Received 31 March 2016; accepted 17 July 2016; published 20 July 2016
Carbonate rocks constitute about 50% of the world’s hydrocarbon reservoir rocks, and among these limestones are widely used as raw materials in the chemical, metallurgical and construction industries. The quality and hence the usefulness of limestone deposit is largely dependent on the geological setting and the physico-chemical, mechanical and mineralogical characteristics of the stone. An evaluation of usefulness or appraisal of a limestone deposit entails a geological field investigation and laboratory analyses of representative samples. Naturally, limestone carries varied suite of impurities such as SiO2, MgO and Fe2O3, whose geochemical concentration determines its industrial application(s). Therefore an assessment of its grade through geochemical analyses such as XRF is essential. Most limestone industrial applications consider the carbonate and MgO contents as fundamental criteria for its chemical purity or grade classification.
In accord with the global increase in the applications of geological models for exploration and exploitation of mineral resources; geochemical models revealing limestone’s chemical purity can be used as a tool to appraise the spatio-temporal distribution of limestone purity throughout the deposit. The modelling approach is targeted at locating anomalous concentration(s) of high purity limestone or other pathfinder elements and characterizing the host lithologies. This method may form a basis for comparison of data for limestones of other geological settings all over the earth.
The Ewekoro limestone belt extends to the northwest and beyond Shagamu to the southeastern part of the embayment. Nwajide  reported the limestone reserve estimate of  of about 36 million tons. Reyment  also presented estimated values of chemical constituents of quarried limestones from the Ewekoro as follows: CaO (53%), CO2 (42%), SiO2 (2%), Al2O3 (5%), Fe2O3 (1.4%), P2O5 (0.8%), MgO (0.3%), MnO (0.1%), and minor quantities of Na2O, K2O, TiO2, F and trace of SO3. Although, these geochemical reports are some five decades old and some of the relatively recent related studies include those of  -  and others; but not much has been done on geochemical characterization to re-appraise the spatio-temporal variation of elemental compositions of the carbonates. This study examines the geochemical characteristics of the Paleocene Ewekoro limestone Formation; using results from XRF analysis of samples obtained in order to determine provenance, diagenesis and depositional setting of the study area. The study area lies within Latitudes 6˚47'N to 6˚48'N and Longitudes 3˚38'E to 3˚39'E, the present location of the Shagamu quarry (Figure 1).
2. Geological Setting
2.1. Tectonic Framework
The Dahomey Embayment spans the continental margin of the Gulf of Guinea, covering the Volta delta in Ghana to the west and the Okitipupa ridge/Benin hinge line to the east   . It’s a marginal pull-apart basin or marginal sag basin  that developed in the Mesozoic sequel to the separation of African from the South American plates   . This separation, accompanied by basement fracturing accounted for the early rifting stage during Jurassic to Early Cretaceous and the development of several marginal sub-basins  .
The eastern Dahomey Embayment (Figure 1) has been studied both on outcrop scale as well as from core holes by various workers such as      -  , amongst others. The stratigraphy of the Nigerian sector of the embayment can be broadly divided into two: the Cretaceous Abeokuta Group (comprising Ise, Afowo and Araromi Formations) and the Cenozoic units (comprising Ewekoro, Akinbo, Oshoshun, Ilaro, and Benin Formations)   (Figure 2). However, this study is focused on the Ewekoro limestone Formation.
This formation is made up of fossiliferous shelly limestone of about 12.5 meters thick, which tends to be sandy at the base  . It has been divided into three micro-facies namely: the sandy biomicrite lower unit; the shelly biomicrite grading into biomicrosparite middle unit that consists mainly of pure limestone making up the bulk of the
Figure 1. (a) Geological map of eastern Dahomey Embayment (redrawn after  ; (b) Study area map with NW-SE trending sample locations.
Ewekoro Formation and; the shelly biomicrite and Algal biosparite upper unit  . It’s of Paleocene age based on fossil evidence (foraminifera and ostracods) and deposited in a shallow marine environment   .
