Physicochemical conditions in controlling Au and Ag precipitation of ore-forming processes from the Yindongpo and Poshan Au-Ag-Pb-Zn deposits, Tongbai region: Constraints from chlorite mineralogy

• The chlorite formed within three stages for both deposits, and show distinct textures recording different fluid composition. • The calculations based on the composition of chlorite reveal similar temperature but certain difference in sulfur fugacity between both deposits. • Geological and geochemistry results show that sulfur fugacity difference and changes are key factors in controlling Au and Ag enrichment. The contemporary Yindongpo Au deposit and Poshan Ag deposit located in the Tongbai district of Henan Province, are large-scale and super-large hydrothermal vein-type deposit, respectively. Except for Au and Ag, both deposits contain economic Pb and Zn by-products. These two deposits are located 2.5 km away from each other and are both hosted in carbonaceous mica-quartz schist and biotite leptynite from the Neoproterozoic Waitoushan Formation. The Yindongpo Au and Poshan Ag deposits share similar ore-forming processes and both have three mineralization stages. To address the ore-forming process and the key factors in controlling the differential precipitation of Au and Ag, this study conducted detailed field geological investigations, as well as systematic texture and compositional studies of chlorite which was well-developed in all three stages, using Tescan Integrated Mineral Analyzer (TIMA), and electron probe microanalysis (EPMA). In the Yindongpo deposit, backscattered Electron (BSE) images show that Stage 1 chlorite has vermicular textures, and Stage 2 chlorite exhibits concentric-zoning textures, whereas, Stage 3 chlorite occurs as homogeneous texture. In the Poshan deposit, Stage 1 and Stage 3 chlorite have similar textures to those in the Yindongpo deposit, whereas, Stage 2 chlorite occurs as aggregates with homogeneous texture. EPMA data show that chlorite from all stages in both deposits are dominated by Fe-chlorite, suggesting relatively low oxygen fugacity of hydrothermal fluids. The substitution mechanism of chlorite is dominated by Al Ⅳ -Si 4+ replacement and Fe 2+ -Mg 2+ substitution, with no Al Ⅵ -(Fe 2+ +Mg 2+ ) substitution, implying that pH of ore-forming fluid was relatively acidic. The temperature, oxygen fugacity ( f O 2 ), and sulfur fugacity ( f S 2 ) of ore-forming fluid calculated using the chlorite geothermometer constraints the parameters of fluid: Yindongpo with temperatures of 240 to 300 °C, lg f O 2 of −46.57 to −36.98, and lg f S 2 of −3.08 to 8.22, and Poshan with temperatures of 240 to 280 °C, lg f O 2 of −46.74 to –32.94, and f S 2 of 0.63 to 5.87. Insignificant changes of temperature and oxygen fugacity but relatively significant changes in sulfur oxygen from early to late stages for both deposits indicating that variations of sulfur fugacity should be the main factor controlling the precipitation of Au and Ag in the Yindongpo and Poshan deposits.