Unravelling the hydro-mechanical evolution of a porphyry-type deposit by using vein structures

Abstract

The Permian San Pedro Cu-(Mo) deposit consists of a porphyry system with associated polymetallic veins in the San Rafael Massif, Argentina. This deposit resulted from the emplacement of a dioritic - tonalitic subvolcanic stock into the Early Permian Choiyoi volcanic rocks. Veins and veinlets linked to the hydrothermal alteration stages (potassic, phyllic and carbonitization) show different attitudes and are predominantly steeply dipping; sequentially, they fill four fracture systems: stockwork (potassic), dilatant WNW- & ENE-trending shear-compressional fractures (phyllic), NNE- & ~NS-striking tensional and shear-extensional fractures (phyllic), and NW/NNW- & WNW-trending tensional and shear-extensional fractures (carbonitization). Because of mechanical anisotropies of the host-rock, alteration telescoping, presence of hydrothermal fluids, thermodynamic constraints, and waning orogenic conditions, vein/veinlet overprinting is conspicuous. Focused hydrothermal fluid flow promoted episodic hydraulic reactivation and hydraulic fracturing, and reveals that differences in vein fabrics do not necessarily represents veins emplaced under different stress regimes. Based on the study of the structure of the area (including available geophysical maps) and its kinematics, the control of structures during vein emplacement, the vein/veinlet overprint sequence, and the thermo and physical conditions at the time of vein emplacement (using fluid inclusion data), it was possible to identify, characterize and track the changing hydro-mechanical evolution of this deposit as well as the progressive exhumation of the area during the time of vein emplacement.