CHEMICALLY ASSISTED FRACTURE GROWTH IN HYDROTHERMAL SYSTEMS: EXPERIMENTAL RESULTS

Field studies in Dixie Valley, NV, reveal distinct deformation, fluid flow, and alteration histories along different fault segments, suggesting feedback between dilatant damage and hydrothermal setting. This feedback may manifest as changing mechanical properties during progressive alteration of host rock or as varying rates of chemically assisted fracture growth across hydrothermal plumes. The role of chemically assisted fracture growth in these environments is poorly constrained, despite the implications for conduit and strength evolution around faults. In order to assess the impact of chemical environment on mode-I fracture toughness (K_IC) and subcritical index (SCI) of rocks commonly encountered in hydrothermal systems, we conducted double torsion load-relaxation tests in different physio-chemical conditions using silicified fault rocks. Chemical environments included: ambient air, DI water, dilute HCl (pH 3), dilute NaOH (pH 12), and 0.1 wt% NaCl solutions. Silicified samples were obtained from the Dixie Comstock epithermal deposit, where intense fault zone silicification extends ~600 m along strike and exceeds 2 m thickness. Samples are 88-97 wt% quartz with minor feldspar, calcite, chlorite, muscovite, oxides, and sulfides and low porosity (<1.6%).

Preliminary results show a reduction in mean K_IC (from 2.94 to 2.37 MPa√m) and a >60% reduction in mean SCI (144.7 to 48.3) in aqueous environments compared to ambient conditions. The reduction in SCI is least in DI water and most pronounced in alkaline and saline solutions, suggesting silica dissolution and cation exchange may facilitate subcritical fracture growth in this material. These results have two implications. First, measurements of fracture mechanical properties conducted under ambient conditions may misrepresent in situ characteristics. Second, the chemical conditions in hydrothermal conduits directly impact fracture growth, implying a connection between strength evolution, fluid flow, and fluid chemistry beyond the more obvious role of mineral dissolution and precipitation. In hydrothermal cells, which frequently contain alkaline solutions and elevated salinity, chemically aided fracture growth may provide a positive feedback mechanism that contributes to conduit localization and interseismic weakening.