Hydrothermal mineral deposits are characterized by sub-concentric envelopes of alteration minerals surrounding the metallic profitable zone. These alteration halos are often used as exploration pathfinders, and their characterization can also be important for mine planning and geometallurgical purposes. This paper introduces a new approach to jointly model alteration halos and their geological controls. Indeed, the geometry of hydrothermally altered rocks is mainly controlled by geological features such as fault systems or unconformities and smaller-scale objects such as fracture networks. The proposed method defines a multicomponent structural skeleton as a support for modeling the boundaries of these halos. For this, an alteration potential field d(x) computed from this structural skeleton is calculated to extract alteration levels as isovalues. As alteration patterns also depend on the petrophysical properties of the host rock, this alteration potential field can be adapted to account for heterogeneous rock material. This process is integrated into a marked point process framework: the skeleton components are simulated by a Metropolis–Hastings sampler, using a dedicated likelihood term to compute the consistency between the current model parameters and the observations. The application of the proposed methodology on a two-dimensional outcrop in Utah, analogous to mineralized hydrothermal sites, demonstrates the ability of the methodology to jointly infer plausible geometries of alteration halos and of the associated geological structures.

Geochemical footprints of unconformity-related uranium (URU) deposits in the Athabasca Basin (Saskatchewan, Canada) have been extensively studied at individual deposit scales, but no regional comparison across multiple deposits has been undertaken. Using a compilation of legacy exploration geochemical data covering 11 world-class deposits in the Eastern Athabasca Basin (95,739 samples, 4,251 drillholes), this study compares alteration footprints across deposits and evaluates their dependence on local geochemical background. A clear typology emerges between the dickite-dominated Northeast, where illitization produces a characteristic decrease in Al2O3/K2O towards deposits, and the illite-dominated Southeast, where dravitization, chloritization and kaolinization generate distinct signatures (increasing MgO/TiO2, decreasing Al2O3/MgO and Fe2O3/MgO, and increasing Al2O3/K2O). These footprints are only meaningful when defined relative to their local background, not a basin-wide average. Basement-hosted deposits display minimal geochemical variation from background in the overlying sandstone. These findings provide area-specific reference values and guide the selection of optimal geochemical ratios for uranium exploration targeting.

Formation of giant unconformity related uranium deposits as discovered in the Athabasca Basin (Saskatchewan, Canada) resulted from the conjunction of several critical elements and in particular architecture, hydrothermal fluid pathways and transient favorable geodynamics. Identification of favorable conditions for the circulation of fluid flow carrying metals and precipitation is crucial for defining pathfinders to guide exploration of economic deposits. The structural and microstructural analysis of relationships between fracture networks and hydrothermal alteration, carried out on two fault systems controlling the distribution of diffuse anomalies and major uranium concentrations, show the first-order role of the structural organization at the crustal and fault zone scale and the coincidence between the location of the hydrothermal alteration and the distributed or unique nature of the fault cores and damage zones.

Assessing uncertainties is an essential step throughout the mineral resource development cycle, from exploration campaigns to development planning and remediation. These uncertainties reflect the scarcity of subsurface data, which need to be complemented by geological knowledge.This paper aims to assess the impact of conceptual variations on the geometry of uranium deposits. For this purpose, a case study was carried out on the interpretation of drilling data in a 2D section, in the context of an unconformity-related uranium deposit in the Athabasca Basin. Based on a reference section from this area, a blank cross-section with synthetic drillcores was produced and given to 32 interpreters. These 32 interpreters are from various backgrounds and are divided between experts in uranium geology (8) and non-experts (24). A set of fifty mathematical criteria is defined and correspond to five categories: mineralized zones, associated altered zones, structural network, geometric relationship between structures and mineralization, and annotations. Individual and group analyses of the defined criteria are performed.Results show that uranium experts and non-experts are difficult to discriminate by the metrics used in this study, but experts tend to filter out the most speculative conceptual models. A generalized parsimony of interpretations is observed for all survey respondents. What also emerges is that fault network connectivity is key to conceptual model discrimination in hydrothermal deposits.

