Geochemical Signature and Metalogeny of BIFs and Associated Iron Ore of Zatua Hills, Haut-Uele Province (DR Congo) ()
1. Introduction
The Northeastern part of the DRC has immense iron resources that can reach tens or even hundreds of billions of tons, capable of initiating its industrialization policy and providing iron-based materials and alloys necessary for the needs of Central Africa, a potential market for road construction and civil engineering works. These resources are mainly related to the itabirite formations contained within the greenstone belt of the NE part of the DRC, including those of Ngayu, Zani, Kilo, Moto, Mambasa, Panga, Tele, and Isiro, which constitute the Kibalian series contained in most of the granitic massifs of the “Upper Congo Granites Complex,” which are divided into an Early Kibalian, consisting of high-grade metamorphic series, basaltic-type volcanic rocks, and tonalite intrusions (>2.8 Ga) with a scarcity of BIFs and metasediments ( Lavreau, 1984 ). And a Late Kibalian, dominated by metasediments, large volumes of BIFs, rare volcanic rocks, and intruded by granitic rocks (>2.45 Ga) ( Lavreau & Ledent, 1975 ; Lavreau, 1980 ). The main accumulations of these BIFs form high and very noticeable topography surfaces in the region, namely: the Tina, Maie, Asonga, Zatua Hills, etc. In this publication, we focus on studying the BIFs of the Zatua Hills, characterized by a relatively smaller plateau with altitudes ranging from 500 to 1089 m. Chains and intermittent hills can rise up to 300 m above the surrounding plateau.
The target area consists of a series of NW-SE trending hills, within which lies the Zatua Hills with a maximum elevation of 1300 m. To the southeast, the area is bordered by the Western Rift Valley, including Albert and Edward Lakes, separated by the Rwenzori Hills (5119 m).
Northeastern DRC occupies both the northeastern part of the Congo Basin and the elevated areas that constitute the western boundaries of the graben (rift) basin. This region, essentially composed of Archean shields, is covered by a series of Neoproterozoic and Phanerozoic formations that are affected by the aftershock of recent Cenozoic tectonic events that gave rise to the East African Rift.
In this study, we will present geochronological data from the various sites visited, with a focus on the metallogenic aspects and its significance for the quality of iron ore for metallurgical industries.
2. Study Area
2.1. Geographical Context
The Zatua Hills, which are the subject of this publication, are located between the Bafwasende Degree Square in the north and the Isiro (Paulis) Degree Square in the south, and are situated between 27˚31'48" and 27˚40'48" East longitude and between 1˚55'48" and 2˚01'48" North latitude. They are located in the Wamba Territory, Haut-Uélé Province in the Democratic Republic of the Congo, approximately 315 km by air from the city of Kisangani (Figure 1).
2.2. Geological Context
Regional geology is predominantly composed of the Archean-age basement formations covering an area of 500,000 km2, including Uganda, South Sudan, and the Central African Republic ( de Wit & Linol, 2015 ; de Wit & Ashwal, 1997 ; Cahen & Snelling, 1966 ). These formations were initially associated with the Tanzanian craton before being separated by the Ruwenzori chain (2 Ga) and the filling of rift basins ( Borg & Shacklet, 1997 ; Lavreau & Navez, 1982 ). These formations include the Bomu amphibolitic and gneissic complex dated 3.4 - 3.7 Ga ( Cahen et al., 1984 ), the West Nile amphibolitic complex dated 3200 Ma - 2728 Ma ( Allibone et al., 2019 ), the Garamba complex, the granitic massifs of the Upper Congo Granites Massifs, which cover a large part of the central area, consisting
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Figure 1. Location map of the study area.
of magmatic, orthogneissic and various magmatic intrusions, in which the Kibalian greenstone belt formations, including BIFs, are associated, and which form the core of the large Archean units in the region ( Lavreau, 1982 ; Cahen & Snelling, 1966 ; BRGM, 1982 ; Cahen et al., 1984 ; Bird, 2016 ; Allibone et al., 2019 ).
