吕梁古元古代袁家村条带状铁建造原始矿物组成及演化机制研究

Banded iron formations (BIFs) are characteristic marine chemical sediments that widely occur in Archean and Paleoproterozoic terranes. BIFs are not only regarded as the most principal source of iron for the global steel industry, but also the REE and isotopic compositions of BIFs have been used as proxies for understanding tectonic evolution of the early Earth, the chemical composition and redox state of the ancient oceans and atmosphere. In order to use BIFs as an archive of the hydrosphere, atmosphere and biosphere at the time when they formed, the primary mineral compositions and diagenetic and metamorphic transformation of these precusor sediments must be assessed. The Paleoproterozoic Yuanjiacun BIF situated in the Lvliang area is the most representative Superior-type BIF in the North China Craton. Considering that the BIF has been subjected to greenschist facies metamorphism and that oxide (magnetite and hematite), silicate, and carbonate facies iron formations are clearly recognized, this BIF offers a good possiblity to conduct a detailed and systematic research on the origin of its minerals. In this regard, we will present detailed information on field occurrence, mineral and textural arrangements of the Yuanjiacun BIF. The geological information is complemented with electron probe microanalytical data (EPMA) in situ element analysis and a variety of isotopic (C, O, Fe, Si) data. The objectives of our study are: (1) to reconstruct the paragenetic sequence for the depositional and diagenetic history of the BIF; (2) to discuss the genesis and formation conditions of different minerals; (3) to deduce the mineral compositions of precursor sediments which can be used as proxies for understanding Earth’s ancient environment; (4) and to ultimately establish the depositional model of various facies of the Yuanjiacun BIF.

条带状硅铁建造（BIF）作为前寒武纪特有的海相化学沉积岩类，因其广泛发育且化学性质稳定，故不仅可作为重要的铁矿石资源，也可用来指示古海洋和大气环境。但由于BIF形成时间古老，且普遍遭受成岩-变质等作用的影响，因而如何有效识别后期演化过程，揭示原生矿物信息，是本申请需要解决的科学问题。华北吕梁地区古元古代袁家村BIF作为国内典型的 Superior型BIF，发育国际上少有完整的沉积矿物相建造（氧化物相-硅酸盐相-碳酸盐相），变质程度低（绿片岩相），是理想的研究对象。本申请拟在详细的野外地质调查和室内岩相学观察基础上，应用矿物原位成分和同位素（C、O、Fe、Si）组成的综合分析方法，重点研究袁家村BIF的结构构造和矿物共生关系，探讨主要矿物的成因和形成条件，划分矿物生成阶段和次序，恢复BIF原始矿物组成，揭示古环境信息，并建立袁家村BIF沉积相的形成演化模式。

条带状硅铁建造（BIF）作为前寒武纪特有的海相化学沉积岩类，因其广泛发育且化学性质稳定，故不仅可作为重要的铁矿石资源，也可用来指示古海洋和大气环境。但由于BIF形成时间古老，且普遍遭受成岩-变质等作用的影响，因而如何有效识别后期演化过程，揭示原生矿物信息，是当前国际上亟待解决的重要科学问题，直接影响对BIF古环境意义的理解。华北吕梁地区古元古代袁家村BIF作为国内典型的Superior型BIF，发育国际上少有完整的沉积矿物相建造（氧化物相-硅酸盐相-碳酸盐相），变质程度低（绿片岩相），是理想的研究对象。本申请在详细的野外地质调查和室内岩相学观察基础上，依据原生矿物的共生组合及产出特征，可将BIF沉积相划分为氧化物相(60 vol%)、硅酸盐相(30 vol%)和碳酸盐相(10 vol%)。氧化物相主要组成矿物为赤铁矿、磁铁矿和石英，进一步可划分为赤铁矿(24 vol%)和磁铁矿(36 vol%)亚相；硅酸盐相BIF 以大量硅酸盐矿物出现为特征，散布于研究区；碳酸盐相主要由菱铁矿和铁白云石组成，主要集中于研究区的南部。依据含铁岩系构造格局恢复原始沉积相，推测沉积相主要呈南北向延展，而自东向西显示出相变规律，西边为碳酸盐相，东边为氧化物相，其间是过渡的硅酸盐相。研究区袁家村组变沉积岩地层自下而上，可划分为三个海侵-海退旋回。BIF的沉积与海侵过程密切相关，沉积相自下而上，一般从氧化物相过渡到硅酸盐相再到碳酸盐相，碳酸盐相BIF对应着海侵高峰。进一步应用矿物原位成分和同位素（C、O、Fe）组成的综合分析方法，重点研究了BIF的结构构造和矿物共生关系，探讨了主要矿物的成因和形成条件，划分了矿物生成阶段和次序，大致推测原始沉积的矿物组成应为无定形硅胶、三价铁的氢氧化物、类似黑硬绿泥石组成的铁硅酸盐凝胶和富Al 的粘土碎屑。这些沉积物在随后的成岩期和绿片岩相的区域变质作用下发生矿物之间的相互转变。BIF中绝大部分含铁矿物如黑硬绿泥石均是在较低氧逸度和硫逸度环境中形成的，且所有矿物共存的水体系为中性到弱碱性环境。