Numerical investigation of transient hydrothermal processes around intrusions: heat-transfer and fluid-circulation controlled mineralization patterns.

International audienceNew insights on the circulation of fluids around magmatic intrusions have been obtained through coupled hydrothermal numerical modelling that takes into account i) a continuous variation of permeability with depth, ii) the period of intrusion emplacement, iii) the physical likelihood of ore deposition using a restricted rock alteration index, and iv) the so-far unexplored pluton floor, and then comparing the results against well-constrained natural cases showing different emplacement depths, high permeability zones (cracked thermal aureoles), faults and plutonic apexes. We show that emplacement depth is a key physical parameter controlling the extent and geometries of advective heat dissipation zones, and that shallow apexes strongly modify the fluid-flow pattern by acting as a focus for convective fluids and mineralization zones. We also show that the cooling phase is not the main convective phase for large plutons commonly associated with long-lived magma emplacement; major advective heat dissipation and mineral deposition zones may also develop before and during the hottest phase of the emplacement, i.e. before magma crystallization. The comparison with natural cases shows that we successfully reproduce, in space and time, the physical conditions required for mineral deposition. In particular, extensional detachment is able to restrain and modify classical fluid-flow patterns induced by coeval intrusion. Finally, even though lacking chemical arguments, we conclude that convection induced by granite emplacement plays a major role in the genesis of granite-related Au deposits. Moreover, the formation of this type of deposit is favoured and controlled by the presence of a fractured thermal aureole around the intrusion

Combination of Numerical Tools to Link Deep Temperatures, Geological Structures and Fluid Flow in Sedimentary Basins: Application to the Thermal Anomalies of the Provence Basin (South-East France)

International audienceIn the Provence basin, south-eastern France, more than 230 Bottom Hole Temperature (BHT) data have been compiled and corrected for transient disturbances to provide a thermal model of this Mesozoic to Cenozoic sedimentary basin. The thermal gradient of the area averages 29.9°C/km (32.5°C/km in all France), but some places show gradients reaching 36°C/km or 22°C/km. To characterize thermal anomalies, a three-dimensional model of the temperatures was built between the surface and 5km depth, allowing us to elaborate sets of thermal maps and cross-sections. The newly identified temperature anomalies may reach temperature difference up to 40°C at 3km depth through the basin. After attempting to find correlations between thermal anomalies and large scale features (Moho depths, sediment cover thickness), it appears that fluid circulation may better explain locations, amplitudes and wavelengths of thermal anomalies along faulted zones. In fact, spatial evolution of anomalous cold/warm zones follow directions of main faulted zones. In addition, it is shown that the account of a depth-dependent permeability allows the superimposition of positive and negative thermal anomalies. Away from permeable zones, thermal anomalies should be explained by conductive processes, among which heat refraction due to thermal conductivity contrasts may be significant. In particular, anisotropy of thermal conductivity of clayey formation is shown to enable the development of thermal anomalies similar to those observed between permeable zones. Evolution of fluid circulation in faulted zones (involving enhanced vertical heat transfer) combined with thick anisotropic sediments (involving enhanced horizontal heat transfer) may explain complex thermal patterns deduced from present-day temperature measurements

Geological and thermal conditions before the major Palaeoproterozoic gold-mineralization event at Ashanti, Ghana, as inferred from improved thermal modelling

