Petrographic Analysis of the Palaeocene-Eocene Phosphate-Bearing Rocks of Eastern Dahomey Basin, Southwestern Nigeria.

Phosphorus is ubiquitous for its essential function in plant growth. Phosphate-bearing rocks include phosphorites and rocks that host phosphate mineral. Phosphorites, rocks containing ≥ 18 % phosphorus-pentoxide (P2O5), are essential sources of phosphorus. Due to the vast application of phosphorites in the manufacture of fertilisers and industrial products, the study of phosphate bearing rocks is crucial. This article examines the mineral component and structure of phosphate-bearing rocks in the Eastern Dahomey Basin, Southwestern Nigeria. Eleven pulverised samples of phosphatic rocks were analysed, employing a microscope, for petrographic studies. Three major types of phosphorites exist in the Eastern Dahomey Basin: Granular (which are composed of oolitic and pelletal structures), Nodular (which comprises cylindrical, rectangular, ellipsoidal shapes), and the vesicular (which either have coarse or ne forms) varieties. The phosphorites contain uorapatite (as the dominant mineral), calcite, quartz, and glauconite. Generally, the phosphorites are fossiliferous (lamellibranches, echinoderms, shell, and bone fragments are observable). The phosphatic limestones are micrite (allochems) hosting collophane (specks of ne-textured uorapatite) and glauconite. While uorapatite occurs in the phosphatic claystone, it is glauconitic, just as the gypsum. The results of the petrographic analysis indicated the inuence of seawater in the deposition of the phosphate-bearing rocks in an oxic environment.


Introduction
Natural phosphorus, which is essential for plants' growth, can be derived from phosphorite. Phosphatebearing rocks include phosphorites. Commonly, phosphorite contains phosphorus pentoxide (P 2 O 5 ) in appreciable quantity. According to Nathan (1984), phosphorites (usually of the non-detrital sedimentary variety) have at least 18 percent (≥ 18%) phosphorus-pentoxide (P 2 O 5 ). A substantial quantity of phosphorites is critical materials in the manufacture of fertilisers, animal feeds, and other industrial products (such as phosphoric acids, cleaning agents, cosmetic products, and paints).
Phosphorites occur in some African basins. However, the most proli c basins for phosphate-rock mining are those in North Africa. The formation of Dahomey Basin's phosphorites dates from the Palaeocene to Eocene. The geological association subtlety varies in African basins. For example, the North African basins commonly have a thick bed of phosphorites interbedded by limestones and marls such as those reported in the Metaloui Basin, Tunisia (Garnit et al., 2017), in Tebessa region, Algeria (Rabah Kechiched et al., 2018). In West African basins, the phosphorites occur in sandstones as phospharenites. However, these phosphatic rocks are associated with sandy clays, limestones, dolomites, claystones, marls, and cherts (Pierre, 2014).
According to Whiteman (1982), the Dahomey Basin (Fig. 1) is a vast sedimentary basin on the Gulf of Guinea's continental margin. This trough extends 440 km from the Volta-Delta (Ghana) in the west to the Okitipupa-Ridge (Nigeria) -also known as Ilesha-Spur-in the east; its 224 km width spans from the Northern onshore margin (in Dahomey, Benin Republic) to the Bathymetric contour (Offshore Dahomey).
In terms of structural con guration, the Dahomey Basin falls within the Romanche (South-eastern Ghana) and Chain (South-western Nigeria) Fracture Zones (Jones and Hockey, 1964;Gbadamosi, 2009).

Sampling locations
Well-labelled samples from the eld totalled eleven (Fig. 2). This work took place at the following localities in the Eastern Dahomey Basin, Southwestern Nigeria: Igbeme, Oja-Odan, Ososun, and Ebiti. Ososun and Ebiti areas possess phosphorite exposures. Each sample of the nodular, granular, and vesicular forms of phosphorites was collected and labelled.

Rock thin section preparation
The thin section slides prepared for the petrographic analysis took place at the Department of Geology, University of Ibadan, Nigeria. Fresh eld samples were washed with distilled water to remove all agglutinated sediments (like sands and clays). Airdrying of the samples took place for four weeks. The photographed air-dried samples (described as hand specimens in Fig. 3.1-3.3) exist for documentation and appropriate matching.
A Logitech sawing machine sized each of the eleven rock samples into smaller dimensions (about 3 cm by 3 cm). The rock samples were mounted by glazing, with an Araldite glue, on new thin section slides.
After that, these mounted-rock-slides (m-r-s) underwent lapping. Lapping is vital in the thin section preparation of rock samples. Carborundum (silicon carbide) served as the active ingredient during lapping (a process of making the surface of the m-r-s hazy). It took a mean time of 5 min to lap each slide. The essence of this is to make the m-r-s transparent for light rays to pass through them.

Petrographic study
The minerals present in the rock were identi ed, as subject to what is contained in the slides, by their unique optical properties, under the petrographic microscope using both transmitted and re ected lights.
Finally, photomicrographs are obtainable using a camera attached to the petrographic microscope (magni cation − 500 X). Furthermore, the photomicrographs were well labelled.

