How much hidden oolitic iron ore-hostedprecious and base metals deposits should beencountered in the Variscan belt? Insightsfrom France and Portugal

National audiencePalaeozoic oolitic ironstones layers are widespread in Europeand North Africa, mainly in external parts of the Variscanorogeny where they have been preserved from intense deformationand metamorphism. These ironstones are mainly of two types,either composed of carbonates and silicates (siderite-chlorite), ormade 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 ooliticiron ore from the Saint-Aubin-des-Châteaux quarry suffered anepigenetic transformation by hydrothermal fluids that led to thedissolution of the iron ore and its replacement by massive sulphidelenses. Such hydrothermal alteration is still localized atthe intersection between the ironstone layer and Variscan rightlateralstrike slip faults. Both, structural constraints, parageneticsequence and geochronological results (Gloaguen et al., 2007;Tartèse et al., 2015) underline 100 Ma of successive fluid flowsand polyphazed hydrothermal alteration of the ironstone. A verysimilar type of oolitic-hosted epigenetic gold deposit has beendescribed 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 distributionof such epigenetic mineralization through these two orogenic segmentsof the Ibero-Armorican arc at least. Since these Palaeozoicironstone layers are frequently hosted within, or close to, thicksandstone formations, outcrops are generally scarce. Moreover,because of the lack of magnetite and a thickness in a range 2to 6 m, these layers are not well targeted by geophysical approaches.Then, development of a specific targeting focused onintersections between faults and palaeozoic siderite-bearing ironstonelayers may lead to several significant new discoveries ofAu-Ag-Sb, Cu-Pb-Zn ore deposits