Granite und Uranvererzungen in den Schweizer Alpen

Zusammenfassung Die Radioaktivität vier hercynischer, granitischer Gesteine schweizerischer Zentralmassive (Mont-Blanc-Granit, Vallorcine-Granit, Giuv-Syenit und Zentraler Aaregranit s. l.) wurde im Gelände (Oberflächen- und vor allem Stollen-Messungen) und im Labor (gammaspektrometrische und fluorimetrische Bestimmungen) untersucht. Jeder Gesteinskomplex zeigt ein spezifisches Uran-Thorium-Variationsfeld. Das Th/U-Verhältnis nimmt mit steigenden Th-Gehalten ab. Die Variation der U- und Th-Gehalte ist petrologisch kontrolliert: es ist eine Zunahme in den Randzonen der Granitkörper und (bei Intrusionsfolgen) in den jüngsten Gesteinen, vor allem in Aplitgranitstöcken, festzustellen. Mehrere Gesteine sind besonders uran- und thoriumreich: Giuv-Syenit, Mittel aus 44 Bestimmungen, 22 ppm U und 66 ppm Th; Mont-Blanc-Granit (45), 19 ppm U/31 ppm Th; Zentraler Aaregranit s. str. (10), 13 ppm U/32 ppm Th. Die Gesteine enthalten zum Teil hohe Anteile leichtlöslichen Urans (bis 90%), wobei hohe Gesamt-Urangehalte in der Regel auch einen hohen Prozentsatz leichtlösliches Uran bedingen.In allen Gesteinskomplexen sind eigentliche Uranmineralisationen (meist mit Pechblende) gefunden worden. Sie sind ausschließlich an Scherzonen, Mylonite und Klüfte gebunden und liegen vor allem in den Randzonen der Gesteinskörper oder in der Nähe besonders uranreicher Gesteine wie Aplitgranite. Die Assoziation Granite/Uranvererzungen zeigt, abgesehen von der alpinen überprägung, große ähnlichkeiten mit den französischen Vorkommen des Massif Central.

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The radioactivity of 4 Hercynian granitic rock units of the Swiss Central Massifs (Mont Blanc granite, Vallorcine granite, Giuv syenite and Central Aare granite s. l.) has been measured in the field (on the surface and in tunnels) and in the laboratory (gamma ray spectrometry). Each rock unit shows a clearly defined field of variation on a U-Th plot. The Th/U ratios decrease with increasing Th contents. The variation is petrologically controlled: the U and Th contents increase in the border zones of the granitic bodies and — in the case of intrusive sequences — in the younger derivates, especially in the aplitic phase. Three of the units show unusually high U and Th contents: Giuv syenite (mean of 44 determinations), 22 ppm U and 66 ppm Th; Mont Blanc granite (45), 19 ppm U/31 ppm Th; Central Aare granite s. str. (10), 13 ppm U/32 ppm Th. The fraction of easily leacheable uranium is, according to fluorimetric determinations, unusually high (up to 90%) in some granites. Generally, the higher the U content, the higher the leacheable fraction.In all rock units uranium mineralisations (mainly pitchblende) have been found. They are confined to shear zones, mylonites and fissures and occur mostly in the border zones of the granitic bodies or in the vicinity of especially uranium-rich rock types, such as aplites. Apart from the effects of the Alpine orogeny the association of uranium mineralisations with granites shows close similarities to the occurrences in the French Massif Central.

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Résumé La radioactivité de quatre massifs granitiques hercyniens des massifs cristallins centraux des Alpes suisses (granites du Mont Blanc, de Vallorcine, et de l'Aar et syénite du Piz Giuv) a été étudiée sur le terrain et en laboratoire (spectrométrie gamma et fluorimétrie). La variation des teneurs en U et en Th est spécifique pour chaque type des roche. Le rapport Th/U diminue lorsque la teneur en Th augmente. L'Uranium s'enrichie d'une part dans les faciès bordiers des massifs granitiques et d'autre part (dans des successions magmatiques) vers les roches les plus jeunes et les plus acides (granites aplitiques). Quelques-unes de nos roches ont un fonds géochimique d'U et de Th élevé: syénite du Piz Giuv 22 ppm U/66 ppm Th (moyenne de 44 déterminations); granite du Mont Blanc 19 ppm U/31 ppm Th (45); granite de l'Aar 13 ppm U/32 ppm Th (10). En mÊme temps, on constate des teneurs élevées en Uranium facilement soluble (jusqu'a 80-90% de l'Uranium total).Dans tout les complexes se trouvent des minéralisations d'Uranium (sans importance écononique actuelle). Elles sont situées surtout aux bords des granites ou dans des roches avec beaucoup d'Uranium soluble («fertiles») et sont essentiellement liées à des structures tectoniques. A l'exception de la tectonique et du métamorphisme alpin et d'une forte érosion post-tertiaire notre association granites/mineralisations ressemble beacoup à celle du Massif Central franÇais.

