Experimental study on fluid migration mechanism related to Pb–Zn super-enrichment: Implications for mineralisation mechanisms of the Pb–Zn deposits in the Sichuan–Yunnan–Guizhou, SW China

Abstract

This study focuses on the problem of Pb–Zn super-enrichment, and is based on a new conceptualisation of fluid–rock interactions → change in fluid properties → Pb–Zn hydrolysis → complex instability. Non-magmatic, epigenetic, hydrothermal Pb–Zn deposits in carbonates were used to perform Pb–Zn hydrolysis experiments under normal pressure. Lead–zinc fluid and carbonate interaction experiments, as well as Pb–Zn hydrolysis kinetics experiments during fluid–rock interaction, were performed analyse the roles of pH and fluid–rock interactions in Pb–Zn transport and precipitation, and to investigate the main reasons for the grade disparity between Mississippi Valley-type (MVT) deposits and Pb–Zn deposits found in the Sichuan–Yunnan–Guizhou region of China. Our results show that a pH = 4 is the threshold value for the large-scale hydrolysis of Pb and Zn. When reactions between acidic solutions containing Pb and Zn (pH = 1.0–6.0) and carbonate rocks (i.e., the surrounding rock) reach equilibrium, the final pH of the solution stabilises at 5.2–6.2, which is not associated with the lithology of the surrounding carbonate rocks (dolomite or limestone), crystallite size or the initial pH of the solution, and is negatively correlated with rock particle size. Fluid–rock interactions cause pH to increase, Pb and Zn to partially hydrolysis, and minerals to precipitate, with the maximum precipitation rate reaching ~20%. When hydroxide precipitates are mixed with fluids containing reduced S, the precipitates are converted into sulphides. Thus, mineralisation fluid pH, the driving force of fluid migration, Pb–Zn hydrolysis, and fluid–rock interactions combine to restrict the formation of Pb–Zn deposits in carbonates. Among these factors, differences in Pb–Zn hydrolysis are the main causes of disparities among deposit grades. This study not only provides an in-depth consideration of the geochemical behaviour of Pb and Zn in a carbonate-hosted hydrothermal system, but also serves to clarify the mineralisation mechanisms of Pb–Zn deposits in the Sichuan–Yunnan–Guizhou region.