New perspectives on sediment-hosted Pb-Zn-Ag-Cu polymetallic mineralisation in the Sanjiang tethys orogenic belt, China: a case study of the Malongchang deposit

The Malongchang Ag-Pb polymetallic deposit is located at the junction of the eastern part of the Tethys mega-tectonic domain and the southwestern margin of the Yangtze Craton. It is a medium-sized Ag-dominant deposit (300 tons of proven Ag metal reserves) associated with Pb, Cu, and Zn and has considerable exploration potential, but its theoretical research and level of exploration remain poorly constrained. Based on systematic in-situ trace element and S-Pb isotope analyses, the following conclusions were drawn: (1) The δ 34 S values range from − 8.44‰ to 3.51‰, with a mean of approximately − 2.47‰, indicating enrichment in light sulphur isotopes. The reduced S is of crustal origin and mainly derived from the thermochemical reduction of sulphate in basin brine. (2) Ore-forming metal materials are sourced primarily from the Kunyang Group folded basement and partially provided by fluids extracting ore-forming materials from the wall rock. (3) Ag occurs mainly in galena. Sphalerite is enriched in Cd and Hg and depleted in Mn, Fe, Co, In, and Sn, similar to the Mississippi Valley-type deposits. (4) The ore-forming formation, resulted from the sequential migration of two fluids and subsequent mixed-fluid precipitation. These findings provide theoretical support and new guidance for prospecting in the deeper and peripheral parts of the Malongchang deposit, and the larger Ag-Pb polymetallic metallogenic region.