Paleoproterozoic Mineralization Research Group, Department of Geology, University of Johannesburg, Johannesburg, South Africa.
Department of Science and Technology - National Research Foundation Centre of Excellence for Integrated Mineral and Energy Resource Analysis, University of Johannesburg, Johannesburg, South Africa.
Geobiology. 2017 Nov;15(6):731-749. doi: 10.1111/gbi.12248. Epub 2017 Aug 28.
We document the discovery of the first granular iron formation (GIF) of Archaean age and present textural and geochemical results that suggest these formed through microbial iron oxidation. The GIF occurs in the Nconga Formation of the ca. 3.0-2.8 Ga Pongola Supergroup in South Africa and Swaziland. It is interbedded with oxide and silicate facies micritic iron formation (MIF). There is a strong textural control on iron mineralization in the GIF not observed in the associated MIF. The GIF is marked by oncoids with chert cores surrounded by magnetite and calcite rims. These rims show laminated domal textures, similar in appearance to microstromatolites. The GIF is enriched in silica and depleted in Fe relative to the interbedded MIF. Very low Al and trace element contents in the GIF indicate that chemically precipitated chert was reworked above wave base into granules in an environment devoid of siliciclastic input. Microbially mediated iron precipitation resulted in the formation of irregular, domal rims around the chert granules. During storm surges, oncoids were transported and deposited in deeper water environments. Textural features, along with positive δ Fe values in magnetite, suggest that iron precipitation occurred through incomplete oxidation of hydrothermal Fe by iron-oxidizing bacteria. The initial Fe -oxyhydroxide precipitates were then post-depositionally transformed to magnetite. Comparison of the Fe isotope compositions of the oncoidal GIF with those reported for the interbedded deeper water iron formation (IF) illustrates that the Fe pathways and sources for these units were distinct. It is suggested that the deeper water IF was deposited from the evolved margin of a buoyant Fe -rich hydrothermal plume distal to its source. In contrast, oncolitic magnetite rims of chert granules were sourced from ambient Fe -depleted shallow ocean water beyond the plume.
我们记录了太古宙第一个粒状铁建造（GIF）的发现，并提供了有关其形成过程的结构和地球化学研究结果，表明这些铁建造是通过微生物氧化铁形成的。该 GIF 位于南非和斯威士兰的约 3.0-2.8 Ga 庞戈拉超群的恩康加组中，与氧化和硅酸盐相的微纹层铁建造（MIF）互层。在与相关 MIF 相比，在 GIF 中观察到铁矿化具有强烈的结构控制。GIF 的标志是由燧石核心包围的瘤状灰岩，周围是磁铁矿和方解石边缘。这些边缘显示出层状穹顶状纹理，外观与微层纹石相似。与互层的 MIF 相比，GIF 富含硅且缺铁。在 GIF 中非常低的 Al 和微量元素含量表明，在没有碎屑输入的环境中，化学沉淀的燧石在波浪基准面以上被重新加工成颗粒。微生物介导的铁沉淀导致在燧石颗粒周围形成不规则的穹顶状边缘。在风暴潮期间，瘤状灰岩被搬运并沉积在更深的水环境中。结构特征以及磁铁矿中正值的 δ Fe 值表明，铁沉淀是通过铁氧化细菌对热液 Fe 的不完全氧化形成的。然后，初始的 Fe-氢氧化物沉淀被后生转化为磁铁矿。与报道的互层深海铁建造（IF）的铁同位素组成相比，瘤状 GIF 的 Fe 同位素组成表明这些单元的铁途径和来源是不同的。据推测，深海 IF 是从远离其源的浮力富铁热液羽流的演化边缘沉积的。相比之下，燧石颗粒的瘤状磁铁矿边缘来源于羽流之外的贫铁浅海环境。
