新疆西天山晚古生代铁矿床的地质特征、矿化类型及形成环境

摘要

西天山成矿带是我国重要的铁多金属成矿带之一,以阿吾拉勒铁成矿带为主体,近年来铁矿勘查工作取得重大进展,相继勘查或发现了查岗诺尔、备战、智博、敦德、松湖、雾岭及尼新塔格-阿克萨依等多个铁矿床,使该地区成为新疆重要的大型铁矿开发基地.这些新发现的铁矿床普遍赋存于安山质熔岩及火山碎屑岩中,规模多数达到大中型,品位较高.总体上,对铁矿床的研究程度普遍很低,成矿环境和成矿规律认识不清,缺乏综合性的总体研究.文章在已有的研究成果基础上,结合笔者研究小组近三年的大量野外调查工作和室内的整理研究,综述了新疆西天山主要铁矿床的地质特征、分布规律、矿化类型.将西天山的铁矿床划分为海相火山岩型和矽卡岩型2个大类,根据矿化类型将海相火山岩型细分为火山沉积型、火山岩浆-热液型、类矽卡岩型3个亚类.初步讨论了西天山铁矿床的成矿地质背景,认为石炭纪晚期可能属于碰撞造山晚期阶段的陆缘弧环境,局部存在挤压-伸展的构造转变,是铁矿形成的有利环境.通过区域铁矿床特征的对比以及与国内外火山岩有关的典型矿床特征的对比研究,认为铁矿床的形成与火山-侵入活动有密切的成因联系,形成时间接近或稍晚于火山活动期;早期阶段以富铁流体(熔体)充填-交代作用成矿为主,晚期热液交代富集成矿,整个成矿过程伴随大量的热液围岩蚀变;成矿物质来源可能以岛弧岩浆作用所携带的深部铁质为主,并含有少量火山-次火山气液交代围岩所萃取的铁质;富铁流体(熔体)可能是由俯冲过程中形成的基性岩浆分异形成的,但具体的形成机制、岩浆起源和演化过程是今后研究的重点.%The Western Tianshan metallogenic belt is one of the significant polymetallic iron metallogenic belts in Chi-a. Notable advances have been made in the exploration of iron ore resources in western Tianshan Mountains, especially in the Awulale metallogenic belt, where several iron deposits have been discovered or explored, such as Chagangnuoer, Beizhan, Zhibo, Dunde, Songhu, Wuling and Nixintage-Akesayi. This region has therefore become an important large-scale iron ore development base area. These newly-found iron deposits, mostly of medium to large size with high-grade, are mainly hosted in the mafic-intermediate andesitic lavas and volcaniclastics. However, these regional iron ore deposits have been insufficiently studied, and hence their mineralization environment and metallogenic regularity are poorly known due to lack of a comprehensive overall study. Based on the previous researches, combined with lots of field investigations and indoor collations in recent three years done by the authors, this paper tentatively makes a comprehensive elucidation of the geological characteristics, distribution patterns and mineralization types of iron deposits in western Tianshan Mountains of Xinjiang. These iron deposits can be divided into two types, namely marine volcanic type and skarn type, and the former can be further divided into three subtypes, i. e. , volcanic-sedimentary, volcanic magmatic-hydrothermal and skarniod subtypes. The authors preliminarily discuss the geological setting of iron metallogeny in western Tianshan Mountains, and hold that this area might belong to the late stage of a collisional orogenic continental environment in Carboniferous period, with the development of the extrusion-extensinal tectonic transition somewhere, and these geological settings were favorable for iron mineralization. Based on comparing the characteristics between iron deposits in this region and typical volcanics-hosted iron deposits both in China and abroad, the authors consider that iron deposits were genetically closely related to volcanic-intrusive activity, and formed contemporaneously with or slightly later than volcanic activity; in the early ore-forming stage, filling-metasomatism of iron-rich fluid (melt) played the major role, and mineralization became intensified by hydrothermal metasomatism in the late stage, accompanied by lots of hydrothermal rock alteration in the whole metallogenic process; mineralizing iron material was probably mainly derived from the depth and from arc magmatism, with minor iron material extracted from volcanic-subvolcanic gas and fluid-wall rock metasomatism; iron-rich fluid (melt) might be created by the mafic magmatic differentiation generated in the subduction; nevertheless, the detailed formation mechanism, magmatic origin and evolutionary process remain the keynote questions deserve further research.