Fundamental Research (Jul 2023)
Fluid-rock interactions leading to the formation of the epithermal Ag-Pb-Zn veins: A perspective of thermodynamic modeling
Abstract
Although vein-type silver-lead-zinc ore deposits have been extensively studied, the factors controlling their formation are still poorly understood and their genesis is a matter of ongoing debate. In this contribution, I present new mineralogical data and the results of thermodynamic modeling that constrain the conditions of metal transport and deposition for the Aerhada epithermal Pb-Zn-Ag deposit (reserves of >1,000 t Ag@58 g/t and 1.0 Mt [email protected]%) in NE China. Three primary paragenetic stages have been identified, the second of which (Stage II) is the main base metal and silver mineralization. Freibergite, argentite, pyrargyrite, and canfieldite are the main Ag-bearing minerals and are spatially associated with an alteration assemblage of quartz-muscovite ± chlorite ± epidote. Dissolution textures and evidence of compositional heterogeneity for freibergite suggest that its decomposition may have redistributed the Ag and contributed in part to the high Ag grade ores in the deposit. Thermodynamic calculations indicate that there was extensive silver ore deposition from a strongly reducing (e.g., ∆log fO2 (HM) of <-8.6 to -2.4) and nearly neutral to weakly alkaline (e.g., pH of 5.5 to 6.8) aqueous fluid at temperatures between 220 °C and 170 °C. These calculations reveal that a reduction in fO2 and decreasing temperature, both as a result of fluid-rock interactions, were the key factors leading to silver and base metal mineral deposition. Further path modeling showed that the sole evolution of a magmatic-derived fluid is capable of forming the large Ag-Pb-Zn veins via fluid-rock interactions, which is contrary to the conclusions of some other studies that the mixture of an externally derived fluid is required to explain their formation. The genetic model for Ag-Pb-Zn ore formation developed in this study is applicable to other polymetallic vein-type deposits in comparable geological settings elsewhere.
