Unconformity-related Uranium Mineralization Research Articles

Graphite does not function as a direct reducing agent for unconformity-related uranium mineralization – Evidence from the graphitic metapelite-hosted Gryphon uranium deposit, northern Saskatchewan, Canada

It is generally agreed that the unconformity-related uranium (URU) deposits in the Athabasca Basin (Canada) resulted from reactions between basin-derived, oxidizing, U6+-bearing fluids and reducing agents carried by basement-derived fluids or lithologies enriched in reducing elements. Based on the observation that most URU deposits are spatially associated with graphite-rich zones in the basement, it is believed that either the graphite itself or hydrocarbons derived from it acted as major reducing agents for U mineralization. Several previous studies downplayed graphite as a direct reducing agent for U mineralization because few or no direct contacts between uraninite and graphite have been observed. However, it remains skeptical whether or not this is due to insufficient observation, and what happened to the graphite if it was not directly involved in U precipitation. In this study, graphitic metapelite samples were collected from the Gryphon URU deposit for detailed petrographic and Raman spectroscopic study. Two types of graphite were identified, an earlier phase (Gr1) that is parallel and deformed together with the schistosity, and a later phase (Gr2) that is post-deformation, both of which are pre-U mineralization. Thorough microscopic examination confirmed that there is no direct contact between uraninite and graphite in the thin sections. Raman spectroscopic studies of the graphite indicate that the structural order of both Gr1 and Gr2 decreases toward the orebody. These observations indicate that graphite was not a direct reducing agent causing the precipitation of uraninite. Hydrocarbons (mainly CH4) produced from in situ fluid-graphite reaction was not responsible for the uraninite precipitation either, because such a process would also have produced textures showing replacement of graphite by uraninite. Instead, the graphite initially present in the host rocks that are now occupied by the orebodies was likely dissolved by the hydrothermal fluids ahead of the precipitation of uraninite. The actual reducing agents causing the precipitation of the bulk of uraninite in the orebodies are likely CH4 derived from ex situ fluid-graphite reaction below the deposit.

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

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.

The Paleoproterozoic Kombolgie Subgroup (1.8 Ga), McArthur Basin, Australia: Sequence stratigraphy, basin evolution, and unconformity-related uranium deposits following the Great Oxidation Event

ABSTRACTProterozoic continental sedimentary basins contain a unique record of the evolving Earth in their sedimentology and stratigraphy and in the large-scale, redox-sensitive mineral deposits they host. The Paleoproterozoic (Stratherian) Kombolgie Basin, located on the Arnhem Land Plateau, Northern Territory, is an exceptionally well preserved, early part of the larger McArthur Basin in northern Australia. This intracratonic basin is filled with 1 to 2 km-thick, relatively undeformed, nearly flat-lying, siliciclastic rocks of the Kombolgie Subgroup. Numerous drill cores and outcrop exposures from across the basin allow sedimentary fabrics, structures, and stratigraphic relationships to be studied in great detail, providing an extensive stratigraphic framework and record of basin development and evolution.Tectonic events controlled the internal stratigraphic architecture of the basin and led to the formation of three unconformity-bounded sequences that are punctuated by volcanic events. The first sequence records the onset of basin formation and is comprised of coarse-grained sandstone and polymict lithic conglomerate deposited in proximal braided rivers that transported sediment away from basin margins and intra-basin paleohighs associated with major uranium mineralization. Paleo-currents in the upper half of this lower sequence, as well as those of overlying sequences, are directed southward and indicate that the major intra-basin topographic highs no longer existed. The middle sequence has a similar pattern of coarse-grained fluvial facies, followed by distal fluvial facies, and finally interbedded marine and eolian facies. An interval marked by mud-rich, fine-grained sandstones and mud-cracked siltstones representing tidal deposition tops this sequence. The uppermost sequence is dominated by distal fluvial and marine facies that contain halite casts, gypsum nodules, stromatolites, phosphate, and “glauconite” (a blue-green mica group mineral), indicating a marine transgression. The repeating pattern of stratigraphic sequences initiated by regional tectonic events produced well-defined coarse-grained diagenetic aquifers capped by intensely cemented distal fluvial, shoreface, eolian, and even volcanic units, and led to a well-defined heterogenous hydrostratigraphy. Basinal brines migrated within this hydrostratigraphy and, combined with paleotopography, dolerite intrusion, faulting, and intense burial diagenesis, led to the economically important uranium deposits the Kombolgie Basin hosts.Proterozoic sedimentary basins host many of Earth's largest high-grade iron and uranium deposits that formed in response to the initial oxygenation of the hydrosphere and atmosphere following the Great Oxygenation Event. Unconformity-related uranium mineralization like that found in the Kombolgie Basin highlights the interconnected role that oxygenation of the Earth, sedimentology, stratigraphy, and diagenesis played in creating these deposits.

