华北克拉通条带状铁建造中富铁矿成因类型的研究进展、远景和存在的科学问题

本文在查阅前人大量资料的基础上,对华北克拉通条带状铁建造中富铁矿的研究历史进行了回顾和总结,将研究历史分为1949年以前,1950~1965年期间,1978~1986年期间,1987~1994年期间和2009年以来5个阶段。重点介绍了鞍本地区、冀东-吕梁地区和河南舞阳地区富铁矿的基本地质特征以及典型富铁矿的研究概况,针对鞍本地区弓长岭二矿区磁铁富矿成因的复杂性,对不同成因观点以及目前已取得的共识进行了详细阐述。目前大多数学者不支持接触交代假说和菱铁矿经变质转化为富铁矿成矿假说,近半数学者支持变质热液成矿假说,半数学者支持混合岩化热液成矿假说。作者在综合分析前人大量资料后,认为变质热液成矿说依据不足,理由有四点:(1)磁铁富矿中往往见有磁铁贫矿的残体;(2)磁铁富矿与蚀变岩紧密伴生,蚀变矿物石榴子石、部分角闪石(透闪石)和部分绿泥石均属非变质热液成因;(3)研究区遭受区域高绿片岩相至低角闪岩相变质作用的时间为2500~2450Ma,而与蚀变矿物石榴石紧密伴生的热液锆石SHRIMP U-Pb定年结果为1840±7Ma,明显小于区域变质作用年龄,据此可将热液作用时间限定于古元古代晚期,相当于大陆地壳伸展阶段;(4)部分热液成因富铁矿利用Re-Os方法定年,除一种属原生沉积成矿外,年龄范围也在古元古代晚期,可作为参考。此种热液是否为混合岩化热液尚缺乏足够证据,故本文暂将其作为古元古代晚期热液。此外,本文对华北克拉通条带状铁建造中富铁矿成因类型及其远景进行了初步总结,认为古元古代晚期形成的磁铁富矿规模属大型矿床,有较好远景;原生较富贫铁矿因褶皱构造产生磁铁矿流变而形成的富铁矿(可能尚有热液叠加)规模较大,具有一定远景;其他类型均为小型规模,不具工业意义。最后,本文指出富铁矿成因研究中尚存在的主要问题,包括早元古代晚期热液的来源;热液的形成是一期还是多期;铁建造遭受区域变质达高绿片岩相时,贫铁矿的围岩变质演化机理等,尚需进一步探讨。

Progress, prospecting and key scientific problems in origin researches of high-grade iron ore of the banded iron formation (BIF) in the North China Craton

Basing on a large number of existing data, history of study on several typical high-grade iron ore deposits of banded iron formation (BIF) in the North China Craton is reviewed and summarized. The history can be divided into five stages: (1) before 1949; (2) between 1950 and 1965; (3) between 1978 and 1986; (4) between 1987 and 1994; (5) after 2009. Then basic geological characters of iron rich ore in Anshan area, Jidong-Lvliang area and Wuyang area are introduced. Meanwhile, study status of iron rich ore in the several areas above are emphatically introduced. In view of the complex origin of the magnetite rich ore in the No.2 mining district in Gongchangling area, researches and common viewpoints on them are introduced in detail. Up to now, most researchers are not agreeable on the iron rich deposit hypothesis of contact metasomatism or transformation from siderite through metamorphism. Nearly half of the researchers agree with the theory of metamorphic hydrothermal mineralization and another half the theory of migmatized hydrothermal mineralization. According to comprehensive analysis of voluminous data from the literature, the authors think there is no enough evidence for the contention of metamorphic hydrothermal mineralization. There are four arguments for the viewpoint: the first is that residual magnetite lean ore can be frequently found in the iron rich ore. The second is that magnetite-rich ore is intimately associated with alteration rocks in which the alteration minerals like garnet, some hornblende (tremolite) and chlorite are not metamorphic hydrothermal origin. The third, age of regional (upper greenschist to lower amphibolite facies) metamorphism is bracketed in the range of 2500Ma and 2450Ma, while the hydrothermal zircons accompanied with the alteration garnet give the U-Pb age of 1840±7Ma (dating by SHRIMP), which is younger than the metamorphism age. Thus the hydrothermal activity was constrained to occur in the Late Paleoproterozoic when the continental crust of the Craton was in extension period. The fourth, Late Paleoproterozoic age has been obtained through Re-Os dating from the high-grade iron ore of hydrothermal origin. As no enough evidence to prove the hydrothermal fluid migmatitic, we tentatively regard it as Late Paleoproterozoic hydrothermal fluid. In addition, origin types and prospecting potential of the high-grade iron ore of the BIF in the North China Craton are discussed. The authors deduce that the magnetite-rich iron ores of Late Paleoproterozoic may form to large scale and are of great potentials. The magnetite-rich ores through rheological deformation due to fold structures in primary higher-grade magnetite-poor ores (probably accompanied with hydrothermal superposition) may reach to biggish scale and of certain potential. Others are generally in small scale and of no economical significance. Finally, the authors put forward some problems in high-grade iron ore research, such as the source of the hydrothermal fluid of Late Paleoproterozoic, single or multistages of the hydrothermal fluid formation, and metamorphic mechanism of the wall rocks of the lean iron ores. All these problems need further theoretical investigation.