An advanced reactive transport simulation scheme for hydrothermal systems modelling

Abstract

Abstract Advanced reactive transport simulations of hydrothermal systems require the modelling of diverse physico-chemical phenomena, many of which have only partially been addressed in previous studies. Utilizing newly implemented capabilities of the CSMP++GEM code, we show the relevance of three phenomena: (1) At the example of alkali feldspars, we investigate how employing non-ideal solid solution models instead of pure mineral phases has a significant impact on the resulting chemical evolution of the system. For example, pH when considering solid solution differs by about 0.5 compared to a simulation with pure feldspar phases. Differences in aqueous K/Na ratios may be relevant for interpretation of the K/Na geothermometer and may, in addition, be biased by discrepancies in published thermodynamic databases as observed in this study. (2) Reactive transport at supercritical conditions (within the limits imposed by the HKF thermodynamic model) is illustrated by how a magmatic-hydrothermal fluid, equilibrated with granitic rock at 600\u202f°C and 200\u202fMPa, is reacted along a diffusion path with a similar rock at 400\u202f°C and 50\u202fMPa. This simple conceptual representation of hydrothermal alteration at the Butte porphyry copper deposit in Montana results in a reactive transport model that closely mimics the observed alteration zonation. (3) Emergent behaviour is observed for a simulation of two-phase vapour\u202f+\u202fliquid flow with partitioning of chemical species between the two phases. In this example, hot vapour front condenses into and displaces liquid, with a two-phase boiling/condensation zone developing in between. At the boundaries between steam and boiling zone, and between boiling zone and liquid zone, the interplay of partitioning of volatile species and different transport properties in the single- and two-phase regions leads to distinct local and transient effects: volatile species such as CO2, H2S and HCl may accumulate or deplete at strongly varying rates, affecting pH, redox state and activity of sulfide. The correct simulation of these effects will be relevant in understanding hydrothermal processes such as ore formation, mineral alteration, and the evolution of high enthalpy geothermal areas.