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

¹ ISTO, Université d’Orléans – CNRS/ISTO-UMR 6113, campus geosciences, 1A rue de la Ferollerie, 45071 Orléans cedex 02, France.
² Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QF, Scotland, UK
³ Université du Québec à Montréal, DSTA, CP 8888, Montréal, Quebec, Canada H3C 3P8

Correspondence: Corresponding author: stanislas.sizaret{at}univ-orleans.fr

During the 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. Fluid 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 in green and purple fluorite are similar to those of monzogranite and granodiorite of the basement; those measured in yellow fluorite involve the granulites and other metamorphic rocks of the basement. Measurements of the Sr isotopic ratio and ³⁴S_CDT 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.

Key Words: Fluorite • Barite • Hydrothermal processes • Early Jurassic • Isotopes • Chaillac