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