The Zatua Hills are part of the Ngayu belt, which is one of several greenstone belts found in the Nyanza-Kibali greenstone belt, which stretches from Tanzania to the Central African Republic.
The most remarkable formations in the region are the BIFs, which occupy vast stretches compared to other geological formations. Their presence is clearly shown on the map represented in Figure 2. These BIFs are considered as true markers that define the different structural entities of the region. In some places, these formations can have significant thicknesses of up to 200 m, sometimes in association with certain deformations (folds, faults, laminations) and form relatively high reliefs. BIFs are sometimes found intercalated at the roof by schist rock and at the wall by basalts.
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Figure 2. Geological map showing the different greenstone belts in which the BIFs of the Zatua Hills are embedded.
Apart from the BIFs, the region is also characterized by the presence of basaltic formations, intrusions of diorite, quartz monzonite and tonalite in greenstone rocks. All these rocks are covered by formations of Upper Proterozoic age (Lindian), Mesozoic formations and the West African Rift formations of Cenozoic age.
3. Methodology
This publication is a combination of a series of studies based on meticulous in situ observations, starting from multiple in situ mapping of outcrops, drilling works, laboratory geochemical analysis, and metallogenic study based on both field and laboratory results.
3.1. Field Stage
The work consisted of identifying the outcrop zones of BIFs, iron ores (enriched parts) and their host rocks using airborne magnetometric data acquired and interpreted by the company Rio Tinto, which once operated in this area, geological survey work on the relevant sites, collecting samples in the field, and mapping outcrop zones.
These data enabled the confirmation of zones with rich BIFs (% iron ≥ 50), poor BIFs (% iron < 50) and iron ores (% iron ≥ 60), not forgetting the poor host rock (% iron < 40).
Several samples were collected at these sites (Figure 3) and some were selected for geochemical analysis in the laboratory.
3.2. Lab Stage
The selected samples were used for geochemical analyses in the laboratory, specifically:
· Geochemical analyses using a Panalytical PW1840 diffractometer using Fe, Co, K filters and alpha (α) radiation to identify the predominant oxides in the drill samples;
· Geochemical analysis using X-ray fluorescence spectrophotometry (XRF) to determine major and minor elements;
· Geochemical analyses using an inductively coupled plasma mass spectrometer
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Figure 3. Various surface samples selected in the Zatua Hills.
(ICP-MS) for the determination of minor elements and trace elements;
· and thermogravimetric analysis to evaluate the hydration rate (loss on ignition) of the samples from the itabirite formations and iron ore.
The geochemical analyses performed concerned twenty-six elements, but we only took into account the elements whose content was different from zero (up to 1 ppm) for all selected samples.
3.3. Interpretation and Discussion of Results Stage
The results of the geochemical analyses have enabled us to make a geochemical interpretation of the quality of BIFs of the Zatua Hills and associated iron minerals, while also considering associated elements that may influence the metallurgical quality of iron in its different stages through the formation of different alloys.
4. Results
Geological mapping and drilling work allowed the collection of numerous samples, among which 24 were selected and subjected to analysis by X-ray fluorescence (XRF), thermogravimetric analysis (LOI), and inductively coupled plasma mass spectrometry (ICP-MS) as detailed in the methodology.
The geochemical analysis results of surface samples are presented in Table 1 and Table 2, and the map below shows the different locations of the collected samples (Figure 4).
4.1. Surface Samples
4.1.1. West Zone of the Zatua Hills
The different samples collected in the Western zone of the Zatua Hills and their results are recorded in the following Tables 1-3:
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Table 1. Results of geochemical analysis by fluorescence (XRF) of oxides of the iron ore samples from the Western zone of the Zatua Hills.
X: Content lower than 0.001 ppm.
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Table 2. Results of geochemical analyses using an Inductively Coupled Plasma Mass Spectrometer of some elements in the iron ore samples from the Western zone of the Zatua Hills.
X: Content lower than 0.001 ppm.