Several types of mineral deposits typically form near an unconformity between a sedimentary basin and underlying basement rocks, and the majority of them are associated with reactivated basement faults. In this study, numerical modeling of fluid flow was conducted for a series of 2D models to focus on how specific geometrical attributes of faults, such as spacing, number, vertical extent, and fault intersections, influence fluid flow. These experiments were conducted for far-field (basin-wide) conditions under both thermal convection and compressive deformation. The new results showed that fluid flow patterns, particularly ingress flow vs. egress flow, are controlled by a combination of different factors under different driving forces. Under a thermal convection driving force, two new factors were recognized to be critical for fluid flow patterns: the difference between footwall and hanging wall thermal conductivities, and the intersection of faults with different dip angles. Under a driving force of compressional deformation at a given bulk shortening, fluid flow patterns are sensitive to fault geometries and attributes. High dip angles, extension of faults from the basement into a basin, and low degrees of compression generally favor egress flow, whereas low dip angles and higher degrees of compression generally favor ingress flow. However, these general trends may change or fluctuate depending on the number, attitude, and spacing of faults. Caution therefore needs to be exercised in determining fluid flow patterns in real geological situations and each situation should be dealt with on a case-by-case basis.

A mineral system approach was applied for the prospectivity analysis of uranium deposits in Uganda initiated by the Ministry of Energy and Mineral Development and the International Atomic Energy Agency. Critical elements for different ore deposit models have been evaluated from desktop studies and re-interpretation of available information including geology, geophysics, and geochemistry. Reconnaissance exploration and field visits through the Rwenzori terrane confirmed its fertility and the existence of favorable architecture and geodynamics that developed during the successive orogens and extension since the Paleoproterozoic. Several uranium occurrences, locally associated to polymetallic geochemical anomalies, were identified in the vicinity of regional granitic batholith emplaced between 1.99 to 1.85 Ga identified as metaluminous to moderately peraluminous, high-K calc-alkaline granites. These massifs represent favorable source rocks from which uranium could be leached by surface-derived oxidizing fluids and transferred towards structural traps or sedimentary basins to form granite-related or basin-related mineralization, respectively. This process may have been operating during several episodes since the Mesoproterozic, and the potential for exploring unconformity-related and sandstone-type deposits is discussed. For instance, surface radiometry of the Neogene rift basins in Western Uganda highlights current uranium transport by the hydrographic drainage while permeable sandstone reservoirs represent favorable traps for sandstone-type uranium mineralization.

The Athabasca Basin region (Saskatchewan, Canada) hosts the highest grade and among the largest tonnage uranium deposits in the world. The formation of unconformity-related uranium (URU) deposits was essentially triggered by intense fluid circulations at the basin/basement interface that were channelized in a complex fault network. The development and the geometry of the fault systems had a first-order impact on the location and the size of uranium deposits. The intense fluid/rock interaction which affected that environment more than 1 Ga ago, however, strongly limits the characterization and quantification of these critical parameters. To overcome this problem, physical properties of fault systems and natural flow rates from active hydrothermal systems available in open databases have been used to define the possible range of values associated to fault permeability and fluid flow in the forming context (geometry/timing) of URU deposits.

Although 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 Canada.

Understanding the structural framework of metallogenic provinces and identification of transient events during which fluid circulation and trapping of mineralization occur is a major challenge for exploration of mineral systems. Analogies between geothermal systems, orogenic gold and unconformity related deposits reveal the role of permeability enhancement during episodes marked by the coupling of tectonic activity and fluid flow. Thus, in the case of Unconformity-Related Uranium deposits of the Athabasca Basin ( Saskatchewan, Canada), a new model can be proposed considering that the interplay between compressional fault reactivation and thermal convection is the main driver of this fluid flow system. Short- lived, deformation-driven fluid flow are recorded by the structural and mineralogical evolution of the reactivated basement-hosted fault system.

The concept of Mineral Systems (McCuaig and Hronsky 2014) is applied to the formation of unconformity-related uranium deposits in Athabasca Basin (Canada). Critical elements defining such systems are identified in terms of (i) crustal architecture of this part of Laurentia, (ii) transient geodynamics as characterized by the progressive evolution of the structures, their interaction with thermal convection and deformation driven fluid circulation and (iii) fertility of the lithologies. The unconformity at the base of the Athabasca Basin acts as a major interface in which fluid flow has transported and precipitated high-grade uranium deposits, in particular at the intersection with basement hosted graphitic bearing steeply dipping fault systems. The application of the Mineral Systems concept provides a baseline to reassess strategies and to identify/prioritize the most appropriate technologies at different stages and scales of exploration, from the early regional assessment to resource estimates. Changes in paradigms may be necessary to include not only information collected from drill core but also the architecture of the main structures at regional scales and their ability during successive reactivations to enhance permeability of the systems through organized and focused fluid-flux. The role and mode of reactivation of these basement faults in connection with the formation of unconformity-related uranium deposits is emphasized and an analogy with Enhanced Geothermal Systems is proposed. In both cases a pre-existing permeability network is stimulated, whether by forceful injection of fluids or by tectonic processes, enabling the creation and maintaining fluid convection at least at the scale of the upper crust, down to the britlle-ductile transition.

