{"title":"马达加斯加南部安帕尼希地区古生代锰质成岩的沉积环境和热液蚀变作用","authors":"","doi":"10.1016/j.gexplo.2024.107579","DOIUrl":null,"url":null,"abstract":"<div><p>The Paleoproterozoic is known to host significant sedimentary manganese accumulations deposited in peri-cratonic areas. The depositional environments, the source of Mn and the geometry of the basins are often poorly understood, although they are key aspects in understanding the origin of these strategic resources. We investigate the Mn-rich graphitic metasediments of the Ampanihy district (South Madagascar) to characterize depositional conditions and to identify potential hydrothermal alteration. The results show that the ore is predominantly composed of Mn silicates (spessartine, rhodonite) with accessory jacobsite and pyrophanite varieties rich in Zn close to the Ampanihy shear zone. Geochemical data suggest that the source material in the eastern domain (Ankara) of the district is more crustal in origin than sediments in the western part of the district (Vohidrakitsy, Bekily). The Ankara deposits have chemical characteristics that are close to many other Paleoproterozoic Mn deposits and can be considered as relatively unaltered and fits with the euxinic model. The western domain of the Ampanihy district shows strong hydrothermal overprinting, with significant enrichment in Zn, Ba and Sb, resulting from post-depositional hydrothermal alteration along the proto-Ampanihy shear zone. This event took place at some stage between the deposition of the sediments (c. 1.9‐1.7 Ga) and the suture of the Vohibory and Androyen domains (c. 0.63–0.60 Ma). Based on our data, we propose a new discrimination plot, (La/Lu)<sub>PAAS</sub> vs Zn/Ni, which helps in the identification of sediment sources and the recognition of hydrothermal overprinting.</p></div>","PeriodicalId":16336,"journal":{"name":"Journal of Geochemical Exploration","volume":null,"pages":null},"PeriodicalIF":3.4000,"publicationDate":"2024-09-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":"0","resultStr":"{\"title\":\"Depositional setting and hydrothermal alteration of Paleoproterozoic manganiferous metasedimentary rocks in the Ampanihy district (Southern Madagascar)\",\"authors\":\"\",\"doi\":\"10.1016/j.gexplo.2024.107579\",\"DOIUrl\":null,\"url\":null,\"abstract\":\"<div><p>The Paleoproterozoic is known to host significant sedimentary manganese accumulations deposited in peri-cratonic areas. The depositional environments, the source of Mn and the geometry of the basins are often poorly understood, although they are key aspects in understanding the origin of these strategic resources. We investigate the Mn-rich graphitic metasediments of the Ampanihy district (South Madagascar) to characterize depositional conditions and to identify potential hydrothermal alteration. The results show that the ore is predominantly composed of Mn silicates (spessartine, rhodonite) with accessory jacobsite and pyrophanite varieties rich in Zn close to the Ampanihy shear zone. Geochemical data suggest that the source material in the eastern domain (Ankara) of the district is more crustal in origin than sediments in the western part of the district (Vohidrakitsy, Bekily). The Ankara deposits have chemical characteristics that are close to many other Paleoproterozoic Mn deposits and can be considered as relatively unaltered and fits with the euxinic model. The western domain of the Ampanihy district shows strong hydrothermal overprinting, with significant enrichment in Zn, Ba and Sb, resulting from post-depositional hydrothermal alteration along the proto-Ampanihy shear zone. This event took place at some stage between the deposition of the sediments (c. 1.9‐1.7 Ga) and the suture of the Vohibory and Androyen domains (c. 0.63–0.60 Ma). Based on our data, we propose a new discrimination plot, (La/Lu)<sub>PAAS</sub> vs Zn/Ni, which helps in the identification of sediment sources and the recognition of hydrothermal overprinting.</p></div>\",\"PeriodicalId\":16336,\"journal\":{\"name\":\"Journal of Geochemical Exploration\",\"volume\":null,\"pages\":null},\"PeriodicalIF\":3.4000,\"publicationDate\":\"2024-09-05\",\"publicationTypes\":\"Journal Article\",\"fieldsOfStudy\":null,\"isOpenAccess\":false,\"openAccessPdf\":\"\",\"citationCount\":\"0\",\"resultStr\":null,\"platform\":\"Semanticscholar\",\"paperid\":null,\"PeriodicalName\":\"Journal of Geochemical Exploration\",\"FirstCategoryId\":\"89\",\"ListUrlMain\":\"https://www.sciencedirect.com/science/article/pii/S037567422400195X\",\"RegionNum\":2,\"RegionCategory\":\"地球科学\",\"ArticlePicture\":[],\"TitleCN\":null,\"AbstractTextCN\":null,\"PMCID\":null,\"EPubDate\":\"\",\"PubModel\":\"\",\"JCR\":\"Q1\",\"JCRName\":\"GEOCHEMISTRY & GEOPHYSICS\",\"Score\":null,\"Total\":0}","platform":"Semanticscholar","paperid":null,"PeriodicalName":"Journal of Geochemical Exploration","FirstCategoryId":"89","ListUrlMain":"https://www.sciencedirect.com/science/article/pii/S037567422400195X","RegionNum":2,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":null,"EPubDate":"","PubModel":"","JCR":"Q1","JCRName":"GEOCHEMISTRY & GEOPHYSICS","Score":null,"Total":0}
Depositional setting and hydrothermal alteration of Paleoproterozoic manganiferous metasedimentary rocks in the Ampanihy district (Southern Madagascar)
The Paleoproterozoic is known to host significant sedimentary manganese accumulations deposited in peri-cratonic areas. The depositional environments, the source of Mn and the geometry of the basins are often poorly understood, although they are key aspects in understanding the origin of these strategic resources. We investigate the Mn-rich graphitic metasediments of the Ampanihy district (South Madagascar) to characterize depositional conditions and to identify potential hydrothermal alteration. The results show that the ore is predominantly composed of Mn silicates (spessartine, rhodonite) with accessory jacobsite and pyrophanite varieties rich in Zn close to the Ampanihy shear zone. Geochemical data suggest that the source material in the eastern domain (Ankara) of the district is more crustal in origin than sediments in the western part of the district (Vohidrakitsy, Bekily). The Ankara deposits have chemical characteristics that are close to many other Paleoproterozoic Mn deposits and can be considered as relatively unaltered and fits with the euxinic model. The western domain of the Ampanihy district shows strong hydrothermal overprinting, with significant enrichment in Zn, Ba and Sb, resulting from post-depositional hydrothermal alteration along the proto-Ampanihy shear zone. This event took place at some stage between the deposition of the sediments (c. 1.9‐1.7 Ga) and the suture of the Vohibory and Androyen domains (c. 0.63–0.60 Ma). Based on our data, we propose a new discrimination plot, (La/Lu)PAAS vs Zn/Ni, which helps in the identification of sediment sources and the recognition of hydrothermal overprinting.
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.