Precipitation Reaction Mechanisms of Mineral Deposits Simulated with a Fluid Mixing Model

Abstract

Nonmagmatic, carbonate-hosted epigenetic hydrothermal Pb–Zn deposits similar to those at the Huize Pb–Zn Mine are widespread across the Sichuan–Yunnan–Guizhou (SYG) polymetallic province. The precipitation mechanisms of these geologically intriguing deposits are an area of interest for many researchers. To simulate the underlying precipitation reaction mechanisms and dynamics of each aspect, a fluid mixing model for metal sulfide precipitation was used in a series of experiments, where solutions that contain Pb/Zn chloride complexes and sulfide were subjected to pH changes, water-rock reactions, and dilutions. Based on the results of these experiments, thermodynamic phase diagrams, and other experimental findings, a fluid mixing genetic model was developed for SYG Pb–Zn deposits, and this model was used to analyze the mechanisms of metal sulfide precipitation. The results indicate that acidic fluids in the form of chloride complexes transported Pb and Zn, whereas sulfide exists in the form of H2S within these fluids. The precipitation of metal sulfides occurs when these fluids undergo changes in pH, water-rock reactions, or isothermal dilution. The pH changes were found to be the most effective method for the induction of sulfide precipitation, followed by dilution and then water-rock reactions. The formation of sulfide precipitates due to pH changes, water-rock reactions, and dilution can be attributed to a single mechanism, i.e., changes in the pH of the fluid. Therefore, changes in pH are the primary mechanism of sulfide precipitation.

How carbonate dissolution facilitates sediment-hosted Zn-Pb mineralization

“…Ore-genesis models for Zn-Pb deposits in sedimentary carbonate rocks have been proposed for many decades (e.g., Jackson and Beales, 1967;Anderson, 1975, Anderson andGarven, 1987;Cooke et al, 2000) and have been used to explain field observations (e.g., Leach et al, 2005;Wilkinson, 2014) and as a basis for reactive transport modeling (e.g., Garven et al, 1999;Corbella et al, 2004); however, experimental studies have mainly focused on the solubility of Zn-Pb sulfide minerals and stability of aqueous Zn and Pb complexes (e.g., Ruaya and Seward, 1986;Bourcier and Barnes, 1987;Barrett and Anderson, 1988;Tagirov and Seward, 2010;Mei et al, 2015Mei et al, , 2016Etschmann et al, 2018Etschmann et al, , 2019Sanz-Robinson and Williams-Jones, 2019). Recently, room-temperature titration experiments by Zhang et al (2019aZhang et al ( , 2019b suggested that hydrolysis is the key factor for Zn-Pb sulfide precipitation, and Zhang et al (2021) emphasized that pH change is the primary mechanism for Zn-Pb sulfide deposition up to 150 °C; however, the role of carbonate replacement has not yet been closely examined experimentally.…”