4.1.2. East Zone of the Zatua Hills
The various samples collected in the Western Zone of the Zatua Hills and their results are recorded in Table 3 and Table 4 below:
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Table 3. Geochemical analysis results using X-ray fluorescence (XRF) of the iron ore oxide samples from the East zone of Monts Zatua.
X: Content lower than 0.001 ppm.
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Table 4. Results of geochemical analyses using an Inductively Coupled Plasma Mass Spectrometer of several elements in iron ore samples from the Eastern area of the Zatua Hills.
X: Content lower than 0.001 ppm.
4.2. Geochemical Analysis of Sub-Surface Data (Drillings)
Samples from 4 selected surveys were analyzed via X-ray fluorescence (XRF) and thermogravimetric analysis (LOI) to determine their mineral evolution and various paragenesis associated with iron ores from prototype BIFs.
The results of the geochemical analysis are presented in the following Figures 5-13.
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Figure 5. Various drilling samples selected in the Zatua Hills.
Borehole Samples Analysis
1) SMZ001
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Figure 6. Lithostratigraphic and geochemical log of Borehole SMZ001.
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Figure 7. Diffractometric analysis showing the dominant crystals identified in sample SMZ001-25m.
2) SMZ006
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Figure 8. Lithostratigraphic and Geochemical Log of Borehole SMZ006.
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Figure 9. X-ray diffraction analysis of the sample showing the predominance of hematite in sample SMZ006-30m.
3) SMZ009
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Figure 10. Lithostratigraphic and Geochemical Log of Borehole SMZ009.
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Figure 11. X-ray diffraction analysis of the sample showing the predominance of hematite in sample SMZ009-82m.
4) SMZ010
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Figure 12. Lithostratigraphic and Geochemical Log of Borehole SMZ010.
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Figure 13. X-ray diffraction analysis of the sample showing the predominance of hematite in sample SMZ010-15m.
The presence of Si and Al reflects a finely-grained kaolinite within aggregates of hematite. Kaolinite and gibbsite are also abundant within microfractures traversing the rock (Figure 10). This sample exhibits a phyllite in the midst of ferruginization process.
5. Discussion
5.1. Geochemistry
The results of the geochemical analyses of all selected samples clearly show that for the BIFs samples whose quartz has been extensively leached and enriched with iron, the iron content varies from 57% to 69%, whereas for the unaltered BIFs, this content does not exceed 50%. This observation also holds true for other deleterious elements or impurities of iron (Al, P, and Si), which decrease in the iron ore samples compared to the BIFs.
These results clearly indicate that the majority of breccia and hematite samples are rich and do not contain Si, Al, and P levels beyond the standard, except for samples MZ001, MZ006, MZ010, MZ012, MZ015, and MZ022 which have higher than standard levels of P and Al. This excess is due to the fact that these samples were collected from fractured zones where the flow of meteoric waters has caused the circulation of secondary elements rich in Si (residual silica and kaolinite), P (solid solution of strengite-variscite), Al (gibbsite, cadwaladerite, and kaolinite), and Cl (cadwaladerite). Evidence of this circulation is the hydration rate of the rock influenced by the value of loss on ignition (LOI) in samples where the percentage of these elements is more pronounced ( Makuku, 2018 ).
The majority of the analyzed samples have a silica content of <1.0%, suggesting that most of the primary silica has been leached from the samples at some stage, likely during the oxidation process leading to magnetite martitization. This leaching is possible along weak zones related to rock joints, fractures, and folding, leading to the removal of primary quartz and concentration of iron minerals such as magnetite, martite, and hematite ( Lascelles, 2017 ).
The secondary minerals in these samples are characterized by their composition in silicon (quartz or chert, goethite, and kaolinite), aluminum (gibbsite, variscite, cadwaladerite, goethite, and kaolinite), and phosphorus (variscite) in the analyses (Table 5). It is also noted that some minerals are hydrated and contribute
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Table 5. Geochemical analyses of surface samples from the Zatua Hills in descending order of iron content.
