Geochemical and Preliminary Sedimentological Analysis of Phosphorites from Togo, West Africa
Phosphatic rocks typically accumulate beneath upwelling systems much like those that created the Monterey Formation of California and related units. Phosphatic rocks exposed in southern Togo represent similar environmental conditions. These lower to middle Eocene phosphatic rocks overlie an attapulgite layer and underlie non-marine gravels, sands, and clays. Samples retrieved from an actively mined phosphate-enriched layer varying from 1 to 10s of meters in thickness consist mostly of moderately to poorly sorted phosphatized pellets, formanifera, and other bioclastic debris. Attapulgite clay occurs interstitially and likely represents an alteration product. Sedimentary structures are not evident in the loosely consolidated samples. However, marine currents likely winnowed and concentrated sediments following phosphatization. Francolite is the main mineral phase present in the samples. Geochemically, the rocks show significant enrichment in P2O5 and CaO. Mean oxide compositions (wt %) are 31.27% P2O5, 43.74% CaO, 9.50% SiO2, 4.30% Al2O3, 2.96% Fe2O3, 0.11 % K2O, 0.19% TiO2, 0.02% MnO, 0.02% MgO, 0.02 % Na2O, and 6.12 % LOI (loss of ignition). Compared to averaged compositions of phosphorite deposits elsewhere around the world, the Togo phosphorites contain four times as much Cd and Cr, three times as much Cu, twice as much V and Zn, and 1.5 times as much Ni. Additionally, the phosphatic rocks are depleted in Zr, U, and Pb with enrichment factors of 0.9, 0.8, and 0.2 respectively. Our analysis shows that Cd, U, Th, and F are incorporated into the francolite lattices by isomorphic substitutions. Other heavy metals such as Cr and Cu are absorbed onto clays, calcite, iron, and manganese. Interestingly, mining operations result in the Cd enrichment of waste waters leading to significant environmental problems. As with some of the Miocene pelletal/oolitic phosphatic rocks of California, the Togo phophorites likely represent sediment accumulated beneath fertile oceanic waters followed by phosphatization resulting in high phosphorous contents and elevated Cd and Cu concentrations. Reworking concentrated the sediments.
AAPG Search and Discovery Article #90088©2009 Pacific Section Meeting, Ventura, California, May 3-5, 2009