Petrography,Geochemistry,Geochronology And The Research Significance Of Wuzhang Granitic Porphyry In Southern Xiangshan,Jiangxi Province

There are distribution of granite porphyry in WuZhang area in South Xiangshan,the characteristics of which are similar to the that of the uranium-producing granitic porphyry in the northern part of Xiangshan.However,there has been no great breakthrough in uranium prospecting in the southern part of Xiangshan.The reasons may be related to the lack of systematic study of granitic porphyry in the southern Xiangshan Mountains.The thesis take the Wuzhang granitic porphyry in the south of Xiangshan as the research objective to launch systematically studies on petrography,geochemistry and geochronology.On this basis,we discussed the study significance by comparing the characteristics of uranium-producing granitic porphyry in the north of Xiangshan with that of the uranium-producing granitic porphyry in the north of Xiangshan.Wuzhang granitic porphyry is exposed in the southern part of Xiangshan uranium field.It is mainly an arcuate rock dike group in Xiangshan town Wuzhang which owns an arc distribution and belongs to Wuzhang rock mass.The east-west length of Wuzhang granitic porphyry is about 8¹0km,the north-south width is about 3⁴ km and the area is about24km².The occurrence of the rock mass is relatively smooth;From the east to the west,the dip angle of the rock mass gradually diminished.The northwestern part of the rock mass mainly contacts with the clastic lava of the ehuling formation and the pyroclastic-sedimentary rock of the daguting formation.There are residual roofs of fragmentary lava in the Wuzhang rock mass found in about the Xiabao area,with the latter intruding into the former as rock branches.The hand specimen of Wuzhang granitic porphyry is flesh red to light flesh red with block-like structure and porphyritic texture.The Porphyry consists of Quartz,Potassium feldspar,Plagioclase and Biotite.The matrix is cryptocrystalline to semi-cryptocrystalline,and the mineral composition is in consistence with that of porphyry.For major elements,Wuzhang granitic porphyry has many characteristics like its rich in sillicon?SiO2=65.71%⁶7.87%,with a mean value of 66.84%?,its rich in alkali?ALK=8.27%⁸.83%,the mean value is 8.55%?,its alkalinity index?AR=2.73³.34,with a mean value of2.93?,its rock saturation index?A/CNK=0.92¹.10,the mean value is 1.02?,its differentiation index of?DI=79.32⁸4.71,with the mean value of 84.49?and its Richman's index of??=2.54³.16,with the mean value of 2.88?.The relationship between the content of K₂O?Na₂O and SiO₂ in Wuzhang granitic porphyry is unobvious.The content of Al₂O₃?CaO?MgO?TiO₂?P₂O₅ decreased with the increase of SiO₂ concomitantly,showing a regular evolution feature.For trace elements,Wuzhang granitic porphyry is rich in ion lithophile element such as Th?U?Pb and REE such as La?Nd,lack of HFSE such as Ba?P?Ti?Sr.There exists similarity between the trace element spider web map and the rare earth distribution pattern graph to some extent;For rare earth elements,the total ree content of Wuzhang granitic porphyry??REE?is 327.7×10^-6473.82×10^-6(the mean value is 415.81×10^-6),which is higher than that in the crust.Light rare earth elements LREE=309.1×10^-6444.58×10^-6(the mean value is 388.98×10^-6),LREE/HREE=12.41¹6.62?the mean value is 14.66?.Chondrites standardization partition pattern diagram of REE has an obvious right-tilting trait,which indicates an obvious fractionation between LREE and HREE in Wuzhang granitic porphyry.The characteristics of major elements,trace elements and rare earth elements are all consistent with that of uranium-producing granitic porphyry in the northern part of Xiangshan?Hengjian,Youfang,Shazhou?.⁸⁷Sr/⁸⁶Sr=0.71740⁰.71789,which is close to the average ⁸⁷Sr and ⁸⁶Sr ratio in the continental crust;_?Nd?t?=-6.3^-8.2,T_DM2=1443¹592Ma,whichisClosetotheageofthecontinental crust;²⁰⁶Pb/²⁰⁴Pb=18.42¹8.46,²⁰⁷Pb/²⁰⁴Pb=15.61¹5.62,²⁰⁸Pb/²⁰⁴Pb=38.91³8.93.From what discussed above,it is suggested that the formation of Wuzhang granitic porphyry is related to the melting of ancient crustal materials.The LA-ICP-MS zircon U-Pb dating results of Wuzhang granitic porphyry are as follows:The average weighted age of ²⁰⁶Pb/²³⁸U in the Granitic porphyry in Shangnan area is135.6±1.6Ma?MSWD=0.67?;The average weighted age of ²⁰⁶Pb/²³⁸U in the Granitic porphyry in Xiabaonan area is 135.2±1.5Ma?MSWD=0.95?.The diagenetic age of granitic porphyry in Wuzhang pluton in southern Xiangshan is 134-136Ma,Belonging to the early Cretaceous.The magmatic material of Wuzhang granitic porphyry has originated from the mesoproterozoic crust.The genetic type of the granitic porphyry is a transitory type between the A type and the S type granite which formed in an intraplate Extensional environment.After comparative studies with the uranium-producing granitic porphyry in the northern part of Xiangshan,we found that Except for the differences in the size of rock mass.The Wuzhang rock body is highly consistent with the uranium-producing granitic porphyry in terms of petrography,geochemistry,chronology,genetic type,tectonic environment,material source and so on,hence it is inferred that Wuzhang granitic porphyry has the possibility of forming porphyry type uranium deposit.