Mineralogical and Geochemical Characteristics of Carbonates and Their Geological Significance to the Fuli Pb-Zn Deposit, Yunnan Province

Abstract

Mississippi Valley-Type (MVT) deposits are among the main types of Pb-Zn deposits that feature carbonate minerals as the main gangue minerals; their formation runs through the entire metallogenic process of MVT deposits. Therefore, carbonate minerals contain rich information on metallogenic fluid evolution and are thus important prospecting indicators. The Fuli Pb-Zn deposit in eastern Yunnan is located in the southeast of the Sichuan-Yunnan-Guizhou (SYG) Pb-Zn metallogenic province, which is the biggest producer of zinc and lead in China and contains more than 400 deposits and over 20 million tons of Pb + Zn reserves. The ore occurs in the interlayer fracture zone of Middle Permian Yangxin Formation Dolomite, and the orebody shape is generally stratiform. The main metal-bearing minerals of this deposit are sphalerite, galena, and pyrite; the gangue minerals mainly comprise dolomite and calcite. Three mineralized stages are observed (the early metallogenic period, the main metallogenic period, and the late metallogenic period) according to the characteristics of stratigraphic output, the intercalated contact relationship of gangue minerals, and the alteration characteristics of the wall rock. To determine the source and properties of the ore-forming fluid and the ore-forming process of the Fuli Pb-Zn deposit, different stages of mineralogy and trace element geochemical characteristics of hydrothermal dolomite were systematically studied. The minerals were observed under microscope and subjected to in situ analysis by LA-ICP-MS and C–O isotope test. The δ18OSMOW value of the dolomite in the metallogenic period was between 13.29‰ and 20.55‰, and the δ13CPDB value was between −4.13‰ and 3.5‰. Dolomite of the metallogenic period mainly came from the dissolution of carbonate wall rocks, while C in the ore-forming fluid came from the wall rocks. A few dolomites showed a trend of depleting δ13CPDB and δ18OSMOW at the same time, implying the influence of sedimentary rock contamination in the mantle multiphase system. The lower δ18O was due to the exchange of O isotopes between the wall rocks and the depleted δ18O in ore-forming fluids. From the early to the later stage of mineralization, the ore-forming fluid changed from alkaline to neutral to weakly acidic due to a decrease in the oxygen fugacity and temperature of the fluid; this change resulted in the precipitation of sulfide and dolomite in the deposit. From the early to the late stages of mineralization, Fe and Mn showed a downward trend. Fe and Mn entered the alkaline environment of the carbonate minerals, while Fe and Mn were released into the acidic fluid, indicating that due to the metasomatism from strong to weak, their metallogenic environment evolved from alkaline to acidic. From the early to the late stage of mineralization, Sr showed an upward trend, which might indicate that the continuous reaction between the hydrothermal fluid and the wall rock continuously released Sr into the fluid. The Fe-Sr and Mn-Sr diagrams show that two kinds of fluid mixing occurred in the ore-forming process. The Fuli Pb-Zn deposit may have formed from mineral precipitation caused by the mixing of the metal-rich, oxidized acidic fluid and the sulfur-rich, reduced alkaline fluid. The results show that the Fuli Pb-Zn deposit belongs to MVT deposits.