Hydrothermal properties of complex aqueous solutions, and applications to ore-forming systems

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Geologic fluids control numerous processes within the Earth driving chemical reactions, transporting heat and material, and promoting melting and magmatism. Modern analytical methods reveal that these fluids are commonly complex brines with high concentrations of diverse solutes. In ore-forming settings in particular, complex brines dissolve and transport elements to form mineral deposits. However, geochemical processes involving complex, saline fluids are mostly interpreted qualitatively owing to the lack of quantitative models for the properties of such fluids. Thus, there is a critical need to develop models to predict and model quantitatively the physical and chemical properties of complex brines.The objective of this work is firstly to develop new thermodynamic models to characterize the compositions and mineral solubilities of complex, geologic brines based on data obtained from fluid inclusions in minerals tiny droplets of fluids trapped within minerals as they grow and secondly to apply these methods to analyze fluids from two major ore-forming settings. This work will involve new thermodynamic models for brines which will be built into computer programs. This work will also involve field- and laboratory-based studies of fluids degassed from magmas in the deep roots of porphyry systems, and of saline brines derived from seawater evaporation in the context of base-metal deposits.This work will provide researchers with the tools to explore the compositions of geologic fluids in diverse settings, and to relate fluid composition to hydrothermal alteration and mineral precipitation. The field-based portions of the study will address two major questions: How fluids evolve during the waning stages of magma crystallization, and how variations of seawater composition through time affect compositions and ore-forming potentials of evaporite-derived brines.The work described here is novel because at present, fluid properties are still mostly interpreted using approximations based on H2O-NaCl. This work will represent a major step forward in allowing robust characterization of fluids. Computer programs implementing these models will facilitate applications of these new methods to illuminating the properties of real fluids in many settings. The applications of these new approaches to characterizing magmatic and basinal fluids will provide new insights into the physical and chemical processes at work in these settings. These outcomes will be beneficial to Canada because hydrothermal fluids are crucial to the formation of economic mineral deposits. This research will provide new insights into the nature of these fluids and how they react with rocks to form mineral deposits. Thus, broadly, this research will improve our understanding of the fundamental controls on ore formation.