High Temperature Melting Experiment On Precambrian Metamorphic Rock Uranium Ore Field And Its Geological Significance From Xiangshan,Jiangxi

Amount the 13 granite and volcanic-type uranium ore fields in southern China,12 of them was yielded upon the Pre-Cambrian strata with high uranium content in the basement.Therefore,many researchers believe that uranium probably derived from deep crust in South China for the granite-type and volcanic-type uranium deposits formed in the Cretaceous-Paleogene.In addition,the study on petrogenesis of porphyroclastic lava have been hot debated over half century,in which quartz broken mechanism was the focus.Xiangshan uranium ore field is located in the southern Jiangnan orogenic belt with Pre-Cambrian middle to low metamorphic basement.The melting experiments were carried out on phyllite which was the mainly composition of Neoproterozoic Shenshan Formation,the Kuli Formation and the Shangshi Formation.The molten samples were cut into thin sections and observed under microscope,and analyzed by electron probe.Comparison with un-molten rock,the result shows that:（1）As the temperature rises,clay minerals and mica first melt,and the particles become coarser,slightly melted,melted,and intensively melted.The color and shape of the sample also change:（a）dark gray,dull,rough surface;（b）dark gray,dull,visible coarser particles;（c）grayish black,with air voids,slightly metallic luster;（d）grayish black,a large number of voids in the cells.In addition,hydrous minerals melted first,indicating dehydrated metling occur.Besides,under the same temperature,melting degree vary probably because of mineral association fluctuate.（2）Micro-area X-ray spectrum observation of the molten sample,with the increase of temperature,the characteristics of sample boundary interface and crack interface of mineral change:（a）the emergence of rough surface;（b）a lot of fine grained material development;（c）obviously melt and fluid develop and fill in the cracks;（d）-a lot of melt fluid development and visible uneven bubbles.（3）Electron-probe analysis was performed on the melted sample.The unmelted part was more consistent with the original sample composition,and was characterized by high contain of silicon-aluminum;the Si content of the melt part decreased significantly,K₂O/Si O₂ had a significant upward trend.and K₂O/SiO₂ of the molten sample had a trend of inclination to the ore bodies,however,the melt composition ratio gradually approaches to the ore body.To the content of Na in the melt is significantly higher than that of the unmelted part,but as the temperature increases,the proportion of Na elements gradually decreases,and the K element increases.Overall,the melt has high Al,K,Na characteristics.It indicates that the melting of the metamorphic base provides alkaline materials for the alkaline alteration of uranium mineralization,creating favorable conditions for the later formation of uranium deposit.（4）After the molten sample is quenched,the quartz breaks and the melt and fluid fill the fracture,indicating that the widely-developed porphyroclastic lava in the Xiangshan area may be related to the rapid temperature drop.Combining with previous research results,it is believed that the metamorphic basement of the Xiangshan uranium ore field suffer volcanic activity and forms favorable environmental conditions that conducive to ore-forming diagenesis.Structural events and volcanism provide fractures and micro-fracture layers.The melting of the metamorphic base provides a material basis for alkali alteration,creating favorable conditions for the formation of uranium deposit in the later period.