIFs were probably deposited during the peak in BIF abundance between ca. 2.50 and 2.40 Ga (cf. Soh Tamehe, 2020), while BIF deposition occurred at ca. 2.8 Ga on the Ntem Complex (cf. Chombong and Suh, 2013). Hence, large occurrence of ca. 2.50–2.40 Ga BIF-hosted iron mineralization of the Nyong Complex can be economically exploited like their correlative BIF-hosted iron ore grade of the Quadrilátero Ferrífero region in Southeast Brazil. Although magnetite is a valuable pathfinder mineral in geochemical studies, the chemical composition of this mineral has been rarely documented for the Nyong and Ntem complexes BIFs. Therefore, the source and mechanism of Fe mineralization remain unclear to date.
This paper focuses on four BIF-hosted magnetite of the Bipindi iron ore district including the Mewongo, Bibole, Kouambo, and Zambi BIFs located in the central part of the Nyong Complex (Fig. 1a). A combination of in-situ magnetite major and trace element chemistry and whole-rock geochemistry was applied to investigate the ore forming conditions and constrain the magnetite source. This new data contributes to improve our understanding of the magnetite formation and controlling factors for the magnetite composition of the Bipindi BIFs deposited at the margin of the Congo craton.
The geological setting of the Nyong Complex is well documented in previous studies (Soh Tamehe et al., 2021; and references therein). The Complex generally comprises BIFs, gneisses, amphibolites, TTG suite, quartzites, schists, eclogites, serpentinites, metasyenites, metagranodiorites, and metagabbros (e.g., Soh Tamehe et al., 2022; Tsoungui et al., 2020; Houketchang Bouyo et al., 2019; Loose and Schenk, 2018; Lerouge et al., 2006). The volcano-sedimentary succession of the Nyong Complex was deposited at ca. 2 466–2 422 Ma, and undergone metamorphism during the Eburnean-Transamazonian orogeny (ca. 2 080–2 050 Ma) and the Pan-African overprint between 620 and 500 Ma ago (U-Pb zircon; Djoukouo Soh et al., 2021; Soh Tamehe et al., 2021; Nzepang Tankwa et al., 2021; Lerouge et al., 2006; Toteu et al., 1994). Plutonic activity occurred in the Nyong Complex at ca. 2 100–2 000 Ma, and it was associated with charnockite formation at ca. 2 050 Ma and high-grade metamorphism which continued probably up to 1 985 ± 8 Ma (U-Pb zircon; Lerouge et al., 2006; Toteu et al., 1994). Loose and Schenk (2018) constrained the age of eclogite-facies metamorphism at 2 093 ± 45 Ma (SHRIMP U-Pb zircon) for the Nyong Complex. The pressure and temperature conditions of metamorphism have been constrained at 16–25 kbar and 800–850 ℃ (Houketchang Bouyo et al., 2019). These eclogites are further associated with charnockites and mafic granulites of ca. 2 050 Ma age (SHRIMP U-Pb zircon; Loose and Schenk, 2018).
The Nyong Complex BIFs are commonly interbedded with volcanic and sedimentary rocks, which have regionally experienced amphibolite- to granulite-facies metamorphism (Nzepang Tankwa et al., 2021; and references therein). The contact between these BIFs and associated rocks is generally sharp and well-defined without sign of discordance (e.g., Deassou Sezine et al., 2022; Djoukouo Soh et al., 2021; Soh Tamehe et al., 2018). The Nyong Complex BIFs are characterized by relatively thin layers (mesobands), with a thickness varying between 1 and 5 km. The mesobanding structure consists of rhythmically alternating microlayers of Fe-oxide and quartz (band width of 0.1 to 3 mm). The dominant structure in these BIFs is more likely to be a tectono-metamorphic foliation overprinting primary sedimentary structure (Swiffa Fajong et al., 2022; Moudioh et al., 2020; Ganno et al., 2016), corresponding to the NE-SW trending S1 foliation with gentle (15°–20°) to steeply dips (~70°), towards the NW. The Nyong Complex BIFs have recorded a D2 deformation event associated with the development of C2 ductile shear planes and F2 isoclinal folds. Veins and joints have been identified in these BIFs, and correspond to the latest D3 deformation event (e.g., Soh Tamehe et al., 2019; Ganno et al., 2017).
