Physicochemical modeling of the coupled behavior of gold and silver in hydrothermal processes: Gold fineness, Au/Ag ratios and their possible implications

An estimation of gold fineness deposited from aqueous sulphide–chloride solutions in pyritic ore with varying (Au/Ag)ore has been obtained using thermodynamic modeling (at T = 200–500 °C and P = 1000 bar). Silver sulphide with Au–Ag solid solution or Au–Ag solid solution are only stable under conditions of the Py–Po(–Mt) and Hem–Py (–Mt) buffers depending on Au and Ag concentrations in the ore-forming system. The predominant Au and Ag species in aqueous sulphide–chloride solutions depend on temperature, pH and chloride concentration and these solutions can be grouped as follows:Type I: high chloride (mHCl > 0.0l, mNaCl = 1 to 5), high temperature (300–500 °C) solutions in which AgCl2− and AuCl2− are dominant; I: acidic, near-neutral, medium chloride (mNaCl ≈ 1), moderate to high temperature (200–400 °С) solutions in which AgCl2− and AuHS0 are dominant; II: eutral, high chloride (mNaCl = 1 to 5), high temperature (300–500 °С) solutions in which AgCl2− and Au(HS)2− are dominant; V: ne, variable chloride (mNaOH > 0.01, mNaCl = 0 to 5), moderate to high temperature (200–500 °C) solutions in which Au(HS)2− and Ag(HS)2− are dominant; and : o near-neutral, low chloride (mNaCl < 1), low to moderate temperature (< 300 °C) solutions in which AuHS0 and AgHS0 are dominant. id and near-neutral high- and medium chloride solutions (types I–III) are distinguished by high Ag-carrying capacity and low values of (Au/Ag)aq ≈ 10− 7 to 10− 2. The highest Au-carrying capacity and high (Au/Ag)aq ratios (≈ 10− 3 to 1) are typical of alkaline solutions of any chloride concentrations (type IV) and low chloride solutions of variable acidity (type V). n the physicochemical conditions of Au–Ag mineralization and mineralogical–geochemical characteristics of pyritic ores of hydrothermal gold deposits of different types are summarized. Wide ranges of gold fineness and low (Au/Ag)ore ratios of Au–Ag epithermal, Au–Cu-porphyry and Au–Cu-skarn deposits are related to the participation of acid or near-neutral high- or medium chloride ore-forming solutions (types I–III). High fineness gold and high values of (Au/Ag)ore ratio of Au–Te, Au–As, Au–Sb, Au orogenic and Carlin-type deposits are explained by participation of alkaline (type IV) or low chloride (type V) sulphide solutions. Gold fineness and (Au/Ag)ore ratios in the sulphide ores may be characteristics that determine the type of ore solution and its evolution in the process of hydrothermal deposit formation.