Primary ultra-high-grade Au mineralisation driven by sustained fracture dilation and fluid flow

Ultra-high-grade Au (UHG Au) is a texturally distinct vein-hosted Au mineralisation style in epithermal and orogenic Au deposits. The localised hyper-enrichment of Au appears to defy the Au transport capacity of typical ore-forming fluids; thus, the genesis of UHG Au remains debated. In this contribution, the genesis of UHG Au was investigated at the Beta-Hunt Au mine (Kambalda, Western Australia). The Beta-Hunt deposit hosts ∼2640 Ma, structurally controlled orogenic Au mineralisation with two distinct Au mineralisation styles: (1) low-moderate grade Au, occurring as isolated fine grains hosted within a hydrothermally altered shear foliation and (2) coarse-grained UHG Au hosted solely by albite-quartz-carbonate extension veins overprinting the foliation. We employed a multi-disciplinary methodology, conducting characterisation of vein morphology, microtextural analysis, μ-XRF mapping and 3D synchrotron X-ray tomography. Our observations show that UHG Au and associated gangue minerals precipitated within a sustained fluid-filled open space formed by progressively dilating, low aspect-ratio (length/width) fractures during a single or few opening-infill cycles. The low aspect-ratio geometry of UHG Au-hosting veins implies that viscoelastic wall rock deformation contributed to the dilation of the veins, establishing a stable mineral growth environment. Our observations deviate from the expectations of the current orogenic Au system framework, which involves quasi-instantaneous fracturing and growth of veins, via rapid infilling during fluctuating pressure and chemical conditions. We explain our observations through a viscoelastic fracture growth model, which couples the generation of mineral growth space with sustained fluid flow for steady nutrient supply, extending the scope of current models for the genesis of UHG Au.