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Article Published In Vol.6 (March-April-2018)

Petro-Geochemical and Statistic Studies of the Nimba Region in the Republic of Guinea

Pages : 256-272, DOI: https://doi.org/10.14741/ijmcr/v.6.2.2

Author : Mohamed Samuel Moriah CONTE, Abdellah BOUSHABA and Ali MOUKADIRI

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Summary: Guinea is described as a geological scandal, due to the richness of its soil, its subsoil, and its complexity. But this geology is poorly studied and under exploited and the region of Nimba hardly escapes to this fact. Located in the south-east of the country, Nimba is about 60 km long and 25 km wide, it belongs to the Precambrian domain (Archean basement (Craton) and Proterozoic) and is characterized by volcano-sedimentary rocks. In addition to mineral diversity, shear, brecciation and fault structures are observed in the rocks (Nion District), while they are not observable in rocks of other parts of the region. Large deposits of iron (magnetite, hematite, goethite) occupy the summits of the region. Granites exhibit magmatic differentiation whereas amphibolites occupy the MORB domain set up by rifting. The major elements exhibit perfect positive and negative linear relationships, while the rare earths exhibit perfect positive linear relationships. The PCA of the major elements confirms the differentiation of granitic rocks while the amphibolites do not show differentiation due to weathering.

Objective:  Geological cross sections of the Nimba range, petrographic studies of the different rocks and mineralogical description of these rocks were realized. And this to know the context of setting up of the region using the results of chemical analysis, the knowing of the different correlations which allow to visualize a space with p dimensions using spaces of smaller dimensions of the Geochemical data.

Materials and Methods: This study in the Nimba region was done in two stages. The first stage performed in the field (period from November 25 to December 20, 2015 and from January 23 to February 15, 2017); it permits to collect the samples, the realization of two geological cross sections (one in the South near the border between the Republic of Guinea and Liberia, and one in the North in the fan zone of the region). The second step, carried out in the laboratories, with the digitalization of the two (2) geological cross sections (using Canvas version: 11Build 1252), then a microscopic description of the samples and the analysis of the different spectroscopies and diffractograms of the rocks of the Nimba region (using the Bruker Raman SENTERRA type spectroscope and the Pert Pro Panalytical X-type diffractometer). The rocks were analyzed geochemically by ICP-AES and ICP-MS. Finally, the geochemical data is processed by statistical methods (using Statgraphic Centurion software version _16.1.11).

Results: The Nimba region contains two (2) groups of rocks, the first group clear and  SiO2 rich  (Gneiss, Granites, and Quartzites) and the second group is dark and SiO2 poor (Muscovitites, Amphibolites). But il also contains Itabirite-type iron deposits (Banded Irons Formations: BIFs) which occupy the main peaks of the Nimba region chain. These rocks contain minerals that are abundant in some and rare in others or even absent (Table 1). Next, they reveal that the quartzite facies are characterized by open muscovite crystals in the form of “S” slits and crystals of quartz, K feldspar and “C” plagioclases that give the “C / S” structures and hence the formation of “Shears Zones” (District of Nion, photo 2r). Amphibolitic rocks (Nion District) reveal both a breccia zone and a fault zone (Figure 2w and 2x). The breccia zone constitutes the phase S1 which is filled by the grunerite and the fault zone constitutes the phase S2 filled with microcrystals of quartz, magnetite, feldspars and grunerite. This phase S2 is characterized by the displacement of the blocks due to the fault activity. On the other hand, in the district of Gbié, the amphibolites do not contain zones of breccias or of faults, but they reveal the arrangement of the minerals (figure 2z and 2aa). The spectroscopies and diffractograms of the different rocks of the Nimba region confirm the results obtained from the petrographic description but also the appearance of new minerals (Table 2). The chemical rocks analyses of the Nimba region reveal that they are sub-alkaline. The amphibolites are tholeiitics, occupying the area of MORBs, with REE spectra (normalized to chondrites of N-MORBs derived from depleted upper mantle) showing depletion in LREE and almost flat in HREE. The granitics rocks are calco-alkaline, peraluminous, marked by the decrease of minerals ferromagnesian, ferro-titanic oxides and plagioclase with an increase of the alkali feldspar content. The spectra of REE  of granitic rocks  chondrite normalized at the chondrite  show both negative in Eu anomalies suggesting on one hand that the plagioclase was  fractioned  and positive  anomalies in EU that indicates on the  other hand that the plagioclase has not been fractioned. All these spectra present the enrichment in LREE and a poverty in HREE. The Bravais-Pearson correlation of the major elements of amphibolitic and granitic rocks shows that Al2O3 does not correlate with other major elements and P2O5 correlates only with TiO2. The rest of the major elements have both positive linear relations and perfect negatives with P-Values less than 0.05. The Pearson-Pearson correlation of REE of amphibolitic and granitic rocks shows perfect positive linear relationships with P-Values less than 0.05. The Principal Components Analysis (PCA) of the major elements of amphiboles and granites has three groups of major elements, the first formed of SiO2, Na2O and K2O which correlate with each other to the component 1. The second formed of MgO , TiO2, MnO, Fe2O3t and CaO correlate with each other at component 1; these groups are anti-correlated with respect to component 2. The third group of major elements consists of Al2O3 and P2O5 which correlate neither with component 2 nor with the first two groups of major elements. Only TiO2 correlates with P2O5. Finally, the amphibolites have an affinity with the second group and do not exhibit magmatic differentiation due to weathering and weather conditions. The granitic rocks have an affinity with the first group, they are distributed according to the magmatic differentiation. The Principal Component Analysis (PCA) of REE of amphibolitic and granitic rocks has two groups: the first formed of Ce, La, Pr, Nd, Sm, Gd, and Eu which correlate with each other at component 1. The second, Tb, Dy, Er, Ho, Tm, Yb and Lu that correlate with each other at component 1. These two groups of chemical elements are not anti-correlated to component 2, which is confirmed by the hypothesis of the linear correlation method.

Conclusions: The region of Nimba belongs to the precambrian domain, it is largely correlated with those of Brazil and Venezuella. It is formed by metavolcanic and metasedimentary rocks: quartzitic rocks characterized by ‘C / S’ shear and zoned zircon; grunerite amphibolites (cummingtonite family) rich in iron and associated with large iron deposits in fault and breccia zones. These amphibolites are located in the MORB field, which was emplaced by rifting, while the granitic rocks were emplaced by magmatic differentiation. This region is rich in iron deposits that occupy the main peaks and whose establishment was made by precipitation of a silico-ferruginous gel in a closed basin, virtually protected from other terrigenous inputs. This precipitation of dissolved iron results from an enrichment in atmospheric O2 characteristic of the Archean and Paleoproterozoic.

Keywords: Mineralogy, Petrography, Geochemistry, PCA, Shear-zones, Itabirite iron deposits, MORB, Precambrian, Nimba region, Guinea.

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