新疆西天山备战基性-超基性岩矿物地球化学研究及其对铁成矿作用的制约

Mineral geochemistry of Beizhan mafic-ultramafic rocks,West Tianshan Mountains,Xinjiang:Constraints on genesis of Beizhan iron deposit

文章对备战铁矿区内的基性-超基性岩、中基性岩脉、矿体围岩和铁矿石中的辉石、橄榄石、金云母、铁钛氧化物进行了电子探针分析与显微特征研究。金云母辉石橄榄岩中辉石的化学成分在w(SiO2)-w(Al2O3)图中均落于亚碱性系列区域,在w(Al2O3)-w(Na2O)-w(TiO2)图中,辉石主要落在拉斑玄武岩系列区域,表明该区岩浆经历了拉斑玄武岩系列演化。备战金云母辉石橄榄岩中橄榄石的w(FeO)较低,介于19.22%~23.79%,w(MgO)较高,介于37.35%~41.30%,Fo变化介于0.74~0.79,属于贵橄榄石。橄榄石中较低的w(FeO)表明其形成于较高的氧逸度环境,而岩浆的拉斑玄武岩系列演化一般发生于低氧逸度条件,综合分析岩浆经历了从低氧逸度到高氧逸度变化的过程。随着岩浆的演化,橄榄石Fo与w(Ni)由负相关变为正相关又变为负相关关系,其中负相关关系表明在岩浆演化过程中橄榄石与粒间硫化物熔浆发生过Ni-Fe交换反应,从另一方面可以认为岩浆中含有丰富的硫,岩浆中的高硫特征很可能是备战磁铁矿为较纯磁铁矿(低Ti)的原因之一。此外,成矿岩浆具有高氧逸度特征,铁钛氧化物固溶体在亚固相条件下的氧化作用使固溶体发生分离以及铁磷络合物的发育等因素是造成矿区磁铁矿为较纯磁铁矿(成分接近分子式Fe3O4)的原因。金云母辉石橄榄岩和铁矿石中均发育金云母,表明铁矿与基性-超基性岩的源区都是富含挥发分的。该区铁钛氧化物主要有4种:较纯磁铁矿、含钛铬磁铁矿、铬铁矿、钛铁矿。金云母辉石橄榄岩中的较纯磁铁矿(低Ti),与铁矿石中磁铁矿成分类似,表明两者具有成因联系。结合金云母辉石橄榄岩的显微特征分析,在岩浆阶段曾有一期富铁镁岩浆的加入,这很可能为备战铁矿的形成提供了主要的铁质来源。

In this paper,electron microprobe analyses and microscopic characteristics of pyroxenes,olivines,phlogopites and Fe-Ti oxides from mafic-ultrabasic rocks,intermediate-mafic dikes,wall rocks and iron ores are presented.Combined with pyroxenes falling in the area of subalkaline series in the w(SiO2)-w(Al2O3)diagram and pyroxenes dominantly plotted in the area of tholeiite series in the w(Al2O3)-w(Na2O)-w(TiO2)diagram,the authors hold that the magma in this area has experienced the evolution trend of tholeiite series.The olivines from phlogopite pyroxenes peridotite display low w(FeO)content ranging from 19.22%to 23.79%and high w(MgO)content ranging from 37.35%~41.30%,as well as variable Fo of 0.74~0.79.The lower w(FeO)content in olivine indicates that it was formed under a higher oxygen fugacity condition;however,the evolution trend of tholeiie series of magma generally occurred under the low oxygen fugacity condition,which suggests that magma underwent a process from low oxygen fugacity to high oxygen fugacity.With the evolution of magma,the relationship between Fo and w(Ni)in olivine changed from negative correlation to positive correlation and then to negative correlation.The negative correlation between Fo and w(Ni)indicates that olivine might have experienced Ni-Fe exchange reaction with intergranular sulfide melt during the magmatic evolution.On the other hand,it can be considered that the magma was rich in sulfur,and the high sulfur feature in the magma seems to be probably one of the reasons why the magnetite from the ore district is pure magnetite(low Ti).In addition,the ore-forming magma had the characteristics of high oxygen fugacity,thus the solid solution separation caused by the oxidation exsolution under the subsolidus condition and the occurrence of iron-phosphorus complex seem to be the reasons that the magnetite in the mining area is relatively pure magnetite(the composition is close to the molecular formula Fe3O4).Phlogopite pyroxene peridotites and iron ores both contain phlogopites,indicating that the sources of iron ores and mafic-ultramafic rocks were all volatile-rich.There are four kinds of Fe-Ti oxides in this area,i.e.,pure magnetite,titanochrome magnetite,chromite and ilmenite.The composition of the relatively pure magnetite(low Ti)from phlogopite pyroxenes peridotite is similar to that of magnetite from iron ore,indicating that there is a genetic link between them.The analysis of the microscopic characteristics of phlogopite pyroxene peridotite suggests that there was addition of iron-and magnesium-rich magma to the ore-forming magma,which probably provided the main source of iron for the Beizhan iron deposit.