Siliciclastic associated banded iron formation from the 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa
Siliciclastic associated banded iron formation from the 3.2 Ga Moodies Group, Barberton Greenstone Belt, South Africa
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DOI：
10.1016/j.precamres.2012.12.003
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发表时间：
2013-03-01
影响因子：
3.8
通讯作者：
Foellmi, Karl B.
中科院分区：
文献类型：
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作者：
Bontognali, Tomaso R. R.;Fischer, Woodward W.;Foellmi, Karl B.
大多数条带状铁建造（BIF）沉积模型的提出是基于保存完好的晚太古代和古元古代BIF的观察。努力推动从更年轻的继承中获得更深的理解，受到了早太古代BIF特征的高变质等级的阻碍。这项研究的重点是一个独特的发生保存完好，并contextualized BIF从早太古代（类似于3.2 Ga）穆迪集团，在巴伯顿绿岩带，南非。穆迪BIF与细粒和交叉分层砂岩薄互层，表明碎屑沉积物供应减少时的沉积。在穆迪的BIF中，燧石以结核的形式存在，从未观察到与硅质岩物质直接接触，但总是与铁矿物有关。这一观察结果表明，导致燧石和铁矿物形成的过程是耦合的。未风化穆迪BIF内的主要富铁矿物为赤铁矿和磁铁矿，较少出现铁碳酸盐相（主要为铁白云石）。岩石学纹理显示，赤铁矿构成早期矿物相，而磁铁矿和铁白云石显示纹理指示晚成岩或变质起源。碳质颗粒与磁铁矿晶体密切相关。这些含碳相可能是参与三价铁前体相生产的微生物保存的有机物质，尽管很难排除菱铁矿热分解为磁铁矿和有机碳化合物的非生物过程的来源。尽管如此，一系列的纹理，矿物，和价态支持的观点，成岩稳定BIF矿物反映了在成岩过程中还原流体的相互作用的三价铁相。这些模式通常在较年轻的太古代和古元古代铁地层中观察到，并意味着在一系列古环境和较长的太古代时间间隔内，铁和硅循环中的过程具有连续性。(C)2013爱思唯尔有限公司版权所有。
Most models proposed for banded iron formation (BIF) deposition are based on observations of well-preserved Late Archean and Paleoproterozoic BIF. Efforts to push the understanding gained from younger successions deeper in time have been hampered by the high metamorphic grades that characterize Early Archean BIF. This study focuses on a unique occurrence of well-preserved and contextualized BIF from the Early Archean (similar to 3.2 Ga) Moodies Group, in the Barberton Greenstone Belt, South Africa. The Moodies BIF occurs thinly interbedded with fine-grained and cross-stratified sandstones, indicating deposition during times of decreased clastic sediment supply. In the Moodies BIF, chert is present as concretions, and is never observed in direct contact with the siliciclastic material but is always associated with iron minerals. This observation suggests that the processes leading to the formation of both chert and iron minerals were coupled. The dominant iron-rich minerals within unweathered Moodies BIF are hematite and magnetite, with less common occurrences of Fe-carbonate phases (mainly ankerite). Petrographic textures reveal that hematite constitutes an early mineral phase, while magnetite and ankerite display textures indicative of a late diagenetic or metamorphic origin. Carbonaceous particles are present in close association with the magnetite crystals. These C-bearing phases may be the preserved organic matter of microbes involved in the production of the ferric iron precursor phases, though it is difficult to rule out an origin from abiotic processes involving thermal decomposition of siderite to magnetite and organic carbon compounds. Nonetheless, the range of textures, mineralogies, and valence states supports the view that diagenetically-stabilized BIF mineralogies reflect the interaction of ferric iron phases with reducing fluids during diagenesis. These patterns are commonly observed in younger Archean and Paleoproterozoic iron formations, and imply a continuity of processes operating in the iron and silica cycles across both a range of paleoenvironments and long intervals of Archean time. (C) 2013 Elsevier B.V. All rights reserved.