Characteristics of alkali feldspar granite in tungsten (tin) deposits of Nanling region

Abstract：The early Yanshanian granites related to tungsten and tin deposits in Nanling region have been considered to be biotite granite, monzogranite, albite granite etc. However, microscope and electron microprobe analyzes of the granite related to tungsten mineralization in Yaogangxian, Shizhuyuan, Xitian, Dengfuxian, Limu and Meiziwo show that all the feldspars of the granite consist of K-feldspar and albite (An<5), which suggests that the name should be alkali feldspar granite. The albite of greisen inclusions in the alkali feldspar shows the character An<3. There is no obvious vertical regular variation or zoning of the compositions of the albite and granite within the depth range of 1000m. On the contrary, the plagioclases of the granite inclusions (xenoliths), some granite porphyry dykes, giant batholiths, and Triassic and Paleozoic monzogranite or granodiorite have a wide range of An value of 0～93, belonging to oligoclase, andesine or even basic plagioclase. The alkali-feldspar granite that contains two kinds of alkali feldspar was usually formed from the rich-volatile (Li-F) magma, with the extensive development of liquid immiscibility at the top of the granite, which caused the enrichment and mineralization of W, Sn, Bi, Mo in the hydrothermal-magma. The An value of the albite in alkali-feldspar granite could be used as an indicator for evaluating granite in search for W-Sn mineralization, because the value is obvious lower than that of barren granite.