Mineralization and Exploration Perspectives in the Oudiane Elkharoub Zone, Birimian Domain, Reguibat Shield, Mauritania ()
1. Introduction
The West African Craton is subdivided into three distinct domains (Figure 1), [1] [2] [3] [4] . 1) In the north the Reguibat Shield outcropping in Mauritania, Algeria, and Morocco, constituted by Archean formations (3.0 - 2.7 Ga) and Paleoproterozoic (~2 Ga). 2) In the South the Man or Leo Shield formed by the Archean series of the Liberian Shield and the Paleoproterozoic formation of the Baoule-Mossi Domain covering Ghana, Ivory Coast, Guinea, southern Mali, Burkina Faso, western Niger, these Paleoproterozoic formations are generally referred to as Birimian. 3). In an intermediate position between the two dorsal, two buttonholes, the Kayes buttonholes in western Mali and the Kedougou Buttnhole located on either side of the border Senegales-Malian, these buttonholes are formed exclusively by Birimian series.
There are five principal geological entities within the borders of Mauritania (Figure 2). One of these, the Reguibat Shield, is located to the north of the country. It hosts Archean and Paleoproterozoic metamorphic sequences and granites that form the northwestern limit of the West African Craton (WAC) [7] [8] . Further to the south, the Craton’s margins coincide with the Mauritanide Belt (Figure 2), a Hercynian tectonic belt consisting of Neoproterozoic and Paleozoic metamorphosed sediments and volcanoclastic formations, including
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Figure 1. Geology of the West African Craton. Modified from [5] [6] .
parts of the underlying basement [9] . To the east of the Mauritanide Belt lies the vast Mesoproterozoic [10] to-Paleozoic Taoudeni Basin. Finally, to the west lies the northern limit of Mauritania-Senegal-Gambia-Conakry basin, a Mesozoic-Cenozoic continental margin-type basin that overlies a Paleozoic (and probably Archean) basement.
Mauritania is a significant source for gold, iron and copper ores in Africa continent. It was the second-highest ranked exporter and producer of iron in Africa in 2012, being preceded only by South Africa. In addition to iron, Mauritania also produces cement, copper, crude oil, gaz, gold, gypsum, quartz, salt and steel [11] .
The Birimian terrains (2156 ± 10 to 2067 ± 12 Ma) of the West Africa Craton are one of enormous interest, both for their fundamental potential in terms of geological research and in terms of mining. Indeed, they are some of the oldest lands in the Craton, and are affected by important orogenies. The Birimian rocks host many gold deposits and occurrences of the West Africa Craton. They were formed and accreted during the Eburnean orogenic cycle, which took place between 2.25 Ga and 1.80 Ga. The Birimian hosts some of the main types of such deposits in West Africa Craton, and these are directly associated with granitic plutons. This type of environment is, in turn, the supplier of relevant metals such as Au, Ag, Pb and Zn, which are associated with many types of deposits (copper porphyries, perigranitic veins, skarns, etc.). As such, they are major economic targets. In addition, perigranitic deposits contain gold, which has become the subject of abundant and extensive geological discussion, leading to varied and novel terminologies. The Mauritanian Birimian terrains are, however, yet to be properly known.
The study Zone is located in the Gallaman in the central part of Reguibat Shield (Figure 3), which has been stable since about 1700 Ma [1] and dominates the north third of Mauritania’s geological surface. This choice is based on the fact that despite the scare amount of research on the study zone, as a part of Reguibat Shield, the Oudiane Elkharoub Zone has the same characteristics as the lager area it constitutes, possessing many precious and base metals. The objective is to figure out the anomalies for precious and base metals and define the type of mineralization (Auriferous, Polymetallic etc.). This work contributes on the one side to a better understanding and planning of subsequent research activities and exploration, on the side to improvement of geological knowledge on Oudiane Elkharoub Zone which presents all the specificity to host all type of mineralization.
