Geology , Mineralogy and Geochemistry of the Oligocene Oolitic Iron Ore of the Continental Terminal Formation , Kandi Basin , North-East Benin

The Oligocene Continental Terminal Formation of the Kandi Basin contains high grades of iron mineralization (~56.72% Total Fe). The microscopic study under the polarized and reflected light showed that the iron ore consists of silicate minerals (quartz 50% and zircon 1%) and non-silicate minerals (goethite 30%, hematite 7%, magnetite 3%, pyrite 1%, chalcopyrite 1%, blende 3%, galena 3%, scheelite 1% and gold 2%). The X-rays fluorescence shows that the iron ore is characterized by various elements, such as Fe2O3 (57.91% to 91.33%), SiO2 (3.07% to 33.19%), aluminum (2.94% to 7.74%), vanadium (0.04% to 0.11%), phosphorus (0.79% to 2.29%) and sulfur (<0.3%). The deleterious elements grade is above the permissible limit in metallurgy (0.05% 0.07% for phosphorus and 0.1% for sulfur). Their high grades indicate that the Kandi Basin iron ore characteristics are not favorable for steel manufacturing despite its good vanadium contents (0.04% to 0.11%). However, it could be used for the cast iron manufacture. Spectrometric analysis by atomic absorption confirms the presence of low-grade gold associated to the iron ore (from 0.006 to 0.015 ppm). The comparative study of discontinuous stratiform iron ore of the Kandi Basin with other oolitic iron ores in exploitation from other countries such as Brazil, Australia, China, Russia, Uganda and the United States shows that iron ore of the Kandi Basin can be mined despite its high silica content. How to cite this paper: Fatiou, A.K.I., Konaté, M., Yessoufou, S., Glodji, C.L.A., Heckmann, M. and Saley, H.G. (2019) Geology, Mineralogy and Geochemistry of the Oligocene Oolitic Iron Ore of the Continental Terminal Formation, Kandi Basin, North-East Benin. International Journal of Geosciences, 10, 491-512. https://doi.org/10.4236/ijg.2019.104029 Received: March 3, 2019 Accepted: April 27, 2019 Published: April 30, 2019 Copyright © 2019 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access A. K. I. Fatiou et al. DOI: 10.4236/ijg.2019.104029 492 International Journal of Geosciences


Introduction
The ore classification takes into account several parameters such as grade, quantity, nature and shape.Each parameter corresponds to an important technical criterion taken into account for the evaluation of the economic potential of the ores.
Mined since prehistoric times, oolitic iron ores are important sources of iron [3].This metal represents for human being the most important element among all elements of the periodic classification of Mendeleïev [4].
Oolitic iron ores of continental origin are exploited as those of marine origin [5].Of all the 175 known oolitic iron ores in the world, only a few have undergone detailed mineralogical and geochemical analysis [6] [7].
Very few studies have been conducted on oolitic iron ores in West Africa [7].These iron ores are particularly abundant in the formation of the Continental Terminal of Iullemmeden basin.They have many levels of high iron content that can be mined economically [8] [9].
In northeastern Benin and southwestern Niger (Figure 1), the Kandi Basin contains the most high graded iron ore of Benin Republic with estimated reserves of more than 300 million tonnes [10].Known incorrectly as the "iron deposit" of the Kandi Basin [11], iron ore is especially important in the Continental Terminal Formation.It has been mined traditionally for more than a century [12].This formation, Oligocene in age [13], forms a subhorizontal sedimentary layer ranging from massive and oolitic goethite to ferruginous sandstones [14].It is a large detrital spread with ferruginous concretion and kaolinite [15].Iron ore extends over thousands of km 2 in northwestern Nigeria and northeastern Benin [14] [16].
The Kandi Basin iron ore has not received adequate attention in terms of mineralogical, metallurgical and geochemical studies.Most of the executed exploration surveys are preliminary and engaged in the reserves assessment of the iron ore, hence, the necessity of the current work.
The aim of this preliminary survey is to improve the geological knowledge of the oolitic iron ore of the Continental Terminal Formation.More specifically, the objectives deal to determine the mineralogical, geochemical and metallurgical characteristics of iron ore in the Madékali-Loumbou-Loumbou areas.International Journal of Geosciences Figure 1.Geographical location of the study area in west Africa (Source: [28]; modified).