3. Materials and Methods
This work involved a geological field survey of the study area, a geochemical analysis and a computer-based geo-modelling to evaluate the CaO and SiO2 distribution in the limestone formation. The field work entailed examination and logging of quarry sections at Shagamu and 83 representative samples were obtained for laboratory analysis. Sampling was done from bottom to top at distinct limestone beds in fifteen sections in a NW-SE traverse (Figure 1(b)) and readings of geographical location obtained using a GPS device.
Sample preparation and analysis were done in the field-based laboratory of West African Portland Cement, Shagamu. Samples were washed, air dried, ground to powder form and homogenized; thereafter 2 g of each sample was mixed with spectroflux powder and 0.6 g of LiNO3 salt in an agate mortar. The mixture was poured into fusing containers on a burner within M4 fluxer equipment and switched on for fifteen minutes to produce fused pellets. Fused pellets produced were analyzed for major elements using an X-Ray fluorescence machine (ARL 9900 XP). Loss on ignition (LOI) was determined separately by calculating weight loss after heating 2 g of each sample in a furnace for 1 hour at 1000˚C.
Figure 2. Stratigraphy of the Dahomey Embayment (After  and  ).
The geochemical results obtained (Table 1) were further evaluated using a computer programme (SurferTM) to generate geochemical models for the distribution of CaO and SiO2 (the main component and impurity in limestones respectively). To achieve this, the latitudinal and longitudinal readings were scaled to the X and Y-axes respectively, while the corresponding chemical concentrations (in wt%) of CaO and SiO2 were scaled to the Z-axis for each of the beds. Through a statistical algorithm called kriging, the GPS readings and concentrations of CaO and SiO2 were interpolated, resulting in a spatio-temporal distribution of these oxides within the various beds through a grid-based contouring of their concentrations. This variogram mathematically express the variance of the geochemical concentration in each of the beds giving a series of surface geochemical models that may constitute baseline information for further exploitation of the deposit.
4. Results and Interpretations
4.1. Field Relationships
Field study revealed five limestone beds, labeled E to A spanning the 15 sections logged (Figure 1 and Figure 3); although a sixth bed, F (which is quite silty with large amount of quartz) was recognized belonging to the underlying Araromi Formation. Bed E is the oldest bed among the limestone units. It’s a light brown sandy algal bed with fossils embedded in sparry calcite cement. Bed D is light brown, shelly biomicritic and inter-fingered Bed E which is more friable, sandy algal biosparite, with the cementing material mostly sparry calcite. Bed C is dark grey, calcareous unit that comprised algal biomicrite facies. Bed B is grayish to brown in colour, hard with greater tenacity than the overlying bed and comprised sandy algal biomicrite embedded in micrite cement. Bed A is a red phosphatic limestone (typifying the intra-sparite facies) that is crystalline textured with localized quartz fillings within vugs and caves induced by migrating acidic fluids.
Table 1. Major elements composition (wt%) of Limestones from the Ewekoro Formation exposed at Shagamu.
4.2. Major Elements Distribution
The concentrations (in wt% of oxides) of major elements in the limestone samples are shown in Table 1. The
Figure 3. A representative lithologic section of the Ewekoro Formation (Fm) observed at Shagamu Quarry.
limestone samples analyzed are enriched (>1 wt%) in CaO (33.7 - 59.99 wt%, average 47.29 wt%) and slightly depleted to enriched in SiO2 (0.47 - 31.21 wt%), Al2O3 (0.47 - 3.23 wt%), MgO (0.39 - 3.15 wt%), Fe2O3 (0.51 - 3.43 wt%) and SO3 (0.10 - 2.27 wt%). However, they are depleted (<1 wt%) in the alkalis (Na2O and K2O), TiO2, MnO, P2O5 and Cr2O3. LOI varies between 17.64 - 45.10 wt percent. The values of CaO and MgO tend to decrease down the Formation with increasing silica content probably due to the presence of non-carbonate input transported from adjacent continental sources.
4.3. CaO and SiO2 Variations in the Ewekoro Limestone Formation
A marked inverse negative correlation exists between SiO2 and CaO contents of the limestones (Figure 4). A generalized geochemical variation model for the area representing the SiO2 distribution is depicted in Figure 5. A bed-by-bed spatio-temporal geochemical distribution of the concentrations of CaO and SiO2 is discussed below in super-positional order.
