Physical-chemical properties and metal budget of Au-transporting hydrothermal fluids in orogenic deposits

We use a set of analytical techniques for fluid inclusion analysis to determine the bulk properties of the ore fluids from five orogenic gold deposits. The most common ore fluid is a low salinity (0.4-6.5 wt% NaCl) two- to three-phase aqueous-carbonic fluid. Its Th(total) range is 200-400 °C, and total homogenization occurs by bubble or liquid disappearance. A less common aqueous fluid of similar bulk salinity and Th(total) is also documented. Microchemical data show that the ore fluid in the five deposits is chemically uniform and made of Na, subordinateK and B and minor to trace amounts of Cu, As, Li, Sr, Rb, Ba, Cs, Sb and Au (range 0.5-5 μg/g). Thermodynamic modelling carried out for one of the studied deposits indicates that the ore fluid was in equilibrium with vein minerals at the time of gold deposition, and that its vapour phase was distinctly enriched in Au, B, As and Sb. The proposed mechanism of gold precipitation is a combination of fluid decompression and boiling. Combining these results with 3D geometrical reconstructions of one of the studied deposits (Sigma), we estimate that a minimum ore fluid volume of c. 0.1-1.0 km³ is sufficient to generate a world-class orogenic deposit.