Indosinian Granitic Magmatism And Mineralization In The Miao’ershan Area, Northern Guangxi

The Miao’ershan-Yuechengling batholith is located in the western Nanling W-Sn metallogenic belt with an outcrop area of more than 3000 km2. Available geochronological data indicates that this complex was formed by repeated intrusive activities in the Silurian-Devonian and late Triassic. Many W-Sn(Mo) deposits occurred in the interior and periphery of the complex pluton. The geological exploration began early and was barely focused on the exploration and evaluation work of the Niutangjie, Changgangling and some other deposits. Little research work has been done with most deposits. Therefore, this area has become an ideal place to do research on pre-Yanshannian magmatism and mineralization. Based on the previous studies, we selected Gaoling quartz-vein type tungsten deposit and Yatoushui quartz-vein and skarn type tungsten deposit as our research target. Based on systematic study of petrography, geochemistry, mineralogy and mineralization characteristics, we discuss about the Indosinian granitic magmatism and mineralization in the Miao’ershan area.Gaoling quartz-vein type tungsten deposit is located in southern margin of the Miao’ershan-Yuechengling batholith. The granitic rocks closely associated with tungsten mineralization can be divided into two facies according to the contents of phenocrysts:medium-to fine-grained porphyritic two-mica granite, and medium-to fine-grained two-mica granite. Whole rock chemical analyses indicate that the granitic rocks have high SiO2 and are metaluminous to slightly peraluminous, which belong to typical highly fractionated S-type granites. They have relatively low total REE contents and are enriched in LREE, Cs, Rb, Th, U, Pb, depleted in HREE, Ba, Sr, Ti. In-situ LA-ICP-MS zircon U-Pb chronology indicates that the granitic pluton was emplaced at 224.9±1.4 to 220.2±1.6 Ma. Furthermore, we analyzed the Sm-Nd isotopic compostions in scheelite concentrates and obtained a Sm-Nd isochron age of 212±20 Ma (MSWD=0.52). The result suggests that the tungsten mineralization in Gaoling depositis the response of the Triassic tungsten mineralization event in the western Nanling Range. The detailed observations of ore minerals under optical microscope and EPMA show the Gaoling tungsten deposit was formed in two mineralization stages:(1)wolframite-scheelite-quartz vein stage; (2) scheelite (wolframite)-sulfide-quartz vein stage. SNd values of the scheelite concentrates range from -8.9 to -9.4, which is correspond with the crustal origin of typical highly fractionated S-type granite and indicates that the parental granite was formed by the partial melting of the crustal basements.Yatoushui tungsten deposit is located in the middle part of Miaoershan-Yuechengling batholith, which is closely associated with the medium-to fine-grained two-mica granite. Bulk-rock chemical analyses indicate that the granite has high silica contents and is stongly peraluminous, also with affinities of typical highly fractionated S-type granite. They also displays medium-low total REE contents and enriched in LREE, Cs, Rb, Th, U, Pb, depleted in HREE, Ba, Sr, Ti. Zircon grains from the granite have obvious core-rim textures. In-situ LA-ICP-MS U-Pb dating of the inherited zircon cores yields an age of 439.7±4.3 Ma, which illustrates the zircon cores were formed during the Caledonian period, the U-Pb dating of the magmatic zircon rims yields an age of 219.9±4.3 Ma, which indicates that the granitic pluton was emplaced during the Indosinian period. Yatoushui tungsten deposit consists of two mineralization styles:scheelite skarn and quartz-vein types. The skarn-type scheelite mineralization is the most important type in Yatoushui tungsten deposit, which can be divided into three stages:early prograde skarn stage, late retrograde skarn stage and quartz-sulfide stage. While the quartz-vein type mineralization has two stages:wolframite-scheelite-quartz vein stage and scheelite (wolframite)-sulfide-quartz vein stage. The compsitions of the ore-forming fluid changes greatly in different stages. Both the formation of the skarn-type and quartz-vein type mineralzation from the Yatoushui tungsten deposit and the occurence of core-rim textures from the zircon grains show an excellent example for studying multiple-aged granitic magmatism and mineralization events in the Nanling Range.