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

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

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、碾盘及风原无矿化伟晶岩中,黑电气石生长环带发育,单偏光下呈蓝绿色-棕色,具有较高的Mg、Ti、Sc含量,较低Li、Mn、Zn含量,δ^(11)B值变化较大(约-19.00‰~-12.00‰)。街子沟富铍伟晶岩中黑电气石单偏光下呈蓝色-草绿色,Ti、V、Sc含量低于无矿化伟晶岩中黑电气石,δ^(11)B值约-16.00‰~-12.00‰。丰庄伟晶岩具弱铌钽矿化,黑电气石具有核-边结构,单偏光下呈蓝色-深蓝色,比无矿化和铍矿化伟晶岩中黑电气石含有更高Li、Zn含量,δ^(11)B值-21.92‰~-14.75‰。在蔡家沟Li矿化伟晶岩中,黑色-深蓝色电气石Li_(2)O含量达到约1.0%,成分上过渡至黑电气石-锂电气石系列,具有最高Li、Mn、Zn含量,而Mg、Ti、Sc、V含量极低,其δ^(11)B值-18.96‰~-16.89‰。硼同位素分析揭示碾盘、风原及丰庄伟晶岩中存在两类电气石,Ⅰ类电气石富重B同位素,可能为较早从伟晶岩熔体中结晶形成;而Ⅱ类电气石具有更负的δ^(11)B值,可能为伟晶岩流体出溶时由流体中晶出。流体出溶导致围岩中形成热液电气石,其δ^(11)B值与伟晶岩中Ⅱ类电气石相似,表明出溶流体与围岩的作用并未导致显著的B同位素分馏。但伟晶岩与围岩之间的作用,使得Mg、Ti、V进入伟晶岩,同时Li、B、Al进入围岩。在研究区内,从无矿化至锂矿化伟晶岩,随伟晶岩分异程度增加,岩浆成因的黑电气石中Li、Mn、Zn、F含量升高而δ^(11)B值降低,表明黑电气石的Li、Mn、Zn及F含量和B同位素组成可以有效指示伟晶岩矿化类型。

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 δ^(11)B values(-19.00‰~-12.00‰).In the Jiezigou Be pegmatite,the tourmaline which is blue to grass green in plane-polarized light exhibits δ^(11)B 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 δ^(11)B 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 Li_(2)O content up to 1.0%.In this pegmatite,the tourmalines with δ^(11)B 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 δ^(11)B 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 δ^(11)B 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 δ^(11)B.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.