International audienceHeat transfer processes before a major mineralizing event in the Paleoproterozoic continental crust of southern Ghana are the subject of a detailed regional thermal modelling study. The area of the Ashanti belt is the most mineralized for gold in West Africa, and it is believed that prior to the main gold mineralization event, crustal-scale thermal processes may have played a critical role in ore deposit formation. The thermal regime before and after the crustal shortening events that affected the region during the Eburnean orogeny (2130–1980 Ma) is calculated just before the main mineralization event which corresponds to late orogenic hydrothermal gold deposit formation. Measured thermal properties of lithological units are incorporated into the model, which is geometrically constrained by field studies. Computed pressure–temperature paths, compared to thermobarometric data, allow calibration of the model parameters. The temperature-dependence of thermal properties and the influence of compaction on vertically varying conductivities have been considered in the crustal-scale thermal modelling. The predictions from this model indicate that the most probable mantle heat flow value at this time in the region of the Ashanti belt is not, vert, similar30 mW m−2. Such a value is two to three times higher than present-day values below stable cratonic areas and could be considered as an upper limit in geodynamic models of the Paleoproterozoic mantle. This relatively high value might be related to the thermal input from a Paleoproterozoic mantle event such as a mantle plume, as previously suggested by several authors to explain metallogenic crises in West Africa. The P–T paths inferred from numerical modelling also allow discrimination between extreme geological scenarios. Indeed, the results of the modelling suggest that the basement is likely to be of continental rather than of oceanic type in the Ashanti area

Deep temperatures in the Paris Basin using tectonic-heat flow modelling

International audienceThe determination of deep temperatures in a basin is one of the key parameters in the exploration of geothermal energy. This study, carried out as part of the CLASTIQ-2 project, presents a 3 temperatures in the Paris Basin derived through a thermal-tectonic forward modelling method, calibrated using subsurface temperature values. The temperature dataset required for the calibration was compiled in 2007 as part of the CLASTIQ-1 project. The temperature measurement dataset is largely composed of BHT (some 2443 values). These BHT measurements required correction due to the thermal disturbance created during drilling. After correction, which was carried out using the Instantaneous Cylinder Source (ICS) method, 494 corrected BHT (BHTx) values were available for the modelling of the Paris Basin. In addition to these BHTx, some 15 DST measurements that are considered as close to the thermal equilibrium (i.e., ±5°C) were added to the temperature calibration values. According to this dataset of BHTx and DST, the average gradient in the Paris Basin was calculated as 34.9°C/km when the surface temperature is fixed at 10°C. The temperature values collected were then used to calibrate the tectonic-heat flow modelling. The model was computed at the lithospheric scale but focused on the temperature field in the sedimentary basin fill. The model takes into account the geodynamic evolution of the last 20 My, the heat production, and the specific heat conduction of each defined sedimentary layer. The result is a 3D thermal block that is presented in the form of isodepth maps. The results are strongly influenced by thermal conductivity variations such as those due to differences in sediment composition while faults create some more localised influences. The presence of anomalously radiogenic bodies beneath the basin, and/or by variations in lithosphere thickness resulting in possible heat production anomalies strongly influence the thermal variations the Paris Basin. The Alpine Orogeny created a slight temperature increase in the south-eastern part of the basin and inhomogeneities in the lithology of the basement generating additional sources of variation in the sedimentary pil

Tectonic Regime as a Control Factor for Crustal Fault Zone (CFZ) Geothermal Reservoir in an Amagmatic System: A 3D Dynamic Numerical Modeling Approach

Crustal fault zones provide interesting geological targets for high-temperature geothermal energy source in naturally deep-fractured basement areas. Field and laboratory studies have shown the ability of these systems to let fluid flow down to the brittle–ductile transition. However, several key questions about exploration still exist, in particular the fundamental effect of tectonic regimes on fluid flow in fractured basement domains. Based on poro-elasticity assumption, we considered an idealized 3D geometry and realistic physical properties. We examined a model with no tectonic regime (benchmark experiment) and a model with different tectonic regimes, namely a compressional, an extensional and a strike-slip tectonic regime. Compared to the benchmark experiment, the results demonstrate that different tectonic regimes cause pressure changes in the fault/basement system. The tectonic-induced pressure changes affect convective patterns, onset of convection as well as the spatial extent of thermal plumes and the intensity of temperature anomalies. Driven by poro-elastic forces, temperature anomalies around vertical faults in a strike-slip tectonic regime have a spatial extent that should be considered in preliminary exploratory phases

Spatial and temporal distribution of Cu-Au-Mo ore deposits along the western Tethyan convergent margin: a link with the 3D subduction dynamics

