Natural phosphorus, which is essential for plants' growth, can be derived from phosphorite. Phosphate-bearing rocks include phosphorites. Commonly, phosphorite contains phosphorus pentoxide (P2O5) in appreciable quantity. According to Nathan (1984), phosphorites (usually of the non-detrital sedimentary variety) have at least 18 percent (≥ 18%) phosphorus-pentoxide (P2O5). 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, with a crustal abundance of 0.12%, can be found in these forms with their corresponding proportion in the world: marine sedimentary deposits (75%), igneous, metamorphic and weathered deposits (15–20%), and guano deposits, derived from biogenic sources (2–3%). However, the remaining (2–8%) are unaccounted for (McKelvey et al., 1953; Jasinki, 2015; Abou El-Anwar et al., 2017). While about 200 minerals with 1% or more P2O5 content are known, apatite is dominant in phosphorite (Palache et al., 1951; Altschuler et al., 1958; Deer et al., 1962). Apatite has a chemical formula, Ca5 (PO4, CO3)3 (OH, F, Cl). Apatite can exist in these forms—due to anionic substitution for the carbonate component in apatite—which are denoted by the corresponding prefixes, in a manner that follows the chemical formula stated earlier: Hydroxyapatite, Fluorapatite, and Chlorapatite, however, if a carbonate mineral such as calcite substitutes (through a partial replacement of carbonate ions) for some phosphate components, carbonate-fluorapatite (known as francolite) is formed. Also, other minerals, among a plethora of varieties, that host phosphorus are brushite (HCaP04. 2H20), monetite (HCaP04), whitlockite (3Ca3(P04)2), crandallite (CaAl(P04)2(OH).5H20), wavellite (Al3(OH)3(P04)2.5H20), taranakite (K2Al6(P04)6(OH)2.18H20), millisite ((Na, K) CaAl6(P04)4(OH)5.3H20), variscite (AlP04.2H20), and strengite (FeP04.2H20).
Phosphorites occur in some African basins. However, the most prolific 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 configuration, the Dahomey Basin falls within the Romanche (South-eastern Ghana) and Chain (South-western Nigeria) Fracture Zones (Jones and Hockey, 1964; Gbadamosi, 2009). Consequently, different parts of the Dahomey Basin, with a similar lithostratigraphy, are found in Ghana, Togo, Benin Republic, and Nigeria.
Phosphate-bearing rocks exist in the Dahomey Basin's Nigerian section and are within the Ososun (spelt as Oshosun in some other literature) and Ilaro formations. The Ososun formation phosphorites are bioclastic, based on their composition of skeletal fragments that form bone beds within the formation (Abimbola et al., 2002; Adesanwo et al., 2010). In the Dahomey Basin, the phosphorites commonly occur as interbeds with limestone and very thin beds (laminae) of gypsum. In most cases, the overlying rock is a shale with some clays as surficial deposits (Antolino, 1968; Abimbola et al., 2002). Besides, phosphorites as intrapelites and intraclasts within cross-bedded grits—that is typical of the Ilaro formation— occur in Eastern Dahomey Basin, Southwestern Nigeria (Adesanwo et al., 2010).
While discussions about the Eastern Dahomey Basin's rock stratigraphy are extensive, little information (such as the Russ (1924) study of phosphorites' composition) exists about the petrographic description the phosphorites in this region. Under a thin section using a microscope, rocks' study offers insights about the mineral content and rocks' arrangement. Shreds of evidence from Adesanwo et al. (2010) provided some information about the mineral make-up and configuration of the phosphorites at Ososun. Notwithstanding, details about phosphatic rocks in other areas (such as Oja-Odan and Igbeme) within the basin want. This academic article examines the detailed petrography of phosphatic rocks in four locations within Southwestern Nigeria. Plus, it elucidates the mineralogical and palaeontological components for suggestive depositional conditions of these rocks.
Figure 1: Outcrop geology of Dahomey Basin showing stratigraphy (Kaki et al., 2013. 1 - Alluvium (Recent); 2 - Benin-Ijebu Formation (Pleistocene/Upper Miocene); 3 - Ososun Formation (Middle Eocene); 4 - Imo Shale (Lower Eocene/Paleocene); 5 - Araromi Shale and Turonian Sandstone/Afowo Formation (Upper Cretaceous); 6 - Pre-Cambrian Crystalline Basement; 7 - Core hole or water well; 8 - Dry well; 9 - Oil well; 10 - Faults; 11 -Water depth contour.