Financial support from the Spanish DGICYT (project PB88-0003) is gratefully acknowledged;
as is the support by PAN and CSIC of exchange visits within the frame of the PAN-CSIC Cooperation Agreement. Thanks are due to Prof. D.A. Spears for chemical analyses of inorganic elements carried out at the Department of Earth Sciences;
University of Sheffield (Sheffield;
UK). We acknowledge INCAR’s (Oviedo;
Spain) staff dealing with coal preparation and coal petrography (particularly Mr. Manuel Fernattdez) for sink-float separations and petrographic analyses. The authors would also like to thank Drs. J.R. Castaiio and F. Goodarzi;
who reviewed the manuscript and provided constructive and valuable comments;