Geology and Geochemistry of Selected Gold Deposits in the Ailaoshan Metallogenic Belt, China: Origin of Ore-Forming Fluids

The formation of the Ailaoshan metallogenic belt was the result of: the Neoproterozoic super mantle plume, the Indosinian and South China blocks in the Late Triassic after the Paleo-Tethys Ocean closure, and Oligocene-Eocene continental-scale shearing related to the India-Eurasia collision. It is one of the most important Cenozoic gold ore province in the world. In this paper, the geological characteristics, isotopic geochemistry, and geochemical data of ore-forming fluids of four large-scale gold deposits in the Ailaoshan metallogenic belt (Mojiang Jinchang, Zhenyuan Laowangzhai, Yuanyang Daping, and Jinping Chang’an) are comprehensively compared. The features of host-rock alteration, metallogenetic periods and stages, geochronology, fluid inclusion, and C-H-O-S-Pb isotopes of gold deposits are summarized and analyzed. The gold mineralization in the Ailaoshan metallogenic belt occurred mostly in 50–30 Ma, belonging to the Himalayan period. The gold mineralization is closely related to silicification, argillation, carbonation, and pyritization due to the strong mineralization of hydrothermal fluid, the development of alteration products, and the inconspicuous spatial zonation of alteration types. The ore-forming fluid is mainly composed of mantle fluid (magmatic water) and metamorphic fluid (metamorphic water). The ore-forming materials of the Jinchang, Chang’an, and Laowangzhai gold deposits mainly originate the host-rock strata of the mining area, and the carbon is more likely to from marine carbonate. The carbon in the Daping gold deposit from the original magma formed by the partial melting of the mantle. Pb isotopes have characteristics of crustal origin, accompanied by mixing of mantle-derived materials and multisource sulfur mixing, and are strongly homogenized.