Features of rock-forming minerals of Yanshanian mineralized granite in the polymetallic deposit concentration zone in southern Hunan and their implications for granite generation

Abstract：There are three types of Yanshanian mineralized granite in southern Hunan, and the chemical compositions and species of their ore-forming minerals are notably different. The characteristics of variations in chemical compositions of the minerals all indicate a mantle-crust magma mixing origin. Systematic analyses show the following：(1)Hornblendes belong to Ca-hornblende.(2)Biotites in most early-stage MC-type granite are Mg-biotite; those in late-stage CM-type granite are mainly meroxene; and biotites in C-type granite are mainly sideophyllite and meroxene, most of which zinnwaldite. The variation in biotite composition of the three types of granite shows a linear relation, implying a genetic relation.(3)Plagioclases in the three types of granite are significantly distinguished. Plagioclases are mainly andesine in MC-type granite, oligoclase in CM-type granite and albite in C-type granite. The zoning structures of plagioclases are developed to different degrees, with the zoning of plagioclases in the early-stage CM-type granite best developed.(4)Alkali plagioclases in CM-type granite and its dark micro-inclusions are mainly K-rich orthoclase, and a few grains of plagioclase in inclusions are anorthoclase, indicating higher temperatures of plagioclase crystallization. Alkali plagioclases in C-type granite are relatively K-poor microcline and Na orthoclase.(5)The characteristics of rock-forming minerals and their compositional variations indicate that the formation of mineralized granites is related to crust-mantle magma mixing. The magmatic evolution that generated MC-and CM-type early-stage granites was mainly magma mixing, while the magmatic evolution that formed CM-type late-stage granitoids and C-type granitoids might be fractional crystallization in addition magma mixing.