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International Journal of Earth Sciences

, Volume 108, Issue 6, pp 1857–1877 | Cite as

A geochemical and micro-textural comparison of basalt-hosted chalcedony from the Jurassic Drakensberg and Neoarchean Ventersdorp Supergroup (Vaal River alluvial gravels), South Africa

  • Elisabetta GliozzoEmail author
  • Bruce Cairncross
  • Torsten Vennemann
Original Paper
  • 52 Downloads

Abstract

South African basalt-hosted chalcedonies provided an ideal case study, to (a) achieve information about the origin of mineral-forming fluids, the process of chalcedony formation and weathering alteration and (b) to verify if the geochemical and micro-textural characterization of chalcedony allowed multiple provenances to be distinguished. For the first time, Drakensberg chalcedonies from the Karoo Jurassic basalts and Windsorton chalcedonies from the alluvial environment of the Vaal River (traversing the Allanridge basaltic–andesitic lavas) were investigated by optical microscopy, laser ablation inductively coupled plasma mass spectroscopy and isotope ratio mass spectrometry (16O, 18O). The results showed that the compositional differences observed in Drakensberg and Windsorton chalcedonies could not be explained by changes in host rocks composition. The tholeiitic basaltic–andesitic lavas of the Allanridge Formation (Pniel Group, comparison term for Windsorton samples) and the tholeiitic basaltic–andesitic lavas of the Golden Gate Unit (Drakensberg Group, comparison term for Drakensberg specimens) proved to be very similar, also in terms of alteration degree. Conversely, both geochemical and oxygen isotope compositions clearly supported a relatively low temperature, hydrothermal origin for all the investigated chalcedonies, further indicating that differences mostly occurred during their formation (fluid circulation). Moreover, several characteristics suggested deposition in non-equilibrium conditions, although oxidizing conditions were most likely. Weathering processes were effective in the alluvial environment only, leading to a wider compositional heterogeneity of Windsorton chalcedonies with respect to Drakensberg samples. Lastly, a correlation between color and chromophores (Fe and Mn) amounts was lacking while a correlation between the geochemical composition and the texture was clearly observed in several specimens.

Keywords

Chalcedony and agate Volcanic host Alluvial weathering Hydrothermal fluids LA-ICP-MS Oxygen isotopes 

Notes

Acknowledgements

The authors are indebted to the two anonymous reviewers for providing insightful comments which improved the quality of our manuscript. One of us (BC) acknowledges that this research was partly funded by the NRF (National Research Foundation) and CIMERA (the DST-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis). Any opinions, findings and conclusions or recommendations expressed are those of the author, and therefore, the NRF and CIMERA do not accept liability in regard thereto.

Supplementary material

531_2019_1737_MOESM1_ESM.tif (50.1 mb)
Supplementary material 1 Binary plots showing the geochemical variation of SiO2 wt% contents with respect to significant minor elements in Drakensberg (A–F) and Windsorton (G–L) samples. In Windsorton binary diagrams (G–L), black full circles indicate Windsorton samples while grey full circles in the background represent the Drakensberg Group samples. The outlier sample Win6 has been omitted (TIFF 51340 kb)
531_2019_1737_MOESM2_ESM.tif (63.2 mb)
Supplementary material 2 Geochemical profiles on samples Win10, 11 and 12. Macro (scanner) images have been provided per each sample. Along Fe and Mn profiles, B = Brown; W = white; O = orange; G = grey; Bk = blackish (TIFF 64761 kb)
531_2019_1737_MOESM3_ESM.tif (63.2 mb)
Supplementary material 3 Geochemical profiles on samples Win13, 14 and 15. Macro (scanner) images have been provided per each sample. Along Fe and Mn profiles, O = orange; B = Brown; G = grey; Bk = blackish (TIFF 64761 kb)
531_2019_1737_MOESM4_ESM.xlsx (139 kb)
Supplementary material 4 (XLSX 139 kb)

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Copyright information

© Geologische Vereinigung e.V. (GV) 2019

Authors and Affiliations

  1. 1.Department of Earth, Environment and Physical SciencesUniversity of SienaSienaItaly
  2. 2.Department of GeologyUniversity of JohannesburgJohannesburgSouth Africa
  3. 3.Institut des Dynamiques de la Surface TerrestreUniversité de LausanneLausanneSwitzerland

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