新疆西天山晚古生代主要磁铁矿床(点)成因类型与成矿过程探讨

新疆西天山晚古生代磁铁矿带是中国重要的铁矿带,其成矿地质背景与成因类型一直以来存在很大的争论.本文在已有研究成果的基础上,结合大量野外调查资料和室内研究工作,对西天山主要磁铁矿床的成因类型、成矿背景、成矿规律进行了详细研究.研究表明,主要磁铁矿床矿石矿物Sr、Nd、Pb同位素基本落入洋陆俯冲碰撞下的岛弧环境;矿石形成年龄介于火山岩与中酸性侵入岩之间,接近于火山岩年龄,矿石与火山岩具有密切的成因联系.矿床总体归为海相火山岩型铁矿,可划分出3个亚类:火山喷溢型、火山-次火山热液型、火山喷溢-热液叠加型,不同亚类矿床具有不同的矿体、矿石特征.铁矿石的形成与俯冲带流体的交代作用有着密切的关系,早期为富铁岩浆交代后分异结晶作用的产物,后期则为火山热液沿断裂、裂隙交代、卸载的产物.

A tentative discussion on the genetic type and ore-forming process of main late Paleozoic magnetite deposits in West Tianshan Mountains, Xinjiang

Absract: The late Paleozoic magnetite ore belt is an important iron metallogenic belt in West Tianshan Mountains, Xinjiang. For a long time, there has been controversy concerning the metallogenic setting and genetic type. Based on previous researches in combination with lots of field investigations and indoor studies conducted by the authors in the past three years, this paper deals tentatively with the geological characteristics, distribution patterns and metallogenic types of iron deposits in West Tianshan Mountains, Xinjiang. The Sr-Nd-Pb isotopic compositions of ore minerals show that the metallogenic setting was the island arc under the subduction-collision of ocean and land. The metallogenic age of iron ores was younger than that of volcanic rocks but older than that of intermediate-acid intrusive rocks in the same ore district. The iron material might have come from the volcanic rocks, where their partition models were very similar in trace elements and REE. These iron deposits all belong to marine volcanic type, which can be further divided into three subtypies: volcanic effusion type, volcanic-subvolcanic hydrothermal type, superimposition type of volcanic effusion and volcanic hydrothermal solution. The geological features of different subtype deposits are not different. The iron ores were relatied to the metasomatism of fluid along the zone of subduction. Early iron ores were probably derived from the fractional crystallization of Fe-rich magma after replacement, and the late ores might have resulted from the metasomatism and precipitation of volcanic hydrothermal solution along faults and fractures.