| The sediment-hosted uranium deposit in Ruoergai is also calledCarbonaceous-siliceous-pelitic-type uranium deposits in China. It’s located in the southof Mianlue-A’nyemaqen suture zone, and in the joint between Qinling-Dabie orogenicbelt、Yangtze plate and Songpan-ganzi fold zone. Previous research on magmaticrocks in the Ruoergai is poor, there is no unified understanding about the genetic ofmagmatic rocks、structural evolution and formation epoch. Magmatic activities inRuoergai may have close realationship with uranium mineralization during thewestern Qinling orogeny.Therefore, this paper choose the intermediate-acid magmatic rocks in Ruoergai asthe main research object. Petrological, geochemical study and zircon U-Pb chronologywork disclose the realationship between tectonics evolution, magma activities anduranium mineralization. the main conclusions are as following:1. The volcanic rocks in Ruoergai are enriched in radioactive elements Th andlarge ion lithophile element (LILE), and depleted in high field strength element(HFSE),such as Nb, Ta, Ti characterized. The spider diagram of trace elements showenrichments of Ba、Th、U、Pb and depletions of Nb、Ta、Sr、Ti. The Nb/La ratiosare0.28~0.54, Hf/Ta ratios are7.07~8.52, La/Ta ratios are25.13~41.68. Thesegeochemical characteristics show that the volcanic rocks in Ruoergai formed in anisland-arc environment or active continental margin. Partial melting result from mantlewedge metasomatism by the subducted sediments in oceanic bottom formed the islandarc volcanic rocks as seen now in the Ruoergai ares.2. The volcanic rocks of Langmusi Formation formed during219.7~214.4Ma;and Caibaoshan Formation formed during212.5~208Ma; the diorite in Jiangzhaformed during218.9~214.8Ma; the granite in Chonger formed in213.6Ma. Thedevelopment of these volcanic rocks, diorites and granits may be as the response of western Qinling orogenesis during220~210Ma.3. There are three groups age of536.9±5.75Ma,774.3~803Ma and2668~3226.4Ma different form the Ruoergai magmatic rocks. These ages are agreed withsuch events as the formation of Shangdan ocean, Bikou volcanic activitiess and theformation age of Kongling complex group. These ages suggested that Ruoergai hadbelonged to the Yangtze Plate during neoarchean to midarchean, and had experiencedthe supercontinental convergence and the collapse events of the Rodinia.4. Mianlue-A’nyemaqen suture zone has experienced oblique arc collisionprocesses from east to west during230～200Ma. This implies that this area still in theactive continental and margin arc environment in208Ma,, belonging to the early stageof the continental collision rather than after collision or after the collision.5. The petrological and geochemical and geochronological study on themagmatic rocks in Ruoergai area described the tectonic-magmatic evolution history,and give implications on the uranium mineralization in this area:(1) This area hasPrecambrian basement with primary uranium enrichment;(2) And then, it experiencedrift valley development during Cambrian–Devonian, and formed a set of submarinesedimentary formation enriched in uranium;(3) Late Indosinian orogenic eventsduring220~210ma leaded to the subduction in island arc-active continental marginenvironment, and then to continent-continent collision;(4) Intra-continetal extensionin Ruoergai during Yanshan Period developed a series of fault-depressed basins, andled to exchange between shallow crust and deep mantle, provided the ore-forminghydrothermal fluids for uranium mineralization. |
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