华北南缘早前寒武纪赵案庄铁矿床的成因矿物学研究

The early-Precambrian Zhaoanzhuang apatite-bearing iron deposit, hosted in ultramafic rocks in Zhaoanzhuang, Wuyang area in the southern part of the North China Craton, is characterized by low-Ti magnetite, different from commonly Ti-rich magnetite of magmatic origin. It has long been controversial on the origin of the deposit. Magmatic, hydrothermal origins and modified banded iron formations (BIF) have all been proposed for the formation of this deposit. Among them, the most debated questions are: 1) whether the ultramafic rocks in Zhaoanzhuang are magmatic rocks; 2) how the low-Ti magnetite formed and their genetic relationship with the ultramafic rocks. Genetic mineralogy study is the key to investigate the origin of this Zhaoanzhuang iron deposit with such complex mineral assemblages. This proposed project will conduct in-situ major and trace elemental analyses on the magnetite, apatite, olivine and pyroxene from the Zhaoanzhuang iron deposit, together with systematic inclusion studies and in situ Sr-Nd isotopic analyses on apatite and O isotopic analyses of magnetite mineral separates. By studying the chemical variations of the constituent minerals from bottom to top and the host of titanium, we could provide constraints on the physical and chemical conditions of the ore-forming process, and the origin of low-Ti magnetite. The ultimate goal is to establish the metallogenic model of the Zhaoanzhuang iron deposit. This project will not only provide a more complete understanding on the metallogenic theory of magnetite-apatite deposits, but also provide scientific supports for regional mineral exploration.

华北南缘早前寒武纪赵案庄铁矿是赋存于超镁铁质杂岩中富含磷灰石的磁铁矿矿床，磁铁矿具有低Ti特征，不同于典型岩浆型铁矿床中磁铁矿富Ti。长期以来，其成因存在岩浆型、热液型和BIF叠加改造型等不同认识。争议焦点是：超镁铁质杂岩是否属于岩浆成因？低Ti磁铁矿如何形成、是否与超镁铁质杂岩有成因联系？解析矿物组成复杂的赵案庄铁矿的成因，开展成因矿物学研究是最佳途径。本项目拟在详细的钻孔编录及岩相学观察基础上，以磁铁矿、磷灰石、辉石和橄榄石等典型矿物为研究对象，通过矿物微区原位元素组成和包裹体岩相学及成分分析，结合磷灰石原位Sr-Nd和磁铁矿单矿物O同位素组成，查明不同矿/岩石中矿物的成分变化规律及Ti的赋存状态，揭示矿物的成因，探究磁铁矿贫Ti的机制，建立赵案庄铁成矿模式。本项目的实施不仅为丰富磁铁矿-磷灰石成矿理论提供重要素材，而且可以为华北南缘前寒武纪铁矿找矿预测提供科学依据。

华北南缘早前寒武纪赵案庄铁矿是赋存于超镁铁质杂岩中富含磷灰石的磁铁矿矿床，磁铁矿具有低Ti特征，不同于典型岩浆型铁矿床中磁铁矿富Ti。长期以来，其成因存在岩浆型、热液型和BIF叠加改造型等不同认识。争议焦点是：超镁铁质杂岩是否属于岩浆成因？低Ti磁铁矿如何形成、是否与超镁铁质杂岩有成因联系？本项目开展了详细的矿物学工作，获得如下主要结论：1>超镁铁质杂岩中的橄榄石具有岩浆成因的Fo值、斜方辉石具有岩浆成因的成分，表明赵案庄超镁铁质岩石属岩浆成因，但并不是典型地幔橄榄岩；2>磷灰石具有高F、高REE总量、高Sr同位素组成和负εNd(t)值，指示岩浆热液成因；3>磁铁矿分为两种成因类型：低δ18O值（3.0-3.6‰）和高δ56Fe值（0.219-0.319‰）的岩浆成因磁铁矿和高δ18O值（4.4-4.5‰）的岩浆热液成因磁铁矿；4>磁铁矿低Ti的原因：一是氧逸度条件的改变，使得Ti元素以钛铁矿矿物独立形式存在；一是原始岩浆具有低Ti特征；5>提出Fe-C-O的铁矿浆在高温 (800-900℃) 条件下结晶出低Ti磁铁矿的成因模式。本项目的实施不仅为丰富磁铁矿-磷灰石成矿理论提供了重要素材，而且为华北南缘前寒武纪铁矿找矿预测提供了科学依据。