Resumen de Tesis Doctoral: Caracterización mineralógica y geoquímica de los lignitos de la cuenca Terciaria de As Pontes (Provincia de La Coruña)




The basin infill of the As Pontes Tertiary strike-slip basin (A Coruña, NW Spain) mainly consists of terrigenous sediments and lignites. This contribution deals with establishing the depositional environment of these lignites, as well as with the analysis of their range, maceral composition and paragenetic mineral sequence. The major and trace elements in the lignites and their affinities were also determined. The integration of the available sedimentological, stratigraphical, petrographical, mineralogical and geochemical data allowed to establish: a) The depositional and geochemical controlling features on sulphur accumulation. b) A model for the lignite formation and evolution. The successive, tectonically forced evolutionary stages in the basin generated diverse lignite kinds with different characteristics. The lower coal seams are characterised by highest sulphur and early diagenetic mineral content. These coals were formed in small size, restricted swamp-lacustrine environment with alkaline and highly mineralised syngenetic pore solutions. The basin evolved later to more pore - water dilute low-lying swamps with larger areal extent and lower mineralised syngenetic solutions. As a consequence, the middle and upper coalseams were deposited under decreasing alkalinity and sulphur content, closely related with the basin evolution. The upper coal seams show lower sulphur content and the highest detritical mineral content, in comparison to the underlying seams. The correlation between the mineralogical and geochemical parameters of the As Pontes lignites showed the following element associations: a) major organic affinity for Ca and Mg, and minor affinity for S, Mn and B; b) major aluminosilicate affinity for Al, K, Na, Ti, Bi, Cr, Cs, Cu, Ga, Hf, Li, Mo, Nb, Pb, Rb, Sn, Th, Tl, U, V, Zr and REE, and minor affinity for As, B, Ba, Be, Ge, Mn, P, Sb, Sc, Sr and Se; c) sulphide affinity for Fe, S, As, Co, Ni and Zn; and d) phosphate affinity for P, Ba and Sr. The evolution and the quality of the coal was controlled mainly by: a) Tectonic activity, which controlled subsidence, drainage evolution and basin infill trends. b) Climate, which controlled water supply and evaporation. c) Parent rock, which influenced on detritical mineral supply and water hydrochemistry. d) Syngenetic solutions pH and Fe2+/SO42 - rate, which influenced diagenetic minerals formation and sulphur content and forms in the lignites.