晚太古代五台绿岩带金岗库层控BIF与VMS矿床成因联系及沉积环境

The Wutai Neoarchean granite-greenstone belt, located in the north of the Shanxi province, is one of the most representative greenstone belts in the North China Craton. It is characterized by the occurrence of the oldest S-Fe volcanic-hosted massive sulfide (VMS) deposits (e.g., the Jingangku 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.

本项目以晋北五台金刚库矿床作为解剖对象，紧抓金刚库组赋存火山岩型块状硫化物（VMS）硫铁矿和条带状（BIF）铁矿为其特色；以系统的矿床地质学研究为基础，系统解剖其地质地球化学特征；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的形成动力学背景、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。

金岗库矿床位于华北克拉通中部造山带五台绿岩带，发育典型的 VMS 与BIF 共生现象；本项目对金岗库矿床的地质与岩石地球化学特征进行系统研究，开展流体包裹体分析实验，揭示金岗库矿床流体系统特征，通过同位素年代学测 试厘定金岗库矿床的成矿时代，并在以上研究的基础上探讨金岗库硫化物矿石与磁铁石英岩的共生特点与成矿动力学模式。金岗库矿床的矿体受地层及岩性控制，呈扁豆状、层状-似层状产出，赋矿围岩为五台绿岩带金岗库组的磁铁石英岩、斜长角闪岩、斜长片麻岩和云母石英片岩。矿石中金属矿物组合为黄铁矿-黄铜矿-磁黄铁矿-磁铁矿，矿石主要呈半自形-他形粒状结构和块状、条带状构造，围岩蚀变为绿泥石化和绢云母化。经 PAAS 标准化的金岗库磁铁石英岩显示轻稀土相对亏损，具有 La、Y 和Eu 的正异常，并且 Y/Ho 比值为 31.6，指示海底热液和海水是金岗库矿床的成矿物质来源。磁铁石英岩缺乏 Ce 的负异常，说明矿床沉积时海水处于缺氧环境。对斜长角闪岩的原岩恢复结果表明其原岩为拉斑玄武岩，可能形成于岛弧环境。LA⁃ICP⁃MS 锆石 U⁃Pb 定年显示变基性火山岩的原岩形成于 2500Ma，说明金岗库矿床形成于新太古代晚期，此时五台地区处于岛弧构造环境中。流体包裹体研究表明，金岗库矿床含矿磁铁石英岩中流体包裹体较发育。根据包裹体测温和激光拉曼分析结果显示，金岗库矿床变质流体体系的成分模式为H2O⁃NaCl⁃CO2⁃CH4±N2±H2，变质峰期为中高温（322 ℃～473 ℃）、低盐度（2.2%～6.74%）热液流体，并叠加少量中高温（291 ℃～470 ℃）、高盐度（37.4%～55.79%）的岩浆热液流体；峰后阶段为中低温（225 ℃～302 ℃）、中低盐度（4.03%～11.81%）的热液流体。金岗库矿床赋存的磁铁石英岩和硫化物矿体紧密共生，具有相同的成矿时代、物质来源和变质变形历史。综合以上研究认为金岗库矿床的成矿作用与海相火山喷流沉积有关。