Numerical simulations of geochemical processes can help in the design of exploration programs for a variety of geological environments. Chemical modelling can be used to predict (i) the maximum metal-carrying capacity of hydrothermal fluids, (ii) the alteration and vein assemblages that will form during fluid flow along faults and aquifers, and (iii) the likely sequence of mineral deposition and relative abundance of mineral phases precipitated in the trap environment. We have numerically modelled the transport and deposition of 20 chemical components in 4 high-temperature (250°C) metalliferous brines (10 wt% eq. NaCl), focusing on processes that may occur in sedimentary basins. The results show that sign ificant amounts of base metals (> 1 ppm) can be transported in acidic oxidised, alkaline oxidised, and acidic reduced brines. Alkaline, reduced brines are predicted to be incapable of carrying sufficient quantities of base metals to be important in the formation of sediment-hosted base-metal deposits. Hematite-bearing quartz sandstones can be unreactive aquifers for oxidised metalliferous brines, allowing metals to remain in solution during transport. Potassic and/or propylitic alteration assemblages are predicted to form when the brines interact with mafic volcanics. High-temperature mineralised brines can potentially migrate long distances (> 1 km) along faults without dumping their base-metal load, provided the fluids remain chemically isolated from the surrounding wall rocks (e.g. silica mantling). Cooling and/or boiling of metal-rich brines during cross-stratal transport may result in the precipitation of barren quartz veins with traces of hematite or pyrite, depending on the oxidation state of the initial fluid. For the trap environment, mixing of metalliferous brines with anoxic seawater is predicted to be efficient at precipitating Zn-Pb-rich massive sulphide mineralisation. Mixing of the same brines with oxidised seawater can result in the formation of siliceous Cu-Ag-rich exhalites. Replacement-style Zn-Pb-rich sulphide mineralisation is predicted to precipitate when reduced acidic brines interact with pyritic dolomitic sediments. Complex alteration assemblages are predicted to form concurrently with base-metal sulphide deposition when the replacement process involves oxidised brines. Our chemical modelling technique allows rapid testing of various hypotheses about trap sites and depositional mechanisms. It is probably most useful in the project generation stage of a regional exploration program.
