Triaxial stress and failure modes in hydrothermal mineral systems

Abstract Rock mechanics experiments show that the magnitude of the intermediate principal stress significantly affects rock failure. Since triaxial stress states (no principal stress is zero) are ubiquitous in the crust, and polyaxial axial states (all three principal stresses are different) are general, the magnitude of the intermediate principal stress should have an important effect on hydrothermal mineralisation. For example, extensional veins or dykes in vein-hosted gold or porphyry deposits may have multiple orientations when the intermediate and least principal stresses have similar magnitudes, or single orientations when the intermediate and maximum principal stresses are similar. The Griffith-Murrell triaxial fracture criterion with a tensile cutoff can be used to illustrate the effects of the intermediate principal stress on failure. At the lowest values of mean stress, the criterion suggests that only extensional failure can occur. At low–intermediate values of mean stress, either extensional or shear failure may occur: extensional failure is favoured when the intermediate and maximum principal stresses have similar magnitudes. At higher mean stresses, shear failure will occur at lower values of pore fluid pressure and differential stress when the magnitudes of the intermediate and minimum principal stresses are similar. KEY POINTS The intermediate principal stress has significant effects on rock strength, which should be considered for understanding failure in hydrothermal mineralising systems, because stress states in the Earth are generally triaxial and polyaxial. The magnitude of intermediate principal stress relative to those of the other principal stresses affects the range of orientations of extensional fractures, for example in stockworks and sheeted veins. At low values of mean stress, only extensional failure occurs. At low to intermediate values of mean stress, extension is favoured over shear failure when the magnitudes of intermediate and maximum principal stresses are similar. At high values of mean stress, shear failure will occur at lower values of pore fluid pressure and differential stress when the magnitudes of the intermediate and minimum principal stresses are similar.