东秦岭花岗伟晶岩中电气石地球化学特征及成矿指示意义

东秦岭地区是我国重要的花岗伟晶岩区及稀有金属成矿区.电气石在东秦岭各类花岗伟晶岩中广泛发育,通常在无矿化伟晶岩、铍矿化及锂矿化伟晶岩中呈黑色-深蓝色.本文旨在通过各类伟晶岩中电气石的对比研究揭示电气石地球化学特征对东秦岭伟晶岩矿化类型的指示作用.本文所研究电气石为作为东秦岭各类伟晶岩贯通矿物的黑电气石系列.在双峰村、碾...

Geochemistry of tourmaline from granitic pegmatites in East Qinling and its implications for mineralization

The East Qinling region is one of the most important pegmatite provinces that host rare-element deposits in China.Tourmaline is a ubiquitous mineral in various types of pegmatites in this area and commonly exhibits black to dark blue colour in barren,Be and Li pegmatites.In this study,we investigated the compositional variation of black tourmaline from barren to Li pegmatites to demonstrate that the geochemical characteristics of the tourmaline can potentially serve as a mineralization indicator.The investigated black tourmaline belongs to schorl series which is ubiquitous in most granitic pegmatites in East Qinling.In barren pegmatites at Shuangfengcun,Nianpan and Fengyuan,the tourmaline exhibits distinct sector zoning and strong pleochroism(bluish green to brown in plane-polarized light)and contains relatively high Mg,Ti and Sc,but low Li,Mn and Zn.The tourmaline from these localities also shows variable δ¹¹B values(-19.00‰~-12.00‰).In the Jiezigou Be pegmatite,the tourmaline which is blue to grass green in plane-polarized light exhibits δ¹¹B varying between-16.00‰and-12.00‰,and contains lower Ti,V and Sc than the tourmaline from the barren pegmatites.In the Fengzhuang pegmatite with slight Nb-Ta mineralization,the tourmaline shows distinct sector zoning and is blue to dark blue in plane-polarized light.The tourmaline from the Fengzhuang pegmatite contains higher Li and Zn than the tourmaline from the barren and Be pegmatites and exhibits δ¹¹B values ranging from-21.92‰to-14.75‰.In the Caijiagou Li pegmatite,the composition of the black to dark blue tourmaline evolves to schorl-elbaite series,with a Li₂O content up to 1.0%.In this pegmatite,the tourmalines with δ¹¹B values ranging from-18.96‰to-16.89‰have the highest Li,Mn and Zn concentrations,while they have extremely low Mg,Ti,Sc and V contents.The results of boron isotopic analysis show that two types of tourmalines exist in the Nianpan,Fengyuan and Fengzhuang pegmatites.The Type I tourmaline is characterized by relatively heavier B isotope that is likely crystallized from the pegmatite-forming melt at an earlier stage,whereas the TypeⅡtourmaline displays more negative δ¹¹B values,which is likely precipitated from the exsolved fluids from the pegmatites.Such fluids also resulted in formation of hydrothermal tourmaline in the country rocks.The hydrothermal tourmaline exhibits δ¹¹B values comparable to those of TypeⅡtourmaline,indicating no significant B isotopic fractionation caused by the interaction between the exsolved fluids and the country rocks.Nevertheless,the interaction between pegmatites and their country rocks resulted in addition of Mg,Ti and V into the pegmatite,and meanwhile migration of Li,B,and Al into the country rocks.In the study area,from barren to Li pegmatites,with increasing differentiation degree of pegmatites,the Li,Mn,Zn and F contents of schorl tourmaline increase and exhibit negative correlation with tourmaline δ¹¹B.This suggests that Li,Mn,Zn and F contents and B isotopic compositions of schorl tourmaline can indicate various pegmatite-hosted rare-element mineralization in East Qinling.