Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal-As-Sb-Au mineralization during Hercynian strike-slip deformation.

In the Saint-Aubin-des-Châteaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Grès armoricain Formation. The hydrothermal overprint produced pervasive and massive sulphidation with stratoid pyritized lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. Following the massive sulphidation stage (Fe-As, stage 1), subsequent fracturing allowed the deposition of base metals (stage 2) and Pb-Sb-Au (stage 3) parageneses in veins. The dominant brittle structures are vertical extension veins, conjugate shear veins and strike-slip faults of various orders. All these structures are filled with the same paragenetic sequence. Deformation analysis allows the identification of structures that developed incrementally via right lateral simple shear compatible with bulk strain affecting the Central Armorican Domain. Each increment corresponds to a fracture set filled with specific parageneses. Successive hydrothermal pulses reflect clockwise rotation of the horizontal shortening direction. Geothermometry on chlorite and arsenopyrite shows an input of hot hydrothermal fluids (maximum of 390-350°C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300-275°C). Lead isotopes suggest that the lead source is similar for all hydrothermal stages and corresponds to the underlying Neo-proterozoic basement. Lead isotope data, relative ages of deformation and comparison with neighbouring deposits suggest large-scale fluid pulses occurred during the whole Hercynian orogeny rather than pulses restricted to the late Hercynian period. The vicinity of the Hercynian internal domain appears as a key-control for deformation and fluid flow in the oolitic ironstones which acted as a chemical and structural trap for the hydrothermal fluids. The epigenetic mineralization of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in BIF-hosted gold deposits