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S U M M A R YClassical progressive unfolding makes a distinction between pre-folding, post-folding or synfolding magnetization. It yields the direction of magnetization only in the two first cases. To determine the direction of synfolding magnetization, possible different unfolding percentages during remagnetization have to be assumed according to the site. The small circle approach of Surmont et al. not only leads to this direction, but also to the dip value at each site during remagnetization. In the Cévennes area, extensive palaeomagnetic study reveals syntectonic remagnetization enabling the reconstruction of the shape of a fold for the time of the magnetic overprinting as well as analysis of the rotations undergone by some sites after the remagnetization.

Summary Palaeomagnetic dating techniques have been applied to determine the age of fluid migration that produced the Mississippi Valley‐type (MVT) Pb–Zn–Ba–F deposits in the Cévennes region of southern France. 15 sampling sites in two gently deformed areas around the Largentière and Croix‐de‐Pallières mines on the Cévennes border were selected for palaeomagnetic study. They yielded a very well‐defined direction of remagnetization corresponding to an Early‐Middle Eocene age. This remagnetization cannot be related to the formation of magnetite as a result of the transformation of smectite to illite because the latter has been well dated as a Mesozoic event. The magnetic overprint in this area is related to a chemical phenomenon during fluid migration. The age of remagnetization corresponds to a major uplift in the Pyrénées mountains, located to the south of the Cévennes. This implies that fluid migration occurred from the south to the north as a result of hydraulic head established in the Pyrénées orogenic belt during orogenesis and suggests that the MVT deposits in the Cévennes region formed from a gravity‐driven fluid system as described by Garven & Freeze (1984a,b).