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

Minéralisations et Circulations péri-granitiques :<br />Modélisation numérique couplée 2D/3D, Applications au District minier de Tighza (Maroc-Central).

couplant transfert de chaleur et circulation de fluide. L'objectif principal de ce travail est de tester la nature du lien génétique entre l'intrusion et le processus de minéralisation. La première série de résultats s'appuie sur une comparaison avec des exemples naturels de gisements bien connus : i) L'hydrodynamique et la localisation des zones probables de minéralisation sont fortement dépendantes de la profondeur de mise en place du pluton. Au-dessus de 4.5km de profondeur de mise en place, le seuil de perméabilité de 10-16 m² est atteint et les cellules convectives peuvent créer des zones de décharge additionnelles où des minéralisations peuvent avoir lieu ; ii) Pour toutes les profondeurs d'emplacement, la zone en dessous du pluton n'est pas favorable à la précipitation minérale ; iii) Les apophyses focalisent les fluides convectifs et les zones de minéralisation autour d'elles ; iv) La phase de refroidissement n'est pas la phase majeure de convection. La zone advective principale et celle de haute favorabilité peuvent se produire avant et pendant la phase la plus chaude d'emplacement, avant que le magma ne cristallise complètement; v) Les détachements sont capables de fortement modifier et de re-localiser les flux convectifs déclenchés par une intrusion syn-tectonique; vi) Les conditions physiques favorables à la minéraliser sont produites pendant une durée courte autour de la phase la plus chaude de l'intrusion. Même si les arguments chimiques sont absents, la circulation de fluide (induite par la mise en place de magma) joue un rôle principal dans la genèse des gisements d'or associés aux intrusions. De plus, la formation de ce type de gisement est favorisée par l'occurrence d'une auréole thermique fracturée autour de l'intrusion. La seconde série de résultats concerne l'étude du cas naturel de la minéralisation W-Au de Tighza (Jebel Aouam) au Maroc Central. Une campagne d'acquisition de données gravimétriques, l'inversion données et l'utilisation de logiciel 3D, ont permis d'obtenir la géométrie 3D complexe du pluton de Tighza. Les résultats sont les suivants : i) la zone probable de la minéralisation apparaît au début de la mise en place du magma dans la zone perméable (veine W1) et s'étend pour remplir W1 et couvrir la région autour du pluton pendant la phase la plus chaude de mise en place; ii) lors du refroidissement, la zone probable est réduite et limitée à la zone perméable (W1) pendant 0,6 Ma; iii) L'application de la température de fermeture isotopique de la muscovite et de la biotite avec la distribution du R²AI montre que les âges de refroidissement entre la minéralisation au niveau de la veine W1 et l'intrusion ne sont pas séparés de plus de 0,10 Ma. Ceci est confirmé par la datation absolue de la minéralisation de Tighza et permet de discuter la fiabilité des âges obtenus pour la minéralisation dans la veine W1

Peri-granitic circulations and mineralization : 2D/3D coupled numerical modeling, applications in the mining district of Tighza (Central Morocco)