The Kpwa-Atog Boga, Bibole, Kouambo, and Mewongo BIFs are located in the central part of the Nyong Complex, where iron ore occurrences have been recognized (Fig. 1b; Djoukouo Soh et al., 2021; Kwamou et al., 2021; Soh Tamehe et al., 2018; Ganno et al., 2017). All these BIFs host hard iron ore type with magnetite being the major iron mineral. However, the replacement of magnetite by martite is commonly observed in the Bibole and Kouambo BIFs, which might result from near-surface oxidation (Djoukouo Soh et al., 2021). The Kouambo BIFs host low-grade iron ore (TFe < 40 wt.%), with unknown resource (Ganno et al., 2017). The iron ore grade and total resource of other BIFs are not defined yet. In all BIF-hosted iron occurrences, the major gangue mineral is quartz and minor chlorite and pyrite that are distributed as veins of few millimeters to few centimeters in thickness along fractures in magnetite.
The BIF-hosted iron ore deposits are located in the northern, central, and southern sectors of the Nyong Complex (Figs. 1a–1b). The BIF-hosted iron ore deposits of northern Nyong Complex comprise the Ngovayang, Sanaga, Kelle-Bidjoka, and Gouap deposits (Swiffa Fajong et al., 2022; Nzepang Tankwa et al., 2021; Soh Tamehe et al., 2019). The latter deposit is the most studied iron deposit of the Nyong Complex although its iron ore resource is not defined yet (Soh Tamehe et al., 2021, 2019). The Gouap deposit hosts soft and hard iron ore types. The soft iron ore type comprises hematite-martite and magnetite-hematite in western Gouap, whereas the hard type consists of magnetite in its southern part. The Sanaga, Ngovayang, and Kelle-Bidjoka deposits host hard magnetite as the main iron ore mineral. The Sanaga iron deposit contains a total inferred resource of 82.9 Mt @ 32.1% Fe). The iron ore resource of the two latter deposits is still unknown. The gangue minerals comprise quartz, amphibole, and pyroxene in all these iron ore deposits.
The Zambi, Anyouzok, and Mamelles BIF-hosted iron deposits lies to the central and southern Nyong Complex, respectively. The Zambi deposit hosts low-grade magnetite ore (TFe < 52 wt.%; Ganno et al., 2016), which is planned to be in production within the next five years. The Anyouzok deposit contains magnetite ore resource of 96.9 Mt at 34.92 wt.% Fe indicated and 79.4 Mt at 35.04 wt.% Fe inferred. The Mamelles deposit is one of the least studied iron deposit of the Nyong Complex although a prefeasibility study revealed an estimated resource of 632.82 Mt @ 35 wt.% Fe, with an annual production of 4 Mt/year at the first stage. Iron ores of this deposit are mainly composed of martite, goethite, and quartz, with minor amounts of magnetite, kaolinite, and halloysite (Teutsong et al., 2021).
The studied BIF-hosted iron ore samples comprise the Mewongo, Bibole, Kouambo, and Zambi BIFs, which are located in the Bipindi iron ore district (Fig. 1b). Little is known about this iron ore district situated at the central part of the Nyong Complex.
The Mewongo BIFs are poorly documented. These BIFs are associated with amphibolites and crosscut by dolerite dykes (Fig. 2a). Geochemical studies of the Mewongo amphibolites revealed their basaltic protoliths and tholeiitic signatures, suggesting that they were formed in an island geotectonic setting (Kwamou et al., 2021). LA-ICP-MS U-Pb zircon dating of these rocks yielded a metamorphic age of 2 050 Ma (Kwamou et al., 2021).
The Bibole BIFs are generally interbedded with quartz-chlorite schists and phyllite (Fig. 2b). The BIFs are characterized by relatively thin layers of 1–3 m thick. Geochemical studies of the Bibole BIFs revealed that they were deposited in a submarine volcanic arc environment, similar to the Algoma-type BIFs (Djoukouo Soh et al., 2021). These BIFs were deposited at ca. 2 466 Ma and experienced metamorphism and metasomatism at ca. 2 078 Ma during the Eburnean-Transamazonian orogeny (SIMS zircon U-Pb dates; Djoukouo Soh et al., 2021).
The Kouambo and Zambi BIFs are found as discontinuous lenses associated with garnet-pyroxene gneisses, chlorite-schists, and epidote-amphibolites (Fig. 3). The rock sequences form a NE-SW trending ridge of about 8 km long (Moudioh et al., 2020; Ganno et al., 2017). The depositional age is not yet constrained.
