东秦岭卢氏稀有金属伟晶岩的绿柱石矿物学特征及其指示意义

Mineralogical characteristics and significance of beryl from the rare-element pegmatites in the Lushi County,East Qinling,China

绿柱石是重要的铍矿石矿物,记录了花岗伟晶岩型稀有金属矿床的成岩成矿过程。东秦岭伟晶岩区是我国重要的稀有金属产地之一。本文调查了东秦岭卢氏伟晶岩区中的南阳山矿区(703号脉锂矿化伟晶岩和302号脉铍矿化伟晶岩)、七里沟-前台矿区(前台锂矿化伟晶岩)、蔡家沟矿区(大西沟和韭菜沟锂矿化伟晶岩)和瓦窑沟矿区(西山沟和瓦窑沟铍矿化伟晶岩)的伟晶岩内部结构分带,认为东秦岭稀有金属伟晶岩主要为过铝质LCT型伟晶岩,属于稀有金属(REL)类REL-Li亚类。其中,703号脉、韭菜沟和大西沟伟晶岩属复杂型锂辉石亚型,前台伟晶岩属钠长石-锂辉石型,302号脉、瓦窑沟和西山沟伟晶岩属绿柱石型绿柱石-铌铁矿亚型。电子探针结果表明绿柱石富碱金属,贫铁和镁。绿柱石元素替代机制包括通道-八面体替代、通道-四面体替代以及通道中碱金属阳离子间的置换。西山沟和瓦窑沟绿柱石的替代机制分别是Na(Fe2+,Mg)-1Al-1和NaLi-1Be-1。302号脉、前台、大西沟和韭菜沟绿柱石的替代机制为(Na,Cs) Li-1Be-1。703号脉绿柱石的替代机制包括NaFe2+-1Al-1、NaCs-1和(Na,Cs)Li-1Be-1。绿柱石的Cs2O含量和Na/Cs值揭示伟晶岩分异演化程度序列(由低至高)为瓦窑沟矿区→302号脉铍矿化伟晶岩→蔡家沟矿区→前台→703号脉锂矿化伟晶岩。铍矿化伟晶岩岩浆分异演化程度低于锂矿化伟晶岩岩浆。背散射图像显示绿柱石内部分带多样,包括均一结构、条带状、正/反蚀变边、补丁分带和复杂不规则分带。与铍矿化伟晶岩相比,锂矿化伟晶岩产出的绿柱石内部分带复杂多样,反映更为强烈的液相不混溶和交代作用。随伟晶岩岩浆分异演化程度升高,绿柱石FeO含量降低,内部分带更为复杂,发育蚀变边结构、补丁分带和不规则分带等。绿柱石FeO含量和内部分带特征可作为花岗伟晶岩分异演化程度的潜在指示标志。锂矿化伟晶岩中绿柱石的化学组成和内部分带特征表明岩浆就位时是高度分异演化的稀有金属伟晶岩岩浆。大西沟、韭菜沟和前台锂矿化伟晶岩岩浆就位后未经历明显分异演化过程,而南阳山703号脉伟晶岩岩浆就位后经历了较充分的分异演化,导致稀有金属的进一步富集。锂矿化伟晶岩的稀有金属成矿机制是结晶分异和液相不混溶。

Beryl which is one of the most important beryllium minerals records the rock-forming process and rare-element( REL)mineralization of granitic pegmatites. East Qinling Lushi pegmatite district is a famous rare-element producing area in China. This study investigated the internal zonation patterns of the pegmatite dykes from the East Qinling,including No. 703 Li-mineralized pegmatite and No. 302 Be-mineralized pegmatites in Nanyangshan,Qiantai Li-mineralized pegmatite in Qiligou-Qiantai,Daxigou and Jiucaigou Limineralized pegmatites in Caijiagou,Xishan’gou and Wayaogou Be-mineralized pegmatites in Wayaogou. These pegmatites belong to peraluminous LCT-family and REL-Li subtype. The Nanyangshan No. 703,Jiucaigou and Daxigou pegmatites belong to complex type and spodumene subtype,while the Qiantai pegmatite belong to albite-spodumene type. The Nanyangshan No. 302,Wayaogou and Xishan’gou pegmatites belong to beryl type and beryl-columbite subtype. The EMPA results show that beryls are enriched in alkali and poor in iron and magnesium. The substitution mechanisms of beryl from the REL pegmatites in the East Qinling are channel-octahedral,channel-tetrahedral and alkali cations in channel. The substitutions of beryl from the Xishan’gou and Wayaogou are Na( Fe2+,Mg)-1 Al-1 and NaLi-1Be-1,respectively. The beryls from the No. 302,Qiantai,Daxigou and Jiucaigou pegmatites mainly display substitution of( Na,Cs) Li-1Be-1,while the substitutions:( 1) NaFe2+-1Al-1,(2) NaCs-1 and( 3)( Na,Cs) Li-1Be-1 were revealed for the No. 703 pegmatite. The Cs2O contents and Na/Cs values indicate a sequence of the decreasing degree of fractionation and evolution of pegmatites,which is No. 703 → Qiantai →( Daxigou and Caijiagou)→ No. 302 →( Xishan’gou and Wayaogou). Li-mineralized pegmatitic magma is more highly fractionated than those of beryllium mineralization. The internal zonation patterns of beryls are homogeneous grain,layered zoning,alteration borders,patches and complex irregular zoning in BSE images.Compared with the Be-mineralized pegmatites,the beryls from the Li-mineralized pegmatites show more complex internal zonation patterns,reflecting strong liquid immiscibility and replacement. With the increase of degree of fractionation and evolution of pegmatitic magma,the FeO contents of beryl decrease and the internal structures of beryl become more complex,such as alteration border and irregular zoning. The Fe O content and internal structure of beryl could be potential indicators of the degree of fractionation and evolution of pegmatites. The chemical compositions and internal structure features of beryl suggest that Li-mineralized pegmatitic magma is highly fractionated REL pegmatitic magma when it is emplaced. The Daxigou,Jiucaigou and Qiantai Li-mineralized pegmatites did not undergo obvious fractionation,while the No. 703 pegmatites experienced fractionation and evolution after emplacement to finish further REL-enrichment. The lithium mineralization mechanisms are fractional crystallization and liquid immiscibility.