L’hydrodynamique et la probabilité de minéralisation (R²AI) autour des intrusions magmatiques ont été étudiées par modélisation numérique couplant transfert de chaleur et circulation de fluide. L’objectif principal de ce travail est de tester la nature du lien génétique entre l’intrusion et le processus de minéralisation. La première série de résultats s’appuie sur une comparaison avec des exemples naturels de gisements bien connus : i) L’hydrodynamique et la localisation des zones probables de minéralisation sont fortement dépendantes de la profondeur de mise en place du pluton. Au-dessus de 4.5km de profondeur de mise en place, le seuil de perméabilité de 10-16 m² est atteint et les cellules convectives peuvent créer des zones de décharge additionnelles où des minéralisations peuvent avoir lieu ; ii) Pour toutes les profondeurs d’emplacement, la zone en dessous du pluton n'est pas favorable à la précipitation minérale ; iii) Les apophyses focalisent les fluides convectifs et les zones de minéralisation autour d’elles ; iv) La phase de refroidissement n'est pas la phase majeure de convection. La zone advective principale et celle de haute favorabilité peuvent se produire avant et pendant la phase la plus chaude d’emplacement, avant que le magma ne cristallise complètement; v) Les détachements sont capables de fortement modifier et de re-localiser les flux convectifs déclenchés par une intrusion syn-tectonique; vi) Les conditions physiques favorables à la minéraliser sont produites pendant une durée courte autour de la phase la plus chaude de l'intrusion. Même si les arguments chimiques sont absents, la circulation de fluide (induite par la mise en place de magma) joue un rôle principal dans la genèse des gisements d'or associés aux intrusions. De plus, la formation de ce type de gisement est favorisée par l'occurrence d'une auréole thermique fracturée autour de l'intrusion. La seconde série de résultats concerne l’étude du cas naturel de la minéralisation W-Au de Tighza (Jebel Aouam) au Maroc Central. Une campagne d’acquisition de données gravimétriques, l’inversion données et l’utilisation de logiciel 3D, ont permis d’obtenir la géométrie 3D complexe du pluton de Tighza. Les résultats sont les suivants : i) la zone probable de la minéralisation apparaît au début de la mise en place du magma dans la zone perméable (veine W1) et s'étend pour remplir W1 et couvrir la région autour du pluton pendant la phase la plus chaude de mise en place; ii) lors du refroidissement, la zone probable est réduite et limitée à la zone perméable (W1) pendant 0,6 Ma; iii) L’application de la température de fermeture isotopique de la muscovite et de la biotite avec la distribution du R²AI montre que les âges de refroidissement entre la minéralisation au niveau de la veine W1 et l'intrusion ne sont pas séparés de plus de 0,10 Ma. Ceci est confirmé par la datation absolue de la minéralisation de Tighza et permet de discuter la fiabilité des âges obtenus pour la minéralisation dans la veine W1.Coupled hydro-thermal numerical modeling has been used to simulate the hydrothermal fluid flow regime and the mineralization probability (R²AI) around plutons. The main objective behind this work is to test the nature of the genetic link between mineralization and intrusions. The first series of results comes from comparison with well-constrained mineral deposits: i) Fluid circulation and mineralization patterns are strongly dependent of the emplacement depth of the pluton. Deep seated plutons emplaced below 10 km do not induce an advective heat dissipation. For emplacement depth less than 4.5 km, the permeability threshold of 10-16 m2 is reached and second order convection cells may create additional discharge zones where mineralization are expected; ii) For all emplacement depths, the pluton floor zone is not favorable for mineral deposition; iii) The apexes strongly modified the fluid flow patterns by focusing convective fluids and mineralization zones around them; iv) The cooling phase is not the main phase of convection for large pluton often associated with long-lived magma emplacement. Major advective heat dissipation and mineral deposition zone may also occur sometime before and during the hottest phase of emplacement; v) Extensional detachments faults are able to delocalize and strongly modify classical fluid flow patterns induced by coeval intrusion; vi) Favorable physical conditions for mineral deposition are encountered around middle crust pluton, during a short time span bracketing the hottest phase of intrusion. We conclude that, even if chemical arguments are absent, fluid circulation induced by granite emplacement plays a key role in the genesis of granite-related Au deposits. Moreover, formation of this type of deposit is promoted and controlled by the occurrence of a fractured thermal aureole around the intrusion. The second series of results deals with the W-Au granite related Tighza deposits (Jebel Aouam, Morocco). Based on gravimetric data, inversion, and 3D modeling software, we were able to construct the most probable complex geometry of the Tighza pluton. The 3D geometries of the pluton and major fractures (W1 vein) were injected in the hydro-thermal modeling procedure. The results are: i) the probable zone of mineralization appears at the beginning of magma emplacement within the permeable zone (W1 vein) and extends to fill up W1 and covers the area around pluton at the hottest phase; ii) During the cooling phase, the story was reversed; the probable zone was reduced and restricted in the permeable zone (W1) during 0.6 Myr of cooling; iii) Application of isotopic closure temperature of muscovite and biotite coupled with R²AI distribution shows that the cooling ages between mineralization in W1 veins and the intrusion are not separated by more than 0.10 Myr. This is confirmed by the absolute dating obtained for Tighza Au mineralization and allows discussing the significance of older ages obtained for the mineralization in W1 veins

