The genesis and ore-controlling regularity of interbedded orebodies controlled by tectonics and lithofacies in Lutangba, Gejiu, Yunnan Province

The Gejiu tin deposit in Yunnan Province is a world-class super-large tin-polymetallic deposit, and the interbedded deposit (cassiterite-sulfide) is one of the important deposit types, but its genesis has been debated. The mineral fabric and geochemical characteristics of the interbedded orebodies of No.10 Ore Group at different levels of the Lutangba section in the Gaosong ore field were systematically studied to further discuss the genesis and provide theoretical basis for deep exploration of this type of deposit. The study results show that there is big difference between primary ore and oxidized ore. In the primary ore of the interbedded orebodies, the metallic sulfides are mainly pyrrhotite, pyrite, sphalerite, chalcopyrite, etc; the gangue minerals are mainly tremolite, fluorite and a few quartz, etc. In the oxidation ore of interbedded orebodies, most cassiterites are reddish-brown and irregular, and a small amount of cassiterite is hypidiomorphic, with a particle size of 0.07~0.3 mm according to the analysis of heavy sand. Comparing the primary ore and oxidized ore at different elevations, we found that the loss of SiO₂, A1₂O₃, TiO₂, MnO, MgO, CaO, Na₂O, K₂O, P₂O₅ and ignition loss are greater than that of primary sulfide ores in general, except that the content of S and FeO in oxidized ores is less than that of primary ores. The analysis results of the main elements at different elevations of the primary ore show that CaO and P₂O₅ increase from the bottom to the top, while MgO, S and ignition loss show a wave-like change from small to large and then smaller. In addition, the metallic sulfide in the primary ore has obvious zonation, with copper mineralization in the lower part and zinc mineralization in the upper part. The oolitic colloidal pyrite and pores formed by early high temperature exhalation can be seen in the upper part of the primary ore, while the lower part is mostly automorphic to hypidiomorphic pyrite. We hold the opinion that the interbedded orebodies in Lutangba mainly are related to post-magmatic gasification hydrothermal of the late Yanshanian period, and are obviously controlled by the tectonics and lithofacies. In the Indosinian period, the north-south compression formed multiple interlayer detachment structures between the calcareous dolomite and limestone strata, which are the important ore-hosting structures. The NE faults (such as the Lutangba fault) formed in the late Yanshanian period are the important passable structures. During the post-magmatic gasification hydrothermal transgression through NE faults, gently inclined orebodies were formed in the multiple interlayer detachment structures, and finally, steep inclined orebodies formed in the faults. The widely developed karstification in this area has a great influence on mineralization, which further enlarges the ore-hosting structure formed in the Indosinian period before mineralization, and accelerates the oxidation of interbedded orebody margins after mineralization.