Hydrothermal alterations in the Echassières granitic cupola (Massif central,france)

Detailed petrographic and mineralogic investigations of an albite-lepidolite granite at Echassières (Massif Central, France; scientific deep drill program) shows the existence of hydrothermal stages which are closely related to the magmatic and structural history. According to fluid inclusion data, K-Ar datations and ¹⁸O/¹⁶O-D/H compositions of secondary minerals, two successive hydrothermal periods have been recognized. The early one (273–268 million years) produced a series of aluminous phyllosilicates: muscovite, pyrophyllite, donbassite, tosudite, kaolinite which are observed as vein deposits (<10 mm wide) and alteration products of primary minerals in wall-rocks. The vein system was sealed by monomineralic assemblages during a cooling period (400–150°C). This early hydrothermal alteration stage was controlled by interactions of rock with low salinity (1–10 wt% NaCl equivalent) fluids expelled from the granitic body during the cooling processes. The chemical properties of these fluids were the following: low pH, very low Mg and Fe and high Li, Na and K contents. Thermodynamic calculations show that the sequence pyrophyllite, Li-bearing donbassite, tosudite is mostly temperature dependent. From the chemical composition of secondary minerals and isotopic data it can be deduced that these fluids, which have a meteoric origin, have been expelled from the granite body during its cooling period and after interaction with it at high temperature. The late hydrothermal stage corresponds to deposits of fluorite and Fe-Mg rich illite (151 million years) in subvertical fractures. Temperature conditions did not exceed 250° C and fluids came through the surrounding metamorphic rocks into the granitic body. IIlite/smectite mixed-layer minerals have been identified in subvertical fractures which were opened during Tertiary periods. In the host micaschists, successive hydrothermal alterations took place during the cooling of the Beauvoir granite. Early magmatic fluids interacted with these micaschists. Locally, the metamorphic assemblage is replaced by a metasomatic one. Secondary topaz and (F, Li)-rich mica crystals were formed over a range of 450 of 150°C. Later hydrothermal fluids reacted with the country rocks to form phengite-biotite, chlorite-illite and kaolinite over a range of 300 to 150°C. Illite/smectite mixed-layer minerals crystallized in the roof micaschists and within the Beauvoir granite during the Tertiary alteration period. Meteoric water invaded open fractures producing supergene alteration mineral assemblages.