Geochemical Characterization of the Paleocene Ewekoro Limestone Formation , SW Nigeria : Implications for Provenance , Diagenesis and Depositional Environment

The usefulness of limestone as an industrial raw material is dependent on the level of its purity, largely controlled by diagenesis and the depositional setting. Limestone samples (83) obtained from the Ewekoro Formation exposed in quarry sections at Shagamu were analyzed using X-Ray Fluorescence spectrometry (XRF). A geochemical characterization of result was done to evaluate the purity levels and the implications on provenance, diagenesis, and depositional setting of the limestone. Five limestone beds from quarry sections (15 locations) were logged. Beds “E”, “D”, “C”, “B”, and “A” comprised sandy algal in sparry calcite cement, sandy biomicrite, algal biomicrite, sandy algal biomicrite, and the intra-sparite facies respectively. Results of major elements chemistry indicated concentrations (in wt%) as follows: CaO (33.71 59.99), MgO (0.39 3.15), Al2O3 (0.47 3.23), Fe2O3 (0.51 3.43), SiO2 (0.47 45.98), SO3 (0.10 2.27), K2O (0.02 0.28), Na2O (0.00 10.0), TiO2 (0.00 0.27), P2O5 (0.02 0.92), MnO (0.01 0.06) and Loss on ignition (17.64 45.20). Geochemical result showed that the samples are enriched in CaO, while SiO2 varies widely, likely due to hinterland input. Plots of SiO2 versus CaO concentrations depicted a negative correlation attributable to chemical diagenetic processes. Diagenesis of the carbonate in form of reversible replacement of SiO2 with CaO and vice versa occurred under a shallow marine condition. This negative correlation between these oxides (CaO and SiO2) dictates zones of high purity limestones that are of desirable industrial applications. Relatively high silica content in the northwestern part is possible indication of outlets to the continental or inland areas where clastic sediment source may have been prominent. Ratios of Ca/Mg and Mg/Ca indicated a relatively low rate of evaporation of sea water and palaeo-salinity conditions marked by precipitation of limestone. Intermittent increase in palaeo-salinity and sea water evaporation level gave rise to the magnesian limestone. O. A. Ehinola et al. 62


Introduction
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 SiO 2 , MgO and Fe 2 O 3 , 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 [1] reported the limestone reserve estimate of [2] of about 36 million tons.Reyment [3] also presented estimated values of chemical constituents of quarried limestones from the Ewekoro as follows: CaO (53%), CO 2 (42%), SiO 2 (2%), Al 2 O 3 (5%), Fe 2 O 3 (1.4%),P 2 O 5 (0.8%), MgO (0.3%), MnO (0.1%), and minor quantities of Na 2 O, K 2 O, TiO 2 , F and trace of SO 3 .Although, these geochemical reports are some five decades old and some of the relatively recent related studies include those of [4]- [7] 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).

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 [8] [9].It's a marginal pull-apart basin or marginal sag basin [10] that developed in the Mesozoic sequel to the separation of African from the South American plates [11] [12].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 [9].
This formation is made up of fossiliferous shelly limestone of about 12.5 meters thick, which tends to be sandy at the base [1].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 Ewekoro Formation and; the shelly biomicrite and Algal biosparite upper unit [1].It's of Paleocene age based on fossil evidence (foraminifera and ostracods) and deposited in a shallow marine environment [16] [17].

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 SiO 2 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 LiNO 3 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.The geochemical results obtained (Table 1) were further evaluated using a computer programme (Surfer TM ) to generate geochemical models for the distribution of CaO and SiO 2 (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 SiO 2 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 SiO 2 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.

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.

Major Elements Distribution
The concentrations (in wt% of oxides) of major elements in the limestone samples are shown in Table 1.The

CaO and SiO 2 Variations in the Ewekoro Limestone Formation
A marked inverse negative correlation exists between SiO 2 and CaO contents of the limestones (Figure 4).A generalized geochemical variation model for the area representing the SiO 2 distribution is depicted in Figure 5.
A bed-by-bed spatio-temporal geochemical distribution of the concentrations of CaO and SiO 2 is discussed below in super-positional order.

CaO and SiO 2 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 SiO 2 ranged from 1.68 to 6.34 wt%. Figure 6 is the 2-D and 3-D model views of CaO and SiO 2 .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 SiO 2 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.

CaO and SiO 2 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  A 2-D and 3-D model views of CaO and SiO 2 concentration is shown in Figure 7. Peaks of CaO and SiO 2 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 SiO 2 was observed.

CaO and SiO 2 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 SiO 2 is 1.98 -22.23 wt%.Geochemical variation models of CaO and SiO 2 indicated a decline in CaO (with small peaks in southern part) concentration with an accompanying enrichment in SiO 2 at both the southeastern and northwestern ends of the study area (Figure 8).

CaO and SiO 2 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.SiO 2 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.SiO 2 concentration is very low in this bed; a peak concentration apparent in the mid-western part coincided with    the CaO lowest concentration.A notable increase in CaO concentration was noticed in compensation for the decline in SiO 2 concentration, a trend similar to that of the aforementioned beds.

CaO and SiO 2 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 SiO 2 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 SiO 2 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.

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 SiO 2 against CaO for all the samples analyzed clearly revealed a similar and unique trend marked by increase in CaO with corresponding decrease in SiO 2 contents and vice versa.This SiO 2 -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, CaCO 3 production dominates as shallow marine conditions become fully established and silica supply from the hinterland reduces.Hence the higher the SiO 2 input, the more the continental influence, whereas the CaCO 3 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 [18].Fe 2 O 3 is usually  and thus leached away [18].SO 3 is low probably because anoxic conditions prevailed in such quiet, low energy environments and there is rapid rate of sulphate reduction.

Implications of Ca/Mg and Mg/Ca Ratios for Palaeo-Salinity
Todd [19] 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 [19].The Ca/Mg ratio has implications for the stability conditions of the depositional environment that led to the formation of the carbonate(s) [20].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.

Conclusion
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 SiO 2 and CaO showed a general distribution of purity level of the limestone that is applicable to mining operations.The limestone is rich in CaCO 3 with varied inputs of other oxides like SiO 2 , MgO, Fe 2 O 3 , etc.A negative correlation resulted from SiO 2 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 CaCO 3 precipitation.However, occasional rise in sea water evaporation and salinity levels resulted in the formation of magnesian limestones.

Figure 5 .
Figure 5.A generalized SiO 2 (in wt%) distribution models of the study area.

Figure 6 .
Figure 6.Geochemical variation models of SiO 2 (in wt%) and CaO (in wt%) for bed E.

Figure 7 .
Figure 7. Geochemical variation models of SiO 2 (in wt %) and CaO (in wt%) for bed D.

Figure 8 .
Figure 8. Geochemical variation models of SiO 2 (in wt%) and CaO (in wt%) for bed C.

Figure 9 .
Figure 9. Geochemical variation models of SiO 2 (in wt%) and CaO (in wt%) for bed B.

Figure 10 .
Figure 10.Geochemical variation models of SiO 2 (in wt%) and CaO (in wt%) for bed A.

Table 1 .
Major elements composition (wt%) of Limestones from the Ewekoro Formation exposed at Shagamu.

Table 2 .
[19]sification of calcium and magnesium contents of the Ewekoro limestone (After Todd[19]).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)