Evolution Of Leucogranites In The Cuona District,South Tibet And Related Rare Metal Mineralization

Cuona district is located in the eastern Himalayan orogen,south Tibet,at the boundary of the Higher Himalayan crystalline-rock sequence（HHS）,and The Tethyan Himalayan Sequence（THS）,the north-south extension of South Tibet detachment system（STDS）through the district.The leucogranites have been interpreted to be primary melts that were produced by the partial melting of the subducted HHS in depth,and subsequently fractional crystallization associated with the exhumation of the partially molten rocks.Cuona district includes Cuonadong,Kuqu,and Cuona plutons.The margins of the plutons are low-grade metamorphosed sedimentary,the leucogranites in the dome are exposed in the granitic gneiss and slate,granitic pegmatite dikes（including aplite in the dike margins）intruded the leucogranites.The research is about leucogranitic rocks of the three plutons in the cuona district,including two-mica granite,muscovite granite,albite granite and pegmatite（aplite）,as well as skarn and nearby altered rocks.In this study,whole rock major-trace-element compositions and mineral compositions and textures indicate the fractionation trend of leucogranites and the type of rare matal mineralization.Monazite and columbite U-（Th）-Pb chronology clarifies the rock-and ore-forming ages and indicates the relationship between multiple mineralization.Meanwhile,the indicator minerals which show the metallogenic specialization of rare matel estimate the metallogenetic potentiality in leucogranite.These leucogranitic rocks have high Si O₂（～73 wt.%）,Al₂O₃（～16 wt.%）,and total alkali contents（～9 wt.%）,and with A/CNK ratios of 0.95-1.2.Moreover,these leucogranites have low total REE concentration and Chondrite-normalized REE patterns all have negative Eu anomalies.In Cuonadong dome,whole-rock major and trace element data and mineral compositions reveal strong differentiation and fractionation trends for the two-mica granite,muscovite granite,and pegmatite-aplite dikes.Albite,muscovite,and garnet contents increased as the leucogranitic magma evolved,whereas K-feldspar and biotite contents decreased.Muscovite in aplite has higher Rb₂O（～0.4 wt.%）,Nb（～219 ppm）,and W+Sn（～737 ppm）contents,and lower Nb/Ta ratios（～6）than muscovite in the two-mica and muscovite granites.Garnet varies in composition from almandine in the leucogranites to spessartine in the aplites.Similarly,the whole-rock parameters and mineralogical characteristics of the Kuqu leucogranitic rocks reflecting the evolution of the two-mica granites through the muscovite granites to the albite granites.Muscovite in albite granite has higher Rb（～2345 ppm）,Cs（～514 ppm）,Nb（～254 ppm）and Sn（～767 ppm）,and rare metal minerals such as Li-Cs beryl and Nb-Ta oxide appear in albite granite.In leucogranite-pegmatite of Cuona district,the rare metal-bearing minerals include beryl,columbite-group minerals,tapiolite,Nb-Ta-bearing rutile,Pyrochlore-microlite,cassiterite,and wolframite.Beryl,scheelite,and cassiterite occur in the skarn and altered rocks（i.e.,chloritized）rocks along the margins of the leucogranites in Cuonadong dome.There are multiple magmatic activities and associateed mineralization in Cuona district.The U-Th-Pb dating results of monazite in Kuqu show the earliest magmatic activity,mainly at ca.25-24 Ma,and there is also leucogranitic magmatism at ca.20Ma.Monazite U-Th-Pb dating reveals two episodes of leucogranitic magmatism in the Cuonadong dome at ca.20 and 17 Ma,columbite U-Pb dating indicates two stages of rare-metal mineralization at ca.17 and 14 Ma（including tapiolite）.Columbite U-Pb dating in the Cuona dome shows the last rare-metal mineralization at ca.12 Ma.Rare-metal mineralization in the Cuonadong dome can be divided into two types:magmatic Be-Nb-Ta mineralization related to pegmatiteeaplite dikes and hydrothermal Be-W-Sn mineralization hosted in the skarn（associated with the two-mica and muscovite granites）.It is inferred that magmatism was related to the exhumation of the Higher Himalayan Crystalline Basement,with the degree of differentiation of the associated leucogranites being the key factor controlling the type of mineralization.Be-Nb-Ta mineralization in the Kuqu is also closely related to highly differentiated leucogranite and pegmatite（aplite）.Meanwhile,Kuqu has the potential to offer Li,Sn elements when mineralized.The Cuona dome pegmatite is related to Be-Nb-Ta mineralization,and tapiolite has been discovered.