论华北克拉通南缘霍邱群变质作用、形成时代及霍邱BIF铁矿成矿机制

On the metamorphism of the Huoqiu Group,forming ages and mechanism of BIF and iron deposit in the Huoqiu region,southern margin of North China carton

安徽霍邱铁矿位于华北克拉通南缘,是一个大型BIF铁矿田,矿体均赋存于一套新太古代中高级变质作用的含铁建造中,自下而上可分为A+B矿带和D矿带:前者为变粒岩-片岩-磁铁石英岩建造;后者为片岩-大理岩-赤铁(镜)铁石英岩建造。本文利用现有的矿床勘查、物化探和地质科研资料,对该矿田的地质背景、矿体赋存条件、构造型式进行深入剖析,推断矿区周边及深部构造格局与控矿因素,研究分析霍邱铁矿含铁岩系特征,进行含矿层位的准确划分、厘定和对比,深入研究含铁岩系的平面分布范围和延深,对矿田控矿构造进行综合分析。运用LA-ICP-MS技术获得霍邱群斜长角闪岩最老年龄为2.8Ga左右,代表原岩形成年代。同时获得晚期混合花岗岩的侵入年龄为1.8Ga左右。通过Hf同位素测定,获得各变质岩系锆石的Hf同位素组成,表明最老的锆石Hf模式年龄可达3.5Ga前后。根据区域年代学资料和霍邱铁矿含铁岩系特征,本文正式提出"霍颍运动"这一概念,作为皖西北新太古代2.7Ga前后在霍邱-颍上-寿县-蒙城一带发生的构造事件和成矿作用,以区别于以往沿用蚌埠运动描述本区霍邱群变质作用和成矿事件的提法。最后结合本区含铁建造特征,初步判定霍邱铁矿为一种介于阿尔戈玛和苏必利尔型之间的过渡类型铁建造。

The Huoqiu iron ore field in Northwest Anhui Province is located in the North China craton. As a large BIF iron ore field, the ore bodies are resident in a middle-high grade of metamorphism formation in the period of Neo-Archean, forming a banded silicon-iron series from north and south. The main ore bodies can be divided into two sub-belts from bottom to upper layers, i.e., the A＋B ore belt, consisting of granulite-schist-magnetite-quartz formation;and the D ore belt, consisting of schist-marble-hematite-quartz formation. Combined with the geological background, geophysical and geochemical exploration, ore-forming conditions and structural form of the iron deposit, we present a comprehensive understanding on structural types, speculation surrounding, deep tectonic and ore-controlling factors as well as characteristics and distribution of this colossal BIF ore filed in the Huoqiu region. Using LA-ICP-MS techniques, we obtained the oldest U-Pb age of ca. 2.8Ga for plagioclase amphibolites as the forming age of original rock and 1.8Ga for magmatitic granites. The Hf isotopes were also obtained from zircons separated from the Huoqiu Group. We measured an oldest Hf model age of 3.5Ga in zircon. According to the characteristics of this BIF and iron ores, this paper firstly proposed the concept of ＂Huoying Movement＂, accounting for tectonic event with the metamorphism and metallogenesis in the Neo-Archean around 2.7Ga in the Huoqiu-Yingshang-Shouxian-Mengcheng regions, Northwest Anhui, which is differed from the former Bengbu Movement occurred in Northeast Anhui for description on both metamorphism and metallogenesis in these two regions. Finally we regard the iron deposit and BIF in the Huoqiu region as a transitional iron formation between Algoma and Superior types according to the volcanic-sedimentary characteristics in the Huoqiu Group.