A total of twenty-two BIF iron ore samples were collected from field outcrops throughout the Bipindi iron ore district, including seven Mewongo BIFs (MWB), six Bibole BIFs (BOB), five Kouambo BIFs (KOB), and four Zambi BIFs (ZAB) samples. Proper care was taken to collect unweathered rocks for petrographic, mineralogical, and geochemical analyses.
All the studied BIFs are composed of magnetite as the principal iron mineral and quartz and less amphibole, chlorite, biotite, and pyrite as the gangue minerals (Fig. 4). Biotite is common in the MWB (Fig. 4b), while chlorite is mainly found in the BOB and ZAB (Figs. 4e, 4k). Amphibole is abundant in the KOB and ZAB (Figs. 4h, 4k), while pyrite is common in the BOB and ZAB relative to other BIFs (Figs. 4f, 4l). The replacement of magnetite by martite is evident in the MWB and ZAB (Figs. 4c, 4l). Magnetite appears as the major constituent of Fe-rich layers (> 70 vol.%) and is less abundant (< 10 vol.%) in Si-rich layers, which predominantly comprise quartz. Magnetite is characterized by its dark gray and subhedral to anhedral shape with a grain size up to 300 µm. Some magnetite crystals often host quartz and amphibole inclusions (Figs. 4h, 4k).
Quartz is present as subhedral to anhedral crystals with size up to 500 µm. It generally forms irregular aggregates and is rarely found as individual grains within the Fe-rich layers (Figs. 4b, 4k). Inclusions of biotite, chlorite, and magnetite are common in quartz crystals.
Polished thin sections were prepared for selected BIF samples in order to perform major and trace element analyses. Major element compositions of BIF magnetite were determined using an electron microprobe (JXA-8230) at the State Key Laboratory of Continental Dynamics (SKLCD), Northwest University, Xi'an, China. The instrument was operated at an acceleration voltage of 15 kV, beam current of 10 nA and beam diameter of 1 μm. Microprobe standards of natural and synthetic phases were supplied by SPI Company.
In-situ analyses of trace elements for BIF magnetite were further performed at the SKLCD using a ASI RESOlution 193 nm excimer laser ablation (LA) system coupled with an Agilent 7900 inductively coupled mass spectrometer (ICP-MS) instrument. Helium was applied as a carrier gas, and argon was used as the make-up gas and mixed with the carrier gas via a T-connector before entering the ICP. Each analysis incorporates an approximately 20 s background acquisition followed by 45 s of data acquisition from the sample. Analytical spots (60 μm) were ablated by 160 successive laser pulses (4 Hz). Element contents were calibrated against multiple-reference materials (GSE-1G, BCR-2G, BIR-1G, BHVO-2G, and NIST610) using 57Fe as the internal standard (Liu et al., 2008). Every 8 sample analyses were followed by three analyses of GSE-1G, BHVO-2G, and BCR-2G as quality control to evaluate the quality of analyses. Off-line selection and integration of background signals, and time-drift correction and quantitative calibration were performed by ICPMSDataCal (Liu et al., 2008).
The whole-rock geochemical analyses were performed at ALS (Australian Laboratory Services) in Guangzhou, China. Prior to analysis, each rock sample was powdered to < 200 mesh using an agate ball mill. Major elements were determined with wavelength X-ray fluorescence (XRF) spectrometry using a Shimadzu EDX 700 spectrometer with an analytical precision of < 3%. The XRF analyses were conducted using pressed-powder discs with the international standard JG-2 as a reference. Then, loss-on-ignition (LOI) was determined after igniting the sample powder at 1 000 ℃ for about an hour. Trace and rare earth elements were analyzed by inductively coupled plasma mass-spectrometry (ICP-MS) using an Agilent 7700e spectrometer, with an analytical precision of ~5%–10%. Finally, a sample powder of ~50 mg was dissolved in a combined lithium borate and aqua regia solution. Internal standard JG-2 was routinely analyzed to monitor analytical quality.
A total of 30 analyses were performed on magnetite samples from the Bipindi BIFs-hosted iron ore, and the analytical results of major elements are presented in the Electronic Supplementary Material Table S1.
The EPMA data indicate that the MWB magnetite shows high iron content (expressed as FeO), ranging from 91.87 wt.% to 93.48 wt.%. The Al2O3, SiO2, and Cr2O3 contents are low in the MWB magnetite (≤ 0.05 wt.%). The contents of TiO2, Na2O, and MgO are low, with values ≤ 0.07 wt.%. The MWB magnetite contains low concentrations of MnO (≤ 0.08 wt.%). The K2O and CaO contents are very low in the MWB magnetite (≤ 0.02 wt.%).
