Geochemical Characteristics And Genesis Of The Giant Talc Deposit In Guangfeng, Jiangxi Province

Talc, Mg3(Si4O10)(OH)2, a layered silicate mineral, is a very important industrial mineral with characteristics of chemical stability, high melting point, excellent insulation properties, fire resistance and can be used in paper, rubber, plastics, paints, ceramics, household cosmetics. The talc deposits are produced mainly in the former Soviet Union, China, America, Korea, Austria and other countries. In China, the talc deposits are concentrated maily in Jiangxi, Liaoning, Shandong and so on. There are three genetic types of talc deposits. First, magnesium-rich carbonate or clay rock metasomatised by silicon-rich hydrothermal fluid, such as Germany Gopfersgriin talc deposit. Second, ultrabasic-mafic rocks altered by silicon-rich hydrothermal fluid, such as west Africa Cameroon Boumnyebel talc deposit. Third, sedimentary talc deposit.The Jiangxi guangfeng talc deposit is located in Guangfeng, Shangrao City, the interchange of Jiangxi, Fujian and Zhejiang provinces, tectonically located in the suture zone of the lower Yangtze Plate and the residual South China Basin. It is superlarge scale talc deposit with reserves of thousands of tons, and is still mined in openpits. Ore bodies occurred in the siliceous dolomite and (false) oolitic siliceous shale or limestone of Neoproterozoic Dengying Formation, with false oolitic cherts or phosphate layer in the bottom, false oolitic siliceous limestone in the roof. The ore bodies are layered and bedded, folded together with the bottom sedimentary phosphate rocks. The rock types of the mining area are relatively simple, mainly sedimentary rocks. Metamorphic and magmatic rocks are not developed, with the outcropof Yanshanian lamprophyre veins. There are comprehensive studies on this deposit. In this study, we focused on systematic geochemical studies such as major and trace element, rare earth elements (REE), Sr-Nd isotopic compositions of lamprophyre, talc ore and wall rocks (i.e. dolomite), C and O isotopes of dolomite and zircon U-Pb dating of lamprophyre by LA-ICP-MS.The Jiangxi-Yangcun lamprophyre studied here was formed at about131.6Ma, significantly cutting the early ore bodies. It is characterized by strong enrichment of LILE (Ba, Th, K) and LREE, significant depletion of HFSE (Nb-Ta-Ti)and weak depletion of (Zr-Hf), and high ISr (0.70688to0.70892) and low εNd(t) ratio (-0.11to-1.24). These geochemical characteristics are similar to island arc basalt and the South China Cretaceous basalt.The talc ore consists mainly of oolite granular talc with a small amount of quartz, carbonates and disseminated organic matter. It shows typical sedimentary oolite granular structure. The wall rock dolomite was strongly silicified and consists mainly of dolomite and quartz. The talc ores and dolomites show similar REE patterns, with low REE contents, weakly negative Ce anomaly, and weak enrichment of HREE in relation to LREE. They also show similar Sr isotopic compositions, with87Sr/86Sr ratios of0.7092～0.7101for both the talc and dolomite. The dolomite has a δ13Cv-pDB value of-4.9to1.7%, similar to the range of the contemporaneous seawater. These data indicate that the magnesium in the talc ore and dolomite may share the same source and may have originated from the seawater. However, some of the samples also show positive Eu anomalies, lower δ18Ov-SMOW (14.5to19.7‰), and slightly higher87Sr/86Sr values than the contemporaneous seawater values, and these characteristics may suggest an additional source beside seawater that contribute to the formation of talc ore. We suggest that the required silicon in talc might be provided by silica-rich hydrothermal fluid with slightly higher87Sr/86Sr values than the seawater. In summary, we consider that the Yangcun talc of Guangfeng deposit in Jiangxi Province may have a syngenetic hydrothermal sedimentary origin. Magnesium was mainly derived from magnesium-rich seawater and silicon was mainly derived from silicon-rich and high87Sr/86Sr hydrothermal fluid which was formed by seawater infiltrating and leaching of the old crustal rock strata. After synsedimentary deposition, talc ores have also underwent strong metamorphism and deformation, which formed talc schist with typical schistosity structure.