Coupling deformation and fluid P-V-T-X conditions in orogenic gold deposits

Abstract

Deformation is intimately linked to fluid conditions in fault related ore systems such as orogenic gold deposits. Orogenic gold fluids are characterised by complex CO2-H2O-NaCl fluids with XCO2 values typically between 0.1 and 0.3 and salinities of 3 to 7 wt % NaCl equiv. The typical formation conditions for orogenic gold deposits lie between 1 and 2.5 kbars and 250 to 350 °C (the “orogenic gold window”). The location of CO2-H2O-NaCl solvi typically lies within or close to this orogenic gold window and therefore phase separation or fluid unmixing will be a common process in these deposits. However, the processes that control the fluid pathway from the one phase field to the two phase field may have critical impact on the subsequent behaviour of the fluid. Isothermal processes will result in fluid volume expansion whereas isobaric and intermediate paths along typical geothermal gradients may result in fluid volume decrease. The different P-V-T-X pathways followed will potentially lead to significantly different structural behaviour. An example from the world class Sunrise Dam orogenic gold deposit is used to illustrate different scenarios.