The BOB magnetite has high FeO concentrations of 91.05 wt.%–93.64 wt.%. The contents of Al2O3 and Cr2O3 are relatively low, with values ≤ 0.07 wt.% and ≤ 0.11 wt.%, respectively. The BOB magnetite displays low concentrations of SiO2 and TiO2 with values ≤ 0.08 wt.%. The MgO and MnO display low contents with values ≤ 0.05 wt.% and ≤ 0.08 wt.%, respectively. Other oxides such as K2O, Na2O, and CaO have very low contents in the BOB magnetite (≤ 0.02 wt.%).
The KOB magnetite exhibits high FeO contents of 92.06 wt.%–93.55 wt.%. The Al2O3 contents are moderate ranging from 0.37 wt.% to 0.79 wt.%. The KOB magnetite displays moderately low contents of Cr2O3 and MgO with values varying between 0.02 wt.% and 0.09 wt.%. Concentrations of TiO2 (0.06 wt.%–0.74 wt.%) are relatively low in the KOB magnetite. The contents of SiO2 and MnO are low with values ≤ 0.04 wt.% and ≤ 0.12 wt.%, respectively. The K2O and Na2O contents are very low (≤ 0.02 wt.%).
The ZAB magnetite shows high FeO contents of 92.68 wt.%–93.57 wt.%. Concentrations of Al2O3 and Cr2O3 are moderately low, with the corresponding values ranging between 0.02 wt.%–0.06 wt.% and 0.01 wt.%–0.11 wt.%, respectively. The ZAB magnetite displays low contents of SiO2 (≤ 0.04 wt.%) and TiO2 (≤ 0.02 wt.%). The Na2O and MnO contents are low, with values ≤ 0.03 wt.% and ≤ 0.08 wt.%, respectively. The contents of K2O, CaO, and MgO are very low (≤ 0.01 wt.%).
The dataset of trace element concentrations of magnetite from the Bipindi BIFs-hosted iron ore includes 40 analyses, and the analytical results are given in the Electronic Supplementary Material Table S2.
The magnetite crystals from MWB exhibit higher contents of Fe (736 159 ppm–754 245 ppm), Al (244.13 ppm–1 212.94 ppm), Si (538.41 ppm–10 751.95 ppm), Cr (22.89 ppm–1 055.81 ppm), Ti (76.43 ppm–260.89 ppm), Mn (321.56 ppm–387.88 ppm), and Mg (90.98 ppm–294.05 ppm). The MWB magnetite shows wide range of K (1 ppm–101.71 ppm) and Na (0.20 ppm–222.44 ppm). They have moderately low concentrations of Zn (21.96 ppm–33.73 ppm), V (12.92 ppm–23.35 ppm), Ni (15.54 ppm–19.06 ppm), Ga (2.67 ppm–4.05 ppm) and Co (2.23 ppm–3.61 ppm). Contents of Cu, Sc, Rb, Sr, and Zr are close to or below detection limits.
The magnetite grains from BOB also have large range of Fe (747 821 ppm–752 620 ppm), Ti (373.56 ppm–2 861.10 ppm), Si (902.39 ppm–1 675.56 ppm), Al (364.52 ppm–726.76 ppm), Cr (209.49 ppm–388.92 ppm), Mn (303.54 ppm–830.98 ppm), and Mg (108.36 ppm–302.74 ppm). They have relatively high contents of Zn (40.09 ppm–52.99 ppm), Cu (7.83 ppm–59.76 ppm), and Ni (15.77 ppm–26 ppm). Concentrations of V (8.64 ppm–28.44 ppm), and Co (4.86 ppm–6.11 ppm) are moderately low. The BOB magnetite is depleted in other elements except Ca, Na, and K.
Magnetite in KOB has higher concentrations of Fe (638 935 ppm–748 889.88 ppm), Mg (269.11 ppm–19 688.04 ppm), Si (604.44 ppm–52 004.13 ppm), Al (2 896.77 ppm–4 209.49 ppm), Ti (646.71 ppm–941.39 ppm), Cr (129.22 ppm–963.59 ppm), and Mn (120.77 ppm–662.52 ppm). They display a wide range of K (1 ppm–1 430.48 ppm) and Na (1 ppm–170.66 ppm), and relatively high concentrations of V (39.48 ppm–49.07 ppm) and Ni (32.06 ppm–41.55 ppm). The KOB magnetite show moderately low concentrations of Co (5.51 ppm–6.27 ppm) and Zn (6.47 ppm–25.35 ppm). Except for Ca, the contents of other elements (e.g., Cu, Sc, and Zr) are very low.
