Aspects of U-Th fractionation in Tertiary limestones and calcretes of Dhofar, southern Oman

Bernhard Pracejus, Narasimman Sundararajan, Salah Al-Khirbash, Talal Al Hosni, A. Ebrahimi, Shakil Al-Bulushi, Ali Al-Lazki, Musadem Al-Maashani

Research output: Contribution to journalArticle

Abstract

This paper presents findings from a comprehensive geochemical and geophysical re-examination of known radiogenic anomalies in Tertiary limestones and (sub-)recent calcretes of southwestern Dhofar in the Sultanate of Oman. U-Th-enrichments seem to be associated with deep-rooted fault systems that cross-cut Corg-rich shales at depths of some 800–1000 m, which generally show elevated gamma-ray levels in southern Oman and act as the initial geochemical trap. Metals and radiogenic elements, such as K (max 1945 ppm), U (max 44 ppm), and Th (max 26 ppm) mobilised from these rocks and emplaced higher up in the faults must have constituted radiogenic lineaments at and near the surface (observed in a different but difficult to access location). However, successive weathering partially obscured such anomalies through further re-mobilisation/-mineralisation processes within the calcretes that also enriched Sr and V. In these carbonates, uranium correlates positively with Sr but not very well with V, while thorium shows moderate positive correlations with Sr and V. Both U and Th are also not present (i.e., below the detection limits of a few ppm) in a second sample group that represents background conditions. Being much more immobile than U, Th remained closer to the original western fault positions of the examined site (outlined by magnetics and VLF-EM during the surveys), while uranium moved down-dip over the plateau and through underlying sub-horizontal strata towards the eastern fault system. Here, supergene Sr-enriched calcite preferentially incorporated U, thus reflecting the observed U-Th fractionation.

Original languageEnglish
JournalChemie der Erde
DOIs
Publication statusAccepted/In press - Jan 1 2018

Fingerprint

Oman
Uranium
Calcium Carbonate
limestone
Fractionation
fractionation
uranium
anomalies
Thorium
shales
Carbonates
thorium
weathering
strata
Weathering
calcite
Gamma rays
plateaus
carbonates
examination

Keywords

  • Calcrete
  • Dhofar
  • Limestone
  • Oman
  • Uranium-thorium fractionation

ASJC Scopus subject areas

  • Geophysics
  • Geochemistry and Petrology

Cite this

Aspects of U-Th fractionation in Tertiary limestones and calcretes of Dhofar, southern Oman. / Pracejus, Bernhard; Sundararajan, Narasimman; Al-Khirbash, Salah; Al Hosni, Talal; Ebrahimi, A.; Al-Bulushi, Shakil; Al-Lazki, Ali; Al-Maashani, Musadem.

In: Chemie der Erde, 01.01.2018.

Research output: Contribution to journalArticle

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AU - Pracejus, Bernhard

AU - Sundararajan, Narasimman

AU - Al-Khirbash, Salah

AU - Al Hosni, Talal

AU - Ebrahimi, A.

AU - Al-Bulushi, Shakil

AU - Al-Lazki, Ali

AU - Al-Maashani, Musadem

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N2 - This paper presents findings from a comprehensive geochemical and geophysical re-examination of known radiogenic anomalies in Tertiary limestones and (sub-)recent calcretes of southwestern Dhofar in the Sultanate of Oman. U-Th-enrichments seem to be associated with deep-rooted fault systems that cross-cut Corg-rich shales at depths of some 800–1000 m, which generally show elevated gamma-ray levels in southern Oman and act as the initial geochemical trap. Metals and radiogenic elements, such as K (max 1945 ppm), U (max 44 ppm), and Th (max 26 ppm) mobilised from these rocks and emplaced higher up in the faults must have constituted radiogenic lineaments at and near the surface (observed in a different but difficult to access location). However, successive weathering partially obscured such anomalies through further re-mobilisation/-mineralisation processes within the calcretes that also enriched Sr and V. In these carbonates, uranium correlates positively with Sr but not very well with V, while thorium shows moderate positive correlations with Sr and V. Both U and Th are also not present (i.e., below the detection limits of a few ppm) in a second sample group that represents background conditions. Being much more immobile than U, Th remained closer to the original western fault positions of the examined site (outlined by magnetics and VLF-EM during the surveys), while uranium moved down-dip over the plateau and through underlying sub-horizontal strata towards the eastern fault system. Here, supergene Sr-enriched calcite preferentially incorporated U, thus reflecting the observed U-Th fractionation.

AB - This paper presents findings from a comprehensive geochemical and geophysical re-examination of known radiogenic anomalies in Tertiary limestones and (sub-)recent calcretes of southwestern Dhofar in the Sultanate of Oman. U-Th-enrichments seem to be associated with deep-rooted fault systems that cross-cut Corg-rich shales at depths of some 800–1000 m, which generally show elevated gamma-ray levels in southern Oman and act as the initial geochemical trap. Metals and radiogenic elements, such as K (max 1945 ppm), U (max 44 ppm), and Th (max 26 ppm) mobilised from these rocks and emplaced higher up in the faults must have constituted radiogenic lineaments at and near the surface (observed in a different but difficult to access location). However, successive weathering partially obscured such anomalies through further re-mobilisation/-mineralisation processes within the calcretes that also enriched Sr and V. In these carbonates, uranium correlates positively with Sr but not very well with V, while thorium shows moderate positive correlations with Sr and V. Both U and Th are also not present (i.e., below the detection limits of a few ppm) in a second sample group that represents background conditions. Being much more immobile than U, Th remained closer to the original western fault positions of the examined site (outlined by magnetics and VLF-EM during the surveys), while uranium moved down-dip over the plateau and through underlying sub-horizontal strata towards the eastern fault system. Here, supergene Sr-enriched calcite preferentially incorporated U, thus reflecting the observed U-Th fractionation.

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KW - Uranium-thorium fractionation

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