Fluid flow through the sedimentary cover in northern Switzerland recorded by calcite–celestite veins (Oftringen borehole, Olten)

Antoine De Haller,Martin Mazurek,Alexandre Tarantola,Jorge Spangenberg

doi:10.1007/s00015-011-0085-x

Antoine De Haller, Martin Mazurek + Show 2 more

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https://doi.org/10.1007/s00015-011-0085-x

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Abstract

Abundant veins filled by calcite, celestite and pyrite were found in the core of a 719 m deep borehole drilled in Oftringen near Olten, located in the north-western Molasse basin, close to the thrust of the Folded Jura. Host rocks are calcareous marl, argillaceous limestone and limestone of the Dogger and Malm. The δ18O values of vein calcite are lower than in host rock carbonate and, together with microthermometric data from fluid inclusions in vein calcite, indicate precipitation from a seawater-dominated fluid at average temperatures of 56–68°C. Such temperatures were reached at the time of maximum burial of the sedimentary pile in the late Miocene. The depth profile of δ13C and 87Sr/86Sr values and Sr content of both whole-rock carbonate and vein calcite show marked trends towards negative δ13C, high 87Sr/86Sr, and low Sr content in the uppermost 50–150 m of the Jurassic profile (upper Oxfordian). The 87Sr/86Sr of vein minerals is generally higher than that of host rock carbonate, up to very high values corresponding to Burdigalian seawater (Upper Marine Molasse, Miocene), which represents the last marine incursion in the region. No evidence for internally derived radiogenic Sr (clay minerals) has been found and so an external source is required. S and O isotope composition of vein celestite and pyrite can be explained by bacterial reduction of Miocene seawater sulphate. The available data set suggests the vein mineralization precipitated from descending Burdigalian seawater and not from a fluid originating in the underlying Triassic evaporites.

Highlights

Veins filled by calcite, celestite and subordinate pyrite have been documented in the Mesozoic sedimentary cover of the Swiss plateau and the Jura (Weibel 1966; Meisser 1997; Stalder et al 1998; Waber and Schurch 2000; Pearson et al 2003), but to date, their origin has not been thoroughly explored
The d13C results are reported relative to the Vienna-Pee Dee Belemnite (V-PDB) standard, whilst d18O values refer to the Vienna Standard Mean Ocean Water (V-SMOW) standards; standardization was accomplished using international standards NBS 19 and NBS 18 (Friedman et al 1982)
Fluid inclusions in celestite are aligned along fractures that never cut calcite grains, suggesting that these fluid inclusions predate the precipitation of calcite

Summary

Introduction

Celestite and subordinate pyrite have been documented in the Mesozoic sedimentary cover of the Swiss plateau and the Jura (Weibel 1966; Meisser 1997; Stalder et al 1998; Waber and Schurch 2000; Pearson et al 2003), but to date, their origin has not been thoroughly explored. The main objective of the study was to constrain the timing, temperature and origin of the fluids involved in the precipitation of the vein minerals, as well as the geodynamic context. This was achieved by petrographic/mineralogical studies, fluid-inclusions microthermometry and analyses of the isotopic composition (C, O, S, Sr) of vein minerals and surrounding host rocks

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