N.B. Samples containing SiO2 > 3.0%, Al2O3 > 3.0%, and P2O5 > 0.18% are considered non-standard samples for metallurgical treatment.
to a high loss on ignition value.
In addition to these observations, the introduction of these secondary minerals leads to a decrease in iron content in the samples. As observed in Table 5, the total iron content decreases from 69.28% to 57.72% simultaneously with an increase in secondary minerals content. The nature and relative proportions of secondary minerals vary from sample to sample and may have significant implications for the industrial value of the exploited ores, particularly in the case of phosphorus.
5.2. Evidence of Elements Derived from the Hydrothermal System
When the silica in BIFs is leached by deep hydrothermal solutions, the system deposits the following elements: K and Al (stilpomelane), Rb, Sr, Ba, S (pyrite and chalcopyrite), Mg (chlorite), Ca (calcite and dolomite), and Cu (chalcopyrite). These solutions cause distal alteration around quartz veins and proximal alteration around sills and dykes (forming hard hematite). The geochemical analyses of surface and drilling samples presented in Tables 1-5 show the presence of these elements, but at values too low to indicate the function of the hydrothermal system as a precursor to the iron ore-enrichment process ( Lascelles, 2007 ). This is because the solubility of silica is only possible under high pressure and temperature conditions favorable to hydrothermal process ( Morre & Maynard, 1929 ; Okamoto et al., 1957 ; Loughnan, 1969 ; Yariv & Cross, 1979 ; Hiemenz, 1997 ; Parr et al., 2003 ; Lascelles, 2017 ).
The supergene process is suggested like a recent one and follows the hypogene process that had already played a role in the solubility of primary silica, erasing the traces of the hypogene process through leaching. Metamorphism, in turn, contributed to the deconvolution and crystallization of hydrated minerals, leading to the leaching of silica and enrichment of BIFs in rich and exploitable iron ores, just like the previous processes.
This observations open the new opportunities of research by making a selection of petrographic, geochemistry, REE and others studies in order to make is place the Genetic Model of BIFS to Iron Ore in Zatua Hills.
6. Conclusion
The various analyses conducted on surface and drilling samples in the Zatua Hills area have revealed that BIFs are composed mainly by enriched Breccia, unenriched Breccia, enriched BIFs and Unenriched BIFs. In various analyses of samples of BIFs with heavily leached quartz enriched in iron, the iron content varies between 57% to 69%, whereas for unaltered BIFs, this content does not exceed 50%. Deleterious elements (Al, P, and Si) are very low in enriched BIFs and iron ores associated, and should not exceed 3% for SiO2 and Al2O3 contents, and 0.18% for P2O5 content.
These deleterious elements coming from fractured zones where the flow of meteoric waters has caused the circulation of secondary elements rich in Si (probably residual silica and kaolinite), P (probably solid solution of strengite-variscite), Al (probably gibbsite, cadwaladerite, and kaolinite), and Cl (probably cadwaladerite). Evidence of this circulation is the hydration rate of the rock influenced by the value of loss on ignition (LOI) in samples.
This leaching is possible along zones of weakness related to rock joints, fractures, and folds, leading to the removal of primary quartz and to the concentration of iron ores (magnetite, martite, and hematite).
Magnetite, hematite, martite are the common iron minerals that exist in the samples and hematite and martite are derived by oxidation of magnetite and leaching of quartz in the BIFs.
Lascelles’ modified hypogene-supergene process associated with metamorphism is suggested, in which meteoric waters and waters from the deconvolution of hydrated rocks by retrograde metamorphism played a major role in leaching silica and enriching BIFs to the exploitable rich iron ores.
The presence of Ti in the drill core samples clearly shows that it is associated with both phyllites and BIFs. Phyllites (chlorites, sericitic schists) are considered to be low-temperature metamorphic rock derived from the argillites found in this sedimentary basin. The affinity of titanium to this type of rock indicates that the associated titanium oxide form would be anatase, which corresponds to this low-temperature and low-pressure metamorphism.