山西袁家村条带状铁建造中赤铁富矿成因的年代学制约

High-grade hematite mineralization is common in banded iron formations (BIFs) worldwide. However, in the North China Craton where Neoarchean–Paleoproterozoic BIFs are abundant, economic high-grade hematite ores are scarce. High-grade hematite ores hosted in the Paleoproterozoic Yuanjiacun BIFs represent the largest occurrence of this type of ore in the North China Craton. The possibility of a lack of prolonged lateritic weathering along ancient unconformities after BIF deposition has been proposed, but can be refuted because of the obviously fault-controlled nature of the orebodies. Absolute age determination of hematite growth is critical for understanding the geological controls on iron ore mineralization. In this study, in order to yield robust age constraints on the Yuanjiacun high-grade hematite mineralization, we will carry out the following lines of studies. On the basis of detailed field observation, systematic microscopic petrographic observation on the external morphology and internal structure of the monazite and xenotime and their mutual relation with other minerals in the high-grade hematite ore will be conducted to identify the occurrence of monazite and xenotime, and further to determine their origin. In situ micro-analysis on the elemental distribution characteristics of monazite and xenotime will be applied to discuss the source of fluid or hydrothermal fluid, and reveal the formation mechanism of monazite and xenotime. High-precision in-situ SIMS U-Th-Pb dating will also be performed to obtain the formation age of monazite and xenotime, in order to determine the age of the hydrothermal and iron-rich mineralization. Our new dataset will be used to reveal the genetic mechanism of the high-grade hematite ores, and further to discuss the fate of the missing high-grade hematite ores in the North China Craton.

条带状铁建造(BIFs)中的赤铁富矿(>60%Fe)是世界上最重要的铁矿资源，我国华北克拉通BIFs规模虽大，但其中罕见具经济价值的赤铁富矿，仅在山西袁家村BIFs中有规模发育。传统观点认为华北赤铁富矿缺失与缺乏长期表生风化淋滤作用有关，然而此观点不能解释袁家村赤铁富矿受断裂控制的特征。赤铁富矿成矿时代的准确厘定是解决其成因机制的关键。本申请以袁家村赤铁富矿为研究对象，在野外工作基础上，通过对富矿石中独居石和磷钇矿外部形态、内部结构及与其他矿物相互关系进行详细岩相学观察，查明它们的赋存状态，确定其成因及与赤铁矿的关系；采用多种原位微区测试技术研究独居石和磷钇矿的元素分布特征，探讨流体或热液来源，揭示其形成机制；采用SIMS高精度原位小束斑U-Th-Pb定年，准确厘定独居石和磷钇矿形成年龄，约束热液活动及赤铁富矿形成时代，揭示赤铁富矿成因机制，探讨华北克拉通赤铁富矿的缺失原因和成矿前景。

条带状铁建造中的赤铁富矿（>60% Fe）是世界上最重要的铁矿资源。我国华北克拉通条带状铁建造分布虽广，但其中罕见具经济价值的赤铁富矿，仅在山西袁家村地区有少量发育。传统观点认为我国华北赤铁富矿的缺失与不具备长期表生风化淋滤的条件有关，然而此观点不能解释袁家村赤铁富矿受构造控制的特征。本项目选择袁家村赤铁富矿为研究对象，在野外接触关系和岩石矿物学研究的基础上，对与赤铁矿共生的独居石和磷钇矿开展了原位U-Pb定年，对与赤铁矿共生的石英开展了流体包裹体分析。研究表明，袁家村赤铁富矿的成矿时代为14.1–13.4亿年，与伸展构造背景有关，成矿流体为富CO2的热卤水（温度313–370℃，盐度约20wt% NaCl equiv），赤铁富矿的形成与深成热液交代有关，而与表生风化淋滤作用无关。通过与国外典型矿床对比，结合前人区域地质研究成果，提出我国华北赤铁富矿早期形成于浅部地壳位置低级变质的条带状铁建造中，其缺失与中—新生代强烈的构造抬升–剥蚀作用有关。该认识对丰富铁富集成矿理论研究，以及指导找矿勘查，具有理论和实际意义。