The evolution of syn-deformational porosity in a marble mylonite over increasing strain: Insights from EBSD and 3-D microcomputed tomography

Fluid flow through ductile shear zones is increasingly recognised as a key control on the localisation, upgrading, and redistribution of hydrothermal ore systems. We investigate how syn-deformational porosity evolves with increasing finite strain in a calcite-rich marble mylonite from the Western Mary Kathleen shear zone adjacent to the Mary Kathleen REE-U deposit (NW Queensland, Australia). Microstructural evolution and pore-network topology are tracked along a natural strain gradient using electron backscatter diffraction (EBSD) and synchrotron micro-computed tomography (3-D micro-CT). EBSD reveals a progressive transition from twin-rich, dislocation-dominated calcite fabrics at lower strain to uniformly fine-grained, foam-like mosaics at higher strain, where grain-size-sensitive deformation (diffusion creep and grain-boundary sliding) dominates and crystallographic preferred orientations weaken. In lower-strain mylonites, pores occur mainly as isolated to weakly connected cavities along subgrain and grain boundaries, concentrated at boundary junctions and locally associated with twin lamellae. With increasing strain and grain-size reduction, porosity reorganises into fewer but larger, high-aspect-ratio grain-boundary networks that link into laterally continuous pore sheets. Micro-CT-derived orientations show that the normals to these sheets cluster near the instantaneous shortening direction, indicating that connected pore sheets are commonly oblique to both the S- and C-planes rather than strictly foliation-parallel. These results demonstrate that finite-strain-driven grain-size reduction can generate transient, strongly anisotropic permeability by organising boundary-hosted porosity into interconnected, sheet-like conduits, providing a plausible microstructural mechanism for deformation-controlled fluid focusing and REE-U-bearing fluid redistribution in carbonate shear zones.