Fluid evolution along the Patterson Lake corridor in the southwestern Athabasca Basin: constraints from fluid inclusions and implications for unconformity-related uranium mineralization

The Patterson Lake corridor (PLC) in the southwestern margin of the Athabasca Basin hosts several high-grade uranium deposits. These deposits are located in the basement up to 900 m below the unconformity surface, raising questions about their affiliation with typical unconformity-related uranium (URU) deposits elsewhere in the basin. Based on cross-cutting relationships four pre- and three syn- to post-mineralization quartz generations were identified. Fluid inclusion analyses indicate that pre-mineralization fluids have salinities ranging from 0.2 to 27.2 wt% NaCl equiv. (avg. 9.0 wt%), whereas syn-mineralization fluids have salinities ranging from 8.8 to 33.8 wt% NaCl + CaCl 2 (avg. 25.4 wt%), with NaCl- and CaCl 2 -rich varieties. The homogenization temperatures ( T h ) of fluid inclusions from pre-mineralization quartz range from 80 to 244°C (avg. 147°C), and from syn-mineralization quartz range from 64 to 248°C (avg. 128°C). Fluid boiling is indicated by the co-development of liquid-dominated and vapour-dominated fluid inclusions within individual fluid inclusion assemblages from the syn-mineralization quartz and is related to episodic fluid pressure drops caused by reactivation of basement faults. Our results indicate that composition and P – T conditions of the ore fluids in the PLC are comparable to those of typical URU deposits in the Athabasca Basin, indicating that the uranium deposits in the PLC formed under similar hydrothermal conditions. Episodic reactivation of basement faults was an important driving force to draw uraniferous fluids from the basin and reducing fluids from the basement to the mineralization sites, forming deep basement-hosted deposits. Supplementary material: Table 1, microthermometric results of type-2 and -5 fluid inclusion assembladges and isolated inclusions from the Patterson Lake corridor, and table 2, microthermometric results of type-6 fluid inclusions from the Patterson Lake corridor are available at https://doi.org/10.6084/m9.figshare.c.5510179 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Structural Controls of Uranium Mineralization in the Basement of the Athabasca Basin, Saskatchewan, Canada

The occurrence of unconformity-related uranium mineralization requires the combination of three components: fluids with the right composition, geochemical traps with the right agents that produce precipitation, and structural traps with the right geometry. In the Athabasca Basin unconformity-related uranium deposits, while basinal brines are commonly accepted as the principal mineralized fluids and graphite and gases (CH4, CO2, and H2S) are well known as the reductants, only few case studies describing structural traps are published. A number of recent works, including numerical modelling, have improved the understanding of the role of inherited shear zones on fluid flow and the development of uranium deposits at a micro- and regional-scale. Nevertheless, there is still a lack of knowledge about the meso- or deposit-scale structural controls that lead to the present (and potentially predictive) localization of uranium deposits along a given shear zone. The present work examines new structural data from drill holes and deals with (i) the identification of mesoscale structural traps that lead to the formation of the Athabasca unconformity-related uranium deposits hosted within the basement and (ii) with the understanding of the role and mode of reactivation of the inherited shear zones. The Sue deposits (McClean Project), the Tri-Island showing (Martin Lake Project) in the Eastern Athabasca, and the Spitfire prospect (Hook Lake Project) in the Western Athabasca have been selected for a detailed analysis of structures and related uranium mineralization. The structural analysis performed brings new insights about the mesoscale structural controls, the role the inherited ductile fabric had on the mode of brittle reactivation and to trap mineralization, and the tectonic regime to which basement-hosted uranium deposits may be associated in the Athabasca Basin.