The ZAB magnetite has comparable high contents of Fe, Al, Si, Ti, Cr, Mg, and Mn to that from other BIF ore, with the corresponding values of 731 053 ppm–750 906 ppm, 1 816.55 ppm–8 451.01 ppm, 631.09 ppm–2 461.07 ppm, 42.09 ppm–3 535.79 ppm, 16.04 ppm–543.36 ppm, 186.87 ppm–2 091.25 ppm, and 147.91 ppm–675.24 ppm, respectively. The ZAB magnetite has the highest concentrations of Zn (22.99 ppm–491.9 ppm), Ni (24.79 ppm–55.46 ppm), and Co (3.20 ppm–24.65 ppm). The contents of V (7.16 ppm–42.71 ppm), Na (1 ppm–31.95 ppm), and K (1 ppm–9.46 ppm) are moderately low. With the exception of Ca, the range concentration of other elements in this magnetite is very low.
Whole-rock major and trace element concentrations of the Mewongo, Bibole, Kouambo, and Zambi BIFs-hosted iron ore are presented in the ESM, Table S3. The contents of all these BIFs are almost similar.
The studied BIFs are characterized by high concentrations of Fe2O3 with values of 56.31 wt.%–0.03 wt.% (MWB), 47.59 wt.%–58.20 wt.% (BOB), 52.52 wt.%–57.43 wt.% (KOB), and 44.59 wt.%–62.90 wt.% (ZAB). The Fe contents of the MWB, BOB, KOB, and ZAB range from 39.8 wt.%–43.2 wt.%, 33.3 wt.%–40.4 wt.%, 36.1 wt.%–39.2 wt.%, and 30.9 wt.%–43.1 wt.%, respectively. These BIFs have moderate SiO2 contents of 37.96 wt.%–41.42 wt.% (MWB), 38.72 wt.%–49.87 wt.% (BOB), 39.11 wt.%–43.90 wt.% (KOB), and 32.34 wt.%–51.66 wt.% (ZAB). The studied BIFs contain low to deficient concentrations of other major elements such as Al2O3 (mean ≤ 1.85 wt.%), MgO (mean ≤ 3.66 wt.%), MnO (mean ≤ 0.48 wt.%), and TiO2 (mean ≤ 0.08 wt.%).
All the studied BIFs have low contents of high field strength elements (HFSE), such as Zr (≤ 17 ppm) and Hf (≤ 0.5 ppm). Compared to other BIFs, the Bibole and Kouambo BIFs display high contents of Ba with range values of 10 ppm–140 ppm (BOB) and 110 ppm–550 ppm (KOB). Other large ion lithophile elements (LILE) have moderate to low contents of Sr (≤ 81.7 ppm) and Rb (≤ 11.6 ppm) in all the BIF samples. The contents of some base metals are moderate in the BOB (Co: 4.2 ppm–7.3 ppm; Cu: 14.9 ppm–90 ppm; Zn: 16 ppm–20 ppm) and ZAB (Co: 2.1 ppm–17.9 ppm; Cu: 4.6 ppm–53.7 ppm; Zn: 17 ppm–201 ppm) compared to other BIF samples.
The origin of magnetite in BIFs has long been debated. Most previous studies have demonstrated that magnetite can be sourced mainly from hydrothermal-derived fluids or continental material (e.g., Manikyamba et al., 1993; Dymek and Klein, 1988). Involvement of magmatic fluids has been proposed for the formation of magnetite in BIF (e.g., Bhattacharya et al., 2007). On the other hand, the common occurrence of magnetite in magmatic and hydrothermal iron ore-forming environments has led to discriminate between magmatic and hydrothermal source for magnetite (e.g., Knipping et al., 2015; Dare et al., 2014). Since the trace element abundance can reflect either the parent magma or resultant fluids (e.g., Song et al., 2021; Hu et al., 2020, 2014; Canil et al., 2016; Chung et al., 2015), the contents of trace elements in magnetite are useful to distinguish between magnetite precipitated from hydrothermal fluids from those derived from silicate melts.
