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

Ordovician oolitic ironstone as chemical and structural trap for Variscan hydrothermal fluids (As,Zn,Cu,Pb-Sb-Au) during deformation: Example of Saint-Aubin-des-Châteaux deposit (French massif Armoricain).

The Saint-Aubin-des-Châteaux quarry provides exceptional exposures of a regionally well-known oolitic ironstone A-layer (OIL) (Chauvel, 1974), intercalated within sandstones, Early Ordovician (Arenig) in age. Within OIL, decameter-long massive sulphide lenses (dominant pyrite-marcasite, arsenopyrite, and residual pyrrhotite) have formerly been considered to be of syngenetic origin. Preserved oolite relic and other petrological and mineralogical evidences show that these massive sulphide bodies result from strong, pervasive sulphidation and metasomatism of the OIL by hydrothermal fluids related to late Variscan fluid circulations. Similar process is only reported for Archean and Proterozoic hydrothermalized BIF gold deposits (e.g. São Bento gold deposit, Brazil). Trace elements, abnormally rich within non-hydrothermalized OIL, were remobilized and concentrated by fluids, triggering crystallization of phosphates such as Sr-Fe-Al phosphate (lulzacite), Sc phosphate (pretulite) and REE phosphates (Moëlo et al., 2002). Other successive hydrothermal polymetallic paragenesis crosscut OIL, massive sulphide lenses and sandstones leading us to propose a three-step hydrothermal model - 1) massive sulphidation Fe-As; 2) Zn; 3) Cu-Pb-Sb-Au. Structural studies lead to distinguish different vein and fault systems filled by quartz and/or associated polymetallic parageneses. Structural studies allow constructing a tectonohydrothermal model assuming: i) a close genetic link with fracturing; ii) the syntectonic formation of the polymetallic parageneses during incremental right-lateral strike slip deformation; iii) the chemical and structural control played by OIL for trapping hydrothermal fluids. Finally, lead isotopic study suggests a common hydrothermal origin with other neighbouring polymetallic deposits and a major participation of fluids leaching a Brioverian crust. Chauvel, J.-J. (1974) Sedimentology 21, 127-147. Moëlo Y., Lulzac Y., Rouer O., Palvadeau P., Gloaguen E. and Léone P. (2002) Can. Mineral., 40, 1657-1673