Uranium-rich diagenetic fluids provide the key to unconformity-related uranium mineralization in the Athabasca Basin

The Proterozoic Athabasca Basin is well known for its unusually large-tonnage and high-grade ‘unconformity-related’ uranium (U) deposits, however, explanations for the basin-wide U endowment have not been clearly identified. Previous studies indicate that U-rich brines with up to ~600 ppm U and variable Na/Ca ratios (from Na-dominated to Ca-dominated) were present at the sites of U mineralization, but it is unknown whether such fluids were developed solely in the vicinity of the U deposits or at a basinal scale. Our microthermometric and LA-ICP-MS analyses of fluid inclusions in quartz overgrowths from the barren part of the basin indicate that U-rich brines (0.6 to 26.8 ppm U), including Na-dominated and Ca-dominated varieties, were widely developed in the basin. These U concentrations, although not as high as the highest found in the U deposits, are more than two orders of magnitude higher than most naturally occurring geologic fluids. The basin-scale development of U-rich diagenetic fluids is interpreted to be related to several geologic factors, including availability of basinal brines and U-rich lithologies, and a hydrogeologic framework that facilitated fluid circulation and U leaching. The combination of these favorable conditions is responsible for the U fertility of the Athabasca Basin.

Fluid P-T-X characteristics and evidence for boiling in the formation of the Phoenix uranium deposit (Athabasca Basin, Canada): Implications for unconformity-related uranium mineralization mechanisms

The Phoenix uranium deposit in the southeastern Athabasca Basin (Canada) is a typical unconformity contact-hosted uranium deposit, characterized by an association with reactivated basement faults, graphite-rich rocks in the underlying basement, and a pervasive clay alteration halo surrounding the mineralized zones. Petrographic results suggest that the mineralizing hydrothermal system was characterized by alternating desilicification and silicification events. Uraninite and clay-size minerals (tourmaline, kaolinite, illite and minor chlorite) mainly precipitated in the desilicification periods whereas hydrothermal quartz (mainly drusy quartz) formed during the silicification periods. Primary fluid inclusions in the hydrothermal quartz are inferred to represent the ore-forming fluids even though quartz did not co-precipitate with uraninite. The coexistence of multiple types of fluid inclusions (liquid-dominated biphase, vapor-dominated biphase, vapor-only and halite-bearing triphase) within individual fluid inclusion assemblages is interpreted to indicate fluid boiling and heterogeneous trapping. Bulk fluid inclusion volatile analysis by mass spectrometry indicates H2O as the dominant species, with less than 1 mol% non-aqueous volatiles that show a compositional trend typical of fluid boiling. Microthermometric and cryogenic Raman spectroscopic analyses indicate that the mineralizing fluids are of H2O-NaCl-CaCl2 ± MgCl2 composition, with salinities ranging mainly from 23.4 to 31.1 wt%. The liquid-dominated biphase inclusions (excluding those interpreted to have resulted from heterogeneous trapping) have homogenization temperatures from 90 to 157 °C. These data are generally consistent with the classical diagenetic-hydrothermal model in which basinal brines that extracted uranium from the basin or the basement were channeled along reactivated basement faults, and precipitated uraninite near the unconformity through reaction with reducing agents. However, the relatively low fluid inclusion homogenization temperatures documented in this paper, together with fluid boiling inferred from fluid inclusion assemblages, suggest that the deposit may have formed in a shallower environment (∼2.5 km) than assumed in the conventional diagenetic-hydrothermal model (>5 km). Abrupt fluid pressure drops during episodic faulting may have resulted in fluid boiling (flash vaporization) changing the fluid pH and promoting uraninite precipitation, with the alternating liquid-vapor conjugates of the boiling system resulting in alternating precipitation of quartz and uraninite respectively.

