弓长岭铁矿二矿区蚀变岩中锆石SHRIMP U-Pb年龄及地质意义

SHRIMP U-Pb geochronology of zircons from the garnet-rich altered rocks in the mining area Ⅱ of the Gongchangling iron deposit: Constraints on the ages of the high-grade iron deposit

国外的富铁矿（TFe含量超过50％）主要来自长期稳定的古老克拉通上早前寒武纪铁建造（BIF）经过后期风化淋滤作用形成的赤铁矿石。虽然我国的华北克拉通等古老地块也发育早前寒武纪BIF，并经历了强烈的变质变形改造，但是由于地块活动性强导致缺乏充分风化淋滤作用的条件，因而赤铁富矿很少，铁矿石以TFe含量30％左右的沉积变质型磁铁贫矿为主。辽宁弓长岭铁矿床位于华北克拉通东北部，是一个大型沉积变质型铁矿床，总体以磁铁矿贫矿石为主，但其二矿区的磁铁富矿（TFe含量大于50％）达大型规模，是我国唯-的大型沉积变质型磁铁富矿。弓长岭二矿区富铁矿是条带状铁建造沉积后受后期叠加改造作用形成的，富矿体成矿时发生了强烈的围岩蚀变，形成以石榴石和镁铁闪石为特征矿物的蚀变岩．这种富含石榴石的蚀变岩在区域上乃至全国的沉积变质型磁铁矿矿床中都是独一无二的，表明其与磁铁矿富矿有密切的成因联系。该蚀变岩中与镁铁闪石、绿泥石、石英、钛铁矿共生有热液锆石。本文从该蚀变岩中分选出了锆石，锆石呈他形至半自形粒状，在阴极发光（CL）图像上呈多孔状、斑块状、补丁状，明暗极不均匀，可见不明显的环带；锆石内包体在背散射图像上呈暗色，长条状或片状自形晶，主要由MgO、FeO、SiO2、Al2O2组成，为绿泥石、铝直闪石和镁铁闪石；锆石LA-ICP-MS原位微量元素分析表明，Hf含量为10672×10^-6 -11822×10^-6，Y为12．58×10^-6-19．41×10^-6，Th为0．32×10^-6-1．48×10^-6，U为425×10^-6-663×10^-6，Th／U为0．001-0．003，Ti为1．63×10^-6-3．7×10^-6，∑REE为10．37×10^-6-20．15×10^-6，球粒陨石标准化的稀土配分曲线上轻稀土强烈亏损，中、重稀土富集，重稀土较平坦，有弱的铕正异常，这些特点表明该锆石为与蚀变岩和富铁矿同时形成的热液成因锆石。利用SHRIMPU-Pb定年方法对该热液锆石进行了年龄测定，获得的上交点年龄为1850±16Ma，MSWD=2．1；10个测点加权平均年龄为1840±7Ma，MSWD=1．6。该年龄代表了富含石榴石的蚀变岩的成岩年龄，因而也可能代表了富铁矿石的形成年龄，因此推测磁铁富铁矿的形成是条带状磁铁石英岩在1．9-1．8Ga时华北克拉通基底隆升与裂谷-非造山岩浆事件所产生的热液交代作用的结果。

Global occurrence of BIF-related iron resources are typically represented by high-grade hematite ores （ TFe 〉 50% ） formed by supergene leaching. Most of the BIF-related iron deposits in China are low-grade iron ores with TFe grade approximately of 30%. The BIFs in China have experienced intense metamorphism and deformation, which led to the majority of iron oxides transforming into coarse-grained magnetites. Though their ores generally contain only 30% TFe, the BIFs are favorable for industrial exploitation by using magnetic beneficiation. The mining area II of the Gongchangling high-grade iron deposit, the largest high-grade magnetite deposit in China, is referred to a kind of sedimentary metamorphosed deposit. The high-grade iron ores were formed by the superimposition and reformation of banded iron formation （BIF）. During the process of the high-grade iron mineralization, the country rocks of the ore bodies were intensely altered into garnet-rich rocks. In BSE images, zircons are observed to be in close association with quartz, ilmenite, cummingtonite and chlorite. The mineral assemblage is distributed along fractures in garnet, indicating that the zircons were formed during the process of retrograde alteration of garnet. Zircons separated from the garnet-rich altered rocks are hypautomorphic-allotriomorphic granular, and display different bright and dark patches in the cathodoluminescence （CL） images and back-scattered electron images and vaguely oscillatory zones in CL images. The inclusions inside the zircons display dark color; long strip and flake shaped euhedral crystal, and are determined as chlorite, gedrite and cummingtonite. In-situ trace element analyses of the zircons by LA-ICP-MS gives Hf from 10672 ×10^-6 to 11822 × 10^-6, Y from 12. 58× 10^-6 to 19. 41 ×10^-6, Ti from 1.63 × 10^-6 to 9.48×10^-6, Th from 0. 32 × 10^-6 to 1.48 ×10^-6 ,U from 365× 10^-6 to 663 × 10^-6, Th/U from 0. 001 to 0. 003, ∑REE from 7.18 × 10^-6 to 20. 45 × 10^-6 Chondrite-normalized REE patterns are MREE and HREE rich and show fiat HREE curves and slightly positive Eu anomaly. All of the above suggest that the zircons from the garnet-rich altered rocks are of hydrothermal origin and formed at the same time as the high-grade iron ore and altered rocks. SHRIMP U-Pb dating of the zircons yield an upper intercept age of 1850 ± 16Ma （MSWD = 2. 1 ） and a weighted mean age of 1840 ± 7Ma （MSWD = 1.6 ）. This age represents the metallogenetie age of the high-grade iron deposit, and matches with the uplift of the early metamorphic basement and rift-anorogenic magmatism in North China Craton at - 1.8Ga. This high-grade magnetite deposit is transformed from BIF by the tectonic thermal event at 1.9 - 1.8Ga.