Numerical modelling of fluid mixings in the H2O–NaCl system application to the North Caramal U prospect (Australia)

Fluid mixing is an important process in many geological environments where fluid circulation is active. Mixing may result in mineral precipitation, change of fluid composition and temperature. Fluid mixing has been often invoked for gas-poor fluids from microthermometric data plotted using Th–Tmice diagrams. The aim of this work is to generate theoretical Th–Tmice diagrams for different kinds of isobaric fluid mixing in the system H2O–NaCl. Three kinds of fluid mixing are considered: (1) isothermal mixing whereby the temperature is controlled by the host rock; (2) adiabatic mixing whereby the temperature of the mixed fluid before trapping results only from the enthalpy of the two fluid end-members and their proportion in the mixing process; (3) mixing of fluids with partial heat exchange with the host rock before trapping inside the inclusions. Calculations were carried out with different combinations of input data (salinity and composition of fluid end-members, temperature of the fluid end-members and rock reservoir and degree of heat exchange between the mixed fluid and the rock reservoir before trapping). These different kinds of fluid mixing give several geometrical distributions of microthermometric data in Th–Tmice diagrams and are used as an aid for interpreting the data. A natural case of adiabatic fluid mixing in the North Caramal U showing (Australia) is presented. Modifications of the diagrams resulting from pressure changes are discussed.