Binchuan - Yongsheng Permian Volcanic Rocks And Their Metallogenic Potential Evaluation

Continental flood basalts (CFB) represent major magmatic events, and some are associated with important sulphide mineralization, for example, the Norils'k deposits. About CFB formation there is an active debate both over the extent to which one modle is applicable to all CFB, and whether melting is triggered by lithosphere extension or deep-seated mantle plumes. Emeishan basalts, widely spreading out in the west margin of the Yangzi block, are one of many regions in which Geo-science circles are interested. According to characteristics of its petro-assemblage and petrography, Emeishan basalts can be divided into three-big rock regions- west, middle and east. These basalts are mainly formed in the course of mantle upwelling and widely continental extension through analyzing tectonic setting and pale-geographical setting.Based on the previous study, in this paper the most completely outcropped geological section, the basalts of Binchuan and Yongsheng County with intrusions of Lamagu region and Xilaping region are chosen for Major-Element, Trac-Element and Isotopic Composition study. Applying modem theory of petrology formation, the paper describes systemically some characteristics of Emeishan basalts' petro-geochemistry and magma activation and discusses on mantle source characteristics and magma evolution and magmatism of the late Permian Emeishan CFB. On the basis of this paper's research results, the problem whether the deposits can be formed in Binchuan and Yongsheng County, having compared with the geological setting and viscosity of the Norils'k deposits, is discussed. Through a comprehensive study and comparative analysis of existing datum, some conclusions can be drawn as follow:1. CFB occurring in Binchuan and Yongsheng County can be compositionally subdivided into three types: the high-Mg (MgO=l2.3-27.2%), transitional-Mg (MgO=l 0.4-11.3%) and low-Mg groups (MgO=2.48-7.53%). All three groups show continuous shift in major and trace elements, enriching LREE and LILE. They exhibit similar geochemical features to OIB and are contributed to tholeiitic basalts.2.The high-Mg series approach the primitive mantle compositions, and their magmatic source is derived from the lower mantle, showing similar petro-geochemical features to primitive mantle. So they can be ascribed to lower degrees of partial melting and greater melting depths. On the contrary, the low-Mg volcanic rocks have the lowest (Ce/Yb)cN with significant "crustal source" signatures, indicating the largest melting degrees and shallowest melting depths. So their magmatic source is generated from hybrid magma. The trans-Mg series have transitional characteristics between the high-Mg and low-Mg rocks.3.The ultra-basic intrusions and high-Mg volcanic rocks have higher content of MgO with high Mg# value. The Mg# value is respectively from 0.71 to 0.82 and from0.70 to 0.80, representing primitive magma. But they have dissimilar patterns.4.On the basis of the geological background, in research region the generation of CFB is somewhat temporally related to mantle plume activities, which consistent with what the predecessors make. But this paper think that the mantle plume activities are not resulted from lithosphere extension but result in occurring of the latter. The magma arisen from the mantle plume interacts with lithospheric materials, undergoes different degrees mixing of lithospheric mantle and crustal components and develops a variety of basalts and intrusions. Based on particular researches, this paper makes an approach to the formatting mechanism of high-Mg volcanic rocks.5.Compratived with the metallogenic realities, settings and viscosities at home and abroad, especially with Noril'sk region, this paper estimates the perspective of nickel-copper sulfilde deposits in Binchuan and Yongsheng County and suggests further assignment be carried through in order to expand perspective targets.