ROLE OF RHEOLOGICAL HETEROGENEITIES IN VEIN-ORE LOCALIZATION

Structural studies of the Saint-Salvy zinc deposit and other Hercynian , veinhosted ore deposits in the French Massif Central and Pyrenees re veal a fourstage evolution of mineralized structures under rheological control: (i) localization of potential mineralized areas, guided by t he presence of first-order lithological or structural heterogeneities that caused stress and strain perturbations; (ii) creation of second-o rder heterogeneities, corresponding to indurated shear zones that acte d as rheological discontinuities; (iii) tectonic activation of these s econd-order heterogeneities, opening voids that allowed circulation of hydrothermal fluids and periodic trapping of ore minerals; (iv) rewor king and partial destruction of the mineralized structures, caused by the reactivation of anisotropic surfaces acting as zones of weakness. The interaction between preexisting, first-order heterogeneities and r egional shear strain caused instability, which in turn produced second -order and then lower-order heterogeneities. Such progressively smalle r heterogeneities induced an increasingly focused, centripetal localiz ation of structural disturbances that enabled hydrothermal fluid chann elling. This is the reason that lower-order and late structures prefer entially bear economic mineralization.