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攀西地区超大型钒钛磁铁矿矿床挥发份组成及其C-H-O稳定同位素研究:对挥发份来源和矿石成因的约束

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Keywords: 磁铁矿,挥发份,C-H-O同位素,超大型钒钛磁铁矿矿床,攀西地区

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Abstract:

?四川攀西地区出露攀枝花、红格和白马三个赋含超大型钒钛磁铁矿矿床的镁铁-超镁铁质层状岩体,岩体底板分别与震旦纪灯影组灰岩或古元古代河口组变质沉积-火山岩系接触.围岩组分是否被带入岩浆、并控制磁铁矿的形成尚不清楚.本文采用分步加热质谱法分别测定了三个岩体中不同类型矿石中磁铁矿、磷灰石、单斜辉石和斜长石中的挥发份组成和含量,以及挥发份中CO2的C-O同位素和H2O的H同位素组成.结果显示,磁铁矿中的挥发份主要由H2O和CO2等组成,其他矿物的挥发份主要由H2O,CO2和H2组成.C-H-O同位素测试结果表明,800~1200℃阶段,三个岩体的δ13CCO2值平均为-7.7‰~-13.5‰,δ18OCO2值平均为19.1‰~19.5‰,介于玄武岩和围岩的C-O同位素组成之间,表明有围岩CO2组分加入.400~800℃阶段,三个岩体的δ13CCO2值(平均为-13.7‰~-17.9‰)和δ18OCO2值(平均为16.2‰~19.2‰)均明显降低,与围岩组分更接近.该阶段大量H2O的释出以及较低的δDH2O值(-90‰~-115‰)也表明有来自围岩的流体组分加入.红格、白马和攀枝花岩体的磁铁矿平均释气总量分别为4891,2996和1568mm3STP/g,远远大于其他矿物的平均释气总量(分别为382,600和379mm3STP/g),这表明磁铁矿结晶时岩浆中更富挥发份.这可能是两种原因造成的:一是岩浆早期大量单斜辉石和斜长石的结晶导致残余岩浆富集H2O等挥发份,二是岩浆演化过程中加入了大量来自围岩的流体组分并被晚期结晶的磁铁矿捕获.因此,磁铁矿的结晶可能晚于单斜辉石和斜长石等矿物,而不是岩浆早期结晶的产物.

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