Les minéralisations uranifères localisées à la discordance du Bassin de l’Athabasca (Canada) sur son socle résultent d’intenses circulations de fluides à cette interface. L’histoire géologique du socle antérieure au dépôt du bassin, les caractéristiques de la déformation, les facteurs de contrôle sur les circulations sont exposées par Ledru et al., au service d’une exploration toujours renouvelée.

Deep mineral exploration is increasingly important for finding new mineral resources but there are many uncertainties. Understanding the factors controlling the localization of mineralization at depth can reduce the risk in deep mineral exploration. One of the relatively poorly constrained but important factors is the hydrodynamics of mineralization. This paper reviews the principles of hydrodynamics of mineralization, especially the nature of relationships between mineralization and structures, and their applications to various types of mineralization systems in the context of hydrodynamic linkage between shallow and deep parts of the systems. Three categories of mineralization systems were examined, i.e., magmatic-hydrothermal systems, structurally controlled hydrothermal systems with uncertain fluid sources, and hydrothermal systems associated with sedimentary basins. The implications for deep mineral exploration, including potentials for new mineral resources at depth, favorable locations for mineralization, as well as uncertainties, are discussed.

The 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.

Abstract The structural controls on unconformity-related uranium deposits of the Athabasca basin, Saskatchewan, Canada, are a matter of debate regarding the role of inherited fault systems and their reactivation. This can be related to the lack of outcrops allowing for direct observations, the strong clay alteration halos wrapping deposits that often obliterate structures, and the poor core recovery related to drilling strongly altered and mineralized intervals, which limits observation of structures and reliable oriented measurements. Borehole imaging technology is an invaluable alternative for obtaining oriented data through challenging drilling intervals. The use of borehole Acoustic Televiewer (ATV) has been integrated in recent exploratory campaigns in the Athabasca basin by Orano Canada. Here, we present the inputs and benefits of the use of the ATV in the exploration of unconformity-related uranium deposits and the structural analysis of oriented data from seven inclined diamond drill holes completed in 2016 during the McClean project (Sue deposits). The main objectives were to precisely identify the structural controls of the basement-hosted mineralization, and to test the tool in a well-known site. This work shows the applicability and added value of using televiewer probes to provide reliable oriented data in zones where there is much less information available. The ATV data structural interpretation supports the concept of mineralization of dilational jogs opening during preexisting shear-related foliation under right-lateral reverse fault reactivation. The ATV provides robust oriented data, allowing for a better understanding of the meaning of flat-lying mineralized structures along the Sue trend.

The epigenetic uranium deposits in the eastern part of the Athabasca Basin are classified as unconformity-related ore deposits. Their explicit spatial association to reactivated basement faults is observed within the regional structural NNE trend Wollaston-Mudjatik transition zone, marked by elongated dravite, illite, and chlorite alteration zones. Accordingly, geochemical studies have advocated a circulation and focalization of large amount of one or more fluids to carry and precipitate aqueous chemical materials. At the deposit-scale, the uranium deposits are found mainly at the intersection between two or more fault sets, and described as elongated-like bodies varying in orientation from E-W to NNE direction along the regional transitional zone. Furthermore, some orebodies show a change of orientation and dip of their structures. Thus, what is the hydro-mechanical response of reactivated and inherited fault architecture (e.g., intersection zone) under different stress states (e.g., reverse, strike-slip, and normal faulting regime), and its potential contribution to the shape and orientation of orebodies at deposit scale? Using hydro-mechanical numerical modeling, this project demonstrates the role that fault intersections play in controlling mineralized fluids by examining the various fluid flow patterns observed when reactivated intersected faults are under various stress states. Numerical modeling is performed using 3-Dimensional Distinct Element Code (3DEC). The numerical models are subdivided into two categories: 1) simplified 3-D models of two intersecting faults, 2) 3-D complex models of fault network at different deposits sites (e.g., the Cigar Lake deposit). While the first simple models attempt to evaluate the effects of intersection angle, burial depth, fluid pressure, basin permeability and stress states on the fluid flow patterns; the second models investigate the stress state under which certain orebodies may have formed. Our preliminary results from simplified models show that at defined intersection angles, the fluid flow deviates from the main fault toward the secondary fault at their intersection point. The deviation in fluid flow is referred to the value of intersection angle at which the shear stress varies along the secondary fault, leading to the opening of secondary fault. Additionally, the burial depth does not affect the flow along the basement faults, whereas, the overlying highly permeable basin reduces the horizontal flow along the basement faults toward the intersection zone, and reorients a part of the flow toward the basin. In the complex models (the Cigar Lake model), considering a compressional regime, the E-W fault set is reactivated once the maximum stress is oriented N40W to N65W, which is in agreement with field observations.

