Fracture Mechanical Properties of Damaged and Hydrothermally Altered Rocks, Dixie Valley, NV: Implications for Fault Conduit Development in Geothermal Systems

摘要

Enhanced water-rock interaction in hydrothermal systems changes the textural and mineralogical composition of host rock, affecting the mechanical properties of fault-fracture systems, their permeability structure, and thus the development of hydrothermal convection cells. To characterize the impact of alteration on fracture mechanical properties, we used double-torsion load-relaxation testing to measure subcritical fracture growth indices (SCI) and mode-I stress intensity at calculated fracture growth velocities of 10~(-6) m/s (K_(IC*)) in two pairs of rock of varying chemical alteration from different hydrothermal settings. By comparing distinct hydrothermal alteration assemblages preserved in the footwall of the Dixie Valley Fault, NV, we can infer the relative changes in mechanical properties during progressive deformation and chemical alteration. Our tests indicate that alteration influences fracture mechanical properties to varying extent. Silicification is associated with high K_(IC*) and SCI, while altered samples containing unhealed damage are mechanically weaker. We conclude that changes in mechanical properties are related to the dominant alteration mechanism rather than the degree of alteration. A key control on relative strength of altered rocks is the prevalence of existing damage, in the form of microfractures or dissolution, versus mineral precipitation and healing. Spatial variation in dominant alteration mechanisms across hydrothermal systems likely produces systematic changes in the mechanical properties of fault-fracture conduits and adjacent host rock. Based on the alteration assemblages that we sampled, the mechanical contrast between fault zone and host appears to reverse between deep and shallow portions of the system: precipitation may seal and strengthen fault-fracture conduits in shallow portions of the system, while damage and dissolution at depth may contribute to mechanical localization of fault-fracture conduits.