Abstract 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 T h(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 T h(total) is also documented. Microchemical data show that the ore fluid in the five deposits is chemically uniform and made of Na, subordinate K 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 km 3 is sufficient to generate a world-class orogenic deposit.

Abstract Hydrothermal ore deposits that are exploited for gold include both gold-only deposits, such as orogenic deposits, and gold-bearing examples of the common hydrothermal deposit types that are formed around upper-crustal magmatic centres, in particular porphyry and epithermal deposits. Fluid inclusion data have shown that ore fluids of gold-only deposits are compositionally distinct compared to fluids of other deposit types: Fluids of the latter deposit types are, in contrast, not distinctive, and recent research has been focussed on relating gold content to geochemical and petrological aspects of related magmatic systems and to fluid evolution on migration through the crust. This Special Publication includes an up-to-date summary of thermodynamic parameters of aqueous Au species at high temperatures and pressures, a dataset of fluid inclusion properties and compositions from orogenic deposits of different parts of the world, several comprehensive case studies of different types of gold deposits and their fluids from the USA, Brazil, Egypt, Slovakia and Bulgaria and numerical modelling aimed to define key parameters that affect fluid flow and gold deposition at a range of scales.