Thermal profiles inferred from fluid inclusion and illite geothermometry from sandstones of the Athabasca basin: Implications for fluid flow and unconformity-related uranium mineralization

The Proterozoic Athabasca basin and underlying basement host numerous unconformity-related uranium deposits that were formed from extensive fluid circulation near the basement-cover interface. Although it is generally agreed that the mineralizing fluids were basinal brines, it is still unclear what driving forces were responsible for the circulation of the basinal fluids. Because different fluid flow driving forces are associated with different thermal profiles, knowing the basin-scale distribution of paleo-fluid temperatures can help constrain the fluid flow mechanism. This study uses fluid inclusions entrapped in quartz overgrowths and authigenic illite in sandstones from three drill cores (WC-79-1, BL-08-01, and DV10-001) in the central part of the Athabasca basin as thermal indicators of paleo-fluids in the basin. A total of 342 fluid inclusions in quartz overgrowths were studied for microthermometry. The homogenization temperatures (Th) range from 50° to 235°C, recording the minimum temperatures in various diagenetic stages. Temperatures estimated from illite geothermometry (121 points) range from 212° to 298°C, which are systematically higher than (partly overlapping) the Th values, suggesting that illite was precipitated in hotter fluids following the formation of quartz overgrowths. Neither the fluid inclusion Th values nor the illite temperatures show systematic increase with depth in individual drill cores. This, together with the high illite temperatures that cannot be explained by burial at a normal geothermal gradient (35°C/km), is interpreted to indicate that basin-scale fluid convection took place during the diagenetic history of the basin. Prolonged fluid convection is inferred to be responsible for delivering uranium (extracted from the basin or the upper part of the basement) to the unconformity, where uranium mineralization took place due to redox reactions associated with fluid-rock interaction or structurally controlled fluid mixing.

The Role of GIS in Spatial Modeling of Multi-disciplinary Geoscientific Data for Uranium Exploration over the Kunjar-Darjing Basin, Odisha

Abstract: Application of Geographical Information System (GIS) for identifying the spatial locations of uranium exploration target areas using multi-disciplinary geoscientific data is presented in this paper. The data sets used in this study are Airborne Gamma Ray Spectrometric (AGRS), Aeromagnetic (AM), Satellite images, regional ground gravity and geochemical surveys over one of the promising mobile Proterozoic Kunjar-Darjing Basins to the west of Singhbum Uranium Province (SUP), Odisha, India. Analysis of the geochemical data indicated unconformity related uranium mineralization along the unconformity between Kunjar-Darjing sediments and the S-type per-aluminous Tamparkola granite with labile uranium. All the data sets are processed and interpreted independently in terms of geology based on characteristics such as intensity, frequency and texture of the images generated. Various ratio maps generated from AGRS data were used as training points for spatial modeling by building relationships (topology) with the structures and geology interpreted from the magnetic and gravity datasets. Index overlay method is adapted in spatial modeling. The study shows that integrating the geological, geophysical, geochemical and other geodata in a GIS environment provides valuable guidelines for geological mapping as well as identifying target areas for uranium exploration. The GIS study facilitated in identifying potential target areas for uranium exploration along the regional faults D1 and D2 around the villages Kelo, Tamra, northeast of Kunjar, Nuarali and Betajharan.

