Reactive transport numerical modeling of intermediate sulfidation epithermal deposit: A case study of Haopinggou Ag-Au-Pb-Zn deposit, Henan province, China

The Haopinggou Ag-Au-Pb-Zn deposit is the only deposit that simultaneously contains Au and Ag-Pb-Zn vein-type ores at the Xianyu ore field in Xiong'ershan District, Henan Province, China. The early-stage gold-bearing pyrite-quartz veins are cut or surrounded by late-stage silver-bearing Pb-Zn-sulfide veins. However, there is controversy whether these two-stage veins were formed from distinct fluid systems associated with discrete mineralization events or via hydrothermal evolution processes of individual mineralization events. To study the metallogenic dynamics of how the Au and Ag-Zn-Pb veins were formed at the same depth in the Haopinggou deposit under these two distinct metallogenic models, we established a series of reactive transport numerical models. We studied the influence of the temperature of the hydrothermal fluid, fault permeability, and HS⁻, Au⁺, and Ag⁺ concentrations on the mineralization of Au, Ag, Pb, and Zn. Based on the model results, two distinct mechanisms causing Au and Ag to precipitate at the same depth has been established: (1) Under the assumption of the single hydrothermal fluid metallogenic model, the deep part of the early Au precipitation will be overlapped by the shallow part of the late Ag precipitation due to temperature and permeability decreases, causing Au and Ag to precipitate at the same deep depth; (2) Under the assumption of the distinct hydrothermal fluids metallogenic model, the shallow part of the early Au precipitation will be overlapped by the late Ag precipitation due to high concentration of HS⁻, causing Au and Ag to precipitate at the same shallow depth. The metallogenic mechanisms behind these two controversial understandings indicate that the deeper parts of the Haopinggou deposit have a high metallogenic potential for gold or silver.