Hydrothermal magnetite formed at lower temperature has low contents of Al and Ti compared to magmatic magnetite, which contains elevated values of these elements (e.g., Nadoll et al., 2014). The latter magnetite is abundant in silicate melts and exhibits high partitioning coefficients between this mineral and the silicate melt at high temperature (e.g., Canil et al., 2016). On the plot of Ti versus Al (Canil et al., 2016), all the magnetite samples from the studied BIFs cluster into the hydrothermal field (Fig. 5a), suggesting that the MWB, BOB, KOB, and ZAB magnetite are hydrothermal in origin. This result is comparable to that of the magnetite from the Kunlun and Anshan BIFs of the North China Craton (Fig. 5a; Zhou et al., 2017; Dai, 2014). On the other hand, the contents of Co, Cr, and Ni are significantly lower for hydrothermal magnetite than magmatic magnetite (e.g., Zhao et al., 2018). Hence, the low contents of Co, Cr, and Ni of the magnetite from the studied BIFs indicate a hydrothermal origin for the MWB, BOB, KOB, and ZAB magnetite. This is consistent with the plot of V versus Ti of Knipping et al. (2015), where all the studied magnetite samples plot near the hydrothermal field (Fig. 5b). Except the Kulun BIF magnetite (Zhou et al., 2017), similar hydrothermal origin is noticeable for the Anshan BIF magnetite (Fig. 5b; Dai, 2014). Base metals (e.g., Ba, V, Co, Cu, Ni, and Zn) are commonly derived from low-temperature hydrothermal fluids (cf. Rosière et al., 2021). Hence, we suggest that low-temperature hydrothermal solutions were responsible for the transport and deposition of such elements in magnetite from the studied BIFs. The wide range of these base metals in the studied magnetite (Table S2) could be attributed to various degree of evolution of fluids (cf. Chen et al., 2015).
As expected, high Fe contents occur for the analyzed magnetite grains using both LA-ICP-MS and EPMA (Tables S1–S2). However, high Al, Si, Mg, and Mn contents determined by LA-ICP-MS are much higher than the EPMA, which is attributed to the presence of quartz and other silicate inclusions in the analyzed magnetite grains (cf. Nadoll et al., 2014). All magnetite grains from the MWB, BOB, KOB, and ZAB have almost similar continental crust normalized multi-elemental patterns (Fig. 6), except for Zn and Ga, indicating that they may share a common source.
Whole-rock major and trace elements of BIFs are used widely to constrain the source of Fe in BIFs (e.g., Liu et al., 2018; Spier et al., 2008). Choi and Hariya (1992) have proposed abundance of SiO2 and Al2O3 to distinguish hydrothermal and hydrogenous deposits. On a Si-Al plot of Choi and Hariya (1992), all the BIF samples cluster in the hydrothermal field (Fig. 7a), indicating that the MWB, BOB, KOB, and ZAB are chemical precipitates from hydrothermal activity. This is consistent with their high Fe/Ti (mean > 600) and Fe/Al (mean > 75) ratios, indicating the involvement of hydrothermal fluids during chemical deposition of the studied BIFs (e.g., Gurvich, 2006). Such interpretation is also supported by the low Co/Zn ratios of the studied BIFs (mean of 0.20, 0.30, 0.43, and 0.12 for the MWB, BOB, KOB, and ZAB, respectively), which is comparable to that of hydrothermal deposits (Co/Zn: 0.15; Toth, 1980). It should be noted that similar low Co/Zn ratios are noticeable for the magnetite from the MWB (0.10–0.11), BOB (0.12–0.14), KOB (0.25–0.85), and ZAB (0.05–0.14). Compared to other studied BIFs, the high content of Co, Cu, and Zn in the BOB (Co: 4.2–7.3; Cu: 14.9–90; Zn: 16–20) and ZAB (Co: 2.1–17.9; Cu: 4.6–53.7; Zn: 17–201) indicates more hydrothermal input in the BOB and ZAB. This is in good agreement with the common presence of pyrite grains in the BOB and ZAB with respect to other BIFs (Figs. 4f, 4l). Moreover, all the studied BIF samples plot near low-temperature hydrothermal fluids (0.1%) and seawater fields in the Sm/Yb versus Eu/Sm diagram of Alexander et al. (2008) (Fig. 7b). Although the plot of Sm/Yb versus Eu/Sm is applied for BIFs, this plot demonstrates similar distribution for both BIF ore and magnetite minerals (Fig. 7b), suggesting that such diagram can be applied to iron ore minerals (cf. Silveira Braga et al., 2021). This result indicates the influence of low-temperature hydrothermal fluids during the formation of magnetite from the MWB, BOB, KOB, and ZAB. Overall, the evidence for the source of magnetite based on trace element chemistry is consistent with the interpretation drawn from the whole-rock geochemistry of BIF ore sample.
