CHEMICAL MASS-TRANSFER MODELING OF ORE-FORMING HYDROTHERMAL SYSTEMS -CURRENT PRACTICE AND PROBLEMS

Chemical mass transfer modelling is a numerical approach to predicting the progress of multicomponent fluid-rock reactions in natural hydrot hermal systems, based on thermodynamic data for minerals and fluid spe cies. Quantitative simulations of hydrothermal wall-rock alteration an d ore-mineral deposition can be used to develop and test geological mo dels for ore formation, which are a basis for mineral exploration and resource assessment. In practice, the application of thermodynamic mas s transfer modelling to specific ore-forming systems faces two main pr oblems, even for the limiting assumption of local equilibration betwee n fluids and minerals. The first problem relates to the geometry of fl uid-rock interaction, where the scheme of the model must be adjusted t o the geological question of interest, One-dimensional reactor models, defining reaction progress as a function of integrated fluid flux alo ng a path through a chemically reactive rock, do not necessarily provi de 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 st ructures like veins or faults. The efficiency of metal extraction at t he ore deposition site is commonly determined by a sensitive balance b etween fluid focusing on one hand, and wall-rock reaction promoting or e-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 fl ow direction in partially channelized systems can be described by a tw o-dimensional 'multibox' model. The second problem relates to the exte nt and quality of thermodynamic data. Explicit consideration of Fe-bea ring silicate solid solutions is essential for predicting realistic ma ss balance in complex fluid-silicate-sulphide reactions associated wit h hydrothermal ore formation. Although a high level of internal consis tency of multicomponent data sets has been achieved by simultaneous fi tting of multiple thermodynamic and equilibrium measurements, signific ant inconsistencies persist among the most comprehensive data compilat ions published for the three main groups of species relevant to hydrot hermal ore deposits: aqueous species in high-temperature aqueous brine s; silicate/oxide/carbonate minerals including Fe-Mg silicates; and su lphur-bearing minerals including sulphides and sulphates. Discrepancie s 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 th at of sulphides and aqueous S-C-O-H species, This analysis is prelimin ary but illustrates the importance of field-testing of thermodynamic d ata, particularly for hydrothermal conditions between 150 degrees and 450 degrees C, which lie between the high temperatures of reliable equ ilibrium 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).