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).
Résumé
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.