河北大庙蚀变斜长岩及矿石中锆石成因、形成机制及年代

The well-known Damiao-type iron deposits occur in the Damiao anorthosite complex in Chengde area, Hebei province. In addition to the orebodies composed of disseminated iron ores, there are also abundant orebodies composed of massive ores that generally fill the vertical fractures of the altered white-colored anorthosite as irregular dykes or veins with various shapes and sizes. The wall-rcok alteration is spatially related to vein-like mineralization, but it is not clear whether they have a genetic relationship or not. Determination the age of alteration and mineralization is of vital importance. This application is based on newly discovered zircon in the fracture of ilmenite of the altered white-colored anorthosite, zircon in massive ores, and baddeleyite in the unaltered dark-colored anorthosite. Systematic petrographical observation will be carried out to ascertain the internal structure and external morphology of zircon and baddeleyite, and then to identify the genesis. A variety of micro and in-situ analyses of major and trace element characteristics of zircon and associated mineral assemblages will be carried out to explore the formation of albitization and zoisitization of the anorthosite, Zr and Si source of zircon, the influence of volatile on the activity of Zr and Si, and then to reveal the mechanisms for generating zircon; Specific selection and high-precision zircon dating methods will be carried out to determine the age of alteration and high-grade mineralization, and then to discuss their relationship. The application can provide important support both to the regularity of ore formation and to deep prospecting for Damiao-type iron deposits.

冀北大庙斜长岩体中产出著名的大庙式铁矿。大庙式铁矿除浸染状贫铁矿外，还有大量富铁矿脉充填在钠黝帘石化斜长岩中的断裂裂隙中。钠黝帘石化与脉状富铁矿空间关系密切，但它们是否具有成因联系尚不明确，确定蚀变年代和成矿时代是关键。本申请以新发现的产于钠黝帘石化斜长岩中钛铁矿裂隙的锆石、富铁矿脉中的锆石、未蚀变斜长岩钛铁矿中的斜锆石为主要研究对象：通过锆石和斜锆石的外部形态、内部结构及与其它矿物的相互关系的系统岩相学观察，查明锆石和斜锆石的赋存状态，确定其成因；采用多种微区原位测试技术研究锆石及相关蚀变矿物组合的主微量元素特征，探讨钠黝帘石化蚀变流体性质、锆石Zr和Si来源、挥发份对Zr和Si活动性影响，揭示锆石的形成机制；采用针对性的单矿物挑选方法和高精度测年手段，测定锆石年代，确定蚀变与富铁矿化及蚀变与岩体成岩的时间关系，探讨它们的成因联系，为总结大庙式铁矿成矿规律、指导深边部铁矿找矿勘查服务。

河北大庙斜长岩体中产出著名的大庙式铁矿，大量富铁矿脉充填在钠黝帘石化斜长岩中的断裂裂隙中，但成岩、成矿与蚀变之间是否有成因联系尚不明确。本项目在野外接触关系观察的基础上，通过蚀变斜长岩中斜锆石、矿石中斜锆石和锆石、蚀变斜长岩中锆石的成因、形成机制和年代学的研究，探讨了岩演化中Fe-Ti-P超常富集与贯入式富铁矿的形成、斜长岩钠黝帘石化蚀变性质及成岩、成矿、蚀变的成因联系等科学问题。项目取得以下结果：（1）发现岩浆演化晚期阶段的斜长岩-富铁辉石岩韵律层，富铁辉石岩与贯入式富铁矿脉具有渐变过渡的特征，说明母岩浆演化过程中Fe-Ti-P的不断富集是填隙熔体缓慢结晶分异作用的结果，贯入式块状矿体代表了残余岩浆演化最晚期阶段的产物，从而厘定了大庙斜长岩母岩浆演化过程中Fe-Ti-P的富集机制，丰富了基性岩浆系统Fe-Ti-P成矿理论。（2）块状铁矿石和蚀变斜长岩中锆石都为交代成因，Zr都来源于矿石钛铁矿共生的斜锆石，Si都来源于围岩中斜长石的蚀变，其中块状矿石中的锆石为高温阶段铁矿浆侵入早期与围岩交代反应时形成，蚀变斜长岩中的锆石铁矿浆演化晚期岩浆热液中结晶形成，说明传统上认为化学性质稳定的Zr元素在热液流体中仍然具有活动性。（3）斜长岩中斜锆石Pb-Pb年龄1736±3Ma为成岩年龄，块状矿石中斜锆石Pb-Pb年龄1734±7Ma为成矿年龄，块状矿石中锆石U-Pb年龄1721±5Ma为蚀变年龄，它们在误差范围内一致，说明蚀变和矿化具有密切的成因联系。本项目研究表明，大庙贯入式富铁矿为岩浆-热液过渡成因，该认识对丰富基性岩浆系统中Fe-Ti-P富集理论，指导深部找矿勘查，具有理论和实际意义。