Abstract. The Perrefitte anticlinorium district in the Paleozo[c province of the Pyrenees contains abundant mineral occurrences, prospects, and old mines in the upper volcano-sedimentary layer of the Ordovician and Devonian sediments which have been metamorphosed in the greenschist to amphibolite facies. Although of different lithologic environments, the mineralization shows, as everywhere in the Pyrenees, many common characteristics. However, the Arrens deposit outcropping over 5 km on both limbs of a syncline displays specific characteristics: abundant barite with lateral Zn (Pb) Ba zoning, exceptionally thick subreef (?) footwall limestone overlain by black siltstone, abundant Ba silicate, in particular celsianite, locally cymrite, and siliceous, sedimentary-hydrothermal gangue.Due to deformation, such as tight folds and flow cleavage, the paleogeography of the Arrens geological environment could not be precisely reconstructed. However, the deposit is very likely located in an unstable shelf zone. Lithology underwent many changes close to the synsedimentary faults including at Arrens a limestone shelf facies and some small basins of black siltstone, at the bottom of which mineralization occurs. As all the mineralization of the Pyrenees Paleozoic, the deposit is of the exhalative or hydrothermal sedimentary type and bears some analogies with other exhalative deposits throughout the world. (The word "exhalative" is etymologically related to gases. Although "hydrothermal" is a more exact word as it pertains to hot water, "exhalative" will be used in this paper, being of most current usage.)