The Comparative Study Of Diagenetic,Metallogenic Characteristics And Tectonic Environment Of Two Kinds Of Pegmatites In China

The pegmatite deposits are distributed both in orogenic system and continent block domains,and mainly in orogenic system.In this dissertation,we chose Jiajika rare metal deposit,Zhongzuo muscovite deposit and Madi rare metal granite as typical deposits.The contrast research about characteristics of diagenesis,mineralization and tectonic environment between pegmatites in orogenic system and continent block domains was made on the basis of new technology and new methods,such as lithium isotope,boron isotope and U-Pb aging of monazite.We obtained some basic understanding as follows.The micas from two-mica granite of Jiajika are magmatic micas,they are extremely rich in Al.The biotites belong to siderophyllites(but close to protolithionite),and muscovites belong to pure muscovites(not rich in Li).According to test data,we make use of total-aluminium geobarmeter and titanium content geothermometry to calculate pressure and temperature of rock forming.The results show that the emplacement pressure is vary from 430MPa to 560MPa,the average pressure is 480MPa;the crystallization temperature is vary from 480 ? to 550 ?,the average temperature is 520?.In addition,according to the chemical compositon of biotites,we calculated the oxygen fugacity of magma-hydrothermal system is between 10-17 to 10-18.The result also shows that the oxygen fugacity is become bigger from south to north.R cluster analysis of trace elements shows that Li,Rb,Ti,W and Mn have closest relation with metallogeny of rare metal.The changing of rare elements content suggest that the north of the granite has higher metallogenic efficiency than the south.Therefore,the north area of the granite should be pay more attention.According to the study of strontium,neodymium,hydrogen and oxygen isotope,the source of magma derived from the Triassic Xikang group sandstone and mudstone as representatives of the crust partial melting.The ?-7Li value of No.3 pegmatite vein is from-1.70‰ to+3.79‰,and the ?7Li value of Jiajika two-mica granite is from-1.56‰ to +0.90‰.The ?7Li value of No.3 pegmatite and two-mica granite are both have characteristics of higher content of lithium and lower ?7Li value.The ?7Li value of No.3 pegmatite is very close to Jiajika two-mica granite.This can not only add new evidence for homology of them,but also provide suggestion that metallogenic fluid of pegmatite come from two-mica granite.The crystalline age of Quyang pegmatite is about 2506Ma,which belongs to Neoarchean.The age is obviously older than pegmatites in orogenic system.This shows that the formation of pegmatites are closely related to the evolution of tectonic environment.The chemical composition of tourmaline from electronic probe and X-ray powder diffraction show that the type of tourmaline is dravite(but close to schorl).The?11B value of Zhongzuo tourmaline is from-10.5‰ to-7.3‰,which shows the dravite in pegmatite veins was forming in the process of assimilation blending between magmatic melt with surrounding rock,that the temperature is high(700??600?),Fe?Mg in surrounding rock provide an necessary source for the forming of tourmalines.The oxygen isotope value of pegmatite change from 11.400‰ to 13.00‰,hydrogen isotope value change from-67‰ to-96‰.The fluid source maybe mix with metamorphic water and magmatic water.The precise U-Pb crystalline age of monazite of Madi rare metal granite is 175.6Ma,which belongs to Yanshanian.Madi rare metal granite is high-K(Calc-Alkaline)peraluminous S-type granite.According to the study of strontium and neodymium isotope,the source of magma derived from the crust partial melting.The lithium content of Madi rare metal granite vary from 181ppm to 1022ppm,?7Li value are vary from +2.99‰ to +5.83‰.Madi rare metal granite has different characteristics with Jiajika two-mica granite.Madi rare metal granite has higher lithium content and higher ?7Li value than Jiajika.Comparative study shows that the above deposits have following similarity and difference.In the aspect of similarity,Jiajika two-mica granite and Madi rare metal granite are both high-K(Calc-Alkaline)peraluminous S-type granite.The differences are as follows.Firstly,in terms of metallogenic types and temperature-pressure condition of ore-forming,Jiajika is rare metal pegmatite and Zhongzuo is muscovite pegmatite.Jiajika deposit was formed in condition of low oxygen fugacity,and lower temperature-pressure condition than Zhongzuo.Secondly,in terms of crystalline age,Jiajika was formed in the late indosinian,Quyang was formed in Neoarchean and Xinglong was formed in Yanshanian.Jiajika represents mature pegmatite during the evolution of earth,however,Zhongzuo and Madi represents early pegmatite during the evolution of earth.Thirdly,in terms of lithium isotope,Jiajika has characteristic of high 6Li,and Madi has characteristic of high 7Li.We preliminarily consider that the pegmatites which derived from ancient strata have characteristic of high 7Li.Jiajika has characteristic of high 6Li may be associated with Triassic basement.In terms of tectonic environment,pegmatite-type deposits need relative stable tectonic environment.While continent block domains has stable tectonic environment,but it is too stable to keep continuous magmatic activity.So continent block domains is not avail to the formation of pegmatite deposits.Although orogenic system has active tectonic activity,this make the ore magma evolution can be sustained.Relative quiet period after orogeny concentration tends to form large and superlarge pegmatite deposits.Based on the study of metallogenic tectonic background of pegmatite lithium deposits,the author put some suggestions that tectonic dome,anticlinorium,synclinorium,intersection of fractures often mineralization.It was pointed out that the coupling and identification of pegmatite rare metal deposits and orogenic process,the comparison of the rare metal deposit in China and foreign is the further study orientation of the metallogenic tectonic background of pegmatite lithium deposits.