Rocks sourced from active geothermal systems can have unique responses to deformation, due to unique alteration mineralogy and complex microstructure. The current state of understanding of mechanical behaviour of rocks under varying stress conditions is well established on suites of rocks with simple mineralogy and microstructure. Brittle failure can increase porosity and permeability and generate seismicity, whereas inelastic deformation in the ductile regime will decrease porosity and will likely decrease permeability, and generate no or distinct low frequency seismicity. Many studies have focused on the behaviour of siliclastic and carbonate rocks to establish the transition form brittle to ductile behaviour. The geothermal systems in New Zealand, and many other areas, are hosted in mainly volcanic rocks, limiting the applicability of current data and knowledge to these systems.

We present results from laboratory triaxial deformation and strength testing of drill core sampled from a deep geothermal reservoir. We have used our findings to construct failure criteria based on our investigations and compared them to the in-situ and induced stress conditions that may lead to macroscopically brittle or ductile deformation of the host rock. Our results show that under the current stress conditions at the Rotokawa geothermal field the host rock behaves in a brittle, rather than compactive, fashion. Under these in-situ stress conditions brittle fracture generation dominates over cataclastic pore collapse, resulting in a rock mass with suitable macroscale permeability for fluid extraction. Our results also show that the rock strength is typically too high for the induced stresses during drilling to initiate borehole breakout. This is supported by borehole observations revealing very little borehole damage in the host rock.