Study On Nb-Th Rich Accessory Minerals In Suzhou A-type Granites

Suzhou A-type granites are located about 8 km west of Suzhou,China,with an exposed area of about 50km2.The geodetic tectonics are located on the west side of eastern China,the Western Pacific plate and the subduction zone of the continental plate of China,in the Yangtze-Qian tang Paleozoic Depression Zone and the southeast of the tension fault of the Yangtze River.In Suzhou A-type granites,high level mineralization of rare metals minerals such as Nb,Ta,Zr,Hf,and Th has been observed,Nb and Ta especially.Previous studies about Suzhou A-type granites covered many research fields in petrology,mineralogy,ore deposits,and geochemistry.However,comparing with international studies on mineralogy of A-type granites and studies of other series of A-type granites in the east of China,the study of mineralogy in Suzhou A-type granites has been relatively insufficient.Therefore,in this thesis,I investigated the accessory minerals and the mineral combinations of Suzhou A-type granites,and focused in Nb-rich("hydroplumbobetafite")and Th-rich(thorbastnaesite)minerals.In this study,granites sampled from Suzhou Lingyanshan,Tianpingshan,Gaojingshan and Xiaowangshan,most of them are biotite granites and albite granites.The dominates of Nb-rich and Th-rich accessory minerals in research field are ferrocolumbite,"hydroplumbobetafite",thorite and thorbastnaesite.In this thesis,the state of occurrence,chemical composition,crystal structure,evolution mechanism and other characteristics of "hydroplumbobetafite" were studied by X-ray diffraction(XRD),Fourier Transform Infrared spectroscopy(FT-IR),Electron Probe Micro-analysis(EPMA).The research focused on some important points of"hydroplumbobetafite",such as the existence form of water,the mutual substitution of A and B cations,the element evolution trend of Ti to Nb,crystal chemistry occupation of Silicon,and the crystallization variation with heating conditions.By discussing the unit cell variation and chemical composition changes with cations ionic radius of the pyrochlore supergroup minerals,revealed there are some relations between cations and crystal structure.Thorbastnaesite,on the other hand,the studies mainly concentrated on its occurrence state,phase transitions and mineralizition mechanism.Combining with the asymmetry of the mineral metasomatism,differences of the species and content of the elements of each rings,and geochemical characteristics of Suzhou A-type granites,I could inferred that thorbastnaesite is secondary minerals which metamorphosed from thorite.This procedure can be assumed as low-to-medium temperature and F-and CO32-rich fluids metamorphism.