Multi-stage fluid incursion in the Palaeozoic basement-hosted Saint-Salvy ore deposit (NW Montagne Noire, southern France)

Abstract

The Saint-Salvy vein-hosted Zn (+Ge) deposit occurs in an E–W fault system which flanks the southern margin of the late Variscan Sidobre granite, and cross-cuts Cambrian black shales of the Palaeozoic basement. Comprehensive mineralogical and geochemical studies of vein samples have revealed four mineralizing events (M1–M4) related to late and post-Variscan tectonic events. A further late-stage event may be related to weathering. M1 (=skarn deposits) and M2 (=patchily mineralized quartz veinlets) are associated with granite emplacement. Quartz contains low salinity, H 2O CO 2 (NaCl)-dominated fluids (⩽6wt% NaCl equiv.) of relatively high temperature (300–580°C), trapped under moderate to high pressure. Estimated M1 fluid δD and calculated fluid δ 18O plot within the metamorphic water field. There appears to be no involvement of magmatic fluids. By contrast, M3 (= barren quartz) and M4 (= zinciferous economic mineralization) stages have H 2O CO 2 NaCl CaCl 2 fluid inclusions with high salinities (23–25 wt% NaCl equiv.) and low temperatures (∼ 80–140°C), which were trapped under low-pressure conditions. The high salinity and NaCl + CaCl 2 content of both M3 and M4 indicates that their parent fluids leached evaporitic salts. M3 fluids are meteoric water dominated, falling close to the meteoric water line (δD and δ 18O averaging −64 and −8‰, respectively). M4 fluids have highly distinctive δD averaging −101‰, and calculated fluid δ 18O varying from −1.2to+7.1‰. The unusually low δD composition of M4 suggests the involvement of “organic” fluids, in which H is derived directly or indirectly from organic matter. The relatively high δ 18O of M4 fluids indicates that considerable isotopic exchange with sedimentary material took place, displacing the δ 18O from the meteoric water line. The data imply interaction of meteoric waters with evaporite and hydrocarbon-bearing sedimentary sequences, most probably the adjacent Aquitain Basin. The main economic mineralization (M4 stage) took place during a tensional event, probably coincident with the Lias-Dogger transition. Calculated δ 34S H 2S of M4 sulphide (+5.4to+8.2‰) is almost identical to δ 34S of local Cambrian sulphides (+4.7to+9.4‰) suggesting a genetic link. Abundant siderite associated with M4 sphalerite has δ 13C ranging from −2.6to−4.4‰ indicating that carbon was sourced from sedimentary carbonate mobilized by, or equilibrated with the hydrothermal fluid. Late-stage sulphides exhibit extraordinary and highly distinctive δ 34S . Sphalerite has extremely low δ 34S (−42.5to−50.5‰), whereas pyrite has an extraordinary large range from −33.2‰to+74.3‰. Closed system sulphate reduction is held to be responsible for the extremely high δ 34S : whereas more open system reduction produces the very low values. The coincidence of isotopically low δ 13C(−7.6to−11.9‰) for co-genetic calcite suggests the involvement of organic matter in the reduction process.