新疆智博铁矿区富铁熔岩及其对铁成矿的约束

Awulale iron metallogenic belt, an important reservoir of iron resources, is located in the west Tianshan, Xinjiang, NW China. Although considerable investigations have been carried out for the carboniferous submarine volcanic-hosted iron deposits within Awulale iron metallogenic belt, the genesis of this type of iron deposit remains controversial, and the genetic relationship between volcanic rocks and iron mineralization is inexplicit. Recently, some fresh volcanic rocks with compositional spectrum from mafic to intermediate to felsic are recognized in the periphery of the Zhibo ore district. These volcanic rocks, bearing iron-rich lava breccia fragments and magnetite, are coeval with the mineralization of the Zhibo iron deposit. Iron-rich lava breccia fragments are composed of albite phenocryst and iron-rich matrix. These volcanic rocks, iron-rich lava breccia and iron ores likely make up an association, which provides an ideal opportunity to investigate the relationship between magmatic process and iron mineralization. Consequently, a comparison of the volcanic rocks, iron-rich lava breccia and iron ores in major/trace elements and Sr-Nd isotopes will be presented, as well as magnetite/albite from them will also be compared in major/trace elements (in situ EPMA, La-ICPMS and X-ray mapping), in an attempt to discuss the genetic relationship between them. As a consequence, the relationship of magmatic evolution and iron mineralization will be clarified, and the mineralization mechanism of magnetite ores in this area will be constrained.

新疆西天山阿吾拉勒铁成矿带是我国重要的铁矿资源带，前人在此做了不少工作，取得了一系列成果，但对该区石炭纪海相火山岩型铁矿形成机制尚存在较大争议，且对于与铁矿密切相关的火山岩、以及铁矿的形成与火山岩演化的关系研究较少。新疆智博铁矿区外围发育一系列与铁矿同时代的组份连续完整的火山岩，这些火山岩中广泛发育磁铁矿，部分火山岩中发育富铁熔岩角砾（由富铁基质和钠长石斑晶组成），这与矿区的铁矿石一起构成了一套火山岩-富铁熔岩角砾-铁矿石组合。本项目拟对这套火山岩-富铁熔岩角砾-铁矿石组合开展对比研究，包括三者的元素、Sr-Nd同位素地球化学以及单矿物（磁铁矿、钠长石）的原位元素地球化学研究（电子探针、激光探针等离子质谱和X射线能谱扫面分析），厘定三者可能存在的成因演化关系，进而为该区铁矿的形成机制提供新的制约。

海相火山岩型铁矿是一种重要的铁矿类型，然而该类型铁矿的成因认识尚未达成共识。本项目以新疆智博铁矿火山岩、富铁岩屑、铁矿石为研究对象，主要进行了地质特征、岩石地球化学、矿物地球化学以及年代学研究，厘清了三者的成因联系，完善了该类型铁矿理论模型。具体认识如下：智博中基性火山岩为一套成分连续变化的拉斑-钙碱性岩石，形成于314±2Ma。火山岩中单斜辉石斑晶的反环带(Mg#0.72-0.84)和斜长石斑晶的高MgO边，以及不同岩性间截然接触界线，表明有多次来自深部基性岩浆的补给，这为铁矿形成提供了持续的铁质来源。富铁岩屑主要发育于火山碎屑岩中，由钠长石和富铁基质组成，自形钠长石在富铁基质中呈斑晶或交织状分布。电子探针分析显示富铁岩屑中钠长石端元组分类似于火山岩中钠长石端元组分，两者均为岩浆成因钠长石。富铁基质中FeO含量变化较大，从24.89%到87.13%连续变化，具有从富铁岩浆逐渐演化为磁铁矿的趋势；富铁基质SiO2含量最高为50.04%，与安山岩类SiO2含量相近，与矿区赋矿岩石SiO2含量也相符，表明了铁的富集与安山质火山岩的演化密切相关。富铁基质中高Ti磁铁矿和低Ti磁铁矿同时发育，总体上成分不均一，且钠长石呈细针状，为浅成-超浅成低压下快速结晶的产物或为火山喷发情况下快速冷凝结晶所致。钠长石-磁铁矿富铁岩屑的发育是一种碱铁效应的表现，碱铁效应对于海相火山岩型铁矿的形成很可能起到一定作用，具有重要意义。相比中基性火山岩和围岩中的磁铁矿，矿石中的磁铁矿具有较低的Al、Mn、Ti、V含量，暗示铁矿石中的磁铁矿形成的温度比前两者低，而氧逸度比前两者高，结合矿石中磁铁矿氧同位素组成δ18OSMOW集中在1‰-3‰之间，表明智博铁矿形成于岩浆作用主导的晚期岩浆/岩浆-热液环境，后期低温热液作用对铁的成矿作用影响有限。此外，备战铁矿的金云母辉石橄榄岩的研究，表明该区铁矿的形成与深部基性岩浆有一定关联。综上所述，智博铁矿为海相火山岩浆-热液型铁矿。