广西大厂地区黑云母花岗岩中电气石的化学组成及其对岩浆热液演化的指示

Chemical composition of tourmaline in the biotite granite,the Dachang district: Insights into magmatic-hydrothermal evolution

广西大厂地区笼箱盖黑云母花岗岩与区内晚白垩世锡多金属成矿作用在时空上密切相关。岩相学特征表明,笼箱盖黑云母花岗岩中的电气石可以分为三类:1)浸染状电气石;2)石英电气石囊;3)电气石石英脉。本文利用电子探针和激光剥蚀等离子体质谱系统测定三种不同产状电气石的化学组成。分析结果显示,三种产状的电气石均具有高的Fe/(Fe+Mg)和Na/(Na+Ca)比值,主体属于碱基亚类铁电气石。浸染状电气石为岩浆晚期结晶,其Fe/(Fe+Mg)比值变化于0.85~0.94,随着岩浆分异,电气石逐渐富集Li、F、Fe和Sn等元素。与浸染状电气石相比,石英电气石囊中早阶段电气石具有低的Fe/(Fe+Mg)比值,高的V、Co和Sr含量,可能反映了岩浆演化晚期出现的不混溶富硼熔/流体对早期黑云母和长石的交代作用,从而使囊中早阶段电气石继承部分被交代矿物的化学组成特征;石英电气石囊中晚阶段电气石的化学组成变化较大(如Li、F、Mg、Al、V、Fe和Zn),与热液成因电气石的推论一致。与浸染状和囊状电气石相比,石英脉中的电气石具有高的Fe/(Fe+Mg)和Na/(Na+Ca)比值;微量元素组成与囊状电气石相似。就成矿元素锡而言,三种产状的电气石均具有相对高的锡含量,与其他地区锡成矿花岗岩中电气石的成分特征相似。但是,从岩浆晚期到热液阶段,大厂地区电气石的锡含量并没有显著升高,可能反映了早期岩浆热液流体对熔体锡有限的萃取作用。

Tin-dominant polymetallic mineralization in the Dachang district is tempo-spatially related to the Late Cretaceous Longxianggai granitic pluton.Three types of tourmaline in the Longxianggai granitic pluton have been identified in this study:1)subhedral disseminated tourmaline in the granite matrix(DT-type);2)subhedral to anhedral tourmaline in the quartz-tourmaline nodule(NT-type);and 3)tourmaline aggregates in the quartz vein(VT-type).We performed major and trace element analysis by electron microprobe(EPMA)and laser ablation inductively coupled plasma mass spectrometry(LA-ICP-MS)on these tourmaline crystals.Compositionally,these tourmalines are characterized by high Fe/(Fe+Mg)and Na/(Na+Ca)ratios,and belong to the alkali group and schorl-dravite solid solution series.The DT-type tourmaline is interstitial between grains of plagioclase,K-feldspar,quartz and biotite,formed during the late stage of granite consolidation.With fractionation of granitic magma,DT-type tourmaline is progressively enriched in Li,F,Fe and Sn.The early-stage NT-1 tourmaline has lower Fe/(Fe+Mg)ratios but higher concentrations of V,Co and Sr compared to those of DT-type tourmalin,which could be attributed to tourmaline replacement of early biotite and feldspar.By contrast,the late-stage NT-2 tourmaline is characterized by variable compositions(e.g.,Li,F,Mg,Al,V,Fe and Zn),consistent with the speculation of hydrothermal origin.The VT-type tourmaline has relatively high Fe/(Fe+Mg)and Na/(Na+Ca)ratios,and similar trace element compositions to those of NT-type tourmaline.Almost all of analyzed tourmalines have comparable Sn concentrations to those of tourmalines from other Sn-bearing granites.However,the absence of further Sn enrichment in VT-type tourmaline in the Dachang district possibly implies the restricted partition of ore-forming elements into early hydrothermal fluids which contribute to tourmaline-quartz veins.