Numerical modeling of hydrocarbon generation in the Douglas Formation of the Athabasca basin (Canada) and implications for unconformity-related uranium mineralization

The Proterozoic Athabasca Basin in Canada is known for its world-class, high-grade uranium deposits developed near the unconformity between the basin and the basement. The Douglas Formation in the upper part of the preserved strata in the basin contains total organic carbon (TOC) of up to 3.56wt.%. Previous studies of organic matter maturation and hydrocarbon inclusions indicate that oil and gas have been generated from this formation, and that some hydrocarbons found in the unconformity-related uranium deposits were derived from the Douglas Formation. This study aims to evaluate how the oil and gas generation processes may have affected the fluid overpressure development in the basin, and whether or not the hydrocarbons generated in the Douglas Formation could migrate downward to stratigraphically lower intervals and eventually to the sites of mineralization near the unconformities. We carried out a series of numerical experiments to examine fluid overpressures, flow directions, temperatures, and oil and gas generation processes using a two-dimensional conceptual model derived from a geologic cross-section from the basin center to the eastern margin. An additional 5km strata were added to current basin stratigraphy to account for observed paleogeothermal data. Variation studies were undertaken to account for uncertainties in the lithologies of the eroded strata, and the wide ranges of possible permeabilities of different lithologies and kinetic parameters of oil and gas generation. It is found that, if moderate permeabilities are used in the modeling for each lithology (known as the base model), oil and gas generation processes contribute little to the development of fluid overpressure, and fluid pressure in the basin is close to hydrostatic regardless of whether or not hydrocarbon generation in the Douglas Formation is included in the modeling. However, if permeabilities are assigned values one order of magnitude lower than in the base model, significant fluid overpressures are developed in the eroded strata in the upper part of the model. In the base model, oil generated in the Douglas Formation may migrate downward, driven by an overpressure zone situated above the Douglas Formation, but gas migrates upward. In the low-permeability model, however, the overpressures developed above the Douglas Formation are so high that both oil and gas generated in the Douglas Formation migrate downward. The numerical modeling results thus indicate that it is hydrodynamically possible for oil and gas generated in the Douglas Formation to migrate to the base of the basin and reach the sites of the unconformity-related uranium deposits.

A domain-based digital summary of the evolution of the Palaeoproterozoic of North America and Greenland and associated unconformity-related uranium mineralization

Studies of the evolution of the Earth require a better understanding of regional geological geology, particularly for earth scientists who do not have broad, domain-specific knowledge. The Palaeoproterozoic geology of North America and Greenland has been summarized at domain level using the database system developed for the IGCP 509 project “Palaeoproterozoic Supercontinents and Global Evolution”, with additional geochronological information from the DateView database. A series of diagrams illustrate and contrast the geochronology and isotopic evolution of the Trans-Hudson Orogen and its bounding older crustal domains. These are accompanied by several time-space correlation charts and time-slice tectonic reconstructions showing the lithostratigraphy of the region. The origin and setting of uranium mineralization in Canada is discussed in order to illustrate the knowledge potential captured within these databases.

Tectonic history of the North American shield recorded in uranium deposits in the Beaverlodge area, northern Saskatchewan, Canada

