Isotopic (S, Sr, Sm/Nd, D, Pb) evidences for multiple sources in the Early Jurassic Chaillac F-Ba ore deposit (Indre, France)

Abstract : During earliest Jurassic, a widespread hydrothermal event occurred in Western Europe producing large veins and stratiform F-Ba-Pb-Zn ore deposits. Previous work argued about genetic processes involving circulation of mineralising brines. Two main alternative genetic models are proposed. The first one proposes a convection of brines through the crust to produce ore deposits, the second an early infiltration of brine in the basement followed by expulsion during Mesozoic extension. In the Northern French Massif Central, new data on the F-Ba Chaillac deposit suggest that the genesis of these mineralising brines requires a new discussion. Located in the Northern French Massif Central, the Chaillac barite and fluorite ore deposit is an exceptional site where a stratiform deposit is rooted onto a vein. The ore deposition is split in two stages: 1) precipitation of green and purple fluorite within the vein (Fg-p stage), with associated fluid inclusions indicating 135°C for deposition from a low salinity fluid, and 2) yellow fluorite and barite stage (Fy-Ba) filling the vein and forming the stratiform deposit. Fluids inclusions depict a mineralising brine at 110°C. The 87Sr/86Sr and 143Nd/144Nd isotopic ratios measured in the fluorite are compared to those of French Massif Central rocks. The ratios of green and purple fluorite are similar to monzogranite and granodiorite hosted in the basement; those measured in yellow fluorite involve granulites and metamorphic basement. Measurements of the Sr isotopic ratio and 34SCDT in barite and D in fluorite fluid inclusions suggest a deposition process by the mixing of a hydrothermal fluid with meteoric water. At the scale of the Northern Massif Central district, the successive hydrothermal fluid salinities are highly contrasted as in Chaillac deposit. We propose that the two types of hydrothermal fluids have been produced by the boiling of a single fluid at depth.