Geological Setting
The sedimentary series of the Kandi Basin overlie the Pan-African basement separated by a major unconformity through a basal conglomerate or conglomeratic coarse sandstone [14].The formations in the study area consist of dominant terrigenous sandstone rocks including from the bottom to the top (Figure 2), the lower Paleozoic deposits overlain in the Sendé sector by Cretaceous deposits on which overlie through an erosional unconformity the Continental Terminal sediments presumed Oligocene in age [13].The Lower Paleozoic detrital series begins with the Wéré fluvio-glacial formation.The latter is spatially restricted to the fault-bounded paleovalley, marking the western edge of the Kandi Basin.About 500 meters in thickness [17], the sedimentary infilling of the fault-bounded paleovalley consists of conglomerate and polygenic breccia with large boulders scattered in a sandy-argillaceous ferrugineous matrix.These conglomeratic deposits move towards the northeastern (sectors of Goungoun and Guéné) to polygenic breccias of clast-supported to matrix-supported type [18].Wéré upper sandstone deposits are vertically and laterally relayed by tidal to subtidal deposits of the Late Ordovician -Lower Silurian Kandi Formation [19].
The Kandi Formation (Figure 2), with about 80 m in thickness, consists of an alternation of sandstone and clayey-siltstone [17].This Kandi Formation is overlain by coarse sandstone deposits with subordinate silts and clays of the lower Cretaceous continental formation of Sendé.The latter is separated from Kandi's one by a ravinement surface [14].Sendé's Formation is overlain by the Continental Terminal Formation which is dominated at the bottom by a kaolinic They are found on the banks of the river of Niger, Sota and Alibori (Figure 3).They consist of quartzitic sandstones and clays [14].
The Kandi major fault N20˚ trending, controls the west-to-east spatial distribution of alluvial, fluvial and marine detritic facies and the sedimentary geometry structured in a synclinal half-graben shape [17].

Material and Methods
The methodology of this study includes four main steps: 1) The first step focus on the description of the outcroping rocks in the study area and the realizing of structural cross-sections.2) The second stage consists of analyzing and measurements of the sedimentary and tectonic structures.During this step, a total of 150 iron ore samples were collected and 60 of them were subjected to laboratory analysis.
3) The third step involves laboratory works which consisted of microscopic study (30 thin sections in transmitted light and 10 polished sections in reflected light) and geochemical analysis (10 samples by X-ray fluorescence at Activation Laboratory Ontario in Canada and the other 10 samples by Atomic Absorption Spectrometry at Actlabs Ouagadougou in Burkina Faso).4) In the fourth step, the interpretation of petrographic, mineralogical and geochemical data were carried out.The geochemical data were used to determine the relationships between the main interest oxide (Fe 2 O 3 ) and the other oxides.It is based on the use of MINITAB 14 software which also allows statistical analysis.Regression with "fitted line plot" has been adopted to understand the different relationships that exist between elements.

Field Observations
The oolitic iron ore of the Kandi Basin corresponds to the upper level of the undifferentiated Continental Terminal Formation (Figure 3).It is commonly characterized by the presence of three types of faciological elements such as, a kaolinic layer at the bottom, scattered termitic traces and ferrugineous oolite and pisolite layers at the top.This Continental Terminal Formation is composed of four sedimentary levels.The basal level, about 1 m in thickness consists of microconglomeratic ferrugineous coarse-grained sandstone with unclear cross bedding.It is overlain by a second whitish sandstone level with kaolinic cement gradually moving to variegated kaolin.The third level corresponds to reddish clayey sandstones, about 8 meters in thickness.It is relayed vertically by ferrugineous oolitic horizontal sandstone hosting the iron ore.The ferrugineous mineralization levels are about three meters in thickness.They are often capped by a ferrugineous pisolitic concretion, about 5 m in thickness (Figure 4).
The iron ore is affected by several fractures of many trends.Three types of sub-facies characterize the iron ore: the first one is sandy, ferrugineous, oolitic and horizontal (Figure 4 and Figure 5), the second exibits ferrugineous pisolitic concretions (Figure 6) and finally the third type is poorly sorted deposit due to the posterior remobilization of oolitic ferrugineous sandstones pebbles (Figure 7).

