Mineralogical, magnetic and geochemical data constrain the pathways and extent of weathering of mineralized sedimentary rocks
Mineralogical, magnetic and geochemical data constrain the pathways and extent of weathering of mineralized sedimentary rocks
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DOI：
10.1016/j.gca.2022.11.005
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发表时间：
2023-02-03
影响因子：
5
通讯作者：
Gilbert, Benjamin
中科院分区：
文献类型：
--
作者：
Carrero, Sergio;Slotznick, Sarah P.;Gilbert, Benjamin
硫化岩的氧化风化可通过由此产生的酸度和金属的输出而对流域产生深远的影响。风化留下了矿物转变的记录，特别是涉及较小的氧化还原敏感相，可以为概念和定量模型的开发提供信息。然而，在硫化物沉积岩中，沉积历史、成岩作用和矿化作用的变化可能会改变或覆盖这些微量矿物的分布，从而使风化特征的解释复杂化。在这里，我们展示了将大量矿物学和地球化学技术、微米分辨率 X 射线荧光探针分析和岩石磁性测量相结合，应用于钻芯样本和单一风化裂缝，可以提供能够开发地球化学一致风化模型的数据。这项工作重点关注位于梅萨维德地层内的科罗拉多河上游盆地的一个分水岭，梅萨维德地层是一种沉积砂岩基岩，含有浸染的硫化物矿物，包括黄铁矿和闪锌矿，这些矿物是在成岩作用和随后的岩浆热液成矿作用。综合分析方法揭示了铁 (Fe)、碳酸盐和硅酸盐矿物风化的途径，并显示 pH 值如何控制活跃风化裂隙砂岩中的元素保留或释放。钻芯测井、全岩 X 射线衍射和地球化学测量记录了从深处未风化岩石到地表风化岩石的进展情况。沿断裂面的 1 厘米大小的风化剖面的 X 射线微探针分析与由于原生黄铁矿、铁闪锌矿、绿泥石以及少量菱铁矿和磁黄铁矿溶解而导致 Fe(II) 和 Fe(III) 迁移到酸性孔隙水中的情况一致。这些反应之后是次生矿物的沉淀，例如针铁矿和黄钾铁矾，分别是铁（羟基）氧化物和水合硫酸铁（III）。微尺度分析还有助于解释风化反应，该反应导致了在钻芯顶部和最风化部分观察到的矿物学转变。例如，长石和绿泥石的溶解中和了铁和硫化物矿物氧化产生的酸性，使溶液在两种铁氧化物中过饱和。 X射线光谱显微镜和磁测量相结合表明，Fe(III)产物是针铁矿，主要以涂层形式存在于岩心活跃风化区域的断裂表面上，或者更均匀地包含在岩心顶部的松散风化层中。低温磁力数据揭示了铁磁性硫化铁磁黄铁矿的存在，尽管它以痕量浓度存在，但可以为未风化的硫化物矿物提供定性代表，因为磁黄铁矿的损失与氧化风化的开始有关。磁黄铁矿的损失和针铁矿的形成可以通过室温磁矫顽力的变化来检测，这表明岩石磁性测量可以在许多尺度上确定岩石样品中的风化强度。这项工作提供的证据表明，硫化沉积岩的风化遵循一种地球化学模式，其中硫化物矿物的丰度控制着酸度和溶解元素的产生，而这些元素依赖于 pH 的迁移性控制着它们向地下水和地表水的输出。CO 2022 由爱思唯尔有限公司出版。
The oxidative weathering of sulfidic rock can profoundly impact watersheds through the resulting export of acidity and metals. Weathering leaves a record of mineral transformation, particularly involving minor redox-sensitive phases, that can inform the development of conceptual and quantitative models. In sulfidic sed-imentary rocks, however, variations in depositional history, diagenesis and mineralization can change or over-print the distributions of these trace minerals, complicating the interpretation of weathering signatures. Here we show that a combination of bulk mineralogical and geochemical techniques, micrometer-resolution X-ray fluorescence microprobe analysis and rock magnetic measurements, applied to drill core samples and single weathered fractures, can provide data that enable the development of a geochemically consistent weathering model.This work focused on one watershed in the Upper Colorado River Basin sitting within the Mesaverde Formation, a sedimentary sandstone bedrock with disseminated sulfide minerals, including pyrite and sphalerite, that were introduced during diagenesis and subsequent magmatic-hydrothermal mineralization. Combined analytical meth-ods revealed the pathways of iron (Fe), carbonate and silicate mineral weathering and showed how pH controls element retention or release from the actively weathering fractured sandstone. Drill core logging, whole rock X-ray diffraction, and geochemical measurements document the progression from unweathered rock at depth to weathered rock at the surface. X-ray microprobe analyses of a 1-cm size weathering profile along a fracture surface are consistent with the mobilization of Fe(II) and Fe(III) into acidic pore water from the dissolution of primary pyr-ite, Fe-sphalerite, chlorite, and minor siderite and pyrrhotite. These reactions are followed by the precipitation of secondary minerals such as of goethite and jarosite, a Fe-(oxyhydr)oxide and hydrous Fe(III) sulfate, respectively. Microscale analyses also helped explain the weathering reactions responsible for the mineralogical transforma-tions observed in the top and most weathered section of the drill core. For example, dissolution of feldspar and chlorite neutralizes the acidity generated by Fe and sulfide mineral oxidation, oversaturating the solution in both Fe-oxides. The combination of X-ray spectromicroscopy and magnetic measurements show that the Fe(III) product is goethite, mainly present either as a coatings on fracture surfaces in the actively weathering region of the core or more homogeneously contained within the unconsolidated regolith at the top of the core. Low-temperature mag-netic data reveal the presence of ferromagnetic Fe-sulfide pyrrhotite that, although it occurs at trace concentra-tions, could provide a qualitative proxy for unweathered sulfide minerals because the loss of pyrrhotite is associated with the onset of oxidative weathering. Pyrrhotite loss and goethite formation are detectable through room-temperature magnetic coercivity changes, suggesting that rock magnetic measurements can determine weathering intensity in rock samples at many scales. This work contributes evidence that the weathering of sul-fidic sedimentary rocks follows a geochemical pattern in which the abundance of sulfide minerals controls the gen-eration of acidity and dissolved elements, and the pH-dependent mobility of these elements controls their export to the ground-and surface-water.CO 2022 Published by Elsevier Ltd.