Minéralisations et circulations péri-granitiques (modélisation numérique couplée 2D/3D, applications au district minier de Tighza (Maroc-Central))

L hydrodynamique et la probabilité de minéralisation (R AI) autour des intrusions magmatiques ont été étudiées par modélisation numérique couplant transfert de chaleur et circulation de fluide. L objectif principal de ce travail est de tester la nature du lien génétique entre l intrusion et le processus de minéralisation. La première série de résultats s appuie sur une comparaison avec des exemples naturels de gisements bien connus : i) L hydrodynamique et la localisation des zones probables de minéralisation sont fortement dépendantes de la profondeur de mise en place du pluton. Au-dessus de 4.5km de profondeur de mise en place, le seuil de perméabilité de 10-16 m est atteint et les cellules convectives peuvent créer des zones de décharge additionnelles où des minéralisations peuvent avoir lieu ; ii) Pour toutes les profondeurs d emplacement, la zone en dessous du pluton n'est pas favorable à la précipitation minérale ; iii) Les apophyses focalisent les fluides convectifs et les zones de minéralisation autour d elles ; iv) La phase de refroidissement n'est pas la phase majeure de convection. La zone advective principale et celle de haute favorabilité peuvent se produire avant et pendant la phase la plus chaude d emplacement, avant que le magma ne cristallise complètement; v) Les détachements sont capables de fortement modifier et de re-localiser les flux convectifs déclenchés par une intrusion syn-tectonique; vi) Les conditions physiques favorables à la minéraliser sont produites pendant une durée courte autour de la phase la plus chaude de l'intrusion. Même si les arguments chimiques sont absents, la circulation de fluide (induite par la mise en place de magma) joue un rôle principal dans la genèse des gisements d'or associés aux intrusions. De plus, la formation de ce type de gisement est favorisée par l'occurrence d'une auréole thermique fracturée autour de l'intrusion. La seconde série de résultats concerne l étude du cas naturel de la minéralisation W-Au de Tighza (Jebel Aouam) au Maroc Central. Une campagne d acquisition de données gravimétriques, l inversion données et l utilisation de logiciel 3D, ont permis d obtenir la géométrie 3D complexe du pluton de Tighza. Les résultats sont les suivants : i) la zone probable de la minéralisation apparaît au début de la mise en place du magma dans la zone perméable (veine W1) et s'étend pour remplir W1 et couvrir la région autour du pluton pendant la phase la plus chaude de mise en place; ii) lors du refroidissement, la zone probable est réduite et limitée à la zone perméable (W1) pendant 0,6 Ma; iii) L application de la température de fermeture isotopique de la muscovite et de la biotite avec la distribution du R AI montre que les âges de refroidissement entre la minéralisation au niveau de la veine W1 et l'intrusion ne sont pas séparés de plus de 0,10 Ma. Ceci est confirmé par la datation absolue de la minéralisation de Tighza et permet de discuter la fiabilité des âges obtenus pour la minéralisation dans la veine W1.Coupled hydro-thermal numerical modeling has been used to simulate the hydrothermal fluid flow regime and the mineralization probability (R AI) around plutons. The main objective behind this work is to test the nature of the genetic link between mineralization and intrusions. The first series of results comes from comparison with well-constrained mineral deposits: i) Fluid circulation and mineralization patterns are strongly dependent of the emplacement depth of the pluton. Deep seated plutons emplaced below 10 km do not induce an advective heat dissipation. For emplacement depth less than 4.5 km, the permeability threshold of 10-16 m2 is reached and second order convection cells may create additional discharge zones where mineralization are expected; ii) For all emplacement depths, the pluton floor zone is not favorable for mineral deposition; iii) The apexes strongly modified the fluid flow patterns by focusing convective fluids and mineralization zones around them; iv) The cooling phase is not the main phase of convection for large pluton often associated with long-lived magma emplacement. Major advective heat dissipation and mineral deposition zone may also occur sometime before and during the hottest phase of emplacement; v) Extensional detachments faults are able to delocalize and strongly modify classical fluid flow patterns induced by coeval intrusion; vi) Favorable physical conditions for mineral deposition are encountered around middle crust pluton, during a short time span bracketing the hottest phase of intrusion. We conclude that, even if chemical arguments are absent, fluid circulation induced by granite emplacement plays a key role in the genesis of granite-related Au deposits. Moreover, formation of this type of deposit is promoted and controlled by the occurrence of a fractured thermal aureole around the intrusion. The second series of results deals with the W-Au granite related Tighza deposits (Jebel Aouam, Morocco). Based on gravimetric data, inversion, and 3D modeling software, we were able to construct the most probable complex geometry of the Tighza pluton. The 3D geometries of the pluton and major fractures (W1 vein) were injected in the hydro-thermal modeling procedure. The results are: i) the probable zone of mineralization appears at the beginning of magma emplacement within the permeable zone (W1 vein) and extends to fill up W1 and covers the area around pluton at the hottest phase; ii) During the cooling phase, the story was reversed; the probable zone was reduced and restricted in the permeable zone (W1) during 0.6 Myr of cooling; iii) Application of isotopic closure temperature of muscovite and biotite coupled with R AI distribution shows that the cooling ages between mineralization in W1 veins and the intrusion are not separated by more than 0.10 Myr. This is confirmed by the absolute dating obtained for Tighza Au mineralization and allows discussing the significance of older ages obtained for the mineralization in W1 veins.ORLEANS-SCD-Bib. electronique (452349901) / SudocSudocFranceF