Paragenetic and structural relationships and geochronology of minerals in uranium deposits in the Beaverlodge area, Northwestern Saskatchewan, Canada, reveal six periods of uranium mineralization associated with multi-stage deformation during the Proterozoic. The Saint Louis fault and the Main Ore shear zone are northeast striking, southeast-dipping, oblique-normal and dextral fault systems that have mylonite-dominated footwalls. These faults display evidence of exhumation and episodic structural reactivation at progressively shallower crustal levels, accompanied by fluid flow, hydrothermal alteration, and uranium mineralization.New geochronologic data on uranium mineralization and alteration events record over 2.3Gyr of protracted tectonic evolution of the North American shield. 207Pb/206Pb ages of 2293±17Ma and 2289±20Ma date two minor uranium mineralization stages associated with cataclasite rocks and early tensional veins, respectively, coincident with late Arrowsmith orogenic exhumation. Following emplacement of the Gunnar granite at 2321±3Ma, albite metasomatic alteration of the granite is associated with a third moderate uranium mineralizing event. During the late Paleoproterozoic, exhumation along major faults caused the deformation style to change from dominantly brittle–ductile to brittle at shallower structural levels. Massive brecciation of pre-existing rocks is associated with the fourth and most significant uranium-mineralizing event at 1848±5Ma, coincident with fault reactivation during tectonic shortening and regional metamorphism related to the Trans-Hudson Orogen or more likely post-peak compressional uplift during terminal collision of the Taltson-Thelon Orogen. Subsequently, formation of the Martin Lake Basin and intrusion of mafic dikes are related to the fifth minor uranium mineralizing event at 1812±15Ma reflecting rifting related to late stage Trans-Hudson Orogen. Late mineralized veins during the Mesoproterozoic crustal growth of the Nuna supercontinent are associated with the sixth mineralizing event at 1620±4Ma, coincident with the Mazatzal Orogen and the formation of the unconformity-related uranium mineralization in the proximal Athabasca Basin. Post-mineralization alteration and resetting events of uraninite further record various major orogenic events that have affected the North American shield.

Basin Evolution and Unconformity-Related Uranium Mineralization: The Camie River U Prospect, Paleoproterozoic Otish Basin, Quebec

The Paleoproterozoic Otish Basin, Quebec, hosts several uranium prospects that until recently remained underexplored and poorly understood. In this study, the Camie River U prospect, which shows similar characteristics to high-grade unconformity-related U deposits, is the focus of an integrated basin analysis in the western Otish Basin. Conglomerate and sandstone of the Indicator Formation, which were deposited in at least six depositional sequences, were affected by insignificant early diagenetic compaction and cementation. This allowed the formation of regional peak diagenetic aquifers, which became muscovite altered due to interaction with fluids having δ 18O and δ 2H values similar to those of seawater-influenced basinal brines at 250°C. U mineralization at Camie River occurred at 1721 ±20 Ma based on a 207Pb/206Pb date obtained by laser ablation of uraninite, which coincides with a phase of the Otish Gabbro intrusion that has been dated at ca. 1730 Ma. The intrusive event promoted circulation of U-bearing basinal brines, triggering U mineralization at several locations in the western Otish Basin. Interaction of basinal brines with the Otish Gabbro produced coarse-grained chlorite, tourmaline, and phengitic muscovite, which decreased the fluid-conducting capabilities of diagenetic aquifers and resulted in fault zone- and fracture-dominated fluid flow. Subsequent fluid alteration events produced limited U remobilization, sulfides, sudoite, and siderite between ca. 1670 and 1410 Ma based on mineral paragenesis and 40Ar/39Ar dates of muscovite. Metamorphic fluids having high δ 18O values and temperatures around 300°C accompanied 1.2 to 1.0 Ga Grenville orogenesis and subgreenschist-grade metamorphism in the Otish Basin but were present at low water/rock ratios at Camie River and therefore produced little alteration. Post-Grenville uplift of the Otish Basin likely produced late, low-temperature alteration minerals that have been influenced by recent meteoric water, suggesting that the fault zones and fractures the minerals occupy remain as preferential fluid-flow pathways to the present day. Radiogenic Pb and the characteristic trace elements Mo + W + Nb have also preferentially dispersed from the mineralization along fault zones, fractures, and depositional sequence boundaries, and can be used to explore for Camie River-style U mineralization.

Using VLF-EM to delineate a fracture zone in basement granites for uranium exploration

This article describes a survey using low-frequency electromagnetics (VLF-EM) over an unconformity-related uranium mineralization in the northern part of India's Cuddapah Basin. The uranium occurs in granite close to the contact between basement granite and Srisailam sediments. The uranium mineralization is controlled by the basement fractures containing rich concentrations of coffinite and pitchblende. The basement granite is Palaeoproterozoic age.

