Multiscale controls on cyclic damage and elastic memory in heterogeneous rocks from a porphyry copper system

Understanding damage accumulation under cyclic loading is critical for assessing the stability of deep excavations and heterogeneous crustal rocks subjected to repeated stress perturbations. Subvolcanic and volcanic lithologies typical of porphyry copper systems, and analogous to shallow volcanic crust, exhibit strong mineralogical and structural heterogeneity due to intrusive processes, veining, and hydrothermal alteration, challenging models derived from homogeneous rocks. We present results from uniaxial and triaxial cyclic loading experiments on five lithologies from the El Teniente porphyry copper deposit (tonalite, diorite, porphyritic dacite, veined andesite, and hydrothermal breccia), conducted under confining pressures up to 25 MPa and coupled with continuous acoustic emission (AE) monitoring. Cycles of increasing stress amplitude were used to quantify stiffness degradation and acoustic memory through the Felicity Ratio (FR). Elastic moduli were derived from unloading branches, allowing direct comparison with elastic reversibility frameworks proposed for crystalline rocks. Homogeneous to moderately heterogeneous lithologies exhibit gradual stiffness loss and limited departure from elastic reversibility, whereas strongly heterogeneous rocks display pronounced stiffness fluctuations, early deviation from elastic behaviour, and broad FR dispersion, indicating intermittent strain localization and partial loss of elastic memory. Increasing confinement reduces mechanical and acoustic scatter, highlighting the stabilizing role of lateral stress. Ongoing work integrates photogrammetry-based quantification of grain-size distributions, vein density, vein thickness variability, and alteration intensity. These micro- and mesoscale descriptors are used to explore correlations with mechanical degradation rates and acoustic reactivation patterns observed during cyclic loading. This combined mechanical–microstructural approach aims to clarify how lithological heterogeneity governs the style, rate, and intermittency of cyclic damage in subvolcanic crust, with implications for deep mining stability and stress cycling in volcanic systems.