Intrusion-Related Gold Deposits: new insights from gravity and hydrothermal integrated 3D modeling applied to the Tighza gold mineralization (Central Morocco)

International audienceThe Tighza (or Jebel Aouam) district is one of the most important polymetallic districts in Morocco. It belongs to the Variscan Belt of Central Meseta, and includes W-Au, Pb-Zn-Ag, and Sb-Ba mineralization types that are spatially related to late-Carboniferous granitic stocks. One of the proposed hypotheses suggests that these granitic stocks are connected to a large intrusive body lying beneath them and that W-Au mineralization is directly related to this magmatism during a 287-285 Ma time span. A more recent model argues for a disconnection between the older barren outcropping magmatic stocks and a younger hidden magmatic complex responsible for the W-Au mineralization. Independently of the magmatic scenario, the W-Au mineralization is consensually recognized as of intrusion-related gold deposit (IRGD) type, W-rich. In addition to discrepancies between magmatic sceneries, the IRGD model does not account for published older age corresponding to a high-temperature hydrothermal event at ca. 291 Ma. Our study is based on gravity data inversion and hydro-thermal modeling, and aims to test this model of IRGD and its related magmatic geometries, with respect to subsurface geometries, favorable physical conditions for deposition and time record of hydrothermal processes. Combined inversion of geology and gravity data suggests that an intrusive body is rooted mainly at the Tighza fault in the north and that it spreads horizontally toward the south during a trans-tensional event (D2). Based on the numerical results, two types of mineralization can be distinguished: 1) the “Pre-Main” type appears during the emplacement of the magmatic body, and 2) the “Main” type appears during magma crystallization and the cooling phase. The time-lag between the two mineralization types depends on the cooling rate of magma. Although our numerical model of thermally-driven fluid flow around the Tighza pluton is simplified, as it does not take into account the chemical and deformation contributions, it provides evidence for abandoning the time-lag as an argument for invalidating the existence of a genetic link between older mineralizations and/or hydrothermal events spatially related to younger parent plutons

Role of Hydrothermal Circulation along and above Inherited Basement Structures Relating to Unconformity-Related Uranium Mineralization

International audienceThe reactivation of inherited tectonic structures formed during the Paleoproterozoic Trans-Hudson Orogeny (THO) has played a significant role in generating high-grade unconformity-related uranium deposits in the eastern Athabasca Basin. The role of these tectonic structures is now investigated through a series of two-dimensional hydrothermal numerical models. Two modelling scenarios are considered: (1) models during the THO peak of metamorphism and (2) models with a permeable layer mimicking the presence of the Athabasca Basin, deposited unconformably over the THO basement. In the first scenario, general fluid patterns are strongly affected by the applied permeability configurations. Unidirectional high fluid flow zones (from 10-9 to 10-8 m•s-1) and high thermal gradients (up to 65 °C•km-1) can be observed above and within the deep-seated tectonic structures. In the second scenario, well-established fluid convection cells or unidirectional fluid flow zones are observed within the basin layer, with upflow originating from the core of the deep-seated structures, regardless of the applied fluid pressure regime. These results highlight that these deep-seated structures can efficiently transport fluids and heat towards the upper parts of the crust and the basin. In the second scenario, the loci for preore alteration are then evaluated by computing a rock alteration index based on temperature and fluid velocity constraints. These alteration areas reside along and above the deep-seated structures and are potential regions for structural reactivation during mineralization. These results imply that the analysis of the inherited tectonic structures, combined with the alteration regions, can serve as markers for uranium exploration