Results And Discussion
The photomicrographs (Figs. 3.1-3.4) represent the rock samples from the eld. Phosphatic limestone is observable (Fig. 3.1). The phosphatic limestone at Igbeme is a micrite (calcite mud) (Fig. 3.1 A); it precipitated from seawater with little terrestrial in uence during deposition. The phosphate mineral ( uorapatite) was identi able as scattered specks of brown-coloured collophane (Fig. 3.1 A). The phosphatic limestones at Oja-Odan, unlike those at Igbeme, were fossiliferous. These limestones are allochems (lithoclasts) with peloids of quartz and glauconite (Fig. 3.1 B). Echinoderms and lamellibranches with shell and bone fragments are noticeable in thin sections. Gypsum is present in the phosphatic limestone taken at Igbeme (Fig. 3.2 D). Glauconite is present in the gypsum. The phosphatic shale was fossiliferous (Fig. 3

.2 E) as it contains a fauna-that is insect-like in appearance.
Phosphatic claystone, glauconitic, contains uorapatite granules with heavy impregnation by hematite (Fig. 3.2 F). The phosphatic claystone was non-fossiliferous.
The lithic fragments of different minerals such as calcite and quartz (white) and glauconite (green or greenish-brown) with uorapatite and collophane (brown) make-up the micritic matrix ( Fig. 3.1 B -C).
The presence of glauconite signi es the deposition of phosphatic limestone in a marine environment and seawater in uence in the claystone (Rabah Kechiched et al., 2018).

Granular phosphorite
The granular phosphorites (GP) are predominant in both Ososun and Ebiti areas (Fig. 3.3 G). The term granular, commonly used in the description of sedimentary rocks composed of granules, signify minerals with 4 mm or smaller grain sizes. As observed in the microphotograph (Fig. 3.3 G), high to low spherically rounded granules (with oolitic and pelletal structures), subangular to angular grains, and clasts of uorapatite exist with opaline silica and calcite (that form the micritic matrix). The motley of different grain types indicates high-level turbidity (responsible for the mixing of the grains) in the transport of the phosphatic grains to the site of deposition. Turbulent currents lead to the formation of clasts during the fracturing of the grains. The granular phosphorites are bioclastic (Abimbola et al., 2002;Adesanwo et al., 2010). The rock is light-brown in colour due to iron stains, and it weathers into dirty white (Russ, 1924).
Carbonaceous matter encrustation, which is dark, in colour is present. Also observed in the GP were fossils such as lamellibranches and brachiopod shells.

Nodular phosphorite
The nodular phosphorites (NP) lie at both Ososun (Fig. 3.4 J) and Ebiti (Fig. 3.4 K) areas. They were coarser than the granular type. The nodular phosphorites were in either perfect or imperfect ellipsoidal, spherical, cylindrical, and rectangular forms with a pebble-conglomeratic appearance. In addition to that, nodular phosphorites were dead-whitish or creamy in colour (hand specimen). Fluorapatite appears as a yellowish-brown or brown-coloured nodule which are enmeshed in sparite and glauconite. Their diameters could be as wide as 5 cm. Most of the NP were at-and they had pitted surfaces. Minute lamellibranch, through borings, were responsible for these (Russ, 1924). Compositionally, the nodular phosphorites are similar to the granular type. The matrix consists of aluminium phosphates.

Vesicular phosphorite
As the name implies, the vesicular phosphorites (VP) possess vesicles, which are either lled or empty. Two varieties exist, namely: the ne and the coarse. The ne vesicular phosphorite, a weathered derivative of the granular phosphorite, resides at the boundary between Ososun and Ebiti areas (Fig. 3.3 H). Their textures are similar to pumice. Granules of apatite that dissipated during the GP's precipitation led to the formation of the calcined bone or pumice-like structure (Russ, 1924). Replacement reaction in the apatite granules (dark brown) is responsible for the VP's reticulating appearance of iron and alumina (light brown).
Similarly, the coarse vesicular phosphorite lies at the boundary between Ososun and Ebiti (Fig. 3.3 I). The coarse-type is the altered derivative of the nodular phosphorite. The replacement of calcium ions in Caphosphate by iron and aluminium ions in clayey soils or claystones potentially forms wavellite. Ooids are present; the brown concentric rings depict their presence. The addition of iron affects the redox state of the deposited phosphorite. The colouration of the rock towards green suggests a reducing or euxinic environment of deposition.

Conclusion
The collophane (or specks of uorapatite) found in the Igbeme and Oja-Odan regions indicates a favourable redox condition for the exchange between the phosphate (in the overlying Ososun and Ebiti phosphorites) and carbonate (in the underlying Igbeme and Oja-Odan limestones) as a result of leaching.
The presence of glauconites in the phosphate-bearing rocks (Nodular, granular, and vesicular ( ne and coarse) phosphorites, limestones, and claystone) suggests that the environment of deposition as nearshore marine. In the phosphatic claystone, the presence of ferric oxide (reddish brown) indicates a redox condition towards an oxic (oxidising) environment of deposition.