Formation of uranium-, vanadium- and chromium-bearing reduction spheroids in karst bauxite of the Unterlaussa mining district (Austria)

IF 3.4 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Ferdinand J. Hampl , István Dunkl , Burkhard C. Schmidt , Viktor Bertrandsson Erlandsson , Frank Melcher
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引用次数: 0
Abstract
Reduction spheroids are mm- to cm-scale spheroidal features that are formed by reduction processes and are mainly known from siliciclastic red beds. They consist of a bleached halo around a dark core that contains highly elevated contents of redox-sensitive elements such as U, V or Cr when compared to the host rock. Their formation mechanism is still poorly understood and reduction spheroids in rocks that are not associated with siliciclastic red beds are more than underexplored.
Here, we present geochemical and mineralogical results of U-bearing reduction spheroids hosted in karst bauxite of the Unterlaussa mining district (Austria). The goals of this paper are to (1) better understand the formation mechanism of reduction spheroids in non-siliciclastic host rocks that are not associated with typical red beds, and (2) to shed light on the mechanisms of U enrichment in karst bauxites. To this end, we used laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS), scanning electron microscopy (SEM), micro-X-ray fluorescence (μ-XRF) and Raman spectroscopy.
Among several enriched elements such as ΣREE (La-Lu) up to 2.6 wt% or V up to 7.6 wt%, high contents of U and Cr are most characteristic for the dark cores of the reduction spheroids. Uranium-rich reduction spheroid cores with local contents of up to ∼47 wt% U were identified along with almost U-free cores, which can often be found next to each other. The U-bearing mineral was identified as carnotite and a paragenesis of newly described secondary spherical and acicular Cr-V-Al minerals was found responsible for the high Cr contents in the reduction spheroid cores. The reduction spheroids were formed shortly after the burial of the karst bauxite by an unsorted clay-rich sediment and two possible origins for mobilized U, V and Cr are discussed. In the most likely model, U-, V- and Cr-rich material together with organic matter were admixed during a redeposition of the uppermost zone of the karst bauxite. After the burial of the karst bauxite, the decomposition of the organic matter in this mixing zone locally mobilized elements by mineral dissolution, and organic-rich particles beneath the mixing zone generated locally reducing conditions that immobilized U, V and Cr in the developing reduction spheroid cores. This mechanism is suggested to be of significance for the formation of U-rich karst bauxites in general.
This paper establishes karst bauxite as a host rock of reduction spheroids and describes a possible process of U enrichment in karst bauxites. With this publication we provide evidence that reduction spheroids are not formed by a single mechanism which would allow a priori inferences from their occurrence in the geological record, and we present a model for the formation of U-rich karst bauxites.
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来源期刊
Journal of Geochemical Exploration
Journal of Geochemical Exploration 地学-地球化学与地球物理
CiteScore
7.40
自引率
7.70%
发文量
148
审稿时长
8.1 months
期刊介绍: Journal of Geochemical Exploration is mostly dedicated to publication of original studies in exploration and environmental geochemistry and related topics. Contributions considered of prevalent interest for the journal include researches based on the application of innovative methods to: define the genesis and the evolution of mineral deposits including transfer of elements in large-scale mineralized areas. analyze complex systems at the boundaries between bio-geochemistry, metal transport and mineral accumulation. evaluate effects of historical mining activities on the surface environment. trace pollutant sources and define their fate and transport models in the near-surface and surface environments involving solid, fluid and aerial matrices. assess and quantify natural and technogenic radioactivity in the environment. determine geochemical anomalies and set baseline reference values using compositional data analysis, multivariate statistics and geo-spatial analysis. assess the impacts of anthropogenic contamination on ecosystems and human health at local and regional scale to prioritize and classify risks through deterministic and stochastic approaches. Papers dedicated to the presentation of newly developed methods in analytical geochemistry to be applied in the field or in laboratory are also within the topics of interest for the journal.
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