As discussed above, the combination of major-trace element signatures of BIF magnetite and whole-rock geochemistry of BIF ore sample allow constraining the source of iron ore in BIFs. Iron mineralization at the Bipindi iron ore district has chemical signature of hydrothermal fluids. Because the BIF-type magnetite has formed by chemical sedimentation from seawater, its variable composition is most likely controlled by water composition, and/or temperature, and oxygen fugacity (fO2) (Nadoll et al., 2014).
Low concentrations of Al, Ti, Zr, and Sc in BIFs are generally attributed to deposition in deep seawater (e.g., Liu et al., 2018). This is supported by the experimental work of Li et al. (2017), which envisaged formation of magnetite as a consequence of the reaction of ferrihydrite and Fe+2-rich hydrothermal solutions at great depth in seawater. In this context, the high concentrations of Al (244.13 ppm–8 451.01 ppm) and Ti (42.09 ppm–3 535.79 ppm) in the studied magnetite samples suggest shallow seawater for the BIF deposition. On the other hand, there is a common consensus that Cr enrichment in BIFs is linked to an acidic-induced chemical weathering contemporaneous with intense oxidation of pyrite on the continent while BIFs formed in a marine environment of shallow level (cf. Rosière et al., 2021). Therefore, the relative high Cr content (16.04 ppm–1 055.81 ppm) in the studied BIFs and magnetite can be attributed to addition of detrital components in the seawater from which the MWB, BOB, KOB, and ZAB were deposited.
The contents of some elements (e.g., Al, Ti, and Mg) in magnetite are influenced by the temperature of hydrothermal fluids (e.g., Verlaguet et al., 2006; Nielsen et al., 1994). This led Nadoll et al. (2014) to propose the use of (Al + Mn) versus (Ti + V) diagram to constrain the formation temperature of magnetite. In this diagram (Fig. 8a), most of the magnetite samples from the studied BIFs cluster in the field of 200–300 ℃, suggesting that the magnetite was likely precipitated from fluids depleted in Al and Ti (Nadoll et al., 2014). This result is in good agreement with temperature (100–300 ℃) recorded for hydrothermal magnetite in BIF iron ore deposits (e.g., Nadoll et al., 2014). However, magnetite from KOB and ZAB plot in the field of 300–500 ℃, indicating the contribution of moderate-temperature hydrothermal fluids probably of metamorphic origin in Fe upgrade.
Since V content in magnetite is controlled by fO2, many studies have demonstrated that V can be enriched in magnetite formed from a reduced fluid (e.g., Nadoll et al., 2014). Thus the relative low V contents of magnetite in the studied BIFs (MWB: 12.92 ppm–23.35 ppm, BOB: 8.64 ppm–28.44 ppm, KOB: 39.48 ppm–49.07 ppm, and ZAB: 7.16 ppm–42.71 ppm) imply that magnetite were derived from oxidizing fluids. This is consistent with the plot of Ni versus V (Fig. 8b), along with their higher Ti/V ratios of 5.92 ppm–11.17 ppm (MWB), 43.23 ppm–100.62 ppm (BOB), 16.38 ppm–19.19 ppm (KOB), and 5.88 ppm–82.79 ppm (ZAB) indicative of oxidizing fluids conditions (e.g., Zhou et al., 2017). In summary, the above evidence supports that the Nyong Complex BIFs were mainly deposited in shallow seawater in an oxidizing conditions (e.g., Djoukouo Soh et al., 2021; Soh Tamehe et al., 2018).
The source of iron in the Nyong Complex BIFs has been the focus of researches in recent years based on their whole-rock geochemistry (Djoukouo Soh et al., 2021; Ganno et al., 2017; and references therein). Our petrographic study combined with in-situ chemical signature of magnetite emphasizes the crucial role played by hydrothermal processes in the formation of magnetite from the Nyong Complex BIFs. Magnetite is closely associated with pyrite in the BOB and ZAB, and rarely observed in other BIF samples (Fig. 4). Considering that pyrite is a hydrothermal mineral, its association with magnetite in the Nyong Complex BIFs is an affirmation of derivation from hydrothermal fluids probably during the Eburnean-Transamazonian orogeny. Such interpretation is supported by the wide range of Al and Ti contents of magnetite in the studied BIFs (Fig. 5). The BIF geochemical characteristics along with the chemical signature of magnetite indicate mixture of seawater and low-temperature hydrothermal fluids for the deposition of BIFs (Fig. 7b). Based on the results of this study, we suggest that the magnetite from the Nyong Complex BIFs have similar origin as supported by their common oxidizing conditions and depositional setting in continental margin and submarine volcanic arc environment (e.g., Moudioh et al., 2020; Soh Tamehe et al., 2018).
