The Salsigne Au-As-Bi-Ag-Cu Deposit, France

Abstract

The Salsigne mesothermal Au-As-Bi-Cu-Ag deposit is located in the Montagne Noire, a part of the southern external zones of the French Variscan orogen. This part of the orogen consists of a pile of thrust nappes stacked on the autochthonous Axial zone of the Montagne Noire. The Salsigne deposit is located in and close to the lowest unit, the Fournes nappe. The Fournes nappe is less than 500 m thick and intercalated between more competent terranes, with the large Minervois nappe above and the autochthonous Axial zone below. It is intensely deformed, resulting from several stages of folding, shearing, and brittle faulting. The deposit is hosted in siliciclastic and carbonate rock and consists of many orebodies including crosscutting veins, strata-bound stockworks in sandstone, replacement orebodies in carbonate rocks, and orebodies hosted in thrust faults, particularly in the major thrust which separates the nappes from the autochthonous terranes. A succession of nine hydrothermal events, commonly separated by tectonic events, is recognized. The first four stages, controlled by ductile to ductile-brittle structures synchronous with the emplacement of the nappes, were responsible for stockwork, massive, and disseminated mineralization. They include (1) arsenopyrite-pyrrhotite-gold associated with biotite, (2) quartz-muscovite alteration, (3) massive sulfides (arsenopyrite-pyrrhotite-pyrite) and gold associated with chlorite, and (4) feldspathization. The subsequent stages, controlled by brittle tectonics that frequently reactivated earlier structures, include barren silicification and quartz veins followed by four stages of sulfide precipitation separated by fracturing. Two main types of fluids, characterized through the associated mineral associations, were responsible for mineralization in alternating successive pulses. Type I fluids, responsible for most of the gold and for the development of biotite (450°C) in host rocks of very low metamorphic grade, were reduced, CO 2 -rich, and alkaline, with high H 2 S contents but low f S 2 . There is evidence of large-scale, regional circulation of similar fluids, in particular in rocks that lie several kilometers below the deposit. Type II fluids that produced the massive sulfides associated with chlorite (and several later stages) and mainly redistributed gold, were acid with comparatively high f O 2 and f s 2 (about 2 log units higher than type I fluids), possibly in equilibrium with the country rocks below the deposit. The hydrothermal system was initiated, possibly in relationship to late Variscan magmatism, when the tectonic regime changed from ductile-brittle to brittle conditions and from compressional to extensional regime. Fluids were channeled by a shear mesh created at the base of the nappe pile and in the upper part of the autochthonous terranes by the nappe emplacement. The Salsigne deposit is located along this major (>100 km) shear zone at its intersection with transverse brittle structures and sheared fold structures.