Coal inclusions in sedimentary rocks: a geochemical phenomenon. A review

In many coal-bearing basins, there are numerous coalified fragments of ancient plants (coal-precursors) enclosed in host rocks. Such fragments occur in isolated positions out of the coal beds. In the Russian literature, these coal fragments are named coal inclusions. Coal inclusions are mostly the remains of stems, trunks, and branches, as well as the roots of trees. The review presented covers: (a) definition and classification of fossil woods; (b) relations between coalification and mineralization of fossil woods; (c) some special topics dealing with different and even zonal coalification degree of coal inclusions embedded different host rocks; (d) some historical data on geochemistry of coal inclusions; (e) basic empirical regularities in geochemistry, observed world-wide; (f) some data about chemical nature of humin substance-a precursor of lignite and vitrain matter in coalified wood; (g) results of calculations modelling the Ge enrichment in coal inclusions; (h) economic importance of Ge in coal inclusions; and 0) use of coal inclusion geochemistry for indication of some diagenetic and catagenetic processes, and as a tool for stratigraphic correlation. The most part of the above studies performed during 1934-1972 were outlined in the monograph Geochemistry of coal inclusions in sedimentary rocks [Yudovich, Ya.E., 1972. Geochemistry of coal inclusions in sedimentary rocks. L.: Nauka (Leningrad: Science Pub. House], 84 pp.]. These materials are added to some recent work by Bulgarian and Russian geologists, performed with use of modem analytical methods. It has been shown that: (a) coalified wood may contain very exotic micro-mineral phases, sometimes far unexpected; (b) apart from Ge, coalified wood may contain high concentrations of some other trace elements, which were earlier not detected because of analytical limitations (REE, As, etc.). These special peculiarities can be partly contributed by epigenetic hydrothermal processes. As a summary, the review shows that coal inclusions are unique geochemical phenomenon, sharply different from even neighboring coal beds in trace element content. Among the most extreme elements is germanium, its mean concentration in the ash of coal inclusions being up to 220 times higher than in the ash of coal beds. The most important peculiarity of the coalified wood is the good preservation of the original lignin structures, which may effectively scavenge Ge from solutions, whereas peat-born coals (in beds) contained such structures in far fewer amounts. In addition, a reservoir of dissolved germanium in peat bog waters was of lower concentration than in sediments, which buried the coal inclusions. Finally, the peat bog acidic environment may act as an unfavorable factor. Ge-enrichment can be completed in a time ranging from a few thousand years up to tens of million years. However, if the waters are enriched in Ge, the process can proceed more rapidly and would be completed even under the most unfavorable parameters (compared to the model conditions). This implies that enrichment can take place during the early diagenetic stage. Such a scenario was supported by geologic considerations. Some Canadian and Soviet works performed from 1950 to 1960 along with some recent Russian studies show that Ge in coal inclusions can be of economic interest. Geochemistry and mineralogy of coal inclusions are of great interest and need further detailed study. (C) 2003 Published by Elsevier B.V.