The post-Variscan tectonic-thermal activity in the southeastern metalliferous province of the French Massif Central revisited with K-Ar ages of illite

Abstract This study deals with K-Ar data of illite and illite-rich mixed-layers of Cambrian and Permian metasediments that were subjected to tectono-thermal episodes in the Montdardier and Mas Lavayre ore districts of the southeastern Massif Central (France). The objective was to differentiate events that altered the metal-rich deposits from those that affected only barren host rocks close to the ores. On the basis of combined mineralogical analyses and isotopic determinations of the clay material, successive tectono-thermal events induced illitization at 288 + 10, 246 + 9, 197 + 6, 176 + 6 and 107 + 4 Ma and probably at 136 + 4 Ma. The lack of major geodynamic activities near the studied ore deposits during these times hypothesizes periodic long-distance migrations of fluids in the continental crust to explain their occurrences. Metals seem to have been concentrated at specific places, but apparently they were not deposited or altered during post-Visean tectono-thermal pulses recorded by the associated illite. The tectonic-thermal history of the Les Malines district confirms that repetitive geodynamic re-activations induced local heat transfers. However, the episode at 246 + 9 Ma apparently did not affect the earlier deposited ores, while that at 35-40 Ma found in coffinite at Pierre Plantees was probably not induced by a thermal event, but by an alteration event that did not affect the K-Ar system of the surrounding clay material.