pH-logfo2 and pH-loga for Pb-Zn paragenesis and separation in the Zhaotong lead?zinc deposit

Abstract: Located at the southern center of the Sichuan-Yunnan-Guizhou lead-zinc polymentallic mineralization domain on the southwestern margin of the Yangtze block, the Zhaotong lead-zinc deposit in Yunnan Province is a typical representative in northeastern Yunnan or even in Sichuan-Yunnan-Guizhou area. The deposit is characterized by vertical zoning from coarsegrained pyrite, dark-color sphalerite + ferro-dolomite + quartz, rosy sphalerite + galena + pyrite to fine-grained pyrite from bottom to top of the orebody. These typical features are different from known lead-zinc deposit types discovered both in China and abroad, such as Mississipp Valley Type (MVT), Volcanic Hosted Massive Sulfide Type (VHMS), and Hydrothermal Sedimentation Type (SEDEX). The controlling of pH, logfo2, loga over the main metal sulfides and their assembly as well as mineral zoning will contribute to solving the formation mechanism of lead-zinc association and differentiation and the extreme enrichment of elements. Metallic minerals in the mineral zoning are the main study object, and pH-logfo2, pH-loga phase diagrams of the Zhaotong lead-zinc deposit at ore-forming temperatures (373 K, 423 K, 473 K, 523 K) were drawn. On the basis of these phase diagrams, the characteristics of the mineral zoning in the Zhaotong lead-zinc deposit that are different from features of known lead-zinc deposit types discovered in the world could be explained successfully. It is believed that the association of temperature, oxygen fugacity, component concentration and pH caused the lead-zinc association and differentiation and controlled the precipitation of main metal mineral assemblages and the zonation of mineral assemblages. When the temperature drops, the logfo2 value rises, pH goes up, and the sulfide will precipitate more easily.