2. Geological Overview of the Oudiane Elkharoub Zone
As mentioned above, the Oudiane Elkharoub Zone is in the so-called Ghallaman Group, in the central part of the Reguibat Shield (Figure 3). Rocci [1] has described the occurrences of leptynite, pyroxene gneisses (a diopside gneiss and diospide + hyperstene gneiss) and amphibolite, as well as much rarer outcrops of cipolins and two-mica gneisses.
According to [1] [12] [13] [14] , the Temmimichate Group differs from the Ghallaman Group given the omnipresence of granulitic gneiss. The author indicates that the sebkhat of Ghallaman Group correspond to all outcrop of metamorphic rocks located to the east of large sebkhas of the same name within it. Rocci note the presence of the following: two meca gneisses, amphibolite, pyroxene gneiss and leptynite. The group of sebkhas at the Oudiane Elkharoub Zone emerges in series of sumeridian depressions that converge in their descension from the Alous Tmar hills. For [1] [15] , they differ very clearly from the previous sabkhas due to the great homogeneity of the facies that compose them, as well as to the existence of aluminous-to-hyperaluminous gneisses. The observation on field mapping coupled with the local geophysical map show a geological and structural complexity in the study zone (Figure 4) that reinforces its significance to be an interesting zone to hosting the mineralization. Moreover, its location is particularly special, seen, as it is set between two batholiths with different age: to the west, it is the tmemeimichat Ghallaman granitoid (dated 2150 - 2100 Ma) and to the east the Yetti granitoids (dated 2050 - 1995 Ma) [16] . All these geological and structural complexity make a good argument for the discovery of large deposits in Oudiane Elkharoub zone.
3. Material and Methods
The Oudiane Elkharoub Zone (location coordinate: 542,250, 2,611,070) is located within the central eastern part of Reguibat Shield, in the administrative region (Willaya) of Tiriss Zemmour 450 km to the northeast of an Iron deposit (Kediat Ijil, exploiting by National Industrial and Mining Company, (SNIM) in the city of Zeouarat, northern Mauritania (Figure 5).
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Figure 3. Reguibat Shield lithostratigraphic Units modified after [17] .
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Figure 4. Geological and structural map of Oudine Elkharoub after [13] [14] [15] [16] .
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Figure 5. Placement of Oudiane Elkharoub.
Several comparisons between mapping and sampling were carried out during the study. The purpose of this juxtaposition was to conduct a metallogenic study of area, discover anomalies, and generate a new geological map based on field mapping, control and mineralization (precious and base metals…) style as well as on the relationships between host rocks and the mineralization themselves.
A total number of 66 samples were collected from various outcrops in the field (Figure 6). Sixteen samples were analyzed at the MSALABS (is a Global company
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Figure 6. Samples localization in Oudiane Elkharoub.
providing analytical services to the mining industry, with advanced instrumentation, and an experienced team with a client-focused approach, MS Analytical provides full-service solution, from early stage of exploration to the projects, that have advanced to the process development stage), who employed the Fire as their standard analytical method for gold detection, (According the MSALABS, Fire assay method is to quantitatively determine the content of precious metals by adding flux to smelt ores and metallurgical products. This method has the advantages of good sampling representativeness, wide applicability, and good enrichment effect. It is an important means for the chemical analysis of gold and precious metals). A Fluorescence Spectrometer X-ray was utilized to analyze all samples at the National Agency of Geological Research and Mining Heritage in Mauritania. The interpretation of the geological map, in turn, employed software such as Argis and QGIS. While ioGAS and Microsft Excel were used to shed, light on the results and build anomaly maps and graphs.
