Cu-Bearing Mokama Granite Prospect of the Kibara Belt in the Maniema Province, DRC: A Preliminary Petrography, Geochemistry, and Fluid Inclusion Study ()
1. Introduction
The KIB leucogranites originated from the partial melting “anatexis” of the crustal subducting slab during the Mesoproterozoic collision event between the Congo craton and the Tanzanian block [1] . These leucogranites are sources of numeral metals such as Sn, W, Nb, Ta, Li, REE, Fe, Zn, and Cu (Figure 1). The occurrences of tin oxide group minerals (TOGM), columbite group minerals (CGM) and sulfide group minerals (SGM, Cu mineralization) are typically associated with the post-collision differentiated leucogranites [2] [3] . The enrichment and increase in mobility of metals are ensured by incipient post-magmatic fluid processes such as fluid exsolution, fluid-rock interactions [2] [4] , metasomatic reactions and fluid-fluid mixings (magmatic-meteoric) [5] .
The KIB G4 granites are known for Sn-W mineralization (in the Maniema, Kivu, and great Katanga provinces) and even G4 (G5) pegmatitic granites host Nb-Ta (coltan) mineralization. Few pegmatites such as in the Manono host a huge amount of Li mineralization.
The Mesoproterozoic KIB is overridden by the Neoproterozoic Lufilian (Zambian belt) known for the occurrences of Cu-Cu mineralization [7] [8] [9] .
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Figure 1. A geological map unit of the northern part of the KIB displaying the study area and sampling site (yellow rectangle) [6] .
The sedimentary materials that filled up the aborted rift to form later on the Lufilian belt might come from pre-existing and eroded belt formation such as the Mesoproterozoic (Kibara belt, Karagwe-Ankole belt), Paleoproterozoic (Ubendian belt), Archean (Congo-Kasai craton, Bomu complex) [7] .
The present study was conducted in the northern part of the KIB in Maniema province (DRC), and concerns the Mokama mineralized G4 granite that has intruded metasedimentary rocks (Figure 1) and hosting Cu mineralization. This study also tries to set in an unlikely possibility of the KIB Cu mineralization hosted in the granites or any other unknown (undiscovered) mafic or ultramafic rocks enriched in Cu-Co primary ores, to be the source (feeder) of the Lufilian sedimentary and stratiform Cu (Co) mineralization. This hypothesis incentivizes further studies to be done in order to demonstrate the link between the two above belts when it comes to Cu mineralization.
2. Geological Settings
The Mokama granite is located in the Maniema province in the Democratic Republic of Congo and makes part of the Kibara belt (KIB) and intrudes Mesoproterozoic metasediments made of essentially schists. The chemical-minera-logical-structural (CMS) classification of KIB granites showed three (3) main phases. The first phase is pre- to syn-tectonic and hosts less fractionated and “barren” granites (G1 - G3, Barrage site, Figure 1) and yield ages of 1100 - 1500 Ma. The second phase (this study) and the third phase are post-orogenic and host fractionated and “mineralized” granites (G4, Mokama site, Figure 1, Figure 2) and pegmatitic-granites (G4 - G5) of 950 - 1094 Ma [10] [11] [12] . Granites G1 - G3 are associated with the deformation D1 while granites G4 - G5 are associated with the deformation D2 (Figure 2) [6] [12] .
3. Methodology
About 10 thin sections, 5 polished sections, and more than 50 doubly-sides polished sections (chips) have been prepared and observed under the polarizing microscope (Nikon, Japan) at Inha University (Incheon, Republic of Korea).
The electron probe microscope analysis (EPMA) was performed at Busan National University (Busan, Republic of Korea). This EPMA consisted of a JEOL-JXA-8530F PLUS model and used an acceleration voltage of 15 kV, an acceleration current of 40 nA, and an electron beam of 3 mm. The analysis was conducted with a peak duration of 10 s and a background time of 5 s. EPMA results are not provided here but are used for ore mineral identifications.
X-ray fluorescence (XRF, Zetium, Malvern Panalytical, United Kingdom) for major elements and Fusion ICP-MS for trace elements of the Mokama granite were performed at Activation Laboratories (Actlabs, Ontario, Canada). We have analyzed isotopes including 7Li, 9Be, 23Na, 25Mg, 27Al, 29Si, 39K, 42Ca, 45Sc, 49Ti, 51V, 53Cr, 55Mn, 57Fe, 59Co, 61Ni, 65Cu, 66Zn, 71Ga, 73Ge, 75As, 85Rb, 88Sr, 89Y, 90Zr, 93Nb, 95Mo, 107Ag, 111Cd, 113In, 118Sn, 121Sb, 133Cs, 137Ba, 139La, 140Ce, 141Pr, 146Nd, 147Sm,
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Figure 2. Sub-Saharan tectonic-chronostratigraphic and tectonic-magmatic units showing magmatic events associated with Mesoproterozoic syn-to post-orogenic belts. This study is part of the Kibara belt (KIB) granites associated with the deformation D2 [6] .
