Ore-forming Mechanisms And Prospecting Models Of Typical Granite Type Rare Metal Deposits In South China

There are a lot of rare metal deposits in South China, so it has been called the rare metal resources town of China or even the world, and granite type deposit is the most important rare metal deposit type in this area. Limu Ta-Nb-W-Sn polymetallic deposit and Yichun 414 Ta-Nb-Li polymetallic deposit are two large-size granite type rare metal deposits with obviously lithology zonings in the vertical direction in South China, and are also two ideal objects for researching the ore-forming mechanism and prospecting model of granite type rare metal deposits. In this paper, we have collected the samples from the different lithology zonings of the two deposits and then taken several aspects of studies which including the whole rock lithogeochemistry analysis, H and O isotopic testing, microscopic temperature measurement of fluid inclusions, 40 Ar-39 Ar isotopic dating of lepidolite and muscovite, Re-Os isotopic dating of molybdenite and U-Pb isotopic dating of zircon etc. Meanwhile, we focused on the application of HDAC(Hydrothermal Diamond Anvill-Cell) and Laser Raman Spectroscopy to conduct the homogenization temperature measurement and composition analyses of melt inclusions, respectively. In addition, we have also studied the time and space distribution regularity and the prospecting models of granite type rare metal deposits in South China.The following conclusions are reached based on the above studies.(1) The ore-forming rock mass of Limu and Yichun deposit are all belong to Li- and F-rich peraluminous granites. The granites are enriched in Rb, Th, U, Nb, Ta, P and depleted in Sr, Ti, Y, REE with respect to the primitive mantle. The REE total concentration in the granite varies significantly, and the REE distribution pattern exhibits a strong Eu negative anomaly and a tetrad effect.(2) The homogeneous temperature, salinity, and density in the H2O-Na Cl inclusions at the Limu deposit decrease upwards from the bottom of the deposit. However, the homogeneous temperature and salinity in the H2O-CO2-Na Cl inclusions vary insignificantly in the vertical dimension. For the melt inclusions found at Limu deposit, the initial melting temperature of solid phase and the temperature of complete homogenization are 560~600℃ and 704~853℃, respectively. The pressure and depth of Limu deposit were estimated as 270 MPa and 10 km, respectively. A metallogenic model was built to explain the processes forming Nb and Ta at the magmatic stage and W and Sn at the hydrothermal stage at Limu deposit.(3) At Yichun 414 Nb-Ta-Li deposit, there is no obvious homogenization temperatures change for H2O-Na Cl inclusions, but an appreciable increasing tendency of salinity, from the lower to upper lithology belts; while for melt inclusions, there seems a significant decreasing tendency of the homogenization temperatures and densities, and the composition of daughter minerals changed from muscovite-dominated to Na-feldspar and lepidolite-dominated, all of these showed that the magma had underwent a highly degree of crystallization differentiation. Besides, the pressure and depth of this deposit were estimated as 180 MPa and 7 km, respectively.(4) The metallogenesis of Yichun 414 deposit and Limu deposit are mainly controlled by the crystallization differentiation. However, there is no vein-type W-Sn ore body developed above the granite for the former, due to the seal of the overlying rock mass is better than Limu deoposit.(5) The ages of diagenesis and metallogenesis in Limu and Yichun deposits are 212 Ma ~214 Ma and 151 Ma ~156 Ma respectively, and the former is belong to late Triassic period and the latter is belong to the late Jurassic period.(6) I have divided the prospecting model of rare metal deposits in South China into 3 types: normal type, malposed type, and deficient type “Two levels +Basement”, and built ore prospecting methods for granite type rare metal deposits in South China in terms of geological feature, mineral assemblage, geochemical index, fluid and melt inclusions and so on.