Recherches sur les inclusions intracristallines et les joints intercristallins dans un granite uranifère du Morvan, France

Abstract

The uraniferous monzonitic biotite granite of Dommartin near Château-Chinon intruded a volcanic series of Visean age. It lies under a continuous microgranite roof. Two characteristics are to be distinguished: (1) an important deuteric tardimagmatic alteration with disseminated non-fissural uraninite which may be related to the tightness of the microgranitic roof; and (2) Autunian—Liassic age fracturations with hydrothermal pitchblende vein deposits. The granite is weathered on the surface to a friable sand. It is deeply eroded (down to 200 m), forming a basin in the landscape. The purpose of this paper is to see if there is any correlation between the uraniferous character of the granite, its macrofissuration and alteration on the one hand, and the inclusion content and microfissuration of the crystals' host-rock on the other. Fifty quartz, feldspar and polycrystalline aggregate splinters, submitted neither to mechanical processes (sawing, polishing or abrasion) nor chemical processes, have been studied at high magnifications with scanning electronic microscopy. Quartz splinters have an inclusion content 4 or 5 times higher than in other monzonitic granites. Concentrations of inclusions are found in veils and in the external parts of crystals. The cavities are of average size (1–5 μm) and make up to 0.5–2% of the studied areas. Inclusions in feldspar are more numerous but much smaller and more ramified. Intergranular joints have a smooth topography and are bordered with large pores; their surfaces show a very large number of small inclusions. A great part of the cavities is open and their walls are darkened by deposits. The abundance of inclusions, their concentration at crystal faces, and their medium size and negative crystal shape point to an important role of fluids at various stages. Crystals have suffered mechanical stresses due to tectonics and to thermic shocks; the resulting cracks have been healed by the circulation of fluids rich in trace elements. It may be that, during late stages, the continuation of hot fluid circulation led to the burst of a part of the syngenetic and epigenetic inclusions and to the opacification of their walls.