151Eu, 157Gd, 159Tb, 163Dy, 165Ho, 167Er, 169Tm, 173Yb, 175Lu, 178Hf, 181Ta, 182W, 185Re, 197Au, 205Tl, 208Pb, 209Bi, 232Th, 238U. A 193 nm Argon-Excimer laser (LA-NWR 193) was coupled with a quadrupole mass spectrometer Agilent 7700X. We have used a repetition rate of 5 Hz and a laser energy density of 3.5 - 6.0 J/cm2. The laser beam diameter of the ablated zone was 50μm for the Mokam granitic slab. The fusion methods were utilized to ensure the total dissolution of the rock by acids. For the ICP-MS, the detection limits were Li (5ppm), Be (1 ppm), Na (100 ppm), Mg (100 ppm), Al (100 ppm), Si (100 ppm), K (100 ppm), Ca (100 ppm), Sc (1 ppm), Ti (10 ppm), V (5 ppm), Cr (20 ppm), Mn (10 ppm), Fe (100 ppm), Co (1 ppm), Ni (20 ppm), Cu (10 ppm), Zn (30 ppm), Ga (1 ppm), Ge (1 ppm), As (5 ppm), Rb (2 ppm), Sr (2 ppm), Y (1 ppm), 90Zr, Nb (1 ppm), Mo (2 ppm), Ag (0.5 ppm), Cd (excluded), In (0.2 ppm), Sn (1 ppm), Sb (0.5 ppm), Cs (0.5 ppm), Ba (2 ppm), La (0.1 ppm), Ce (0.1 ppm), Pr (0.05 ppm), Nd (0.1 ppm), Sm (0.1 ppm), Eu (0.05 ppm), Gd (0.1 ppm), Tb (0.1 ppm), Dy (0.1 ppm), Ho (0.1 ppm), Er (0.1 ppm), Tm (0.05 ppm), Yb (0.1 ppm), Lu (0.01 ppm), Hf (0.2 ppm), Ta (0.1 ppm), W (1 ppm), Re (1 ppm), Au (10 ppb), Tl (0.1 ppm), Pb (5 ppm), Bi (0.4 ppm), Th (0.1 ppm), U (0.1 ppm).
The fluid inclusion microthermometry was performed under the heating-cooling stage of Linkam-FTIR 600 which helped to determine ice melting temperature (Tm, ˚C) from which we have deduced the apparent salinity, and the total homogenization temperature (Th, ˚C) of fluid inclusions in the basis of H2O-NaCl system. The stage calibration was performed using synthetic fluid inclusions (aqueous CO2-CH4 bearing inclusion and pure water inclusion) to obtain the triple point (−57.1˚C) of the CO2-CH4 mixed aqueous inclusion and ice melting (0.0˚C) and the critical homogenization temperature (374˚C) of the pure water inclusion. About 3 - 4 single inclusions per assemblage were measured and results were expressed as values of average ± standard deviation.
4. Results
4.1. Rock Petrography and Microscopic Observations
The Mokama Cu bearing granite (DMMOKA-2-7 and DMMOKA-2-9) descriptions of gangue and ore minerals were performed under petrographic and metallographic microscopes, and Raman spectroscopy at Inha and Seoul National Universities, and results showed the occurrences of Cu mineralization as sulfides and oxide ore minerals (Figure 3, Figure 4). The mineralogical compositions of the Mokama granites consist of mainly silicates. This Mokama granite showed a phaneritic texture and consists of quartz, biotite, muscovite, k-feldspar (orthoclase), plagioclases, pyroxene and amphibole (hornblende), and accessory minerals such as zircon, ilmenite, fluorite, topaz, magnetite, and ore minerals including tin oxide group (TOG: Sn-W), columbite group minerals (CGM: Nb-Ta), and sulfide group minerals (SGM: Cu-Zn-Fe-As). Some granites showed relatively big-sized grains (>1 cm) of euhedral quartz and flakes of micas (abundant muscovite, rare biotite), and these grain sizes could display a lateral variation within the granitic body. Such rocks could be considered pegmatitic granites, they were rare and mineralized as well.
