Abstract. This study characterizes the elastic and fluid flow properties systematically across a ductile–brittle fault zone in crystalline rock at the Grimsel Test Site underground research laboratory. Anisotropic seismic velocities and permeability measured every 0.1 m in the 0.7 m across the transition zone from the host Grimsel granodiorite to the mylonitic core show that foliation-parallel P- and S-wave velocities systematically increase from the host rock towards the mylonitic core, while permeability is reduced nearest to the mylonitic core. The results suggest that although brittle deformation has persisted in the recent evolution, antecedent ductile fabric continues to control the matrix elastic and fluid flow properties outside the mylonitic core. The juxtaposition of the ductile strain zone next to the brittle zone, which is bounded inside the two mylonitic cores, causes a significant elastic, mechanical, and fluid flow heterogeneity, which has important implications for crustal deformation and fluid flow and for the exploitation and use of geothermal energy and geologic waste storage. The results illustrate how physical characteristics of faults in crystalline rocks change in fault zones during the ductile to brittle transitions.

Mylonite

Cataclastic rock

Brittleness

Dilatant

10.5194/se-9-683-2018

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Echantillonnage et pétrographie de roches collectées lors de missions scientifiques organisées dans le nord du Tchad

Antoine de Haller Yves Haeberlin Tiffany Tchang

Afin de partager les connaissances acquises et les methodologies etablies au cours de la premiere phase du programme ResEau, des documents techniques de synthese ont ete elabore dans le cadre de sa restitution finale en mars 2016. Ce document, le troisieme de la serie, presente des analyses petrographiques inedites, conduites sur les echantillons de roches collectes lors des missions scientifiques dans les regions nord du Tchad. Il inclut des descriptions detaillees en lame mince de roches d’âge Primaire a Quaternaire, ainsi une caracterisation de leur potentiel aquifere.

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Permeability and seismic velocity anisotropy across a ductile-brittle fault zone in crystalline rock

Quinn Wenning Claudio Madonna Antoine de Haller Jean‐Pierre Burg

Abstract. This study characterizes the elastic and fluid flow properties systematically across a ductile-brittle fault zone in crystalline rock at the Grimsel Test Site underground research laboratory. Anisotropic seismic velocities and permeability measured every 0.1 m in the 0.7 m across the transition zone from the host Grimsel granodiorite to the fault core show that foliation-parallel p- and s- wave velocities systematically increase from the host rock towards the fault core, while permeability is reduced nearest to the fault core. The results suggest that although brittle deformation has persisted in the recent evolution, antecedent ductile fabric continues to control the matrix elastic and fluid flow properties outside the fault core. The juxtaposition of the ductile strain zone next to the brittle zone, which is bounded inside the two mylonitic fault cores, causes a significant elastic, mechanical, and fluid flow heterogeneity, which has important implications for crustal deformation and fluid flow, and exploitation and use of geothermal energy and geologic waste storage. The results illustrate how physical characteristics of faults in crystalline rocks change in fault zones during the ductile to brittle transitions.

Mylonite

Brittleness

Cataclastic rock

10.5194/se-2018-15

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

Antoine de Haller Martin Mazurek Jorge E. Spangenberg Andreas Möri

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.

Tufa

10.7892/boris.70714

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Effingen Member in the Baulmes Quarry (near Orbe, Yverdon): Calcite-celestite veins as indicators of past fluid flow