Uranium mineralization in the Patterson Lake corridor (southwestern Churchill province, Canada) is hosted in the metamorphosed Paleoproterozoic basement covered to the north by the flat-lying sandstone formations of the Athabasca Basin. The mineralization is exclusively contained within inherited ductile structures that were reactivated under a brittle regime. Petrographic and micro-structural studies of drill core samples from the Spitfire discovery (Hook Lake project) reveal the linkages between structural evolution of the basement, alteration and mineralization. During basement exhumation, localization of non-coaxial deformation led to the formation of a large anastomosing shear zone system made of mylonitic rocks. Strain localization associated with fluid circulation induced strong mineralogical and rheological changes, forming discontinuities in mechanical anisotropy. During and post-deposition of the Athabasca Basin after 1.80 Ga, these zones of anisotropy localized brittle reactivation, expressed by a network of micro-fractures later amplified by dissolution processes which enhanced porosity later filled with phyllosilicates and uranium oxides. Cross-cutting relationships between alteration minerals and structures indicate that fluid circulation was active after the basement exhumation. Uranium-bearing fluids moved through the network of micro-fractures. As shown for the Spitfire prospect, fertile structures in the basement below the Athabasca Basin have a combined poly-phase structural and alteration history during which development of ductile shear zones followed by brittle reactivation and dissolution processes led to the formation of superimposed shear and damaged zones in which uranium orebodies are located. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Orogens are traditionally classified according to their tectonic style. Paleoproterozoic tectonics is referred to as "ancient-style tectonics" while Proterozoic tectonics is referred to as "modern-style tectonics". Ancient-style tectonics is characterised by distributed vertical structures and low topography gradients, often associated with diapirism and partial melting. In contrast, modern-style tectonics involve prominent strain localisation and the formation of thrusts, nappes and high topographic gradients. However, the parameters controlling the transition from ancient to modern-style tectonics are poorly understood. To quantify this transition, a combination of 1D and 2D high resolution lithospheric-scale thermomechanical models was conducted. The parameters controlling the strength of the lithosphere (i.e., Moho temperature, strain rate, crustal rheology, crustal radiogenic heat production and role of shear heating) were investigated in detail. Our results show that tectonic style is controlled by the maximum of crustal strength (shear stress). Modern-style tectonics is observed to occur when the maximum of crustal strength is greater than 300 MPa. At the opposite, a maximum crustal strength lower than 300 MPa leads to ancient style tectonic structures. Therefore, crustal rheology, temperature and background strain rate significantly influence the transition from ancient to modern-style tectonics. Shear heating remains a key factor in promoting strain localisation in modern-style tectonics. Crustal radiogenic heat production has a moderate influence by increasing/decreasing the tendency for faulting within the crust. This crustal strength criterion also provides an excellent fit for a second potential proxy: a localisation criterion of ca. 225 degrees C. These two proposed proxies can be used interchangeably to predict the transition from ancient to modern-style tectonics. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Electromagnetic (EM) methods are important geophysical tools for mineral exploration. Forward and inverse computer modeling are commonly used to interpret EM data. Real-life geology can be complex, and our computer modeling tools need to faithfully represent subsurface features to achieve accurate data interpretation. Traditional rectilinear meshes are less flexible and have difficulty conforming to the complex geometries of realistic geologic models, resulting in large numbers of mesh cells. In contrast, unstructured grids can represent complex geologic structures efficiently and accurately. However, building realistic geologic models and discretizing these models with unstructured grids suitable for EM modeling can be difficult and requires significant effort and specialized computer software tools. Therefore, it is important to develop workflows that can be used to facilitate model building and mesh generation. We have developed a procedure that can be used to build arbitrarily complex geologic models with topography using unstructured grids and a finite-volume time-domain code to calculate EM responses. We present an example of a trial-and-error modeling approach applied to a real data set collected at a uranium exploration project in the Athabasca Basin in Canada. The uranium mineralization is closely related to graphitic fault conductors in the basement. The deep burial depth and small thickness of the graphitic fault conductors demand accurate data interpretation results to guide subsequent drill testing. Our trial-and-error modeling approach builds initial realistic geologic models based on known geology and downhole data and creates initial geoelectrical models based on physical property measurements. Then, the initial model is iteratively refined based on the match between modeled and real data. We show that the modeling method can obtain 3D geoelectrical models that conform to known geology while achieving a good match between modeled and real data. The method can also provide guidance of where future drill holes should be directed.