The Cu ore minerals (Table 1) consisted of essential chalcopyrite (CuFeS2) that shows rims of chalcocite (Cu2S), also lately replaced by covellite (CuS) and finally malachite (Cu2CO3(OH)2) in the supergene zones (Figure 3(a), Figure 3(b), Figure 4(f)). The alteration that affected the KIB mineralized granites (especially in the mineralized Mokama granite, Figure 3(c)) consisted of quartz ± muscovite ± albite ( ± sericite) whereas the KIB barren granites showed quartz ± pyrite ± fluorite ( ± chlorite) (Figure 3(d)) or quartz ± albite ± chlorite. The presence and abundance of muscovite can be interpreted as an acidic environment where feldspars were transformed into muscovite (Figure 3(c)) [6] .
The barren granites (from the Barrage site, Figure 1) show abundant euhedral quartz grains and pinkish K-feldspar (orthoclase), while biotite, Fe-oxides, pyroxenes, and amphiboles are accessory minerals.
The Cu mineralization of the Mokama G4 granite in the Mesoproterozoic KIB is considered a late-stage magmatic-hydrothermal sulfide and disseminated
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Table 1. Ore mineral associations and paragenetic sequences in the Mokama granite and peri-granitic quartz veins.
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Figure 3. ((a), (b)) Photo macrographs of the Mokama (DMMOKA-2-9) mineralized granite showing Cu mineralization; (c) Photomicrograph (XPL) of the Mokama granite displaying the hydrothermal alteration of feldspar to muscovite; (d) Photo macrograph of the Mokama (DMMOKA-2-7) granite displaying quartz-pyrite-fluorite in a quartz vein.
within the granitic rock compared to the sedimentary stratiform (and vein-type) Cu-Co of the Neoproterozoic Lufilian belt (LUB) in the great Katanga province In DRC [9] [13] [14] . In the KIB Mokama granites, the hypogene ore mineralization consisted of bornite, sphalerite, pyrite and chalcopyrite (Figures 4(a)-(e)). Bornite and chalcopyrite showed rims that are due to the later replacement by supergene sulfides such as chalcocite and covellite (Figures 4(a)-(c)), and these last ones were later on replaced by oxides such as the green mass of malachite (Figure 4(f)). Some sphalerite grains showed chalcopyrite diseases (Figure 4(d) & Figure 4(e)) and inclusions of silicate minerals (Figure 4(e)) that can be interpreted as associated with relatively high temperature-derived fluids. The occurrence of arsenopyrite is more likely associated with hydrothermal environment.
4.2. Rock Geochemistry
The Mokama G4 granites chemistry (XRF, LA-ICP-MS, Table 2) showed SiO2 (71 wt% - 79 wt%), ASI (1.38 - 3.12 molar), and enriched in Rb (681 - 1000 ppm), Ta (12 - 151 ppm), Sn (43 - 142 ppm), Cu (10 - 4300 ppm), Zn (60 - 740 ppm), U (2.2 - 20.7 ppm) while depleted in Zr (20 –31 ppm), Sr (20 - 69 ppm), Hf (1.3 - 2.0 ppm), Th (2.2 - 18.9 ppm), W (9 - 113 ppm), Pb (5 - 50 ppm), Ge (5 - 10 ppm), Cs (21 - 53 ppm), and Bi (0.6 - 17.4 ppm). The Aluminum saturation index (ASI = 1.7 - 3.1) showed that this Mokama G4 granite can be considered a
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Figure 4. (a)-(d) Photomicrographs (PPL) of the Mokama (DMMOKA-2-9) mineralized granite displaying the presence of sphalerite (SP), chalcopyrite [15] , pyrite (Py), chalcocite (Cc), and covellite (Cov); (e) A photomicrograph (PPL) of sphalerite (Sp) showing inclusions of chalcopyrite and silicates from the Mokama site (DRC; (f) A photomicrograph (PPL) of the Mokama G4 granite showing green mass of malachite (Mal). The sphalerite displays in (a), (d), and (e) chalcopyrite disease features.
peraluminous S-type leucogranites (Figures 5(a)-(c)), ferroan, highly potassic and calc-alkalitic. This Mokama granite is a late (post)-collisional granitoid, and is characterized by high Rb/Sr, Cu/As, Zn/As and (Ta/Zr)N; and low (La/Yb) and Ba/Sr. This rock is also relatively enriched in Rb, Ta, Sn, Cu and REE (mostly HREE which are fractionated by pyroxenes and amphiboles). The ratios (La/Sm)N, (La/Yb)N, (Gd/Yb)N, (Ta/Zr)N and (Eu/Eu*)N are 2.9 - 10.1, 0.1 - 11.5, 0.1 - 0.8, 113.3 - 2120.7, and 0.1 - 1.5 respectively.
