The Magmatic Evolution In Xianshang Area And Its Important Role In The Uranium Mineralization

The Xianshang volcanic-intrusive complexes is the the ore-host rock of the biggest volcanic rock type uranium deposits in China. In recent years, the lead and zinc polymetallic ore-body were found at the bottom of the uranium ore-body. The magmatic evolution of the Xiangshan volcano-intrusive complexes plays an important role in the uranium mineralization. In this paper, the based on the 1:50000 regional geological investigation(Tuoshang, Lugang and Le’an county) and three dimensional geological mapping from the Xiangshan volcanic basin, A combined study of mineralogy, petrology, geochemistry and isotopic geochronology with the field work and the indoors research are carried out in the Xiangshan volcano-intrusive complexes and around related rocks.The geochronological framework and the multiple episodes of magmatism of the Xiangshan volcano-intrusive complexes are established, the petrological and geochemical characteristics of the volcano-intrusive complexes, the Jiaoping rocks, the Le’an rocks and the Qiankou rocks are summarized, and then the restricted relationship of magma evolving process and uranium deposit in the Xiangshan area is discussed.The main results achieved and understanding are as follows:(1) A combined study of zircon U-Pb ages was carried out for the Xiangshan volcanic-intrusive Complexes, South China. SHRIMP and LA-ICP-MS zircon U-Pb dating yields weighted 206Pb/238 U ages of 142.3±1.0 Ma, 139.2±1.7 Ma for the welded tuffs of the Daguding Formation, 138.8±0.8Ma for the welded tuffs of the Ehuling Formation, 136.4±2.5Ma(LA-ICP-MS) and 134.7±1.2Ma(LA-ICP-MS) for Shazhou granite-porphyry. Combined with other geochronological result, the Xiangshan volcanic-intrusive Complexes took place mainly in the 143 ~ 132 Ma, in which the South China Plate were formed in the intraplate extension. Numerous U–Pb spot analyses of magmatic zircons reveal that the volcanic-intrusive Complexes experienced two episode of volcanic activity in the Early Cretaceous. The early volcanic activiy in 143 ~ 136 Ma lead to the formation of the Daguding Formation, and the late volcanic activiy in 135 ~ 132 Ma lead to the formation of the Ehuling Formation. The volcanic activity of the Xiangshan volcanic-intrusive Complexes can be divided into six stages. The first volcanic activity stage lead to the volcanic-eruptional rocks of the Daguding Formation. The second volcanic activity stage lead to the flood volcanic rocks of the Daguding Formation. The third volcanic activity stage lead to the volcanic-eruptional rocks of the Erhuling Formation. The fourth volcanic activity stage lead to the flood volcanic rocks(porphyroclastic rocks) of the Erhuling Formation. The fifth stage volcanic activity lead to the flood sub-volcanic rocks(the Shazhou granite-porphyry). The sixth volcanic-magmatic activity stage lead to the formation of the vein rocks,such as the quartz monzonitic porphyry, dellenite-porphyry, lamprophyre, and late uranium mineralization.(2) A combined study of petrologic and geochemical analysis and Sr-Nd-Pb isotopes was carried out for the Xiangshan volcanic-intrusive Complexes, suggesting that they are characterized by the high concentrations of Si O2, Na2 O and K2 O, and Low concentrations of Ca O and Mg O, with relative enrichment in light REE and LILE, middle negative anomaly Eu, the right slanting of chondrite-normalized REE patterns, and significant depletions in high-field strength elements(e.g., Nb, Ta, Zr, Hf and Ti). The Complexes ever experienced strong crystallization differention, which to the regular and cyclic variation of the chemical composition between the first and second stage volcanic activity. The composition of the whole-rock major-trace elements and isotopes of the different type rocks are similar, suggesting that they are come from same source region. Results of the Sr-Nd-Pb isotopes for the Complexes indicate that they were come from partial melting of the low crust, little adding of the mantle-derived mafic magma, represented by the dioritic-mafic enclaves in the complex.(3) Zircon dating of the Jiaoping unit, the Le’an sequence and the Xiankou sequence around Xiangshan volcanic basin was conducted in this study, the Jiaoping unit give an age of 414.2±4.0 Ma, the Le’an unit、the Taiping unit and the Wangyuan unit of the Lean sequence are formed age of 405.6±1.8 Ma, 403.1±1.8 Ma, 401.1± 6.6 Ma. The Dianyuan unit、the Henggang unit、the Shangdianyuan unit and the Paiqian unit of the Xiankou sequence give ages of 228.9±3.6 Ma, 235.3±4.1 Ma, 234.5±2.8 Ma, 235.5±3.3 Ma.Based on the map of the major trace element tectonic environment and regional geological background research of others, following conclusions can be drawn: Two kinds of granites which are characteristic of gneissic schistosity and none-gneissic schistosity are formed by the Jiaoping unit and the Lean sequence structure setting of the Caledonian movement the main collision and following weak compression, subsequently local extension, In other words, the ages of the Le’an sequence is exactly the time at which the main collision of the Caledonian movement and following weak compression happened, and the age of the Jiaoping unit are formed by structure setting of local extension. The last idea is that the Xiankou sequence is formed by intra plate extensional tectonics during Indosinian.(4) The characteristics of major trace elements and Sr-Nd-Pb isotopic in the granite of Jiaoping Unit, Le An series and Xiankou series are quite familiar with that in the host rock of Xiangshan volcanic – intrusive complexes, close to the space. There may be consistency and similarity to their sources. They are both derived from the lower crustal rocks, but they are contaminated by the mantle. Dark mafic enclaves is found both in Le’an series and Xiankou series.(5) The discovery of uranium-rich rock at Xiankou in Indosinian amended the understanding of the predecessor’s only treat Xiangshan original volcanic magmatism as the uranium source of the Xiangshan uranium-rich magmatism. There are at least twice uranium mineralization in Xiangshan volcanic basin and surrounding areas. The first time, Xiankou uranium-rich rock is formed, and the Luoshan uranium deposits, Wangyuan uranium anomalies and Indosinian lamprophyre uranium outliers are formed in Indosinia. The second time is Yanshanian-Xiangshan volcanic rock type uranium mineralization. From the comparison the uranium mineralization age and the volcano-intrusive complexes diagenetic age in Xiangshan volcanic basin, it is a continuous magmatic thermal fluid system of the diagenesis and mineralization. The uranium enrichment finished in the early diagenesis and mineralization, in the late the further enrichment and mineralization under the factors such as magmatic hydrothermal, juvenile fluids, water-rock interaction the diagenesis and mineralization is conditioned by the magma evolution.(6) The magmatic evolution of the Xiangshan volcano-intrusive complexes and around related rocks plays an important role in the uranium mineralization. With the magma evolution, it is in the late the uranium mineralization concentration, and there is came into being the rich uranium geological body, and a rich-U geological body provided for the abundant ore-forming materials of the uranium mineralization in Xiangshan volcano-intrusive complexes. The magma evolution formed different rock physical interface, which provide favorable metallogenetic geochemical convert surface.The boundary between the Daguding volcanic series and Erhuling volcanic series is the favored site for the the uranium mineralization in the Xiangshan volcano-intrusive complexes.. The results of the Sr-Nd-Pb isotopes indicate the ore-forming materials of the Xiangshan uranium mineralization was most likely generated by the Xiangshan and around related igneous rocks. In short, magma evolution restrict the uranium metallogenic position and source of ore-forming materials.