The Biards Sb-Au-Bearing Shear Zone (Massif Central, France): An Indicator of Crustal-Scale Transcurrent Tectonics Guiding Late Variscan Collapse

Abstract

Structural, textural, mineralogic, microthermometric, and isotopic data obtained on the Biards Sb-Au district, located in the south Limousin area (Massif Central, France) provide new evidence for the tectonic setting of stibnite and gold deposition during the late orogenic Variscan collapse in western Europe. A variety of geologic and geophysical data define a major, crustal-scale, left-lateral strike-slip shear zone that acted as a trap for Sb-Au mineralization. During these movements, fault-fill veins and then extensional veins of limited extent were created along rheologic heterogeneities that focused brittle deformation. Geometry and orientation of both vein sets indicate two main episodes of emplacement of ore related to a progressive wrench movement in response to an counterclockwise rotation of the regional stress axes. Both veins sets are filled by quartz and sulfides in three stages: an early arsenopyrite-pyrite stage with fine-grained quartz, a sphalerite-Pb, Sb sulfosalt-native gold stage with comb quartz, and a late stibnite-berthierite stage with comb quartz. Combined fluid inclusion and stable isotope studies on quartz, muscovite, and sulfides reveal that the three stages result from the continuous evolution of a deep-seated hydrothermal system in which the latest stages mixed with meteoric water. These data argue for cooling and a pressure drop during ore deposition and indicate a shallow level of emplacement for Sb-Au veins. A comparison of the Biards deposits with other upper Paleozoic gold and antimony deposits in the Massif Central, the western Variscides, and elsewhere in the world illustrates their common association with crustal-scale, strike-slip shear zones. The Upper Carboniferous age (305–300 Ma) of the Sb-Au deposits indicates that they formed during the early stages of the postthickening extension of the Variscan lithosphere. Sb-Au–bearing strike-slip shear zones resulted from the late Variscan crustal collapse guided by the very large scale dextral shearing between central Europe, northern Africa, and the northern Appalachians, and associated with the opening of the paleo-Tethys and the closing of the Theic ocean during the late Paleozoic Gondwana-Laurasia translation.