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( , Vallorcine, Giuv s. 1.), ( , , ), ( ). . Th/U . : ( ) , , , : Giuv, 44 22 ppm U 66 Th; (45) 19 U, 31 ppm Th; s. 1. (10) 13 ppm U, 32 ppm Th. ( 90%). , , . , . , , -: . / Massif Central, , , .

     

Design and field test of scintillation probe for γ-logging of small diameter boreholes

Summary A 68-cm long probe of 2.5 cm diameter, containing a NaI (Tl) crystal, photomultiplier, HV stabilization and preamplifier, is described. Special attention is paid to the probe-cable coupling: it is designed to withstand hydrostatic pressures up to 10 atm. For logging of inclined, up to 20 m long holes, aluminium extension pipes assembled by push-button closures are used. The probe, operated together with a portable ratemeter, has a sensitivity of 1200 c.p.s. per mR/h (Ra²²⁶). For quantitative log interpretation the conversion factor was determined to be 0.4 p.p.m. U/c.p.s. Radiometrically determined uranium contents agree remarkably well with results of chemical analysis of cores.Contribution No. 89, Inst. of Geophysics, ETH, Zürich.     

The first deep Quaternary groundwater capture in Switzerland: combined use and environmental benefits

 Foresighted and determined local authorities, purposeful exploration (i.e. by seismic reflection) and extensive testing led to the discovery of a substantial groundwater resource near the community of Seon (Switzerland) at a depth of 268–305 m. Production tests revealed a hydraulic conductivity of ∼5.10–5 m/s, transmissivity of ∼5.10–4 m²/s and a storage coefficient of ∼2% in the aquifer. Pumping up to 1500 l/min is sustainable; the water quality complies chemically and bacteriologically with drinking-water requirements. The residence time of several 10³ years, determined by isotope techniques, guarantees protection from surface contamination. The elevated temperature of 19.5  °C of the produced water enables combined use for drinking water and space heating. The environmental benefits are substantial: the emission reduction amounts up to 780 tons/year CO₂ and 1 ton/year SO₂. Received: 21 September 1998 · Accepted: 10 February 1999     

Temperature predictions and predictive temperatures in deep tunnels

Summary Reliable assessments of the underground temperature are needed for construction projects like deep tunnels, shafts and storage facilities. In areas of pronounced topographic relief, special attention must be given to the influence of three-dimensional topography on the subsurface temperature field. Further parameters for the prediction include ground surface temperature, local heat flow density, geological factors (structure/schistosity, thermal conductivity, erosion rate, water circulation). A first prediction was attempted for the planned Gotthard railroad tunnel (NEAT). This tunnel will have a total length of 56 km with a maximum cover of 2500 m. The temperature predictions were calculated, based on the RIMINI topography array, for points at 1-km-intervals along the planned axis. Maximum temperature (conservative upper limit) is about 45°C; the error of the predicted temperatures could be, in view of the uncertainties involved, in general ca. ±5–10°C.Since little is known at present about the deep water circulation system in the realm of the planned Gotthard tunnel, the prediction calculations assume heat transport by pure conduction. Significant subsurface water flow would mainly lead to the reduction of rock temperatures by cold infiltrations from the surface, as demonstrated by observations in the Simplon and Mont Blanc tunnels. In fact, the deviation of actual measurements (performed right behind the advancing face) from conductive previsions should be used as a predictive tool of large water flows which could be encountered during tunnel construction.     

Siting criteria for heat extraction from hot dry rock; Application to Switzerland

For selecting possible hot dry rock extraction sites for geothermal energy applications, the following criteria have been considered: (i) depth to the crystalline basement, (ii) temperatures at depth, (iii) pattern of regional stress field and (iv) natural permeability (=degree of fracturing) of basement rocks. A contour map of the basement topography is presented. From outcrops at the nothern border of Switzerland (crystalline rocks of the Black Forest massif, mainly granites and gneisses of Hercynian age) the basement dips gently toward the SE under the Mesozoic and Tertiary sediments of the Molasse Basin and reaches its maximum depth (7 km) underneath the front of the Alps. Some 30 km further SE the basement rocks appear at the surface (Aar- and Gotthard-massif, Penninic units), where they are deformed and fractured to a great extent. Temperature-depth profiles have been obtained by model calculations. Locally increased heat product on (in granite batholiths) at the base of the Molasse Basin, combined with the blanketing effect of the overlying sediments, could raise the temperatures to 150–170°C at a depth of 5 km. According to earthquake fault-plane solutions (P-axes) the regional stress field in the area of the Swiss Alps and in its northern Foreland is characterized by the maximum horizontal compression oriented N(150±20)°E in the upper crust.In situ stress determinations (overcoring experiments) show that considerable excess horizontal compressive stress is present in the Alpine crust (up to 200 bar). The deep Alpine tunnels exhibited considerable fracturing of crystalline rocks at depths greater than 1–2 km. Information about the degree of fracturing has also been obtained by refraction profiles. The velocitydepth functions show lower than normal velocities in the uppermost 1.5 km, indicating that the rocks there are fractured. A 30–40 km wide region, running along the axis of the Molasse Basin (which coincides with the majority of the population and most of the industry of Switzerland) would provide the best hot dry rock sites.Paper presented at the Second NATO-CCMS Meeting on Dry Hot Rock Geothermal Energy, 28–30 June 1977, Los Alamos, New Mexico, USA. Contribution No. 198, Institute of Geophysics ETH Zurich.     

