Long‐Term Storage of Meteoric Water in the Variscan Basement: Stable Isotope Constraints ( δD – δ ¹⁸ O )

F‐Ba‐Pb‐Zn (±Ag) deposits near the unconformity between the Armorican and Aquitaine basins formed by mixing surface waters and brines with fluid ascending from the basement. To better constrain the source of this fluid and the conditions of mixing, we analyzed the hydrogen (δD water ) and oxygen (δ 18 O water ) isotopic composition of water trapped in mineral fluid inclusions and the oxygen (δ 18 O quartz ) isotope composition. Results indicate that isotopic equilibrium was not reached during fluid mixing. The isotopic signature of basement fluid is similar/identical to Variscan meteoric water that was trapped and stored in the basement and released at the onset of the Bay of Biscay opening.