The Nyong Complex BIFs-hosted iron ore deposits have similar geological features comparable to those from the North China Craton (NCC; Li et al., 2014). They occur in greenstone belts and are predominantly composed of magnetite as the main ore mineral associated with gangue minerals of quartz, pyrite, and amphibole (e.g., Liu et al., 2018; Ganno et al., 2017). However, the source of magnetite in the NCC BIF-related iron ore deposits is dominated by high-temperature hydrothermal fluids and reducing deep seawater (e.g., Liu et al., 2018, 2014). This contrasts with the contribution of low- to moderate-temperature hydrothermal fluids and oxidized shallow seawater for the magnetite from the Nyong Complex BIF-hosted iron ore deposits. Nonetheless, both BIF iron deposits from NCC and Nyong Complex have low-grade iron ore ranging between 20 wt.% and 50 wt.% TFe (e.g., Ganno et al., 2016; Li et al., 2014). This is in good line with TFe of 39.80 wt.%–43.20 wt.% (MWB), 33.30 wt.%–40.40 wt.% (BOB), 36.10 wt.%–39.20 wt.% (KOB), and 30.90 wt.%–43.10 wt.% (ZAB) for the studied BIFs from the Bipindi iron ore district. Hence, we propose that the interaction of BIFs with low-temperature hydrothermal and later metamorphic fluids might have played a significant role for the transformation of the Nyong Complex BIFs into iron ore deposits although supergene enrichment by meteoric fluids cannot be ruled out (Soh Tamehe et al., 2019).
Iron mineralization of the Nyong Complex is hosted by BIFs, with iron ore occurrence and ore deposits located at the Bipindi iron ore district in the central part of the Complex. The Mewongo, Bibole, Kouambo, and Zambi BIFs consist of magnetite as the main iron ore mineral associated with minor pyrite. The gangue minerals comprise quartz and chlorite. The magnetite has almost similar geochemical characteristics in all these BIFs, with a wide range of Al and Ti. Discrimination diagrams of magnetite indicate a hydrothermal origin of iron ore. The major and trace elements composition of the studied BIF ore samples indicates the contribution of hydrothermal fluids and seawater during the BIF deposition in an oxidizing setting. Conclusively, the Nyong Complex BIF-hosted iron deposits were initially derived from low-temperature hydrothermal fluids and lately overprinted by metamorphic fluids.
ACKNOWLEDGMENTS: This research was supported by the Central South University Postdoctoral Research Fund (No. 22020084). The authors are grateful to Geocam Mining and G-Stones Resources Ltd for their logistical and technical support during fieldwork. We appreciate the assistance of Dr. Wenqiang Yang, Dr. Zhian Bao, and Mr. Kaiyun Chen during EPMA and LA-ICP-MS analyses at SKLCD. Profs. Carlos Rosière and Andrey Bekker are thanked for fruitful discussion. Comments and reviews by two anonymous reviewers are gratefully acknowledged. The final publication is available at Springer via https://doi.org/10.1007/s12583-022-1622-4.| Alexander, B. W., Bau, M., Andersson, P., et al., 2008. Continentally-Derived Solutes in Shallow Archean Seawater: Rare Earth Element and Nd Isotope Evidence in Iron Formation from the 2.9 Ga Pongola Supergroup, South Africa. Geochimica et Cosmochimica Acta, 72(2): 378–394. https://doi.org/10.1016/j.gca.2007.10.028 |
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Landry Soh Tamehe, Huan Li, Sylvestre Ganno, Zuxing Chen, Yanick Brice Lemdjou, Safiyanu Muhammad Elatikpo. Insight into the Origin of Iron Ore Based on Elemental Contents of Magnetite and Whole-Rock Geochemistry: A Case of the Bipindi Banded Iron Formations, Nyong Complex, SW Cameroon. Journal of Earth Science, 2024, 35(1): 16-28. doi: 10.1007/s12583-022-1622-4
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