A gamma spectrometric study of Mont-Blanc granite samples

Summary Gamma-ray spectra of the samples were measured by standard laboratory multichannel spectrometry. The weighted least squares method was applied to the digital output data in order to determine the concentrations of U²³⁸, Th²³² and K⁴⁰ present. U and Th increase towards the rim of the massif (from 4 ppm U and 20 ppm Th in the coarse-grained central facies to 23 ppm U and 42 ppm Th in the fine-grained rim variety), whereas K remains fairly constant at 3.85%.On leave Eidgenössische Technische Hochschule, Zurich (Switzerland)     

Heat flow anomalies and their interpretation

More than 10,000 heat flow determinations exist for the earth and the data set is growing steadily at about 450 observations per year. If heat flow is considered as a surface expression of geothermal processes at depth, the analysis of the data set should reveal properties of those thermal processes. They do, but on a variety of scales. For this review heat flow maps are classified by 4 different horizontal scales of 10ⁿ km (n = 1, 2, 3 and 4) and attention is focussed on the interpretation of anomalies which appear with characteristic dimensions of 10^{(n − 1)} km in the respective representations.The largest scale of 10⁴ km encompasses heat flow on a global scale. Global heat loss is 4 × 10¹³ W and the process of sea floor spreading is the principal agent in delivering much of this heat to the surface. Correspondingly, active ocean ridge systems produce the most prominent heat flow anomalies at this scale with characteristic widths of 10³ km. Shields, with similar dimensions, exhibit negative anomalies.The scale of 10³ km includes continent wide displays. Heat flow patterns at this scale mimic tectonic units which have dimensions of a few times 10² km, although the thermal boundaries between these units are sometimes sharp. Heat flow anomalies at this scale also result from plate tectonic processes, and are associated with arc volcanism, back arc basins, hot spot traces, and continental rifting. There are major controversies about the extent to which these surface thermal provinces reflect upper mantle thermal conditions, and also about the origin and evolution of the thermal state of continental lithosphere.Beginning with map dimensions of 10² km thermal anomalies of scale 10¹ km, which have a definite crustal origin, become apparent. The origin may be tectonic, geologic, or hydrologic. Ten kilometers is a common wavelength of topographic relief which drives many groundwater flow systems producing thermal anomalies. The largest recognized continental geothermal systems have thermal anomalies 10¹ km wide and are capable of producing hundreds of megawatts of thermal energy.The smallest scale addressed in this paper is 10¹ km. Worldwide interest in exploiting geothermal systems has been responsible for a recent accumulation of heat flow data on the smallest of scales considered here. The exploration nature of the surveys involve 10's of drillholes and reveal thermal anomalies having widths of 10⁰ km. These are almost certainly connected to surface and subsurface fluid discharge systems which, in spite of their restricted size, are typically delivering 10 MW of heat to the near surface environment.     

Radioactive heat production in rocks and its relation to other petrophysical parameters

Summary Radioactive heat productionA is a scalar and isotropic petrophysical property independent of in situ temperature and pressure. Its value is usually expressed in HGU units (1 HGU=10^–13 cal/cm³ sec) and depends on the amounts of uranium, thorium and potassium.A varies with rock type over several orders of magnitude and reflects the geochemical conditions during rock formation (magmatic differentiation, sedimentation or metamorphism).In order to assign realistic thermal parameters to deeper-seated rocks correlations with seismic velocity (which can be determined from the surface) have been looked for. In the range characteristic for crystalline rocks of the crust (5–8 km/sec)A is strongly correlated with density and compressional wave velocityv _p:A decreases with increasingv _p orp. From this relationship it is now possible to estimate heat production values for any particular layer of a crustal section from measured seismic velocities. Contrary to earlier belief there is, as shown by experimental determinations, no correlation between heat productionA and thermal conductivityK in igneous and metamorphic rocks. In sediments however, especially in sand/shale sequences, a correlation betweenK andA is most likely: increasing clay mineral content, characterized by increasingA, causes the decrease ofK in these rocks.Contribution No. 111, Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, Switzerland.