Geochemical And Metallogenetical Characteristics Of Nb-Ta-Rb Deposits,South Qilian-Beishan Area,Gansu Province,China

In recent years,the geological environment is appropriate for rare metal mineralization with large field researches and studies by Geological Survey of Gansu Province,in Qilian and Beishan area.Some large,extra-large rare metal deposits have been found,and it is expecting to have breakthrough in prospecting after intensive works.By detailedly geological and modern analysis works,the author studied the Yushishan Nb-Ta and Guobaoshan Rb deposits in Southern Qilian and Beishan area,respectively.The article investigated and researched the characteristics of geological setting,petrology,mineralogy,geochemistry and U-Pb chronology,concluded the age,material source and regularity in Yushishan Nb-Ta and Guobaoshan Rb deposits.The major results and realizations are followed:1.The Nb-Ta deposit is mainly occurred in leptynites in Yushishan area,and Rb in amazonite-bearing granite and granite pegmatite.It is significant to finding rare metal deposits in Southern Qilian and Beishan area,respectively.2.The characteristics of zircons show the magmatic and metamorphic zircons of leptynites in Yushishan area.206Pb/238 U ages range from 810±6Ma to 854±7Ma,the concordia age is 829.4±2.7Ma,which stands for the period of magma intruding.The 206Pb/238 U ages is 768±8Ma~809±7Ma of metamorphic zircons,the concordia age is 795±5.4Ma,which stands for the period of hydrothermal action in post-magmatism.The ages of biotite-bearing monzonitic granite,amazonite-bearing granite and amazonite-bearing granite pegmatite are 1853±16Ma,427±8Ma,406±5Ma,respectively,and the latter two ages reveals that the developing period of magma is 21 Ma from granite to pegmatite.3.The protolith of leptynites is alkalinous medium-acidic magmatite by using petrochemical and geochemical ways in Yushishan area,such as syenitoid.The protolith has high contents of Si O2,K2O+Na2O,Rb,Th,Nb,Ta,Zr,Hf,Ga,Y,and low contents of CaO,MgO,Sr,Ba,Cr,Ni,and the values of ?Eu and Ga/Al are 1.4~3.2 and 3~11,which is similar with A-type granite and shows the tectonic environment of extension.Based on the regionally tectonic evaluation,the intrusive age and geochemical characteristics of syenitoid are related closely with cracking of Rodinia supercontinent.The Guobaoshan granites belong to peraluminous and high-K calc-alkali series,the geochemical characteristics are also similar to A-type granite.The weak tetrad effect characteristic of REEs suggests the height evolution of magma.The magma originates from partial melting of metamorphic rocks in the depth of normal crust(about 30km).The biotite-bearing monzonitic granite indicates a magmatic event of middle Paleoproterozoic in extension setting,and the age of amazonite-bearing granite and amazonite-bearing granite pegmatite indicates the South Tianshan ocean closed before 427 Ma.4.Nb and Ta are mainly occurred in rutile and urdite that originated from hydrothermal metamorphism,in Yushishan leptynite,and Rb being in amazonite in Guobaoshan area.5.Nb and Ta mineralization are followed with forming of rutile by hydrothermal metamorphism.The syenitoid(protolith of leptynites)formed in 829.4Ma and was enriched in Nb and Ta.The metasomatism happened in syenitoid about 795 Ma with the participation of hydrotherm that is rich in volatiles of H2 O,F-and K+.The rutile became crystallization after the forming of F-bearing minerals,followed by the Nb and Ta mineralization.6.Rb is originated from muscovite,one of the main minerals of source of granite.The magma of high F and Rb origined from partial melting of pelite enriched in muscovite in lower crust,the highly evolved magma resulted further enrichment of Rb.With the complexation of F,Rb migrated to the last stage of the developing of magma.Subsequently,the complex of Rb and F hydrolyzed followed by lower and lower temperature.Finally,Rb entered amazonite by isomorphism.7.The leptynites and marble being roof or floor of leptynite are the mark if find Nb,Ta deposits in Souther Qilian area.While,the albitization and amazonite are the clues of finding Rb deposit in Beishan area.