The Source Of Porphyroclastic Lava In Xiangshan Ore Field,Jiangxi Province

The Xiangshan uranium ore-field is the largest volcanic-type uranium ore-field in China,its ore-hosting rock is mainly rhyodacite and porphyroclastic lava,and including small amount of the Precambrian metamorphic rocks and Mesozoic granitic porphyry as well.Although previous researchers have got achievement on the chronology and geochemistry of the Xiangshan uranium ore field.As the research level continues to deepen,The debate about Xiangshan uranium mineralization sources and source characteristics of porphyroclastic lava has never stoppeds.This paper address on this issue using microscopes,electron probes,and laser ablation multiple-receiving plasma mass spectrometers to analyze the morphological characteristics,chemical composition,B-isotope of tourmaline and the geochemical characteristics of the melt inclusions in the quartz in Ruyiting.The preliminary research results are as follows:Tourmalines associated with uranium minerals are generally developed in porphyroclastic lava.The electron probe composition analysis of tourmaline shows that tourmaline is typical schorl.It is characterized by rich elements such as Na and Fe.The volatile components in tourmaline are high,so as the content of B2O3,accounting9.38%-10.04%,and the F content is 0.1%-1.77%.Boron isotopes in porphyroclastic lava were examined by in-situ LA-MC-ICP-MS analysis.The results showed that the content of?¹¹B in tourmaline was?-13.15±0.72?‰^-12.28±0.63‰,mean?-12.72±0.94?‰,indicating that the Xiangshan volcanic-intrusive complex mainly originates from the partial melting of the Precambrian crust basement rocks,and may involve the participation of a small amount of mantle-derived materials.The melt inclusions of quartz in porphyroclastic lava are mainly distributed in isolated and star-shaped,elliptical or irregular.Melt inclusion diameter is more than 10?m,and they would be subdivided into four types:glass melt inclusions?Ia?,daughter-mineral bearing glass melt inclusions?Ib?and crystalized melt inclusions?IIa?,daughter-mineral bearing crystallized melt inclusions?IIb?.Electron probe analysis on quartz melt inclusions shows that the SiO₂ content in the melt inclusion is high,ranging from 50.52%to 77.83%with an average of 59.49%,and the Mgo content:0.01%to 9.59%with mean of 1.59%;CaO content between 0.01%and 0.62%,with mean 0.25%.In addition,the total amount of alkali is high,which is the Na₂O+K₂O up to:between 1.58%and 13.74%with mean5.56%.Liteman?index o is between 0.10 and 12.87 with mean of 3.46.Aluminum alkali ratio[?Na+K?/Al]was between 0.11 and 1.00 with mean of 0.32.In generally,the melt inclusions in quartz illustrate high silicon,high alkali,low magnesium,low calcium,which indicate magma chamb charaters of porphyroclastic lava.Combining previous research results,the high content of B,F and other volatile components in the ore-forming fluid and the high oxygen fugacity circustance in the early stage of the magma make the U-element more likely to form a complex,which is more conducive to the migration and enrichment of U.The uranium metallogenesis of Xiangshan is inseparable from the magmatism.The magmatism ensures the abundant source of uranium mineralization,probably the dynamic conditions as well.