4.3.1. CaO and SiO2 Variation in Bed E
Bed E is 1 - 2 m thick, occurring at depths of 17 - 38 m and 18 - 40 m at the upper and lower surfaces respectively. Table 1 shows that CaO content ranged from 46.33 - 53.39 wt%, while SiO2 ranged from 1.68 to 6.34 wt%. Figure 6 is the 2-D and 3-D model views of CaO and SiO2. A reduction in CaO concentration was noticed mostly in the south-central and northwestern part of the study area. However, an average abundance of CaO (>50 wt%) was maintained in other portions of the bed keeping SiO2 proportions at minima level.
This zone carries the highest concentration and purest form of calcite. These calcite-rich zones could serve as suitable targets for limestone mining works, development and the production of Portland cement.
4.3.2. CaO and SiO2 Variation in Bed D
Bed D is ≈3 - 4 m thick and occurred at depths of 14 - 34 m and 17 - 38 m at the upper and lower surfaces respectively. From Table 1, the CaO contents ranged from 34.22 - 54.22 wt%, while SiO2 ranged from 1.69 - 31.21 wt%.
Figure 4. SiO2 (in wt%) versus CaO (in wt%) plots revealing a marked inverse relationship.
A 2-D and 3-D model views of CaO and SiO2 concentration is shown in Figure 7. Peaks of CaO and SiO2 concentrations were recorded in the southwestern and northwestern flanks respectively. Again, just as in bed E for a decline in CaO concentration there is a corresponding abundance of SiO2 was observed.
4.3.3. CaO and SiO2 Variation in Bed C
This bed is 3.5 m thick occupied depths of 8.6 - 9.3 m and 20.2 - 23.7 m at the upper and lower surfaces respectively. Table 1 showed CaO concentration of 35.84 - 56.52 wt%, while SiO2 is 1.98 - 22.23 wt%. Geochemical variation models of CaO and SiO2 indicated a decline in CaO (with small peaks in southern part) concentration
Figure 5. A generalized SiO2 (in wt%) distribution models of the study area.
with an accompanying enrichment in SiO2 at both the southeastern and northwestern ends of the study area (Figure 8).
4.3.4. CaO and SiO2 Variation in Bed B
This 3 - 5 m thick bed occupied a depth of 7 - 25 m and 10 - 30 m at the upper and lower bedding planes respectively.
SiO2 content is 1.10 - 9.98 wt% and CaO is 48.19 - 59.99 wt% (Table 1). Figure 9 is the 2-D and 3-D model views of the bed B, indicating a uniformly high CaO concentration with a sharp drop in the mid-western part. SiO2 concentration is very low in this bed; a peak concentration apparent in the mid-western part coincided with
Figure 6. Geochemical variation models of SiO2 (in wt%) and CaO (in wt%) for bed E.
Figure 7. Geochemical variation models of SiO2 (in wt %) and CaO (in wt%) for bed D.
Figure 8. Geochemical variation models of SiO2 (in wt%) and CaO (in wt%) for bed C.
Figure 9. Geochemical variation models of SiO2 (in wt%) and CaO (in wt%) for bed B.
the CaO lowest concentration. A notable increase in CaO concentration was noticed in compensation for the decline in SiO2 concentration, a trend similar to that of the aforementioned beds.
4.3.5. CaO and SiO2 Variation in Bed A
This bed is ≈5 m thick, occurring at depth range of 5 - 25 m. For this bed, CaO content is 52.01 - 55.77 wt% while SiO2 is 0.47 - 2.07 wt% (Table 1). Figure 10 is the geochemical model for bed A in 2-D and 3-D views respectively. From this model, CaO indicated a peak concentration in the southwestern part, while SiO2 showed a high concentration running almost diagonally from the NNW part to the SSE part. This is shown by the corresponding “trough” on the model as against the “crests” of high CaO concentration.
