Lay summary
"Orogenic Gold Deposits" are one of the world's largest sources of mined gold and they are a major target for the exploration industry. Despite abundant research in the past, many of the fundamental processes that form these deposits remain poorly understood. Any advances in understanding would be most timely, as the exploration industry is now highly active in orogenic belts worldwide and it relies on genetic models to guide its field programmes. One of the problematic aspects concerns the mechanisms of transport and precipitation of gold in the specific kinds of geochemical settings that form orogenic deposits. Although the thermodynamic stabilities of aqueous gold-complexes are fairly well known, it is not clear how gold solubility is influenced by reactions between the fluid and the wall rocks of the deposits. Parameters such as fluid fluxes, the duration of the mineralizing events, the role of kinetics and the evolution of permeability are unconstrained. In addition, the relative influences of processes that compete with rock-water reactions, such as the thermal evolution of the hydrothermal system or phase separation (boiling) have not yet been evaluated. In the proposed project we intend to simulate numerically the rock-water interaction in a specific, well-characterized case study at the Fenilia Mine, Brusson, NW Italian Alps. The distribution of gold in the Fenilia mine shows a strong, simple dependency on wall-rock lithology and the abundant fluid inclusions in the vein quartz allow the original composition of the gold-transporting fluid to be reconstructed. Existing data on the fluid inclusions will be expanded by laser-ablation-ICPMS analyses of individual inclusions. Detailed underground mapping and petrographic, mineralogical and porosity analyses will be performed to characterize the wall rocks. The recalculated rock and fluid properties will be used as input for the numerical simulations. By using quantitative, fully coupled reactive-transport models that involve fluid flow, mineral-fluid equilibria and kinetic laws, we intend to investigate the influence on gold precipitation of dynamic factors such as distal versus local buffering of chemical parameters, fluid flow velocities and fluxes, kinetics of rock-water reactions, thermal aspects of rock-water interaction, and the role of phase separation (boiling). The results will be published in journals that are accessible to both the scientific community and the mineral exploration industry.