4. Results
The analytical results Table 1, together with on-site observations, indicate samples to have anomalies for certain precious (Au, Ag and Pd) and base (Cu, Zn and Pb) metals and Mn, Cr, Ni, Th and Y. gold ranges from 0.13 ppm to 117.52 ppm, silver between 2.54 ppm to 11.85 ppm and palladium, between 1.67 ppm to 2.12 ppm (Figure 7). As for the base metals (Cu, Zn and Pb), copper ranges from 100 ppm to 1709 ppm; Zinc from 3.97 ppm to 228 ppm; and lead , from 2.5 ppm to 83.74 ppm (Figure 8). In the case of case of chromium and nickel, these vary respectively, between 10 ppm - 1085.70 ppm and 8.5 ppm - 432.54 ppm. Manganese highly anomalous reaches 2417.48 ppm (Figure 9). Thorium ranged between 1.17 ppm to 12.92 ppm, Yttrium ranged between 1 ppm to 34 ppm and finally Arsenic ranged between 10.29 ppm to 240.67 ppm.
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Figure 7. Map showing the anomalies: [(a) gold, (b) silver and (c) palladium].
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Figure 8. Maps showing the anomalies: [(a) copper, (b) zinc and (c) lead].
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Figure 9. Maps showing the anomalies: [(a) chromium, (b) nickel and (c) manganese].
All these values represent very important targets for the exploration of Au, Ag, Pd, Cu, Zn, Pb, Mn, Ni, Cr, Y, and thorium in Oudiane Elkharoub.
The results show that among all samples, those with spatial correlations between gold, silver, copper, zinc and lead are OE14 and OE42 Table 1. The positive results of precious and base metals cans be explained by the intense level of alteration for sulfides in the samples (Figures 10(a)-(f)), which are also clearly visible one the field, seen in the outcropping rocks, thus enabling the observation of gold in the existing quartz veins (Figure 10(a)).
On-site observation also indicated there are two generations of quartz veins the first is parallel with the foliation of host rocks, whilst the second intersects the host rocks foliation, both host mineralization. The chemical analysis of the host rock was only undertaken for gold and did not show any anomalies occurring within, only inside the quartz veins with sulfide alteration (mainly Pyrrhotite, pyrite) and tourmaline that control the mineralization of god inside the shearing zone.
The Chromium anomalies are associated with mafic rocks, especially amphibolite, while the anomalies of manganese are associated with quartzite and gabbro; there are a good spatial correlation between gold and manganese especially in the sample OE10 (quartzite). Good spatial correlation between Nickel, Manganese, copper, yttrium, lead and Chromium in the sample S94 (Gabbro).
Noteworthy is the spatial correlation between gold anomalies and silver, copper, zinc, manganese and lead (Figures 11(a)-(j)) that possess a higher anomaly. According to on-site observation and both chemical and XRF analyses, we can affirm that the presence of polymetallic mineralization in Oudiane Elkharoub Zone is a common occurrence.
The compilation of the geological, geophysical, both chemical and XRF analyses and the interpretation of field mappings showed that the existing mineralization is controlled by the structure (shear zone) that intersects the different lithology, indeed, hydrothermal fluids would use the shear zone along with faults and fractures as pathways for their circulation and for the mineralization’s precipitation. However, the current stage in our model’s implementation requires more investment and further prospection work.
5. Discussion
The comparison between the southern and northern portions of the Birimian formation in the West African Craton points to a few important factors. There is a significant spatial distribution of deposits in the southern Birimian part; the Paleoproterozoic terrane to the south of the West African Craton (Figure 12) is intensely mineralized in various economically important commodities and therefore must be described in further detail. The terrane consists of the Baoule-Mossi domain [2] , the Kedougou-Kenieba. As well as Kayes and Anosongo inliers, comprising relicts of Archean rocks set in a granite-greenstone terrane. Such domains are composed of Paleoproterozoic volcano-sedimentary sequences that underwent reworking during the Eburnean Orogeny [18] [19] . The basement rocks contain linear, volcano-sedimentary and volcano-plutonic greenstone belts.
