Genensis Of Leucogranite In Xuebaoding Area,Northwest Sichuan

The Xuebaoding deposit which located in the Songpan Ganzi orogenic belt,Southwest China,is famous for its coarse-grained mineral crystals and the unique type of W-Sn-Be mineralization.A large number of studies have been carried out on the Xuebaoding deposit,but there are few about the widely developed Leucogranites in Xuebaoding area.The two-mica granite was formed between 211.4 to 229.8 Ma,which is similar to the age of albite granite and coincides with the age of late-Indosinian.The two-mica granite in Xuebaoding area is characterized by its high content of the Si O₂(60.87-71.64 wt.%),Ca O(1.24-3.04 wt.%),Al₂O₃(14.95-16.20wt.%)and Sr(178-546ppm),and low content of Y(6.13-15.6ppm),Yb(0.46-1.18ppm)and Rb(48.2-155 ppm).The A/CNK values of the two-mica granite are relatively high(1.08-1.23),and the Rb/Sr ratios are low.Combining with the weak Eu negative anomaly,all this evidents indicate that the two-mica granites were formed by partial melting of metasediments in the upper crust caused by the fluid-present melting of muscovite.The albite granite contains low contents of P₂O₅(0.01-0.15 wt.%),Mg O(0.01-0.53 wt.%),Ti O₂(0.01-0.17 wt.%)and TFe₂O₃(0.17-0.76 wt.%).The Si O₂(59.50-74.91 wt.%)contents of the albite granite vary greatly;the Rb/Sr ratios and the A/CNK values(1.09-1.63)are high,and the Zr/Hf and Nb/Ta ratios are low.Such characteristics indicate that the albite granite is a leucogranite which experienced high degree of fractionation.The albite granite and the two-mica granite are characterized by similar high TFe O/Mg O and A/CNK ratios and low Zr and Zr+Nb+Ce+Y contents.Combined with their Sr-Nd-Pb isotopic compositions and the B isotopic compositions of three tourmalines in the albite granite,it indicates that they are S-type granites formed by partial melting of metasediments in the upper crust.The forming sequence of the three tourmalines in Xuebaoding deposit is:type II(formed in undercooled B-rich magma)-type I(from immiscible B-rich aqueous fluid)-type III(formed in B-rich hydrothermal fluid).The compositional variations of the three tourmalines show the increasing of the fractionation degree of the albite granite;the increase oxygen fugacity and in decrase trend of salinity of the hydrothermal.The high degree of fractionation which leads to the enrichment of W,Sn,Be,Li,Rb,Cs,F,B,P in the magama and hydrothermal fluids,combining with the evolution of the magamatic-hydrothermal the ore-forming fluid stages and the surrounding Ca-rich strata,form the Xuebaoding W-Sn-Be deposit.