Hydrothermal studies on mineral replacement reactions in the gold-silver-tellurium and copper-iron-sulfur systems.

Abstract

Over the past decades, a number of hydrothermal studies were undertaken on the mineral replacement reactions using hydrothermal method, which mostly proceeded via a coupled dissolution-reprecipitation (CDR) mechanism. However, most of experimental studies have been focused on mineral replacement reactions at relative low pressures and at low to medium temperatures. For hydrothermal mineral deposits set at higher temperature, such as porphyry copper systems or intrusion-related gold deposits, solid-state diffusion may be significant due to the high mobility of the metal ions, and solid-state reactions may compete kinetically with the CDR mechanism. Thus, to investigate the possible interaction between CDR reactions and solid state reactions, we designed a set of hydrothermal studies into the mineral replacement reactions in both Au-Ag-Te and Cu-Fe-S systems. The mineral replacement of sylvanite was studied under hydrothermal conditions, exploring the effects of temperature (160-220 °C), pH (2-10), and redox conditions on the sample textures and reaction kinetics. Sylvanite transformed to Au-Ag alloy and a range of other gold-(silver)-telluride phases as intermediate products, including petzite (Ag₃AuTe₂), hessite (Ag₂Te), an Ag-rich-Te-depleted calaverite-I (Au₀.₇₈Ag₀.₂₂Te₁.₇₄) and a normal calaverite-II (Au₀.₉₃Ag₀.₀₇)Te₂. The textures of products are very complex due to the interplay between solution-driven interfaces coupled dissolution-reprecipitation (ICDR) reactions and solid-state diffusion driven processes. The complex interaction among solid-state diffusion and ICDR reactions under hydrothermal conditions is due to the high solid-state mobility of Ag ion in the Au-Ag-Te system. The hydrothermal synthesis of chalcopyrite was performed via the sulfidation of hematite in solutions containing Cu(I) (as a chloride complex) and hydrosulfide, at pH near the pKa of H2S(aq) under hydrothermal conditions. Due to the large positive volume increase, the sulfidation of hematite by chalcopyrite follows a dissolution reprecipitation mechanism progressing via both direct replacement and also overgrowth. Distinct from other solution mediated ICDR reactions (e.g. the transformation from pentlandite to violarite) (Xia et al. 2009), no distinct porosity structures were observed in the quenched product grains. This is probably due, at least in part to the large volume increase during the reactions. This work investigated the nature of CDR reaction with large volume increase at relative high temperatures and high pressures, and improved our understanding of the physical chemistry of chalcopyrite formation in nature. To explain the transformation mechanism of chalcopyrite and bornite intergrowths, we reported the replacement of chalcopyrite by bornite in solutions containing Cu(I) (as a chloride complex) and hydrosulfide over the temperature range 200-300 °C. Results show that chalcopyrite was replaced by bornite under all studied conditions. The reaction proceeds via a CDR reaction mechanism and with some additional overgrowth of bornite. The bornite product formed at 300 °C for 24 hrs is Cu-rich corresponding to compositions in the bornite-digenite solid solution (bdss) Bn₉₀Dg₁₀, which can exsolve into digenite lamella in a bornite host during the further annealing in the original solution at 150 ˚C and 200 ˚C for 24 to 120 hrs. The exsolution of bdss is another example of solid-state diffusion under hydrothermal conditions.