Palaeozoic oolitic ironstone of the French Armorican Massif: a chemical and structural trap for orogenic base metal–As–Sb–Au mineralisation during Hercynian strike-slip deformation

Abstract

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 pyritised lenticular bodies within the oolitic ironstone layer. These sulphide lenses are spatially associated with strike-slip faults and extend laterally from them. After 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 that 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 mineralisation of Saint-Aubin-des-Châteaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.

Simultaneous resetting of the muscovite K‐Ar and monazite U‐Pb geochronometers: a story of fluids

“…This is in good agreement with the Ôtwo-stage alter-ationÕ that affected the Questembert granite (Tarte`se and Boulvais, 2010), and with the intense hydrothermal activity that affected this part of the Variscan orogeny at the end of the Carboniferous (e.g. Gloaguen et al, 2007;Lemarchand et al, 2011). In these two diagrams, error ellipses are at 1r.…”

Antimony deposits in the Variscan Armorican belt, a link with mafic intrusives?

“…Palaeozoic oolitic ironstones are also known in the region, and are associated with Sb mineralisation with sulfosalts (e.g. Saint-Aubin-des-Châteaux area, Gloaguen et al, 2007). However, magnetic anomalies corresponding to such candidates appear to be too narrow, and the associated sources are not deep enough, to generate the observed geophysical signals linked to Sb deposits.…”

Mylonites of the South Armorican Shear Zone: Insights for crustal-scale fluid flow and water–rock interaction processes

“…Arsenic, for example, is typical of some polymetallic ore deposits, and is associated with massive sulfidation stages (e.g. Gloaguen et al, 2007). At ELL site, U also seems to have been mobilized by fluids, its concentration ranging from 6 to 24 ppm in two samples having the same degree of magmatic differentiation (Table 6).…”

“…Gumiaux et al, 2004). To the East of the NBSASZ, fluid circulation within these sub-meridian faults led to Cu-Zn base metal and Pb mineralization (Gloaguen et al, 2007). The sub-meridian direction of these late faults is also shared by some kilometer-scale quartz veins interpreted as giant tension gashes representing an important fossil hydrothermal system (Lemarchand et al, 2011).…”