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Figure 5. Multicationic plots of KIB granitoid (Mokama) the Maniema province (D.R.C); ((a), (c)) B (=Fe + Mg + Ti) vs A (=Al-(K + Na + 2Ca) (b) R1 = (4Si + 11 (Na + K) − 2 (Fe + Ti)) vs R2 = (6Ca + 2Mg + Al) (I = Muscovite > Biotite, II = Biotite > Muscovite, III = Biotite, IV = Hornblende, V = Orthopyroxene, VI = Clinopyroxene), (1-Mantle fractionates, 2-Pre-plate collision, 3-Post-collision Uplift, 4-Late orogenic, 5-Anorogenic, 6-Syn-collision, 7-post-ororgenic).
4.3. Ore Mineral Associations and Paragenetic Sequences
The Kibara belt mineralized granites (granite s.s., pegmatite, peri-granitic quartz veins) in general and the Mokama granite in particular started by the precipitation of TOG, followed by CGM and ended with SGM of non-economic deposits.
4.4. Fluid Inclusions and Microthermometric Measurements
(ID-inclusions), and CO2-rich fluid inclusions (Figure 6(a) & Figure 6(b)). The microthermometric measurements (Linkam-FTIR 600, Inha University) were carried out essentially on fluid inclusions assemblages (FIAs) hosted in quartz, and consisted of liquid-rich and ID as of primary and pseudo-secondary fluid inclusions (Figure 6(a) & Figure 6(b)).
The results of fluid microthermometry can be used to determine the paleodepths and thermodynamic conditions (apparent salinity, homogenization temperature and pressure) of rocks and ore minerals as well [6] [16] . Microthermometric results (Table 3) of fluid inclusion assemblages (FIAs) hosted in quartz in the Mokama granite show ranges of salinities of 4 wt% - 23 wt% (NaCl equivalent)
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Table 2. Major and trace element compositions of the Kibara belt (KIB) mineralized granitic intrusions and the host rock (schist). Major elements (expressed in wt%) were analyzed by XRF while trace elements (expressed in ppm) by Fusion ICP-MS methods.
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Figure 6. (a) Photomicrographs (PPL) of liquid-rich and ID fluid inclusions hosted in quartz in mineralized granite from the Mokama (DMMOKA-2-9) site in DRC; (b) Photomicrographs (PPL) of CO2-rich and ID fluid inclusions hosted in quartz in mineralized granite from the Mokama (DMMOKA-2-9) site in DRC.
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Table 3. Fluid inclusion microthermometric results of the Mokama granites in the KIB (FIAs = fluid inclusion assemblages, wherein 3 to 4 single inclusions per assemblage)Quartz_Barren granite (n = 25 FIAs).
and homogenization temperatures (Th) of 190˚C - 550˚C. A boiling assemblage in the granite suggests a fluid phase separation occurred at about 380 - 610 bars, and this corresponds to apparent paleodepths of approximately 1 - 2 km (lithostatic model) or 3 - 5 km (hydrostatic model). FIAs hosted in cassiterite (salinities of 2 wt% - 10 wt% and Th of 220˚C - 340˚C) set up the upper limit of SGM including Cu (-Fe-Zn-As) sulfides that probably precipitated at temperatures below 200˚C (Figure 7(a) & Figure 7(b)).
5. Discussions
5.1. The Mokama Mineralized Granite Genesis, Metal Mobilities, and Mineralization Processes
The Mokama G4 granites are peraluminous rocks associated with the Kibaran collisional event, and host huge critical and base metal potentials [10] [12] [18] . The magmatic-hydrothermal fluid processes that affected these granitic intrusions enable the mobility of elements and their precipitations inside and outside (country rock as hydrothermal veins) of the granitic intrusions. This is in consideration of many facts including the circulation of rising hot magmatic-hydrothermal reduced geofluids enriched in metal complexes (mostly chlorine and fluorine ligands, and rarely hydroxyl ligands), fluid-rock interactions with remobilization of metals from the host/country rock (lithologic control), mixing of fluids (hot saline magmatic-hydrothermal and cooler less saline meteoric waters) that possibly induced the oxidization, cooling-fugacity controls, and alteration that might be influenced and controlled the pH [6] .
