关键词: 元古代, 矿物组成, 岩石结构, 条带状铁建造

Abstract: The two oxygenation events in the Precambrian were not only continuously rebuilding Earth’s surficial system but also fostering the evolution of habitable environment for life. The globally distribution of banded iron formations (BIFs) respond to the changes of surficial system. BIFs also record abundand information regarding the evolution of the atmosphere, lithosphere, hydrosphere, and biosphere. They are of great scientific significance, as they provide a crucial window for understanding Precambrian geological environments and evolutionary characteristics. This study focuses on the BIFs in the Griqualand West region, South Africa, and utlizes techniques including X-ray diffractometry, SEM equipped with EDS etc, to conduct a detailed mineralogical investigation. We aim to constrain the mineral assemblages, formation conditions, and genesis of the BIFs. The results show that the BIF consists of iron minerals including hematite, magnetite, siderite, goethite, as well as non-iron minerals such as quartz, calcite, rhodochrosite, and rhodonite. These minerals are composed of nanosized grains and occur as granular or oolitic particles. The BIFs display extremely fine banded laminaes, generally 50-250 μm in thickness. These bands comprise alternating layers dominated by iron-rich minerals and chert or silicate-rich minerals, deposited sub-parallel to one another. Irrespective of grain size, numerous iron-bearing mineral grains show well-developed euhedral morphologies, frequently with banded zoning textures. SEM-EDS mapping reveals the native carbon grains and coupled Fe-Mn-C distributions, which may point to a microbial role in the formation of the BIFs.

Key words: Paleoproterozoic, mineral compostion, texture, Banded Iron Formation