Structural controls on Jurassic gold mineralization, and Cretaceous-Tertiary exhumation in the foreland of the southern Patagonian Andes: New constraints from La Paloma area, Deseado Massif, Argentina

Fluid-rock interaction occurring within high-permeability conduits represented by fault-fracture meshes is well known to control the development of hydrothermal ore deposits. In this study we document the fundamental role of four sets of precursor joints, reactivated as dilational faults displaying variable normal dip-slip and strike-slip components of motion, in the formation of epithermal Au-Ag deposits of economic interest. Paleostress analysis unravels how all the resulting four sets of hybrid extension-shear fractures are kinematically compatible with an extensional stress field characterized by multiple permutations between the σ3 and the σ2 axes. Such permutations are interpreted to be associated with dominantly normal fault reactivation of suitably oriented early joint sets as a result of continued regional extension, and by orthogonal stretching produced by differential downthrow of the hanging-wall blocks of laterally-terminating Jurassic normal faults. Later fault reactivation appears to be associated with far field propagation of Andean stresses, marked by superposed strike-slip and reverse fault slip components recording roughly E-W horizontal compression. This late fault reactivation could be related with a late Early Cretaceous shortening event known to have affected the studied sector of the Deseado Massif. The subsequent tectonic evolution of the study area occurred within the general framework of a slow Cretaceous to Tertiary exhumation recorded by apatite fission track cooling ages. Our new thermochronometric results, recording steady-state cooling at a rate of c. 1 °C/Ma during the last 90 Ma, shed new light onto the post-Jurassic geodynamic evolution of the foreland of the southern Patagonian Andes.