Structural controls on the auriferous shear zones of the Saint Yrieix District, Massif Central, France; evidence from the Le Bourneix and Laurieras gold deposits

At least 2,000 occurrences of gold mineralization, including four economic deposits, occur in the Saint Yrieix district, in the Massif Central, France. Two of these, the Le Bourneix and Laurieras deposits, are currently being mined. The mineralization consists of quartz lenses with sulfides and native gold. The lenses are several meters thick and 300 m in length and occur within a shear zone several kilometers long in para- and orthogneisses of the Hercynian basement.Metallogenic and structural investigations in the Saint Yrieix district and in Le Bourneix mine have made it possible to relate the various episodes of mineralization to the sequence of tectonic events. The lenses of vein quartz were emplaced during phases of extension, their distribution being controlled by anastomosing mylonitic bands formed during a preceding tectonic phase.The mylonitization was accompanied by sericitization and the introduction of disseminated pyrite, arsenopyrite, rutile, and organic matter along the planes of mylonitic foliation. Gold (fine-grained), galena, and sulfosalts were introduced later, impregnating the microsaccharoidal quartz resulting from cataclasis of initially barren vein quartz. Subsequent partial remobilization of the gold-bearing lenses during phases of extension caused coarsening of the gold, with the appearance of argentiferous nuggets.The emplacement of the quartz lenses in brittle-ductile shear zones and the diachronism between the introduction of the vein quartz and that of the gold are characteristic of this mineralization and make it possible to propose guides for exploration.