Mineralogy and Metallogenic Mechanism of Weilasituo and Bairendaba Deposits, Inner Mongolia, China

Weilasituo and Bairendaba are two large-scale silver polymetallic deposits, discovered in the western slope of the south and central sections of Great Hinggan Mountains in recent years. This study focuses on identifying and analyzing the mineral associations and occurrence of silver minerals using scanning electron microscopy-energy dispersive spectrometry and electron microprobe. The analyses suggest that mineral associations change from tungstate and oxide, to diatomic sulphide, to simple sulphide, to antimony sulfosalt mineral, to antimonide with decreasing temperature based on the microscopy observations. The silvers contained in the ore occur in several forms, including mainly visible silver minerals, followed by the lattice silver (isomorphous substitution) and sub-micron inclusion silver. The mineral sequence of visible silver's formation is Ag-bearing tetrahedrite-argentian tetrahedrite-freibergite-diaphorite-freieslebenite-pyrargyrite-dyscrasite. Ag occurs in lattice of chalcopyrite, bornite, chalcocite, pyrite and galena in isomorphism in small amount, and also as the microscopic wrappage in galena. The results, combined with the characteristics of mineral association and fluid inclusions in different mineralizing stages, indicate that W and Sn are transported as wolframic acid and tungstate at the early high-temperature meta-acid oxidizing environment, while Zn²⁺, Pb²⁺, Cu⁺ and other metalions are transported as chloride complexes. After wolframite's precipitation and the changes of metallogenetic fluid physical and chemical condition, the metallogenetic environment becomes slightly alkaline and reductive, and Zn²⁺, Cu⁺ and other metalions form HS^- complexes. The continuous drop of temperature and water-rock interaction lead to the separation of Zn²⁺, Cu⁺ with HS^- to form pyrrhotite, sphalerite etc.. In late phases, Ag⁺ can combine Sb³⁺, Cu⁺, Pb²⁺, Sb^3-, S^2- etc., which results in multiple silver antimony sulfide minerals, boulangerite etc..