ON THE EMPLACEMENT OF EPIGENETIC METALLIFEROUS HYDROTHERMAL MINERALIZATION IN EXTENSIONAL BASINS: TECTONIC STRESS AND KINEMATIC CONSTRAINTS (Invited Presentation)

In rift basins and other extensional settings, normal faults and dilatant joint networks commonly serve as conduits for the transport of metalliferous hydrothermal fluids in the upper crust. However, fossil rifts commonly record temporal changes in the tectonic stress field and the behavior of fault systems over the life of the rift. In exhumed fossil rift basins in New Jersey, Eastern North America, and in Botswana, East Africa that are rich in epigenetic hydrothermal copper mineralization, I have been investigating how temporal changes in tectonic paleostress and kinematics may have influenced the localization of trap sites of the mineralization. In my talk, I will present field structural observations from the basins documenting excellent preservation of fault-zone hosted multiphase hydrothermal alteration events with structural domains of variable brecciation intensities. The mapped structures and hydrothermal alterations provide robust kinematic indicators that suggest that the emplacement of the hydrothermal copper mineralization occurred during a major temporal change in stress state and kinematics from normal faulting regime into transpressive/compressive basin inversion-related stress states. Further, the hydrothermally brecciated terranes of the copper belts show co-occurrence of shear, dilatant, and hybrid failure with localized ductile deformation within the brittle damage zones, indicating seismic and aseismic failure of the fluid-hosting fault zones during the time of mineralization. Altogether, the observations from the two investigated basins suggest that 1) transitions in stress state and fault kinematics in evolving extensional settings play key roles in the emplacement of economic epigenetic hydrothermal deposits, and 2) hybrid reactivation of faults and connected fracture systems facilitated the funneling of the epigenetic hydrothermal fluids into their trap sites.