Hydrogeology, sequence stratigraphy and diagenesis in the Paleoproterozoic western Thelon Basin: Influences on unconformity-related uranium mineralization

The Paleoproterozoic (Statherian) Thelon Formation consists of unmetamorphosed sandstone and conglomerate that was deposited in the Thelon Basin, a continental basin in northern Canada. This study presents the first detailed sedimentological and sequence stratigraphic analysis of the expansive and under-studied western Thelon Basin, and its relation to uranium mineralizing processes in the vicinity of the Boomerang Lake unconformity-related U prospect. Four lithofacies comprise the Thelon Formation in the western Thelon Basin, consisting of breccia, conglomerate, sandstone, and minor mudstone. These are grouped into four facies associations that represent deposition in alluvial fans and braided fluvial streams. Three stratigraphic sequences were identified based on the recognition of widespread surfaces that mark significant periods of subaerial exposure or changes in depositional environment. These unconformity-bounded sequences are composite in nature and can be subdivided into six high-frequency sequences (HFS). Low-accommodation and high-accommodation systems tracts are delineated, instead of the traditional systems tracts that are linked to changes in relative sea level, as no marine deposits are recognized in the Thelon Formation. Two significant strataform paleohydrogeological units were distinguished in the Thelon Formation, based on the relative timing of key diagenetic events such as compaction, grain dissolution, and cementation. Diagenetic aquicludes became isolated from basinal fluid flow due to stylolite-catalyzed early diagenetic quartz cementation, and occlusion of secondary intergranular macroporosity by kaolinite and muscovite, and minor dickite during later peak diagenesis. Diagenetic aquifers conducted U-bearing basinal fluids throughout diagenesis and are marked by an abundance of peak diagenetic dickite. Sequence stratigraphic correlation indicates that diagenetic aquicludes, which were not effective at conducting U-bearing basinal fluids, overlie the unconformity at the Boomerang Lake prospect and extend along the entire strike length of a related exploration trend. In contrast, thick diagenetic aquifers lie approximately 10 km to the north of the Boomerang Lake prospect along a different exploration trend. The western and eastern portions of the Thelon Basin experienced depositional environments and diagenetic conditions distinct from one another, suggesting that they were separate sub-basins. The western Thelon Basin contains fewer of the well-cemented facies that compartmentalized basinal fluid flow than the eastern Thelon Basin, and contains a kaolin-dominated diagenetic mineral assemblage rather than a muscovite-dominated assemblage that is common in the eastern Thelon Basin. In these regards, the western Thelon Basin evolved more similarly to the eastern portion of the Athabasca Basin, where thick, regional, kaolin-rich diagenetic aquifers were fluid conduits for the movement of the U-rich fluids that formed high-grade unconformity-related U deposits.

Migration of brines in the basement rocks of the Athabasca Basin through microfracture networks (P-Patch U deposit, Canada)

The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U deposits in the world, are the result of massive basinal fluid migrations in the basement rocks. This study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths (hundred metres below the unconformity) and their chemical modification (salinity increase) by interaction with basement lithologies. These processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity-type mineralization. Microfracture networks, which opened during the basinal brine stage (ca. 1600–1400 Ma) are interpreted as sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing. The stress field at that stage ( σ 1 = N130–150 °E, subvertical) partly reopened the earlier microcrack networks ( σ 1 = N80–110 °E and N130–150 °E, subvertical) issued from the Trans-Hudson Orogeny late retrograde metamorphic stage (ca. 1795–1720 Ma). The circulation of the two types of fluids (carbonic and brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity-related uranium mineralization in the Athabasca Basin.