Mineralogical Composition of the Kandi Basin Iron Ore
Detailed petrographic analyses allow us to classify the mineralogical composition into silicate minerals and non-silicate minerals (iron oxides and hydroxides, sulphides and native elements).

Under the Polarizing Microscope
In polarized light, non-silicate minerals are characterized by an opacity due to their isotropy.They have a brownish or sometimes blackish color forming mainly the cortex of oolites and pisolites (Figure 8).The silicate minerals are represented by the xenomorphic quartz having a textural variety and an angular to sub-angular shape, suggesting a proximal origin (Figure 9).Incidentally, we find crystals of zircons (Figure 10) and scheelite (Figure 11).
The zircon crystals are hexagonal in shape and have black aureoles.

Under the Metallographic Microscope
Under the metallographic microscope, the opaque minerals correspond to iron oxides and hydroxides, sulphides and native elements.Oxides and hydroxides International Journal of Geosciences        are represented in decreasing proportion by goethite, hematite and magnetite (Figure 12).Goethite is characterized by its dark color sometimes brownish.The transition from black to brownish color marks the transformation of goethite into hematite under the effect of dehydration [2].This transformation is expressed by the below equation: The sulphides are here represented by Galena, blende, pyrite and finally chalcopyrite (Figure 13).
The only native element, highlighted by the present study, is xenomorphic gold (Au), occupying the intergranular spaces of ferruginous sandstones (Figure 14).

Ore Geochemical Analysis
The production of iron ore depends on several geochemical parameters including the total iron content and the contents of various noxious elements that form the gangue [20].

Major Elements Composition and Determination of Total Iron Grade (Total Fe)
The results of the geochemical analysis of the oolitic iron ore from the Kandi Basin are summarized in Table 1.The description of the collected samples as well as their spatial references and corresponding localities are mentioned in Table 2 The results indicate a high Fe 2 O 3 content ranging from 57.91% to 89.39% followed by SiO 2 and Al 2 O 3 with contents ranging respectively from 5.8% to 33.19% and from 2.94% to 7.25% (Figures 15-18).
For classification and quality assessment, iron ores were subdivided into three categories, based on the total iron content [21].This classification is shown in Table 3.
-Determination of total iron grade (Total Fe) In order to appreciate the economic potential of iron ore in the Kandi Basin for further exploration and exploitation, we compared the geochemical data of this study to those of ores in operation around the world.
To determine the total iron content, the main iron oxide (Fe 2 O 3 ) must be converted to the elemental state of iron (total iron).This conversion can be done in the following manner.[22].The regression provide information on the direction, size, and statistical significance of the relationship between the International Journal of Geosciences predictor (Fe 2 O 3 ) and the response (the other oxides).Each coefficient indicates the trend of the relationships between the elements.Moreover, the "fitted line plot" allows us to know the type of relation between two continuous variables: the predictor and the response.Once the "fitted line plot" is created, the response variable is displayed on the y-axis (y) and the predictor variable on the x-axis.This statistical analysis was carried out with the Minitab 14 statistical software that allows to predict the type of relationship between the main oxide of interest (Fe 2 O 3 ) and the other oxides (SiO 2 , P 2 O 5 , SO 3 , V 2 O 5 , TiO 2 , MnO, MgO, Al 2 O 3 , CaO, Na 2 O, K2O and Cr 2 O 3 ).
In the Minitab software, S = standard deviation of the error; R 2 (R-Sq) = coefficient of determination.
From these results, we observed that SiO Analysis of the atomic absorption spectrometry reveals gold occurence in the ferrugineous sandstones of the Continental Terminal Formation (Table 4).We can see that the gold occurence (0.015 ppm in average) is three times higher than its crustal clarke (0.005 ppm).

