Thomas Driesner

Magmatic vapor contraction and the transport of gold from the porphyry environment to epithermal ore deposits

Fluid-phase stability relations combined with thermodynamic modeling using fluid-inclusion analyses and new gold-solubility experiments lead to an integrated geological interpretation linking epithermal gold mineralization and porphyry-style ore formation to the cooling of hydrous magma chambers. The essential chemical requirement for gold transport to low temperatures is an initial excess of sulfide over Fe in the magmatic fluid, which is best achieved by condensing out Fe-rich brine from a buoyant, low- to medium-salinity vapor enriched in volatile S. This vapor can contract directly to an aqueous liquid, by cooling at elevated pressure above the critical curve of the salt-water fluid system. Physical and chemical conditions are matched when magmatic fluid is released through a gradually downward-retracting interface of crystallizing magma beneath a porphyry stock, predicting the consistent zoning and overprinting relations of alteration and mineralization observed in magmatic hydrothermal systems.

Hydrothermal solution structure: Experiments and computer simulations

Publisher Summary Insight into the structure of water and aqueous salt solutions has accumulated slowly from X-ray and neutron diffraction studies, and from the application of various other spectroscopic techniques. X-ray diffraction studies have provided a considerable knowledgebase relating to the hydration of ions at ambient conditions, but many of the data from different laboratories are in poor agreement and many simple systems have still not been adequately studied. The dynamical aspects of ion hydration in water is extensively studied using NMR and more recently, through incoherent quasi-elastic neutron scattering. However, there are few structural data pertaining to the interaction of ionic and molecular species with solvent water at elevated temperatures and pressures. Water is the archetype protic solvent and its fundamental properties such as density, viscosity and dielectric permittivity change dramatically with temperature and pressure. The configurational and dynamical aspects of ion hydration play an important role in homogeneous and heterogeneous equilibrium, and kinetics in aqueous systems. Changes in ion hydration environments with increasing temperature and pressure give rise to changes in water activity, which will, in turn, influence mineral solubilities under extreme conditions. Structural changes in bulk water with increasing temperature and pressure may also cause ion pairing as dielectric screening within the solvent changes. There are many other properties of electrolyte solution behavior at extreme conditions, an understanding of which is inherently based upon a knowledge of the structural features of ion hydration as well as the bulk solvent.

Hydrothermal solution structure

Publisher Summary Insight into the structure of water and aqueous salt solutions has accumulated slowly from X-ray and neutron diffraction studies, and from the application of various other spectroscopic techniques. X-ray diffraction studies have provided a considerable knowledgebase relating to the hydration of ions at ambient conditions, but many of the data from different laboratories are in poor agreement and many simple systems have still not been adequately studied. The dynamical aspects of ion hydration in water is extensively studied using NMR and more recently, through incoherent quasi-elastic neutron scattering. However, there are few structural data pertaining to the interaction of ionic and molecular species with solvent water at elevated temperatures and pressures. Water is the archetype protic solvent and its fundamental properties such as density, viscosity and dielectric permittivity change dramatically with temperature and pressure. The configurational and dynamical aspects of ion hydration play an important role in homogeneous and heterogeneous equilibrium, and kinetics in aqueous systems. Changes in ion hydration environments with increasing temperature and pressure give rise to changes in water activity, which will, in turn, influence mineral solubilities under extreme conditions. Structural changes in bulk water with increasing temperature and pressure may also cause ion pairing as dielectric screening within the solvent changes. There are many other properties of electrolyte solution behavior at extreme conditions, an understanding of which is inherently based upon a knowledge of the structural features of ion hydration as well as the bulk solvent.