Hydrogeochemical exploration for unconformity-related uranium mineralization: example from Palnadu sub-basin, Cuddapah Basin, Andhra Pradesh, India

Hydrogeochemical exploration was undertaken over an area of 3100 km 2 in the northeastern part of Cuddapah Basin to identify target areas for exploring the unconformity-related U-mineralization. Water samples from 626 borewells drilled in the Kurnool Group of the Palnadu sub-basin, Nallamalai Group and basement rocks were collected. For the individual lithological domain, univariate and multivariate statistical techniques were applied to the analytical data on 20 variables, i.e. U, Na, K, Ca, Mg, Cl, SO 4, HCO 3, CO 3, Si, Fe, Cu, Ba, Zn, F, Mn, conductivity, total dissolved salts (TDS), Eh and pH. The R-mode factor analysis establishes geochemical association between U, Na, K, Cl, SO 4, HCO 3, TDS and conductivity. Another important association is between Fe, Mn and Zn. Maps of factor-1 scores (with high loadings on U) and U-contours, clearly delineate the well-established U-mineralization at Koppunuru–Dwarkapuri in the Palnadu sub-basin. Interestingly, the same maps also display another prominent north–south trending anomaly zone extending over a linear stretch of 45 km, commencing in the basement, passing through the Nallamalai Group and continuing into the younger Kurnool Basin in the eastern part. The hydrogeochemical anomaly around Julakallu is significant for targeting unconformity-related U-mineralization as, in this area, the Kurnool Group sediments unconformably overly the most promising Cumbum Formation comprising low-grade metamorphosed carbonaceous pelites.

Polyphase hydrothermal breccias associated with unconformity-related uranium mineralization (Canada): from fractal analysis to structural significance

Sandstone-hosted breccias surrounding unconformity-related uranium ore in the Athabasca Basin, northern Saskatchewan, were mapped and described using drill core. Four main breccia types were distinguished and classified chronologically based on the type of matrix: microcrystalline quartz, sudoite–dravite, Fe–chlorite and polyphase carbonate. Mapping of the breccias has shown that they are controlled by various sets of basement faults, including a major graphite-rich structure. Breccia fragment morphologies were quantified using a fractal dimension analysis technique. The matrix percentage and the shape of the fragment morphometry distribution curve allow determination of the degree of maturity and the nature of fluid–rock interactions for each breccia. Microcrystalline quartz breccias are tectonic and fluid-assisted. Sudoite–dravite, Fe–chlorite and polyphase carbonate breccias were formed by dissolution of quartz. Mature sudoite–dravite and Fe–chlorite breccias have collapse textures formed, respectively, by dissolution opening and fluid-assisted brecciation. Breccias are the expression of basement fault reactivation postdating sandstone deposition (circa 1.7Ga).Each seismic event created structural conduits between variably pressured compartments, inducing fluid circulation from the basement to the sandstone. Fluids flowed more slowly in the strongly fractured sandstones and dissolved quartz to form solution breccias. Interaction of these basement-derived fluids with basinal fluids may have led to unconformity-related uranium mineralization deposition. This may explain the good correlation between breccia bodies and sandstone-hosted uranium ore, especially for the sudoite alteration event.

The age of unconformity-related uranium mineralization in the Athabasca Basin, northern Saskatchewan

Age data are presented for major Athabasca Basin uranium deposits at Cigar Lake, Cluff Lake, Collins Bay, Dawn Lake, Eagle Point, McArthur River, Midwest, and Rabbit Lake, as well as for several minor or undeveloped deposits, including Hughes Lake and Nisto. The best constrained data indicate that almost all the deposits formed in a restricted time interval between about 1330 and 1380 Ma. This range of ages is believed to be real and not the result of uncertainties in the calculation of ages based on discordant data. The one major exception is the recently discovered NiAs-free deposit at McArthur River, for which a well-determined age of 1514 ± 18 Ma (2σ) has been obtained. Even this deposit yields an age in the1330–1380 Ma range for some material. Periods of reworking–redeposition occurred at ~1280, ~1000, ~575, and ~225 Ma. These may be basin-wide, affecting to some degree all the deposits that we have studied. Other times of redeposition are less well determined, but may be present as well. No ages that approach the ~1700 Ma age of the Athabasca Group have been found to date for unconformity-related deposits, and the Athabasca Basin mineralization is unrelated to the ~1750 Ma pitchblende vein deposits in the Beaverlodge Lake area.