How much hidden oolitic iron ore-hosted precious and base metals deposits should be encountered in the Variscan belt? Insights from France and Portugal

Palaeozoic oolitic ironstones layers are widespread in Europe and North Africa, mainly in external parts of the Variscan orogeny where they have been preserved from intense deformation and metamorphism. These ironstones are mainly of two types, either composed of carbonates and silicates (siderite-chlorite), or made of oxides and silicates (magnetite-chlorite). In the first case, ironstones are chemically very reactive to crustal fluid circulations. In the central domain of the Armorican Massif (Western France), it has been demonstrated (Gloaguen et al., 2007) that the oolitic iron ore from the Saint-Aubin-des-Châteaux quarry suffered an epigenetic transformation by hydrothermal fluids that led to the dissolution of the iron ore and its replacement by massive sulphide lenses. Such hydrothermal alteration is still localized at the intersection between the ironstone layer and Variscan rightlateral strike slip faults. Both, structural constraints, paragenetic sequence and geochronological results (Gloaguen et al., 2007; Tartèse et al., 2015) underline 100 Ma of successive fluid flows and polyphazed hydrothermal alteration of the ironstone. A very similar type of oolitic-hosted epigenetic gold deposit has been described by Couto and Moëlo (2011) in the Banjas gold mine (Dúrico-Beirã district, Valongo, Portugal; Couto et al., 1990; Combes et al., 1992). This argues for a widespread distribution of such epigenetic mineralization through these two orogenic segments of the Ibero-Armorican arc at least. Since these Palaeozoic ironstone layers are frequently hosted within, or close to, thick sandstone formations, outcrops are generally scarce. Moreover, because of the lack of magnetite and a thickness in a range 2 to 6 m, these layers are not well targeted by geophysical approaches. Then, development of a specific targeting focused on intersections between faults and palaeozoic siderite-bearing ironstone layers may lead to several significant new discoveries of Au-Ag-Sb, Cu-Pb-Zn ore deposits.