Fluid inclusion data (quartz, cassiterite, fluorite) and the ore chemistry (oscillatory cassiterite, wolframite) showed that the TOGM (such as Sn-W) precipitated earlier, then followed by CGM (such as Nb, Ta, Li), and finally SGM (such
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Figure 7. Plots of homogenization temperatures (Th) versus apparent salinities of fluid inclusions hosted quartz from respectively Barren granites (Barrage site, Figure 1) and Mineralized granites (Mokama site, Figure 1) in DRC. The isobaric curves and salinities were constrained by using a H2O-NaCl model [17] . Results are reported as averages and standard deviations (Avg ± σ1) calculated from 3 - 4 single measured inclusions.
as Cu, Zn, As, Fe) that occurred at the very last stage. In the KIB, TOGM and CGM deposits are economically valuable whereas SGM is still now of non-economic value.
5.2. The KIB Magmatic-Hydrothermal Cu Bearing Granites as an Unlikely Potential Source of the Sedimentary Lufilian Cu Mineralization
The KIB hosts Cu mineralization associated with magmatic-hydrothermal processes, and the LUB has Cu (Co) mineralization related to sedimentary (multi-source materials) processes. The Katanga supergroup relies directly on the KIB basement and has no compelled data and evidence that show the possible material connection between the two belts. Some researchers showed that the LUB sediments originated from many sources from Precambrian and Archean formations [7] [8] [9] [13] . A few questions remained unsolved especially when it comes to addressing the source of Cu and Cu mineralization. The occurrence of Cu mineralization in the KIB belt could possibly be the primary Cu feeder of the LUB by the remobilization of rising mineralizing fluids through the Katanga basin. The fact that the LUB is characterizable by Cu-Co deposits, makes the possibility of considering the KIB as Cu feeder very inconsistent and diminished. The best feeder of both Cu-Co metals would be intrusive mafic or ultramafic rocks; and in the nearby LUB area, such rocks have not been discovered or well documented. Another very unlikely source of LUB Co could be ultramafic dyke (peridotites) intrusions in the Archean Congo-Kasai craton that formed lately serpentinized bodies hosting Ni-Cr (with by Co-V-Zn as by products) in the regolith zones (such as in the Lutshatsha, Mfwamba, and Nkonko massifs) [19] . None of the above hypotheses or scenarios has been proven until today. Thus, despite some attempts to grasp the origin of Cu, the primary source of Co in the LUB remains totally unknown and requires more studies in the future.
6. Conclusion
The Mokama G4 leucogranite in the KIB hosts disseminated mineralization of Cu, and the essential ore minerals are composed of chalcopyrite, chalcocite, covellite and malachite. The hydrothermal pervasive alteration of quartz-muscovite-albite in the Kibara belt contributed to metal mobilities during fluid-rock interactions. Fluid inclusion results showed that this granite was emplaced at 2 - 5 km deeper as paleodepths, the TOGM (and CGM) precipitated earlier at relatively high temperatures (>200˚C) whereas SGM precipitated later at probably below 200˚C. This Mesoproterozoic KIB Cu mineralization could unlikely is the source (feeder) of the Neoproterozoic Lufilian Cu (Co) stratiform mineralization. We recommend more studies to be done in order to establish a possible existence or not of link between the two belts Cu mineralization.
Author Contributions
The Conception of the project, D.K.M.; field works and sampling, D.K.M., J.P.B., P.K.K., and F.M.M.; petrographic and geochemical analyses (EPMA, LA-ICPMS, Raman spectroscopy, and fluid inclusion microthermometry), D.K.M., I.B, C.M.M., and F.M.M.; data validation and curation, D.K.M. and J.P.B.; writing—original draft preparation, D.K.M.; writing—review, editing, D.K.M. and F.M.M.; supervision, D.K.M.; funding acquisition, D.K.M. All authors have read and agreed to the published version of the manuscript.
Fundings
The present study received financial support from the National Institute for International Education and Development (NIIED-2020, grant number NIIED-200807-0012) in the Republic of Korea (D.K.M.) and the BEBUC-Kroëner Foundations for sample transportation.
Acknowledgements
We sincerely thank Jung Hun Seo, Insung Lee, Sangki Kwon, Kanika Mayena Thomas, Makutu Ma Ngwayaya, and Ongendangenda Tienge Albert for constructive discussions related to this study. Thanks to Junhee Lee, TongHa Lee, Yevgeniya Kim, Yuri Choi, Hahyeon Park, Yechan Jeon, and Dedel Milikwini for the material and sample preparations.