Original mineralogical features of a hydrothermalised oolitic ironstone : The deposit of Saint-Aubin-des-Châteaux (Armorican Massif, France).

Despite its large area, the Armorican Massif provided only very few new mineral species: plombogummite, laumontite, bertrandite, natrodufrenite and lulzacite. This last species was recently discovered by one of us (Y. L.) in a sandstone quarry at Saint-Aubin-des-Châteaux (near Châteaubriant, Loire-Atlantique department) (Moëlo et al., 2000), and various studies permitted to reveal original mineralogical features, related to the superimposition of hydrothermal processes on an Ordovician oolitic ironstone interstratified in the sandstone sequence. This ironstone belongs to a very large sedimentary Fe deposit lying at the East margin of the Armorican Massif. At Saint-Aubin, ooliths are constituted essentially by siderite and chlorite, with abundant Srrich fluorapatite and organic matter. During Hercynian orogenesis, faulting controlled hydrothermal processes, which induced pronounced mineralogical changes, especially massive sulphidation of the ironstone (Gloaguen, 2002). Due to the abundance of primitive apatite and other peculiar geochemical features, it permitted the crystallisation of various Sr or lanthanoide phosphates. Lulzacite, Sr2Fe2+(Fe2+,Mg)2Al4(PO4)4(OH)10, is very well crystallised in small quartz veins crosscutting the ironstone, together with siderite and pyrite. It is isostructural with jamesite (Léone et al., 2000). It occurs as massive aggregates (up to some cm3), as well as euhedral crystals up to 1 cm in size. It was formed by short-range remobilisation of synsedimentary apatite (process of lateral secretion). Later, its decomposition lead to the formation of goyazite (with amethyst colour), together with late fluorapatite, and some berthierine. Lanthanoide phosphates were formed directly within the ironstone. The very rare scandium phosphate pretulite, ScPO4, has grown together with xenotime-(Y) in epitaxy on detrital zircon crystals (Moëlo et al., 2002). SEM as well EPMA revealed concentric zonation of pretulite and xenotime crystals, indicative of a multi-stage process. Monazite-(Ce) is present as minute anhedral neoformed aggregates, while detrital crystals are rare. Detrital zircon shows growth zones enriched with Sc-, Y- and HREE- phosphate components. EPMA of zircon and pretulite suggested a complete solid solution according to the heterovalent substitution rule: Zr + Si -> Sc + P. This solid solution has been confirmed experimentally in high temperature conditions (Dubost, 2003). A general metallogenic study is in progress (Gloaguen, 2002; Gloaguen et al., in prep.). After the main Fe-S-(As) stage, a Zn-Pb-(Cu) stage presents some Sb sulfosalts (boulangerite, bournonite and tetrahedrite), as well as traces of electrum. While the formation of lulzacite and subordinated goyazite is directly related to the hydrothermal process acting in the deposit of Saint-Aubin, that of xenotime and monazite begins very early within the ironstone formation, as observed in the neighbouring Fe deposit of Rougé, what may also permit the discovery of new pretulite occurrences. Thus, it seems that hydrothermalism of such ironstones is a major key and control when studying rare minerals associated with ironstones. Moreover, others phosphates have been previously described within ironstones: wolfeite (Fe2+,Mn2+)2(PO4)(OH) (Brousse and Chauvel, 1969), lazulite (Chauvel, 1968) and are not yet recognized in the Saint-Aubin-des-Châteaux quarry. We propose that differences between these phosphates minerals parageneses are controlled by ironstone initial chemistry and hydrothermalism. Such hydrothermalism is favoured and associated with late orogenic context. In this way, the large European palaeozoic ironstone belt is probably an important target for potential new and rare minerals species. Actually, all armorican iron mines are closed and a re-examination of ironstones samples preserved in museums could probably allow the discover of new species. Brousse, R. and Chauvel, J.-J. Bull. Soc. fr. Minéral. Cristallogr. 92, 1969, pp 93-94. Chauvel, J.-J. Mém. Soc. géol. Minéral. Bretagne, 16, 1968, 243 p. Dubost, V. Unpublished Research report, Magistère de Physico-Chimie Moléculaire, Paris XI-ENS Cachan, Institut des Matériaux de Nantes, 2003, 32 p. Gloaguen, E. Unpublished Research report, DEA Géosystèmes, Université d'Orléans, 2002, 41 p. Léone, P, Palvadeau, P. and Moëlo, Y. C. R. Acad. Sci. Paris, IIc, 2000, 301-308. Moëlo, Y., Lasnier, B., Palvadeau, P., Léone, P. and Fontan, F. C. R. Acad. Sci. Paris, 330, 2000, 317-324. Moëlo, Y., Lulzac, Y., Rouer, O., Palvadeau, P., Gloaguen, E. and Léone, P. Can. Mineral., 40, 2002, 1657-1673