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SF (Self sealing of fault and paleo-fluid flow): Synthesis report

Mazurek, M.
Möri, A.
Publication Mont Terri Project, Switzerland, 2014
Collection Mont Terri Project Technical Report; TR 2008-02
Description 65 p.
Abstract Calcite, calcite-celestite and celestite veins were identifed from the bottom of the Opalinus Clay up to the Malm limestone. Most of the sampling effort for veins was focused on the NE trending Main Fault, a thrust structure related to the Jura folding and thrusting, where sub-millimeter thick calcite (±celestite) veins are common. The few accessible (no shotcrete on the gallery wall) NW trending and sub-horizontal celestite-calcite veins located below and above the Main fault were also sampled. Isotopic analyses (Sr, S, O, C) were performed on whole rock carbonate, diagenetic sulfides, evaporitic sulfates (Triassic) and vein minerals (calcite and celestite) from samples distributed along a profile from the Muschelkalk to the Malm. Isotope data suggest that veins formed during a single geotectonic event that allowed fluids to flow across the Opalinus Clay, with the main fluid source located in the underlying Trias. Petrographic data combined with structural information indicate that vein mineral precipitation was syntectonic and most likely occurred during the Jura folding and thrusting. Sulfate-rich fluids were apparently expelled upward from the Trias aquifers (probably Keuper) through active faults and mixed with local fluids. The deep Triassic source is relatively well constrained by the strontium, sulfur, and oxygen isotopes of vein celestite, and the same data suggest the Opalinus Clay porewater was a key contributor for the precipitation of vein celestite. No data are available for the Sr and dissolved sulfate isotopic composition of the groundwater that might have been present in the Dogger-Malm at the time of vein formation and no particular isotopic imprint on the vein minerals from this aquifer has been observed. The involvement of downward flowing Tertiary seawater is not supported by the data and the geology. Some degree of perturbation of the rock properties is observed in the Opalinus Clay in and near the Main Fault. This perturbation apparently reflects the imprint of the fluids that produced the vein mineralizations and consists of a lowering of the δ13C value of the whole rock carbonate towards the values of vein calcite. In addition, weak or not straightforward lowering of the δ18O value and 87Sr/86Sr ratio of the whole rock carbonate, and a depletion of the δ34S of the diagenetic pyrite are observed, but doubt remains on their significance. It follows that whole rock δ13C (and possibly δ18O) profiles across major tectonic structures might be used to highlight past fluid flow in clay-rich rocks. The use of the 87Sr/86Sr ratio of whole rock carbonate might also be effective but it is probably not reliable (or at least understood) in clay-rich rocks due to methodological problems associated with the acetic acid leach technique. It is concluded that the Opalinus Clay apparently acted as a stable seal during most of its evolution (from 170 Ma to present), except during the Jura folding and thrusting (3-10 Ma), when tectonic strain induced transient fluid flows and mineralization in faults cutting across the Mesozoic sequence. Aside from this period of intense tectonic activity, solute transport in the Opalinus Clay has probably been dominated by diffusion, as has been demonstrated to be the case for the present. In today’s situation, no hydrogeological or geochemical perturbations are recorded in the Main Fault area, and this provides arguments in favor of an efficient self-sealing capacity. The present study also brings some new insights concerning the present day porewater composition of the Opalinus Clay. Sulfur isotope results obtained during this study on diagenetic pyrite from the Opalinus Clay show highly positive δ34S values up to +58 per mil V-CDT that indicate almost complete bacterial seawater sulfate reduction in the porewater during early diagenesis. However, other studies showed that present day porewater has a sulfate/chloride ratio similar to today’s seawater and concentrations that approach seawater. It is clear from sulfur isotopes of diagenetic pyrite that the present-day sulfate content of the Opalinus Clay porewater is not inherited from the formation seawater at the time of sedimentation, and isotope data on vein celestite suggest sulfate and strontium might have diffused in the porewater from the underlying Trias evaporite, probably since sedimentation time.
Keywords Mont Terri underground research laboratoryOpalinus ClaySelf-sealing of faultsCalcite-celestite veinsDiagenetic pyriteSr-S-O-C isotopesFluid flowPorewater diffusionPalaeohydrology.
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DE HALLER, Antoine et al. SF (Self sealing of fault and paleo-fluid flow): Synthesis report. 2014

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Deposited on : 2018-01-03

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