Metal complexation and ion association in hydrothermal fluids: insights from quantum chemistry and molecular dynamics

Abstract

Complexation by ligands in hydrothermal brines is a fundamental step in the transport of metals in the Earth's crust and the formation of ore deposits. Thermodynamic models of mineral solubility require an understanding of metal complexation as a function of pressure, temperature and composition. Over the past 40 years, mineral solubilities and complexation equilibria under hydrothermal conditions have been predicted by extrapolating thermodynamic quantities using equations of state based on the Born model of solvation. However, advances in theoretical algorithms and computational facilities mean that we can now explore hydrothermal fluids at the molecular level. Molecular or atomistic models of hydrothermal fluids avoid the approximations of the Born model and are necessary for any reliable prediction of metal complexation. First principles (quantum mechanical) calculations based on density functional theory can be easily used to predict the structures and relative energies of metal complexes in the ideal gas phase. However, calculations of metal complexation in condensed fluids as a function of temperature and pressure require sampling the configuration degrees of freedom using molecular dynamics (MD). Simulations of dilute solutions require very large systems (thousands of atoms) and very long simulation times; such calculations are only practical by treating the interatomic interactions using classical two-or threebody interatomic potentials. Although such calculations provide some fundamental insights into the nature of crustal fluids, simple two-or three-body classical potentials appear to be inadequate for reliably predicting metal complexation, especially in covalent systems such as Sn 2+ , Au 3+ and Cu + . Ab initio MD (i.e. where the bonding is treated quantum mechanically, but the molecular motions are treated classically) avoids the use of interatomic potentials. These calculations are practical for systems with hundreds of atoms over short times (<10 psec) but enable us to predict complexation as a function of pressure, temperature and composition. In this paper, I provide an introductory outline of the computational methods and illustrations of their application to NaCl brines and the complexation of Cu, Au, Sn and Zn.

The dissociation mechanism and thermodynamic properties of HCl(aq) in hydrothermal fluids (to 700 °C, 60 kbar) by ab initio molecular dynamics simulations

“…Murakhtina et al (2006) used density functional theory (DFT) based ab initio MD to calculate the H-NMR chemical shift of HCl(aq), and showed good agreement with experimental results. Sulpizi and Sprik (2008;2010) applied ab initio thermodynamic integration by constraining a dummy proton to calculate the free energy of dissociation of a series of acids ('vertical energy gap' technique); they obtained excellent agreement for logKd (57 ºC) values to within 0.4 log units for several small acid molecules (HCl, H2S, formic acid), and within 2 log units for the wide range of acids investigated. They obtained logKd values of 6.7~7.1 for HCl(aq) at 57 ºC (330 K), in excellent agreement with the IUPAC value of 7 at 25 °C (Perrin, 1982).…”

Evaluating the Changes from Endogranitic Magmatic to Magmatic-Hydrothermal Mineralization: The Zaaiplaats Tin Granites, Bushveld Igneous Complex, South Africa

“…However, the anomalous Cu enrichment within the Lease granite may be related to the exsolution of an aqueous fluid phase. The mobility and concentration of Cu in hydrothermal fluids is largely a function of the salinity of the aqueous fluid because the solubility of a likely CuCl 2 complex is proportional to the Clconcentration of the fluid [80,81]. As the granites of the Zaaiplaats tin field are thought to have intruded at approximately 4 to 5 km depth, it is very likely that the magmas underwent fluid saturation as evident in the abundant manifestations of magmatic-hydrothermal alteration and mineralization described herein [15,28,82].…”

“…The decreasing volume, caused by crystallization, may have led to shrinking of the volume occupied by the magma and partial detachment from the roof, allowing the formation of the marginal pegmatitic zone. The abundance of hematized feldspars, suggests a relatively oxidizing environment, which could have been achieved by fluid mixing or boiling [81,84,85]. Pollard et al [28] showed that these granites evolved in a closed system with little to no external influence, thus oxidation via boiling is the most likely mechanism.…”

“…The solubility of metals improves with increasing volatile concentration and acidity, facilitating their transportation within these magmatic-hydrothermal fluids [81,[86][87][88][89]. Thus, internally derived magmatic fluid, produced by the crystallizing Bobbejaankop granite, would have become enriched in various metals, such as Fe, Al, and Sn.…”

Yttrium speciation in subduction zone fluids from &lt;i&gt;ab initio&lt;/i&gt; molecular dynamics simulations

“…Entropy is an additional driving force for the ion association due to changes in the local solvent structure near the critical point and above as discussed, e.g., by Sherman (2010) and Mei et al (2014) based on AIMD simulations. The dominant effect arises from the translational entropy through hydration changes of the ions during the aqueous reaction.…”