Geochemical Evaluation of Arimogija-Okeluse Limestones, Eastern Dahomey Basin, Southwestern Nigeria

. 10 representative samples of limestone were collected from the Arimogija- Okeluse Axis of the Dahomey Basin and subjected to X-Ray Fluorescence analysis. Major and Trace elements were used to classify and predict the depositional environment of Arimogija-Okeluse limestone. The results for the major oxide composition revealed that CaO content ranges from 47.6-52.31% with a mean value of 50.06%. SiO 2 concentration varies from 1.94-5.24% with an average of 3.36%. Fe 2 O 3 . MgO, and Al 2 O 3 contents have mean values of 1.702%, 1.52%, and 1.09% respectively. Other oxides; Na 2 O, P 2 O 5 , K 2 O and TiO 2 are low in concentration. The Loss on Ignition also varies from 38.9-41.8 with a mean value of 40.43% suggesting a high carbonate content for the limestone. Standard Ca/Mg and Mg/Ca ratios varies from 25.87-37.63 and 0.03-0.04 with a mean of 33.31and 0.031 respectively. The high strontium content reveals a saline environmental condition of formation for the limestone. The results obtained showed that the Arimogija-Okeluse limestone is a Magnesian limestone deposited in a shallow marine environment and suitable for cement production.


Introduction
Limestone is the most abundant commodity with a global industrial use. A wide range of products has been made from limestone and its by-products. Such products include fertilizers, refractory fillers, ceramics and paints. Limestone has been mined for cement production. The suitability of limestone for cement production is a function of its chemical characteristics, hence the reason for this work. A knowledge of the purity status of the limestone in the Arimogija -Okeluse area will guide prospective investors and stakeholders in decision making and thus subsequent establishment of a cement factory in the zone. A good number of limestone occurrence in Nigeria has been reported; [1, 2, 3, 4, 5,] Published works on Arimogija -Okeluse limestone are rare. Very few studies have been carried out on the Arimogija -Okeluse sub-basin. Ehinola et al [5] worked on the depositional environment, geophysical mapping and reserve estimation of the limestone deposit. Ola-Buraimo et al [7] did a Palynological Investigation of a Type Section of Early Maastrichtian Arimogija -Okeluse Shale Sequence. Unlike other research work done on the Arimogija-Okeluse Axis, little or no work has been done on the quality of the limestone. However, this work is aimed at bridging the information gap via geochemical criterion using major and trace element analysis to classify and predict the depositional environment of Arimogija-Okeluse limestone.

Geologic Setting
The Dahomey Basin is a sedimentary basin that extends from south-eastern Ghana in the west to the western flank of Niger Delta in Nigeria. It is bounded in the west by the Ghana ridge, which is an extension of the Romanche Fracture Zone; and on the east, by the Benin Hinge line, a basement escarpment which separates the Okitipupa Structure from the Niger Delta Basin and also marks the continental extension of the Chain Fracture Zone [8]. The Nigeria portion of the basin extends from the boundary between Nigeria and Republic of Benin to the Benin Hinge Line. The stratigraphy of the sediments in the Nigerian sector of the Dahomey Basin is controversial. This is primarily because different stratigraphic names have been proposed for the same formation in different localities in the basin [9]. This situation can be partly blamed on the lack of good borehole coverage and adequate outcrops for detailed stratigraphic studies. The stratigraphy of the entire basin into three chronostratigraphic packages; they are pre -lower Cretaceous folded sediments, Cretaceous sediments and Tertiary sediments [10]. In the Nigerian portion of the basin, the Cretaceous sequence, as compiled from outcrop and borehole records, consists of the Abeokuta Group which is sub-divided into three formational units: Ise, Afowo, and Araromi Formations [11]. Ise Formation unconformably overlies the basement complex and comprises of coarse conglomeratic sediments. Afowo Formation is composed of transitional to marine sands and sandstone with variable but thick interbedded shales and siltstone. Araromi is the uppermost formation and is made up of shales and siltstone with interbeds of limestone and sands The Tertiary sediments consist of Ewekoro, Akinbo, Oshosun, Ilaro and Benin (coastal plain sand) Formations. Ewekoro Formation is made up of fossiliferous, well-bedded limestone while Akinbo and Oshosun Formations are made up of flaggy grey and black shales. Glauconitic rock bands and phosphatic beds define the boundary between the Ewekoro and Akinbo Formations. Ilaro and Benin Formations are predominantly coarse sandy estuarine, deltaic and continental beds [12]. The shalelimestone sequence at Arimogija -Okeluse belong to the Cretaceous Abeokuta Formation which was deposited during the first marine transgressive cycle in the Dahomey basin. The study area lies within longitude 5°33'E -5°43'E and latitude 6°46'N -6°51'N at the easternmost part of the Dahomey Basin with elevation range between 50 and 80 meters. (Fig. 1). The mapped area is part of the Cretaceous Abeokuta Group

Materials and Methods
10 representative samples of limestone were collected from the Arimogija-Okeluse sub-basin. The collected samples were appropriately labelled and georeferenced. These samples were further subjected to geochemical analysis via the X-Ray Fluorescence method after being pulverised and sieved with 65µ mesh. Concentrations of the major elements were determined on fused lithiummetaborate discs by X-ray fluorescence spectrometry (Philips PW1400 Spectrometer) at Activation laboratory using an Rh tube operated at 40 kV and 70 mA. Loss on ignition (LOI) was determined by heating powdered samples for 50 min at 1000 ºC. The concentrations of Ba, Sr, Zr, were also determined on pressed pellets by X-Ray Fluorescence spectrometry at Activation Laboratory (operating conditions: Rh radiation, 70 kV, 40 mA).

