Alteration-related damage thresholds in cyclically loaded rocks from deep mining environments

Hydrothermal alteration exerts a first-order control on the mechanical behaviour of rocks in deep mining environments by modifying mineralogical composition, grain bonding, and internal structures. In porphyry copper systems, quartz–sericite, potassic and chloritic alteration produces strong contrasts between mechanically competent and weak mineral phases, influencing damage accumulation and failure under cyclic stress conditions. However, experimental constraints on how alteration intensity governs mechanical degradation and acoustic response during cyclic loading remain limited. We investigate the mechanical and acoustic behaviour of rocks exhibiting variable degrees of alteration from a porphyry copper deposit. A total of 57 specimens, classified according to their alteration intensity, were subjected to single-cycle and multi-cycle compression tests under unconfined and confined conditions (σ₃ = 15 MPa). Acoustic emission (AE) monitoring was performed continuously to track microcrack activity and damage evolution during loading. Analysis of the unloading modulus throughout cycles reveals a progressive stiffness degradation that correlates with internal damage accumulation. In general, ‘perfect elasticity’, where loading and unloading gradients converge, is only observed at low stress levels, typically between 20% and 40% of the peak strength. These results contrast with previous studies on more homogeneous rocks, where a broader elastic range were reported. Our findings indicate that beyond 40% threshold, the divergence between loading and unloading moduli increases sharply as a function of cycle accumulation. Samples enriched in softer mineral phases (sericite-rich) exhibit distinct acoustic signatures that reflect a more distributive damage mechanism, whereas quartz- and K-feldspar–dominated rocks, characterized by higher mineral hardness, show a greater damage and microcrack accumulation. This is quantitatively supported by the Felicity Effect analysis: under unconfined conditions, rock dominated by harder mineral phases exhibit lower Felicity Ratio (FR) values, indicating significant pre-peak damage. However, the introduction of a 15 MPa confining pressure leads to a homogenization of the FR across all alteration intensities, as the external stress suppresses micro-cracking regardless of the initial mineralogical heterogeneity. Ongoing analysis explores relationships between alteration degree, mineralogical composition, cyclic damage thresholds, and post-test fracture patterns. By integrating mechanical measurements and acoustic emission data, this work aims to clarify how hydrothermal alteration governs damage accumulation and failure processes in heterogeneous rocks subjected to cyclic stressing, with implications for deep mining stability and induced seismicity.