Resumo

OGALA, Jude E. The geochemistry of lignite from the Neogene Ogwashi-Asaba Formation, Niger Delta Basin, southern Nigeria. Earth Sci. Res. J. [online]. 2012, vol.16, n.2, pp.151-164. ISSN 1794-6190.

Major and trace element compositions of lignite from the Tertiary Ogwashi-Asaba formation, Southern Nigeria, have been investigated to determine the prevailing environmental conditions which controlled their formation. Seven samples were obtained from outcrops along river valleys, streams and springs in seven localities; these samples were subsequently analysed using fusion combined with inductively-coupled plasma (FUS-ICP) and total digestion with inductively-coupled plasma (TD-ICP). Geochemical analysis revealed the following concentration range in weight percent (wt. %) for major oxides: SiO₂ (0.04-9.78); Al₂O₃ (0.56-6.40), Fe₂O₃ (0.05-1.22), MnO (0.001-0.004), MgO (0.02-0.11), CaO (0.05-0.15), K₂O (0.01 - 0.04), TiO₂ (0.016-0.299) and S (0.08-0.39). Trace elements indicated the following concentration range in parts per million (ppm): Be (2-5), Zr (4-63), Sr (5-22), Y (13-68), Ba (9-89), V (4-216), Zn (17- 176), Ni (8-28), Co (5-13), Cr (2-31), Cu (1-22) and Ga (1-14). The low silica (<10 wt. %) and alumina contents (<7 wt. %) were explained by very limited detrital input during coal formation. Magnesium oxide and CaO content were relatively low thereby confirming the continental nature of the peat-forming environment. Redox-sensitive trace element ratios (Ni/Co, V/Cr and V/V+Ni) indicated predominantly oxic environments for coal deposition. The low trace element concentrations determined in the lignite samples did not point to any severe environmental impact from coal use.

Palavras-chave : Geochemistry; lignite; Ogwashi-Asaba formation; continental environment; Nigeria.

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