The Characteristics And LA-IPC-MS Zircon U-Pb Dating Of The Pegmatoidal Alaskite In Hongshiquan Area,Gansu Province

The Hongshiquan area in Gansu is located in the western part of the Longshoushan uranium-polymetallic metallogenic belt,which forms the pegmatoidal alaskite uranium ore deposit--Hongshiquan uranium deposit that is rare in our country.In the 80 s last century,the predecessor carry out certain geological prospecting and scientific research in this area,but the research on the pegmatoidal alaskite is relatively weak.In this paper,the ore-bearing host rock in Hongshiquan uranium deposit—the pegmatoidal alaskite—is selected as the object of study.Through the field geological investigation,petrography,major elements,trace elements,rare earth elements,Sr-Nd-Pb isotopes,LA-ICP-MS zircon U-Pb dating and other methods,the characteristics of rock mineralogy,geochemistry and chronology are systematically studied.Based on this,the genetic types,tectonic setting,magmatic source area and the relationship with uranium mineralization are discussed.Through the field geological investigation,it is found out that the pegmatoidal alaskite is controlled by NW faults,with a dip of SE,and the east-west length about 3km,the north-south width ranging from 100³00m,which mainly invades metamorphic rocks in Longshoushan group,and is exposed in the shape of Rock Branch.The fresh side of alaskite is mostly white,the weathered side light yellow with slightly brown.Mainly for the crystal structure,occasionally coarse-medium grain structure,block structure.The main minerals are potash feldspar and quartz,the secondary minerals being plagioclase and muscovite ± biotite,accessory minerals being zircon ± pyrite,with the characteristics of aplogranites and pegmatites.The main element of the pegmatoidal alaskites are characteristic of rich Si?SiO₂= 66.81%⁷7.81%?and rich alkali?ALK=7.46%¹0.36%?,poor Fe?TFe2O3=0.62%⁶.46%?,poor calcium?Ca O=0.24%².03%?,poor magnesium?Mg O=0.29%¹.57%?,weakly peraluminous?A/CNK=0.90¹.19?,relative rich K?K2O/Na2O= 2.02³.30?,rich Fe O?Fe2O3/ Fe O=0.22?,the alkalinity index?A.R.?=3.46⁵.88,the Rittmann Index???=1.73⁴.17,the differentiation index?DI?=89.92.TFe2O3,Ca O,Mg O,Al2O3 are negatively correlated with SiO₂,but positively correlated with DI.The trace elements of the pegmatoidal alaskites are shown to be relatively rich in Rb,Th,U,La,Ce,Nd,Zr,Hf and a relative deficiency in Ba,Sr,Nb,Ta,P,Ti,with particularly high U,Th.The total amount of rare earth elements??REE?= 87.42×10-6³05.02×10-6.LREE/HREE=1.61¹3.63,La N/Yb N=1.51¹6.29,the former is relatively rich.La N/Sm N=1.86⁵.48,Gd N/Yb N=0.34².28,the degree of fractionation of LREE is high compared with HREE.?Eu=0.30⁰.89.The Sr-Nd-Pb isotopic of the pegmatoidal alaskites,the?87Sr/86Sr?i=0.743780⁰.9376 21,which is higher than the average value of the continental crust.The ?Nd?t?=-18.9¹1.3,the variation range is large.The fractionation of Sm and Nd is not obvious,whose correspon ding model age of the depleted mantle Nd is T2DM=2538²739Ma.Pb with the characteristics of high radioactive lead isotopes.Through the LA-ICP-Ms zircon U-Pb isotopic chemical analysis shows that the ages of the pegmatoidal alaskites are 2455²500Ma,which is the product of the NeoarcheanPaleoproterozoic tectono-thermal events.According to the comprehensive research,the pegmatoidal alaskite has the characteristics of S-type granites,which is formed in the post-collision tectonic environment and melted by the dehydrated part of the pelitic rocks of the Longshoushan Group;the mantle material may be added during migration and emplacement;alaskite is closely related to uranium mineralization,which is the main material basis for the latter.