胶东昌邑条带状铁建造铁矿成因及其对前寒武纪地质演化的制约

The Changyi iron deposit, which is located in Jiaodong Block, southeastern North China Craton, is characterized by banded iron formation (BIF) in morphology of ore body and mineral assemblages. BIF iron deposit is the most important part of iron resources and has special implications for Precambrian geological evolution. Therefore, as the largest iron deposit mined in Jiaodong Block, Changyi iron deposit is vital in economic and scientific significance. The Changyi iron deposit is newly mined and few studies have been carried out. Our preliminary studies show that this deposit was probably produced at Mesoarchean as defined by zircon U-Pb age of 2930 Ma of a orthometamorphite. This ore-forming age is much older than other Chinese BIFs mainly produced at Neoarchean, thus the Changyi iron deposit offers a chance to investigate the difference as well as the evolution between Mesoarchean and Neoarchean BIFs. Furthermore, the oldest basement rocks outcrop in the Jiaodong Block were also generated at Mesoarchean, implying that the Changyi iron deposit may have been synchronously produced when the oldest basement formed, therefore this deposit provides a key to understand the forming processes of the Precambrian crust in the Jiaodong Block. In this program, we plan to detailedly study the ore bodies and wallrocks of the Changyi iron deposit in geochronology and geochemistry, with an aim to constrain the ore-forming ages, ore genesis, metallogenic regularity, as well as to investigate the forming processes of the Precambrian crust in the Jiaodong Block.

昌邑铁矿位于华北克拉通东南部的胶北地体，其在矿体形态以及矿物组合上为典型的条带状铁建造（BIF）铁矿。BIF铁矿是铁矿资源最重要的组成部分，同时也是研究前寒武地质演化的最要载体，因而作为胶东地区最大的BIF铁矿，昌邑铁矿在经济上和科学上都具有重要的研究意义。除了勘探报告，前人还未对该矿进行相关的成因研究。初步研究表明，矿区-150m中段两个矿体之间的一个正变质岩侵位于～2930Ma，这表明昌邑铁矿很可能形成于中太古代。该成矿时代比国内多数BIF铁矿的成矿时代老，并与研究区出露的最古老基底岩石的成岩时代接近，这暗示昌邑铁矿不仅可以成为研究中太古代与新太古代BIF铁矿差异和演化的良好载体，还可以为揭示研究区前寒武地壳的形成提供重要信息。因此，本项目拟对昌邑铁矿的矿石及围岩进行详细的年代学和地球化学研究，确定成矿时限和矿床成因，同时恢复古地理环境，探讨研究区前寒武地壳的形成过程。

BIF不仅具有重要的经济价值，同时对理解地球早期特征和演化过程，包括岩石圈、水圈、大气圈和生物圈的组成和相互作用，有着重要的指示意义。昌邑BIF铁矿位于华北克拉通东南部的胶北地体，赋存于下元古界粉子山群中。矿体呈透镜状、交错层理状，并由富硅和富铁条状组成。矿石主要为角闪磁铁石英岩，矿石矿物为磁铁矿，脉石矿物主要为石英、角闪石和黑云母等。矿体的围岩主要为角闪岩类、斜长片麻岩和变粒岩类以及含石榴石的片岩和片麻岩类，在深部被古元古代花岗岩侵入。对这些变质岩、花岗岩以及BIF条带的LA-ICPMS锆石U-Pb年龄测试结果表明，BIF沉淀于2240～2193 Ma之间，并经历了～1864 Ma的角闪岩相变质作用。BIF全岩非常高的SiO2+Fe2O3T含量（平均值92.3 wt.%）表明其主要为化学沉积。然而变化较大的Al2O3（0.58～6.99 wt.%）、MgO（1.00～3.86 wt.%）、CaO（0.22～4.19 wt.%）以及Rb（2.06～40.4 ppm）、Sr（9.36～42.5 ppm）、Zr（0.91～23.6 ppm）、Hf（0.04～0.75 ppm）、Cr（89.1～341 ppm）、Co（2.94～30.4 ppm）和Ni（1.43～52.0 ppm）含量同样暗示碎屑物质的混染，特别是古老陆壳物质的参与。BIF稀土元素PAAS标准化后显示轻稀土亏损、重稀土富集、La和Y正异常，结合远高于球粒陨石的Y/Ho比值（平均36.3），表明BIF沉淀于海水中。此外，明显的Eu正异常（Eu/Eu*PAAS=1.14～2.86）也指示了海底热液的参与。Cr、Co、Ni的协变关系以及Co+Ni+Cu vs. ∑REE 和Al2O3 vs. SiO2图解等表明成矿元素来自热液，可能与海底火山活动有关。.对变质围岩的原岩恢复表明，这些变质岩主要为泥、砂质沉积岩和拉斑玄武岩，拉斑玄武岩产自陆内环境。侵入BIF和变质岩的花岗岩为A型花岗岩，就位于2193～2171Ma，源自太古宙陆壳的部分熔融。大陆拉斑玄武岩和A型花岗岩限定了昌邑BIF沉淀于陆内裂谷环境。而BIF明显的Ce负异常则表明华北克拉通古元古代已受到全球大氧化事件的影响。昌邑BIF及其围岩变质岩和花岗岩指示了华北克拉通古元古代裂解事件和氧化事件，对研究华北克拉通东部前寒武纪演化历史提供了重要依据。