Nature's treasure factory: Geochemical controls on ultra-high-grade orogenic gold deposit formation

Ultra-high-grade gold deposits represent some of the most economically significant yet geologically elusive features of hydrothermal ore systems. They are defined by the presence of coarse visible gold greater than 0.1 mm in size and grades above 20 g/t. Although the fundamental processes governing gold mobilisation, transport, and primary deposition in orogenic systems are well established, these models alone cannot account for the extreme local enrichments that characterise coarse native gold-bearing zones. This review synthesises established geochemical concepts and recent advances in ore genesis understanding to develop an integrated view of how ultra-high-grade gold forms. Evidence indicates that no single mechanism is universally responsible; instead, coarse native gold formation reflects the interplay of fluid focusing and metamorphic recrystallisation processes operating from the source region to the final stages of deposition, and subsequent local remobilisation, exploiting the unique geochemical properties of gold. Key contributors include gold nanoparticles, polymetallic melts, piezoelectricity, and post-depositional upgrading via dissolution-reprecipitation, diffusion along crystal defects, and deformation-enhanced remobilisation. Collectively, these processes highlight that ultra-high-grade zones represent structurally controlled, open, dynamically evolving systems that may be repeatedly overprinted by multiple generations of auriferous fluids and/or remobilisation. Understanding the complex, multi-stage pathways to extreme enrichment provides a foundation for developing more predictive exploration strategies for ultra-high-grade gold deposits that leverage emerging insights into gold geochemistry, nanoparticle behaviour, and deformation-driven mineral reactions.