A review of the coordination chemistry of hydrothermal systems, or do coordination changes make ore deposits?
Abstract The hydration and complexation of metals in hydrothermal fluids are key processes controlling the mobility of elements in the Earth's crust, leading to the formation of ore deposits from which the World's supply of Fe, Mn, Ag, Au, Pd, Cu, Zn, Co, Pb, U, and Mo is mined. In the past 20 years a large amount of in situ spectroscopic data, complemented by increasingly accurate ab initio molecular dynamic simulations, have dramatically improved our understanding of the nature and geometry of the metal complexes that are responsible for metal transport in the upper crust. This new information underpins a "Coordination Chemistry" approach to ore transport and deposition. In order to highlight the unifying principles brought about by the concepts of coordination chemistry, we present a periodic table of metal coordination chemistry in hydrothermal fluids based on a review of the literature and new XAS data on the hydration of the uranyl ion in hydrothermal fluids, and the pressure dependence of Ni(II) and Zn(II) complexing. The different coordination geometries of metal complexes control some of the first order behaviours of these metals in hydrothermal systems. In particular, (i) complexes with low coordination number and open structures (i.e. linear and trigonal pyramidal) have an enhanced affinity for low density, vapour like fluids, relative for example to tetrahedral and octahedral complexes; and (ii) fractionation and changes in solubility are associated with changes in coordination geometry caused by changes in pressure, temperature, and ligand availability. For example, first row divalent transition metals as well as Cd(II) all occur as chloride poor, octahedral complexes at low T, low salinity, and tetrahedral like chloride richer complexes at high T, high salinity in chloride brines. However, the octahedral tetrahedral transition occurs at different conditions for different metals, and this can drive fractionation between geochemical pairs such as Zn/Cd, Fe/Mn and Co/Ni. This review highlights the central role of entropy in driving the formation of metal complexes and changes in coordination geometry as a function of temperature and, to a lesser extent, pressure. Changes in coordination geometry are associated with large changes in entropy. Hence, coordination changes usually occur rapidly as a function of temperature (few 10's of degC) and can result in rapid changes in mineral solubility. These effects need to be taken into account when extrapolating the thermodynamic properties of metal complexes to high temperature and high pressure. A molecular level understanding of metal speciation hence underpins the development of more accurate models of reactive transport over wide ranges in pressure, temperature, fluid composition and physical states.