Multi-dimensional Numerical Simulation of Hydrothermal Fluid Flow and Mass Transport with Application to the Formation of Mineral Deposits

Numerical modeling has proven an efficient predictive tool for testing, comparing and contrasting different geological hypotheses. Computer models simulating fluid flow and mass transport in complex hydrothermal systems can provide considerable insight into how these systems operate to produce economic concentration of metals. This paper presents a finite element algorithm that fully couples transient, multi-dimensional fluid flow, heat and mass (solute) transport in discretely fractured porous media. The numerical method employs non-orthogonal quadrilateral meshes and their geometry, size and orientation can be adjusted freely to best fit complex earth structures in reality, such as uneven surface topography, arbitrarily-shaped geological units, and free-oriented fractures and faults. The McArthur basin hosting the HYC deposit in northern Australia is used as a field example. Its salinity conditions are first considered, followed by other scenarios simulating the Irish-type and the US gulf coast-type salinity conditions. Numerical results indicate that salinity plays an important role in controlling hydrothermal ore-forming fluid migration. High salinity at basin floor (evaporitic conditions) strengthens the thermally-induced buoyancy force and hence promotes free convection of basinal solutions; whereas high salinity at bottom (sedimentary brines) counteracts the thermal function and thus suppresses the development of hydrothermal fluid circulation. Numerical experiments also identify the favorable hydrological conditions for the formation of the HYC deposit and evaluate the similarities and differences of modeling results between two-dimensional and three-dimensional simulations.