Silica and iron mobilization, cave development and landscape evolution in iron formations in Brazil
Silica and iron mobilization, cave development and landscape evolution in iron formations in Brazil
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DOI：
10.1016/j.geomorph.2021.108068
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发表时间：
2022-02-01
期刊：
影响因子：
3.9
通讯作者：
Davis, Reed
中科院分区：
文献类型：
--
作者：
Auler, Augusto S.;Barton, Hazel A.;Davis, Reed
铁质地层展示了地球上一些最古老和最神秘的地貌，目前仅限于地球上的少数地区。原始岩石，带状铁地层（BIF）形成于元古代，并通过化学和生物过程的复杂相互作用不断风化，涉及主要元素二氧化硅和铁的大量活动，导致二氧化硅耗尽（但易碎）的高品位铁体和富含铁的survival duricrust，称为canga。本研究提供了新的数据，并包括对现有文献的全面回顾，纳入了来自未发表报告和新发现的大量数据。它旨在定量分析微观和宏观尺度下铁地层中孔隙度的形态和发展，评估负责铁和二氧化硅动员的地球生物学机制及其在这一不断发展的景观动态中的作用。二氧化硅浸出是潜水带深处孔隙度生成的初始过程，并产生富有成效的含水层，有助于去除溶质并生成初始低密度区。在灰岩和风化矿石之间的浅接触带处，渗透率的差异有利于大量孔隙和空隙的发育。洞穴往往出现在高原和山脊边缘的陡崖底部，并且往往在远离边界的地方缩小规模。景观通过陡崖的退缩而演变，主要是通过洞穴通道的坍塌。洞穴的形态特征是大房间和小通道相互交错，这表明大孔隙最初是从孤立的空隙开始的，它们要么完全出现在灰岩中，要么出现在风化的BIF中，但通常与这些岩石之间的接触有关。亚马逊河卡拉哈斯地区的洞穴显示出最大的尺寸值，无论基岩背景如何。较长的洞穴显示了更多的房间之间的连接，这表明房间的合并主要是通过斜坡互通过程是促进洞穴发展的关键因素。为较大的洞穴计算的分形维数（FD）表明，岩性和FD之间没有关系，尽管FD值在不同地区之间存在差异，卡拉哈斯显示最高。地质微生物学过程在铁（III）还原菌的铁动员中发挥着重要作用。特别是，它通过生成硬化表面，保护洞穴和空洞的墙壁免于坍塌，从而促进空洞的长期稳定。化学和细菌过程的相互作用允许产生高度集成的孔隙和洞穴网络，这些孔隙和洞穴代表用于去除二氧化硅和铁的动员/浓缩的出口。这些空洞是地球化学淋滤的结果，并与景观同步演化，主要是在陡坎水平。这些过程连接孤立的孔隙，促进溶质的疏散，并允许景观的缓慢横向退化。这一缓慢的地质过程现在受到采矿作业的严重影响，因此更有必要寻找减缓和/或再生途径，以保护这一独特的地球生物古老景观。(c)2021爱思唯尔有限公司版权所有。
Iron formations display some of the oldest and most enigmatic landforms on Earth, presently restricted to few areas of the planet. The original rock, the Banded Iron Formation (BIF) was formed in the Proterozoic and has been continuously weathered through a complex interplay of chemical and biological processes involving the massive mobilization of the main elements silica and iron, resulting in silica depleted (but friable) high-grade iron bodies anda Fe-rich surficial duricrust known as canga. This study presents new data and includes a compre-hensive review of the existing literature, incorporating extensive data from unpublished reports and new find-ings. It aims to quantitatively analyze the morphology and development of porosity in iron formations both at micro and macro scales, assessing the geobiological mechanisms responsible for iron and silica mobilization and their role in the dynamics of this ever-evolving landscape. Silica leaching is the initial process of porosity gen-eration at depth in the phreatic zone and results in a productive aquifer that contributes to the removal of solute and the generation of initial low-density zones. The development of numerous pores and voids is favored by the permeability contrast at the shallow contact zone between the canga and the weathered ore. Caves tend to occur at the base of scarps at the limit of plateaus and ridges, and tend to scale down in size away from the borders. The landscape evolves through the retreat of scarps, mostly through the collapse of cave passages. Caves exhibit a characteristic morphology that intercalates larger rooms and smaller connecting passages, suggesting that macropores started initially as isolated voids and occur either entirely in canga or in the weathered BIF, but com-monly are associated with the contact between these rocks. Caves in the Amazonian Carajas region display the largest dimensional values, regardless of bedrock context. Longer caves show a larger number of connections be-tween rooms, suggesting that the coalescence of rooms mostly through slope interflow processes is a key player in fostering the development of caves. Fractal Dimension (FD) calculated for the larger caves demonstrates that there is no relationship between lithology and FD, although values of FD vary between regions, with Carajas dis-playing the highest ones. Geomicrobiological processes play a major role in the mobilization of iron through Fe(III) reducing bacteria. In particular, it promotes the long-term stabilization of voids through the generation of a hardened surface that pro-tects the walls of caves and voids from collapse. The interplay of chemical and bacterial processes allows for the generation of a highly integrated network of pores and caves that represent outlets for the removal of silica and mobilization/concentration of iron. These voids are the result of geochemical leaching and evolve synchronously with the landscape, mostly at scarp level. Such processes connect isolated pores, promote evacuation of solutes and allow for the slow lateral degradation of the landscape. This geologically slow process is now heavily affected by mining operations, which reinforces the need for searching for mitigation and/or regeneration pathways in order to preserve this unique geo-biological ancient landscape.(c) 2021 Elsevier B.V. All rights reserved.