Discussion
The most important elements and compounds to take into account in the study of iron ore are: total iron (Fe), gangue mainly composed of SiO 2 and Al 2 O 3 and noxious elements (phosphorus and sulfur).In addition to these elements, the iron ore contains other accessory oxides such as MnO, MgO, TiO and CaO but            in negligible quantities [2].Phosphorus, aluminum and sulfur are considered as impurities in the steel manufacture process.Their presence in large quantities is an obstacle to the exploitation of iron ore.
For commercial viability, iron ores must have a high total iron content and a very low content of noxious elements.
In the oolitic iron ore of the Kandi Basin, silicon and aluminum are present in significant proportion (11.99% for silicon and 4.51% for aluminum).They are not chemically related to iron but seem to be intimately associated to it.This promotes easy separation of these elements.On the other hand, sulfur and phosphorus are chemically associated to iron but with a weak correlation.The close relation between iron and vanadium ensures agood alloying, a hardness and a strength [21].
For a better mining, the average of the total iron content of an iron ore must be between 30% and 65% [23].Geochemical analyzes have shown that iron ore

Figure 2 .
Figure 2. Lithostratigraphic log of the Kandi sedimentary basin showing the oolitic iron ore at the top (Source: [19]).

Figure 4 .
Figure 4. Cross section of the Tourougo area showing the vertical succession of the Continental Terminal deposits.The uppermost deposits include the oolitic iron mineralisation (Location in Figure 2).

Figure 5 .
Figure 5. Ferrugineous sandstone of the Continental Terminal hosting the iron ore deposit (Madekali area, northeastern part of Kandi Basin).

Figure 7 .
Figure 7. Field observation showing conglomeratic sandstone with ferrugineous rounded clast supported facies and nearly matrix supported facies.These deposits are related to a reworked facies (Madekali sector, northeastern part of Kandi Basin).

Figure 8 .
Figure 8. Photo micrograph showing the oxides and hydroxides seen under the polarizing microscope (Ox1: oxides and hydroxides forming mostly oolites and pisolites cortex, Ox2: oxides and hydroxides forming mostly the binding agent, P: pores, Zr: zircon).

Figure 9 .
Figure 9. Photo micrograph showing the shape of quartz under the polarizing microscope (Qz: quartz).

Figure 14 .
Figure 14.Photo micrograph showing gold observed under the metallographic microscope.

4. 3 . 2 .
Relationship between Iron and Other Major Elements: Regression Analysis and "Fitted Line Plot"This study is done in order to understand the type of relationship between iron and other existing elements in the ore.This analysis generates an equation that describe the statistical relationship between one or more predictors (Fe 2 O 3 ) and the response variable (SiO 2 , P 2 O 5 , SO 3 , V 2 O 5 , TiO 2 , MnO, MgO, Al 2 O 3 , CaO, International Journal of Geosciences

Figure 26 .
Figure 26.Regression analysis with fitted line: M g O versus Fe 2 O 3 .

Table 1 .
Grades of various major elements of the Kandi Basin iron ore (Avg = Average; Spl = samples; Cnt = content).

Table 2 .
Description of collected samples with localities and spatial references.

Table 3 .
[3]eralized percentages of elements of major interest in assessing iron ore quality[3].
2 , TiO 2 , Al 2 O 3 , MgO, CaO, Na 2 O and K 2 O have an inverse relationship with Fe 2 O 3 with a degree of response corresponding respectively to 98.6%, 50.2%, 39.9%, 13.9%, 15%, 7.9%, and 16.4%.Silica thus has a strong correlation and is closely related to iron.This kind of relationship is smilar to that of Koton Karfe oolitic in northern part of Nigeria [2].On the other hand, MnO, P 2 O 5 , Cr 2 O 3 , V 2 O 5 and SO 3 have a direct relationship with Fe 2 O 3 with a degree of variation response of 0.1%, 60%, 0% and 0.2% respectively.Phosphorus thus has a strong correlation and is intimately related to iron, whereas sulfur is weakly associated to it (Figures 19-30).