5.1. Geochemistry, Provenance and Depositional Environment
The silica content varies widely (0.47 - 3.21 wt%, Table 1); an indication that the adjacent basement complex rocks of southwestern Nigeria may have sourced varied amounts of these detrital impurity and/or its dissolved component in the shallow marine environment. Geochemical plots (Figure 4) of SiO2 against CaO for all the samples analyzed clearly revealed a similar and unique trend marked by increase in CaO with corresponding decrease in SiO2 contents and vice versa. This SiO2-CaO negative correlation can be attributed to chemical diagenetic replacement. At the shallow part of the upper continental crust where silica is readily made available through weathering and erosion, calcite reacts with it to form a calc-silicate at low temperature and pressure. Also, at all stages of diagenesis, dissolved silica (derived from dissolution of siliceous tests of marine organisms) replaces calcite. Fluvial silica input often augment dissolved silica in the basin at the unset of shallow marine condition. However, CaCO3 production dominates as shallow marine conditions become fully established and silica supply from the hinterland reduces. Hence the higher the SiO2 input, the more the continental influence, whereas the CaCO3 production signals shallow marine incursion.
The low alumina content confirms a low index of weathering of the alumino-silicates such as feldspars and micas in the adjacent basement areas during transportation and deposition prior to diagenesis  . Fe2O3 is usually
Figure 10. Geochemical variation models of SiO2 (in wt%) and CaO (in wt%) for bed A.
Table 2. Classification of calcium and magnesium contents of the Ewekoro limestone (After Todd  ).
derived from intense chemical weathering of heavy mineral such as the ferromagnesians. Its low value indicates that the environment of deposition is a reducing one that does not favour the precipitation of Iron (II) to Iron (III) and thus leached away  . SO3 is low probably because anoxic conditions prevailed in such quiet, low energy environments and there is rapid rate of sulphate reduction.
5.2. Implications of Ca/Mg and Mg/Ca Ratios for Palaeo-Salinity
Todd  presented a petrogenetic classification of carbonate rocks that involved the standard ratio, Ca/Mg and reciprocal ratio, Mg/Ca. The class limits of the standard ratio, Ca/Mg are: > 100 - 39.0, 39.0 - 12.3, 12.3 - 5.67, 5.67 - 1.86, 1.86 - 1.50, 1.50 - 1.22, and 1.22 - 1.00 expressed as limestone, magnesian limestone, dolomitic limestone, dolomitized limestone, calcareous dolomite, dolomite and magnesian dolomite respectively. Also, the class limits of the reciprocal ratio, Mg/Ca are: 0 - 0.03, 0.03 - 0.08, 0.08 - 0.18, 0.18 - 0.54, 0.54 - 0.67, 0.67 - 0.82 and 0.82 - 1.00 also expressed as limestone, magnesian limestone, dolomitic limestone, dolomitized limestone, calcareous dolomite, dolomite and magnesian dolomite respectively. The standard and reciprocal ratios of Ca and Mg composition of the Ewekoro limestone are shown in Table 2. More than 79% of the samples are classified as “pure” limestone and about 20% are magnesian limestone according to the method of  . The Ca/Mg ratio has implications for the stability conditions of the depositional environment that led to the formation of the carbonate(s)  . Naturally, the Mg/Ca ratio increases during evaporation of sea water, especially under saline environmental conditions. Considering the Ca/Mg and Mg/Ca ratios (Table 2) it can be concluded that the relative rate of evaporation of sea water and the palaeo-salinity condition was low, as such limestone was deposited more at the expense of dolomite. However, intermittent increase in rate of sea water evaporation and salinity resulted to the deposition of the few magnesian limestones.
The geochemical characterization of the limestones of the Ewekoro Formation through XRF analysis shed light on the level of chemical purity, provenance, diagenesis and environment of deposition of the study area. Geochemical variation model of SiO2 and CaO showed a general distribution of purity level of the limestone that is applicable to mining operations. The limestone is rich in CaCO3 with varied inputs of other oxides like SiO2, MgO, Fe2O3, etc. A negative correlation resulted from SiO2 against CaO plots implying replacement chemical diagenesis under a shallow marine setting. Ratios of Ca/Mg and Mg/Ca revealed relatively low sea water evaporation and palaeo-salinity conditions that encouraged CaCO3 precipitation. However, occasional rise in sea water evaporation and salinity levels resulted in the formation of magnesian limestones.
The authors wish to acknowledge the Management of West African Portland Cement, Shagamu for permission to obtain samples and analyzing them.
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