Chemical mass transfer modelling of ore-forming hydrothermal systems: Current practise and problems

Chemical mass transfer modelling is a numerical approach to predicting the progress of multicomponent fluid-rock reactions in natural hydrothermal systems, based on thermodynamic data for minerals and fluid species. Quantitative simulations of hydrothermal wall-rock alteration and ore-mineral deposition can be used to develop and test geological models for ore formation, which are a basis for mineral exploration and resource assessment. In practice, the application of thermodynamic mass transfer modelling to specific ore-forming systems faces two main problems, even for the limiting assumption of local equilibration between fluids and minerals. The first problem relates to the geometry of fluid-rock interaction, where the scheme of the model must be adjusted to the geological question of interest, One-dimensional reactor models, defining reaction progress as a function of integrated fluid flux along a path through a chemically reactive rock, do not necessarily provide a more realistic description of chemical mass-transfer than simple 'titration' models without temporal and spatial dimension, because in most ore-depositing systems the fluids are partially channelized by structures like veins or faults. The efficiency of metal extraction at the ore deposition site is commonly determined by a sensitive balance between fluid focusing on one hand, and wall-rock reaction promoting ore-mineral precipitation on the other, An example of gold and scheelite mineralization associated with mesothermal quartz veins illustrates, how ore-deposition and alteration zoning parallel to the main fluid flow direction in partially channelized systems can be described by a two-dimensional 'multibox' model. The second problem relates to the extent and quality of thermodynamic data. Explicit consideration of Fe-bearing silicate solid solutions is essential for predicting realistic mass balance in complex fluid-silicate-sulphide reactions associated with hydrothermal ore formation. Although a high level of internal consistency of multicomponent data sets has been achieved by simultaneous fitting of multiple thermodynamic and equilibrium measurements, significant inconsistencies persist among the most comprehensive data compilations published for the three main groups of species relevant to hydrothermal ore deposits: aqueous species in high-temperature aqueous brines; silicate/oxide/carbonate minerals including Fe-Mg silicates; and sulphur-bearing minerals including sulphides and sulphates. Discrepancies between thermodynamic predictions and observed brine composition and mineral assemblages in the Salton Sea geothermal system suggest that the redox-scale for rock-forming silicates may be inconsistent with that of sulphides and aqueous S-C-O-H species, This analysis is preliminary but illustrates the importance of field-testing of thermodynamic data, particularly for hydrothermal conditions between 150 degrees and 450 degrees C, which lie between the high temperatures of reliable equilibrium experiments among rock-forming minerals (> 500 degrees C) and the low temperatures of many experiments defining the thermodynamics of aqueous species (mostly < 300 degrees C).