Influence of inherited structures as fluid-thermal conduits applied to the formation of uranium mineralisation in the Athabasca Basin, Canada

International audienceOre deposit formation requires the synchronisation of multiple processes with transportation of fluids and heat. Ancient inherited structures found within the basement of the Athabasca Basin were formed as a result from of orogenic processes (i.e nucleation and accretion processes). It is established that their reactivation played a significant role for the formation of the unconformity-related uranium deposits. The time gap between the inherited structures' inception and mineralisation event is sufficient for any traces of hydrothermal systems to reach steady state. The aim of this study is to determine the role for these inherited structures affecting the hydrothermal dynamics, by using a pre-existing tectonic structure generated from a thermo-mechanical study and incorporating fluid-thermal processes until steady-state conditions. The models are then experimented with permeability fields of increasing complexity in three types of tectonic scenarios. Results indicate that deep-seated structures operate as fluid-thermal conduits that bring fluids and heat towards the upper portions of the crust. In addition, thermal convection occurs at favourable permeability conditions. The experimentation of models in different permeability fields provides a general framework for the Athabasca Basin suggesting the transportation of basement-associated fluids and heat to the surface

Improving prospectivity by numerical modeling of hydrothermal processes

International audience— The formation of " hydrothermal resources " , a term including mineral and geothermal resources, is the result of thermal, hydraulic, mechanical and chemical processes. Accounting for all of them (THMC modeling) is not an easy task since a large number of variables are unknown. However, when only hydraulic and thermal processes are selected, numerical tools such as the " Rock Alteration Index " can be used to predict locations of the most probable mineralized zones. As an example, 3D numerical models of the Tighza pluton (Morocco) demonstrate that computed mineralized zones correspond to those found in the field. Using geological, petrophysical data and measured temperatures, numerical simulation of the Soultz-sous-Forêts geothermal system (France) helped to understand how fluid circulation in the shallow crust is controlled. Besides reproducing temperature profiles, the obtained numerical models were also used to predict the depth and the temperature of a previously suspected anomaly (Rittershoffen area). It turned out that this anomaly (160°C at a depth of 2500m) was confirmed at the same time by temperature measurements in a borehole. Numerical modeling of hydrothermal processes should thus be considered as a predictive tool in exploration strategies

3-D hydromechanical simulation of intersecting faults: Influences on fluid circulation and formation of oriented-orebodies

International audienceAlthough intersection zones of two or more fault arrays have been identified as favorable loci of unconformity- related basin-basement mineral deposits in previous studies, their roles in controlling fluid flow patterns related to formation of oriented-, and structurally-controlled mineral deposits remain unclear. In this study, 3-D hydromechanical models were conducted to better understand the response of a natural fault intersected by a hydraulic fault under different stress regimes. A comprehensive parametric study was carried out to assess the roles of fluid pressure, burial depth, basin permeability, intersection angle, and horizontal differential stress on the hydromechanical response of a system containing two intersecting faults. Depending on basin sediment permeability, fault intersection angle, and horizontal differential stress, the fluid flow either stops, crosses, or diverts toward natural fault. Among the tested parameters, the intersection angle, and horizontal differential stress have the most noticeable effect on the fluid flow in the system. The critical intersection angle at which the fluid flow diverts toward the natural fault varies between ≈27° in normal faulting regimes, to ≈55°, in both strike-slip and reverse faulting regimes. Conversely, burial depth and fluid pressure have no significant effect on fluid flow diversion in the intersection zone. The results are in agreement with field observations related to the unconformity-related uranium deposits found in northern Canad