晚太古代清源绿岩带BIF与VMS矿床的成因联系及沉积环境

The Qingyuan Neoarchean granite-greenstone belt, located in the north of the Liaoning province, is one of the most representative greenstone belts in the North China Craton. It is characterized by the occurrence of the oldest Cu-Zn volcanic-hosted massive sulfide (VMS) deposits (e.g., the Hongtoushan deposit) in China. Recent geochronological data indicate that it also hosts a certain amount of Neoarchean BIF-type iron deposits. Hence, a detailed study on the enigmatic association of VMS deposits with BIFs should be conducted urgently. The key scientific issue is focused on the genetic relationship and depositional environment among them. In this regard, we will investigate the composition of typical VMS and BIF deposits in this belt. Based on detailed field survey, we present the U-Pb zircon age of the relevant supracrustal rocks, which places constraints on the formation age of these deposits, and construct the evolution sequence of volcanic and sedimentary rocks. In addition, major and trace elements whole-rock geochemical analyses and Nd-Hf-S-Fe isotopic analyses on the BIF, VMS, and associated lithologies, and in-situ trace elemental analyses on main minerals of the BIF and VMS, are also conducted in order to decipher the source of materials. Within the context of all these data, we give a comprehensive summary on the ore-forming regularity and finally set up a tectonic sedimentary model for the deposit association. This not only plays an important role in the ore prospecting, but also is significant for understanding the redox state and composition of Neoarchean seawater.

辽北清源晚太古代（花岗）绿岩带，是华北克拉通前寒武纪最典型的绿岩带之一，其中以产出中国最古老的红透山火山岩型块状硫化物（VMS）铜锌矿床为特色。但近年同位素年代学研究表明，该绿岩带还发育同期的条带状（BIF）铁矿。对该绿岩带开展BIF铁矿、VMS铜锌矿时空和成因关系及其形成环境的研究就成为迫切需要解决的科学问题。本申请拟在详细绿岩带地质剖面、典型VMS铜锌矿床和BIF铁矿床研究的基础上，采用锆石U-Pb年代学方法，精确限定成岩成矿时代，建立精细的火山沉积、VMS、含铁硅质岩和BIF的演化序列；运用稀土元素、Nd-Hf和S-Fe同位素、矿物微区原位分析等方法，揭示火山岩-VMS-BIF的形成环境、成矿物质来源及成因联系。通过区域和矿床地质的综合分析，总结区域矿产成矿规律，建立区域VMS-BIF的构造-环境成矿模式。这不仅对指导区域找矿预测，而且对了解晚太古代古海洋环境均有重要的科学意义。

清源新太古代绿岩带，是华北克拉通前寒武纪最典型的绿岩带之一，其中以产出中国最古老的红透山火山岩型块状硫化物（VMS）铜锌矿床为特色。本项目选择绿岩带BIF铁矿、VMS铜锌矿时空和成因关系及其形成环境为关键科学问题，在详细地质剖面、典型VMS铜锌矿床和BIF铁矿床研究的基础上，采用锆石U-Pb年代学方法，精确限定成岩成矿时代，建立精细的火山沉积、VMS、含铁硅质岩和BIF的演化序列；运用稀土元素、Nd-Hf和S-Fe同位素、矿物微区原位分析等方法，揭示火山岩-VMS-BIF的形成环境、成矿物质来源及成因联系。研究表明清原绿岩带及有关矿产均形成于新太古代的2570-2530 Ma，并于2520-2480 Ma，遭受变质作用改造。绿岩带火山岩及其有关矿产均形成于较短的时间范围。其中VMS铜锌矿、黄铁矿与BIF铁矿具有空间相伴、时间相随的特征。通过新太古代清原绿岩带变火山岩组合的岩石学及地球化学研究，表明该绿岩带形成于活动大陆边缘的洋-陆俯冲过程，而区内VMS和BIF共生组合形成于伸展阶段的弧后盆地环境。绿岩带不同层位BIF的Nd-Fe同位素特征表明，清原绿岩带BIF样品的ɛNd(t) = +0.8~+2.0，与27-38亿年期间古海水Nd同位素组成 (ɛNd(t)= +1~+2) 基本一致，证实了同时期海底热液对BIF有一定物质贡献，另一方面也反映了古海洋的ɛNd(t) 值从38亿年至25亿年未发生显著变化。与BIF互层产出的含铁页岩样品均与区内玄武岩类似的ɛNd(t) 值一致，说明其物质主要源自绿岩带基性火山岩风化。清原绿岩带中部太阳沟和上部下甸子硅酸盐相BIF中磁铁矿的δ56FeMg值为-0.65 ~ -0.83‰，证实了它们沉淀之前水体中的Fe2+已经发生了部分氧化。结合区内部分BIF样品 (+3.2 ~ +3.6) 具有与同时期N-MORB型玄武岩 (平均为+3.7) 近似的ɛNd(t) 值，以及含硫化物BIF的平均组分相对于硅酸盐相BIF富集LREE且正Eu异常更为显著，以上信息共同指示同时期热液活动在供应物质的同时，也改变了局部水体的地球化学特征。在此基础上，建立了区域绿岩带VMS-BIF弧后盆地系统成矿模式。这不仅对指导区域找矿预测，而且对了解晚太古代古海洋环境均有重要的科学意义。