Gloaguen, E., Branquet, Y., Boulvais, P., Moelo, Y., Chauvel, J.-J., Chiappero, P.-J. & Marcoux, E. (2007) 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. Miner. Depos. 42 399–422.
Added by: Laurent Cournède (2016-03-10 22:02:30)
|Type de référence: Article
Numéro d'identification (ISBN etc.): 0026-4598
Clé BibTeX: Gloaguen2007
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Mots-clés: banded iron-formation, brittany, chlorite, France, gold deposits, greenstone-belt, lead isotopes, lower ordovician, oolitic ironstone, orogenic gold, shear-zone, systems, temperature
Créateurs: Boulvais, Branquet, Chauvel, Chiappero, Gloaguen, Marcoux, Moelo
Collection: Miner. Depos.
Consultations : 1/493
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In the Saint-Aubin-des-Chateaux quarry (Armorican Hercynian belt, western France), an epigenetic hydrothermal alteration affects an oolitic ironstone layer intercalated within the Lower Ordovician Gres 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 strikeslip 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 degrees C) during the main sulphide stage 1. The subsequent stages present a marked temperature drop (300-275 degrees 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-Chateaux appears to be very similar to epigenetic sulphidation described in banded iron formation-hosted gold deposits.
Added by: Laurent Cournède