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

Abstract

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 (KIC*) in two sets of rock of varying chemical alteration from two different hydrothermal settings: a retrograde alteration assemblage dominated by chlorite-calcite-hematite, and an epithermal setting dominated by intense silicification. By comparing distinct hydrothermal alteration assemblages we can infer the relative changes in mechanical properties during progressive deformation and chemical alteration. Our tests reveal that progressive alteration changes the fracture mechanical properties of the host rock, but that the type of alteration is more important than the alteration intensity. Increasing damage in the form of microcracks weakens the rock. However, silicification is associated with high KIC* and SCI, implying it counteracts the impact of damage. Thus, a key control on the relative strength of altered rocks is the trade-off between the accumulation and preservation of damage versus the precipitation of minerals that heal these flaws. 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. In the alteration assemblages that we sampled, precipitation in the shallow epithermal environment strengthens fault-fracture conduits, whereas unhealed damage and dissolution may contribute to mechanical localization of fault-fracture conduits at depth, producing a depth-dependent inversion in mechanical contrast between conduit and host rock.