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Figure 12. A simplified map of the West African Craton that highlights the distribution of all known mineral deposits listed in the WAMDD. A dashed line indicates the limit of the Craton; the map is modified after [29] [30] [31] and [32] .
The diverse structural architecture of Birimian greenstone belts can also be seen inside the Baoule-Mossi domain. Such belts are linear in form and metamorphosed to lower-greenschist-to-ampibibolite facies, depending on their distance from the granitoid intrusions. Numerous studies on the Eburnean granitoids indicate they were emplaced around 2178 - 2176 Ma (U-Pb) in the Ghanaian Aahanti Belt [18] , 2090 - 2070 Ma in eastern Guinea [19] , 2076 ± 16 Ma in the Mauritania Eglab Shield [20] , 2160 - 2080 Ma in the Kedougou-Kenieba inlier, located in Senegal and Mali [21] , and around 2181 - 2117 Ma in northeast Burkina Faso [22] . These Eburnean granitoids are typical ovoid in form or crop up as composite batholiths, which constitute ca. 70% of the birimian trerane, with a composition that range from TTGs to leucogranite [11] . Some granitoids crosscut the region’s volcanic belts, whereas others intrude the Paleoproterozoic basins or older gneissic complexes.
Birimian supracrustal rock sequences are uncomfortably overlain by Neoproterozoic and Cambrian, continental-to-glacial sedimentary rocks in the east. As in the Baoule-Mossi terrane, three major tectonic events have been defined for the region [23] - [28] . In conclusion, the geological and structural context in northern part of Birimian (which our study Zone belongs) is very similar to the Birimian deposits in the southern part of Birimian domain, and the source of mineralization is the same.
6. Conclusions
The results of chemical and XRF analyses have demonstrated the existence of mineralized zone in gold, copper, zinc, lead, manganese, chromium, nickel, silver, thorium and yttrium, in Oudiane Elkharoub Zone all to the southeast portion of the study zone and with high-grade anomalies. This kind of mineralization essentially found in chlorite-rich diorite, sheared amphibolite, mafic rocks and quartzite. The anomalies, in turn, can be interpreted as resulting from the abundance of sulfide and malachite in the outcropping rocks (quartz vein and host rocks).
There is an important correlation between the sheared zone observed on-site and the mineralization itself. The mineralization’s trend follows an N-S direction, with slight deviation.
The contribution of values denounced by the anomalies of polymetallic elements indicates spatial correlations between the existing gold, copper, manganese, zinc, and lead. A few samples demonstrate spatial correlations between their base metal (copper, zinc and lead) and manganese, while others do so for their base metals and their nickel and chromium.
In Oudiane Elkharoub zone, quartz veins are generated with two different size and orientations, and both of them host mineralization occurrences in more than one host rocks. This aspect of our study zone is also observed in the analyzed region of birimian domain in its southern part: from an economic point of view, the southern part of the West Africa Craton contains gold, copper, zinc, silver, lead and manganese all actively explored and mined within the Birimian terrane [28] . The particular positioning of Oudiane Elkharoub Zone between two batholitic granitoids with different ages is one of the strongest parameters for the interest in this district [33] . Furthermore, the main gold mine in the West African Craton is associated with Eburnean orogeny.
Therefore, the northern section of the Birimian Domain in West Africa Craton requires further study and exploration investment to have the same commodities in southern part of Birimian, given both have the same geological and structural characteristics.
Acknowledgements
This study is part of a PhD thesis of the first author (HM, in progress) under the supervision of Dr. Mohamed Raji and Dr. Malika Adjour (Hassan II University, Casablanca). Our thanks go to Mr. Mohamed Raji and Mme. Malika Adjour for the recommendations and availability. We wish gratefully acknowledge for ARPARM for providing of geological and geophysical, data and for the XRF analysis. Finally, our thanks go to MASALAB for the chemical analysis. We sincerely thank the reviewers and assigned editor of this paper.