Antoine de Haller

Veins in Baulmes show two stages of mineralisation separated by a compressional event that produced stylolites. Both mineralizing stages are characterized by the precipitation of similar minerals; calcite and celestite in the Effingen Member, and calcite only in the Sequanian limestone. In addition to these minerals, Stage 1 veins hosted in the Effingen Member locally bear sphalerite, barite, and pyrite. Stage 1 minerals are affected by the compressional event and minerals are cracked or sheared, while Stage 2 minerals are (almost) unaffected by tectonic strain. Sealing of vein structures by vein minerals is more frequent in Stage 1 than in Stage 2,where drusy textures are the rule. The succession of extensional (veins) and compressional structures (stylolite) might suggests vein formation was related to the stress release occurring during rock failure (earthquakes) while stylolite might record the compressional peak previous to rock failure. The veining and folding appear to be genetically related but the exact chronology of veining within the folding process is not solved at this stage. The story is probably complex, likely with inversions of movement along fault (vein) structures during the folding process. Veins can be split into two families, with the first being oriented NW and showing subvertical dip, and the second being oriented NE, and showing variable dip towards NW to SE. The first family of veins is apparently related to minor dextral and sinistral shears, while the second family trends parallel to the fold axis and it interpreted to correspond to syn-folding bed-to-bed stress accommodation, locally showing sinistral movement. Stage 1 and Stage 2 vein minerals are found in both vein families, which means these were connected at the time of mineralisation,and therefore synchronous. A large part of the veins show Stage 2 mineralisation overprinting Stage 1, thus meaning that even if sealed by minerals, vein structures are longstanding weaknesses that can be reactivated at any time in the future in response to tectonic stress and act as permeable structure. Veins can be followed for a maximum of about 5 meters (maximum about 5 cm thick) and are apparently not organized along major tectonic structures but occur as disseminated clusters, consistent with local stress accommodation during Jura folding. Although no major fault-vein structures were found in the investigated area, it is possible that such structure exist SW of the mine, in the continuation of the WNW-ESE fault-thrust structure that continues to Yverdon-les-Bains. The Baulmes anticline axis shows a dextral offset along this lineament that could probably be related to major fault-vein structures. Isotopic data on vein celestite and calcite can be explained with three fluid reservoirs: 1. the Triasic, probably within the Muschelkalk, rich in sulfate, poor in Sr 2. the Effingen Member porewater, rich in Sr (poorly radiogenic), with positive δ13C values 3. the Malm limestone aquifer, poor in Sr (slightly more radiogenic), containing water with negative δ13C values Sulfur and oxygen isotope data of celestite from Stages 1 and 2 hosted in the Effingen Member can be explained with sulfate that was derived from the Triassic evaporite, most likely from the Muschelkalk aquifer. In contrast, Sr isotopes of vein celestite and calcite broadly mimic the values obtained for corresponding whole rock carbonate and thus suggests the Sr source is local,from the Effingen porewater (and Sr sorbed on clay minerals) or the Malm limestone groundwater, respectively. This scheme is similarly valid for carbon isotopes of vein calcite in the Effingen Member, but not for the Malm limestone, where vein calcite shows negative δ13C values in disequilibrium with the host rock, indicating groundwater contained biogenic carbon in isotopic disequilibrium with the host rock carbonate.

Stylolite

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Origin of fluids in iron oxide–copper–gold deposits: constraints from δ 37Cl, 87Sr/86Sri and Cl/Br

Mineralium Deposita (2006)

Massimo Chiaradia Dave Banks R. A. Cliff Robert Marschik Antoine de Haller

10.1007/s00126-006-0082-6

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The Raúl-Condestable iron oxide copper-gold deposit, central coast of Peru

Antoine de Haller

Cette these de doctorat est dediee au gisement de cuivre-or de type iron oxide copper-gold de Raul-Condestable, localise a 90 km au sud de Lima, sur la cote peruvienne. Le minerai est compose de chalcopyrite, pyrite, pyrrhorite et magnetite, sous la forme de disseminations, remplissages de porosite, remplacements et veines dans des roches amphibolitisees appartenant a une serie volcanosedimentaire du Cretace inferieur. Cette etude a permis de dater la mise en place du gisement (115.2 Ma) et des roches magmatiques associees (114.5 a 116.7 Ma), de definir l'alteration hydrothermale et sa zonation, de decrire en detail la mineralisation et son contexte geologique et de caracteriser les fluides impliques et les mecanismes de depot. La mineralisation s'est formee a une paleoprofondeur de 2-3 km dans un contexte subvolcanique a partir de fluides magmatiques provenant de magmas tonalitiques hydrates. Un melange avec des saumures derivees de l'eau de mer est observe.

10.13097/archive-ouverte/unige:98069

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