Pyrométamorphisme induit par la gazéification souterraine de niveaux charbonneux du Westphalien dans le bassin de Mons (Belgique) [Pyrometamorphism induced by underground coal gasification of Westphalian beds in the Mons basin (Belgium)]

During the underground coal gasification experiment conducted at Thulin (Belgium), partial melting occurred in the surrounding rocks (Westphalian shales and sandstones). Samples were collected at the roof of the coal bed. Minerals which crystallized from the partial melts were identified under the polarizing microscope, with electron probe microanalysis, by X-ray diffraction and scanning electron microscopy. They include cordierite, orthopyroxene, olivine, spinel, cristobalite, mullite, osumilite, K-feldspar, ilmenite, hematite, wollastonite, calcite, andradite, monticellite, pyrrhotite, and chalcopyrite. In these paralavas, the glass proportion ranges from 20 to 40%. In some cases, melting and subsequent crystallization have produced a magnetite-bearing gabbro. This gabbro contains an inclusion of sedimentary rock (relic) in which sillimanite, mullite and corundum were recognized. Metamorphic textures suggest that sillimanite is replaced by mullite, and mullite by corundum. Temperatures in the range of 1083 °C up to 1500 °C have been estimated for the underground coal gasification process by several methods: phase diagrams in the system (FeO, MgO)-Al2O3-SiO2 numerical modelling of gabbro crystallization, cordierite polymorphs crystallization, and metamorphic reactions.