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Mean SiO 2 values of 13.9% and 11.3% for Igunmale and Sokoto respectively [2]. The Silica concentration of Arimogija-Okeluse limestone (3.36%) is low relative to other limestones deposits in Nigeria. However, it is almost at par with that of Sagamu, Ewekoro and Ibeshe with mean values of 5.7%, 8.7% and 14.8% respectively, [4]. This study however established that Arimogija-Okeluse limestone is a low silica limestone, hence it is of high grade

Alumina (Al 2 O 3 ) and Magnesium Oxides (MgO)
Relative to the concentration of CaO, Alumina and Magnesium oxide concentrations are low with mean values of 1.09 % and 1.52 % respectively [2]. The low concentration of alumina suggests a low energy environment. The skeletal debris of marine invertebrate has low magnesium with increasing level in the phyla, [15]. Arimogija -Okeluse limestone is rich in brachiopods, gastropods, bivalves, and shell fragments of echinoids and ostracods which are typical of an open shelf environment [5]. The presence of these invertebrates is suspected to be responsible for the low level of magnesium in the samples. Magnesium concentration is also a function of temperature of formation, and often low in shells living in shallow waters [16]. Consequently, shallow marine environment is therefore proposed for Arimogija -Okeluse limestone.

Classification of Limestone
Pettijohn, [17], have classified limestone. However, the classification of limestone in the study area is made after Todd, [22]. The standard ratio; Ca/Mg ratio and its reciprocal ratio: Mg/Ca was employed by Todd [22] as a parameter for Chemical Classification of limestone. Todd [22] grouped Limestone samples with Ca/Mg ratios with a range of 1.41%-12.30% as Dolomitic Limestone, samples with Ca/Mg ratio of 12.30%-39.00% as "Magnesian Limestone" and Limestone samples having 39.00% to 100% are grouped as "Pure Limestone" Table 3.  (Table 4).

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The Ca/Mg ratio also corresponds to stability condition during the formation of carbonate rock [23]. He pointed out that the degree of salinity increases with decrease in Ca/Mg ratio. Higher values of Ca/Mg ratio of the studied carbonate indicates comparatively less evaporation of sea water and low salinity which prevailed during the formation of limestone in general.

Trace Element Geochemistry
The trace elemental composition reported in ppm (Table 5)  Trace elements analysis has been used in describing the conditions at deposition in specific depositional environment. It is a veritable tool used in the differentiation of shallow water and deep water limestones. Strontium and Manganese are markedly enriched in carbonate sediments. Hence, the enrichment of strontium in limestone is accounted for by the fact that Sr 2+ substitutes readily for the very similar ion Ca 2+. The smaller but appreciable concentration of manganese in carbonates is probably also attributable to similarity in ionic size.

Strontium
Flugel and Wedepohl, [25] reported relatively low Sr values (100-400 ppm) for shallow marine and relatively high Sr content (Oberalm limestone) for deeper water carbonates. Bausch [26,27] also reported a high value of Sr (500-3000 ppm) which depicts a deep marine environment. Shallow water and deep water carbonates also have relatively low Sr (100-400 ppm) and high Sr values respectively [28]. The strontium (Sr) concentration in Arimogija-Okeluse Limestone indicates that the formation of the limestone under higher saline environmental conditions.

Summary and Conclusion
Limestone remains one of the industrial minerals with global use. Limestone is quarried for cement production in some part of Nigeria. However, the suitability of limestone production depends on its chemistry. This helps in the prediction of its purity. The geochemical investigation of the Arimogija-Okeluse limestone was carried out to accomplish a chemical classification with respect to its chemical composition. It was revealed that CaO shows an inverse relationship with SiO 2 . The prevailing concentration of CaO with respect to SiO 2 depicts its suitability for cement production. The Todd chemical classification revealed that the Arimogija Okeluse limestone is the Magnesian type; with Ca/Mg standard ratio range of 25.87-37.63 and a reciprocal ratio Mg/Ca range of 0.03-0.04. The relationship between Alumina (Al 2 O 3 ) and MgO depicts a shallow marine environment for the Arimogija Okeluse limestone. The Trace element geochemistry also predicts the formation of limestone in shallow waters. In general, the results obtained, however revealed that the Arimogija-Okeluse limestone is a Magnesian limestone type deposited in a shallow marine environment and suitable for cement production.