Hydro-mechanical behavior of mineralized faults in granite during fluid injection: Insights from triaxial shear-flow experiments

Mineralized faults—fractures that have accommodated displacement and were subsequently filled or coated with mineral precipitates—are commonly found in hydrothermal regions. Precipitation from circulating fluids or magmatic intrusions can substantially alter fault structure, reduce permeability, and influence frictional stability. Despite their geological importance, the slip behavior and frictional evolution of mineralized faults during fluid injection remain poorly constrained. To address this knowledge gap, we conduct triaxial shear-flow experiments on critically stressed mineralized faults in granite to examine their hydro-mechanical response to fluid pressurization. Deionized water is injected at a constant rate of 0.6 mL·min −1 under different confining pressures of 20 and 30 MPa. Fault slip occurs in two distinct stages: an initial unstable stick-slip phase followed by stable slip, with higher confining pressure promoting the transition. The effective friction coefficient fluctuates during stick-slip but increases progressively during stable slip. Greater hydraulic energy input does not necessarily induce earlier fault reactivation, but it results in greater deformation moment accumulation and higher energy release. Microstructural analysis reveals distinct deformation mechanisms: foliation-like microfractures develop near the fault surface at 20 MPa, while 30 MPa conditions lead to grain crushing and the formation of interconnected fracture networks. These findings advance our understanding of injection-induced slip in mineralized faults and provide insights relevant to geothermal energy extraction, energy storage, and waste disposal.