International audienceAlong the western Tethyan convergent margin, where Tertiary subduction history is well constrained, porphyry, epithermal and skarn ore deposits show a variable evolution of their spatial distribution. Using different and complementary database on European and Middle East ore deposits, three metallogenic episodes have been highlighted: (1) a late Cretaceous - Paleocene phase characterized by a copper mineralization within the Balkan chain and in the Kaçkar mountains (eastern Turkey), (2) an Eocene phase with a few copper ore deposits in eastern Turkey and small Caucasia and (3) an Oligocene - Neogene phase with a more southern distribution along the margin and mainly constituted by epithermal Au systems in the west (Carpathians, Rhodope, Aegean and western Turkey) and by porphyry copper deposits in the east (Zagros). Using paleogeographic tools, it turned out that, in the eastern Mediterranean area, the late Cretaceous - Paleocene and Oligocene - Neogenemetallogenic episodes arecoeval with a significant decrease of the Africa - Eurasia convergence rate, from about 1.5 to 0.4 cm/yr. Indeed, compressional tectonics promote the storage of large volumes of metal-rich magma and the development of an extensive MASH (melting, assimilation, storage and homogenization) zone. When this convergence rate decreases, a stress relaxation occurs in the overriding crust, inducing the ascent of a sufficient flux of this fertile magma and allowing the formation of numerous mineralized systems within the upper crust. The Au-rich Oligocene - Neogenemetallogenic episode in the eastern Mediterranean region is also correlated with an increase of mantle-derived and/or subduction-modified lithospheric mantle components in magmas. This feature may be a consequence of the emplacement of hot asthenosphere at shallow depth related to (1) the development of a wide back-arc region due to slab retreat such as in the Aegean domain and (2) a slab tear and/or a lithospheric delamination, suspected notably in the Carpathians and western Turkey where alkaline to shoshoniticvolcanism occurs. As the behavior of the slab and asthenosphere below the upper plate seems to play a key-role in controlling the distribution of ore deposits, it is worth studying the dynamics of the 3D mantle flow related toslab retreat. Thus, 3D numerical models of subduction dynamics with realistic rheologies have been developed. Around the slab edges, the poloidal (i.e. in a vertical plane) and toroidal (i.e. in a horizontal plane) components of the mantle flow in subduction zone appear to depend on the slab rollback to plate velocity ratio

Formation de corrugations hydrothermales lors de l'altération des roches ultramafiques

International audienceDuring shallow subsurface (< 200 m depth) weathering processes, temperatures may reach several tens of °C as a result of exothermic chemical reactions, such as hydration of olivine in ultramafic rocks or chloritization of biotite in granitic rocks. These mineralogical transformations enhance mineral fracturing, and the growth of fracture networks leads to further reactions and increases the permeability. The subsequent deepening of the weathering front creates new reactions, thus self-maintaining the weathering process over several million years (Myr). For more than 20 Myr, the peridotite massifs of New Caledonia have undergone intense weathering that has produced thick lateritic weathering mantles. The observable undulations of the weathering front and the protrusions of unweathered peridotite, from several meters to several tens of meters high, attest to a corrugated bedrock topography, which may result from inhomogeneous fluid circulation patterns within the coarse, permeable and porous (30-50%) saprolite layer. Combined together, the excess heat (up to ≈ 100°C) and high permeability (10-14 to 2 10-13 m²) within lateritic weathering mantles could potentially trigger hydrothermal convection (buoyancy-driven flow). This was numerically modeled by accounting for temperature-dependent fluid density and viscosity, and for time-dependent and spatially varying parameters simulating the deepening of the weathering front. Modeling the transient evolution of the thermal and flow velocity fields over 10 Myr reveals that hydrothermal convection can be triggered in the weathering lateritic mantles of New Caledonia, even on sloped surfaces where topography-driven flow prevails. Convective cells develop above the weathering front, and the amplitudes of thermal undulations are enhanced when feedback mechanisms between permeability and temperature are accounted for. The models also allow definition of the most probable zones of mineralization and reveal two-dimensional corrugations below which weathering is no longer efficient