Geochronology of unconformity-related uranium deposits in the Athabasca Basin, Saskatchewan, Canada and their integration in the evolution of the basin (original) (raw)
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Mineralium Deposita, 2009
The importance of geochronology in the study of mineral deposits in general, and of unconformity-type uranium deposits in particular, resides in the possibility to situate the critical ore-related processes in the context of the evolution of the physical and chemical conditions in the studied area. The present paper gives the results of laser step heating 40Ar/39Ar dating of metamorphic host-rock minerals, pre-ore and syn-ore alteration clay minerals, and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) U/Pb dating of uraninite from a number of basement- and sediment-hosted unconformity-related deposits in the Athabasca Basin, Canada. Post-peak metamorphic cooling during the Trans-Hudson Orogen of rocks from the basement occurred at ca 1,750 Ma and gives a maximum age for the formation of the overlying Athabasca Basin. Pre-ore alteration occurred simultaneously in both basement- and sandstone-hosted mineralizations at ca 1,675 Ma, as indicated by the 40Ar/39Ar dating of preore alteration illite and chlorite. The uranium mineralization age is ca 1,590 Ma, given by LA-ICP-MS U/Pb dating of uraninite and 40Ar/39Ar dating of syn-ore illite, and is the same throughout the basin and in both basement- and sandstone-hosted deposits. The mineralization event, older than previously proposed, as well as several fluid circulation events that subsequently affected all minerals studied probably correspond to far-field, continent-wide tectonic events such as the metamorphic events in Wyoming and the Mazatzal Orogeny (ca 1.6 to 1.5 Ga), the Berthoud Orogeny (ca 1.4 Ga), the emplacement of the McKenzie mafic dyke swarms (ca 1.27 Ga), the Grenville Orogeny (ca 1.15 to 1 Ga), and the assemblage and break-up of Rodinia (ca 1 to 0.85 Ga). The results of the present work underline the importance of basin evolution between ca 1.75 Ga (basin formation) and ca 1.59 Ga (ore deposition) for understanding the conditions necessary for the formation of unconformity-type uranium deposits.
Economic Geology, 2013
The McArthur River unconformity-related uranium deposit, located near the southeastern margin of the Proterozoic Athabasca basin in northern Saskatchewan, Canada, contains both basement-hosted and sandstonehosted orebodies, one of which is the sandstone-hosted Zone 4 orebody and its alteration system. Early pre-ore quartz cementation forming immediately above Zone 4 produced a 200-m-thick silicified zone within the lower Manitou Falls Formation. Late pre-ore oxidizing basinal fluids (δ 18 O = 1.1-8.9‰, δ 2 H = -70 to -34‰) formed zones of dickite, magnesiofoitite, and sudoite-rich alteration in the Manitou Falls Formation above the deposit. These zones were overprinted by an outer halo of illite and illite-chlorite mixed-layer clay that coincided with illite and clinochlore alteration of the Wollaston Group basement rocks. Circulation of basinal fluids into the fault zone, promoted by fault reactivation, created a reducing fluid (δ 18 O = 3.9-7.8‰, δ 2 H= -62 to -25‰) through basement interaction, which formed a narrow sudoite alteration halo near the faulted unconformity. Mixing of these two contrasting fluids initiated uranium mineralization at ca. 1600 Ma. Post-ore alteration caused isotopic resetting of primary uraninite, broadly coincident with the intrusion of the Mackenzie dikes into the basin, followed by alteration of uraninite to coffinite, poorly crystalline uraninite, and gummite during the Neoproterozoic. Cool (ca. 10°C), oxidizing, meteoric waters (δ 18 O = -15.7 to -13.2‰, δ 2 H = -120 to -93‰) later infiltrated fractures, produced post-ore kaolinite, and caused partial oxidation of uraninite to uranophane.
The Paleoproterozoic (Statherian) Thelon Basin is located in the Churchill Province of the Canadian Shield, formed following theTrans-Hudson Orogeny. Basin formation followed an interval of felsic volcanism and weathering of underlying bedrock.The diagenetic evolution of theThelon lasted about one billion years and was punctuated by £uid movement in£uenced by tectonic events. Early quartz cements formed in well-sorted, quartz-rich facies during diagenetic stage 1; £uids in which these overgrowths formed had d 18 O values near 0% (Vienna Standard Mean Ocean Water). Uranium-rich apatite cement (P1) also formed during diagenetic stage 1 indicating that oxygenated, uraniumbearing pore water was present in the basin early in its diagenetic history. Syntaxial quartz cement (Q1) formed in water with d 18 O from À 4 to À 0.8% in diagenetic stage 2. Diagenetic stage 3 occurred when theThelon Formation was at ca. 5 km depth, and was marked by extensive illitization, alteration of detrital grains, and uranium mineralization. Basin-wide, illite crystallized at $200 1C by £uids with d 18 O values of 5^9% and dD values of À 60 to À 31%, consistent with evolved basinal brines. Tectonism caused by the accretion of Nena at ca.1600 Ma may have provided the mechanism for brine movement during deep burial. Diagenetic stage 4 is associated with fracturing and emplacement of ma¢c dikes at ca. 1300 Ma, quartz cement (Q3) in fractures and vugs, further illitization, and recrystallization of uraninite (U2). Q3 cements have £uid inclusions that suggest variable salinities, d 18 O values of1.5^9%, and dD values of À 97 to À 83% for stage 4 brines. K-feldspar and Mg-chlorite formed during diagenetic stage 5 at ca. 1000 Ma in upper stratigraphic sequences, and in the west. These phases precipitated from low-temperature, isotopically distinct £uids.Their distribution indicates that the basin hydrostratigraphy remained partitioned for 4600 Ma.
Economic Geology, 2010
Previous studies on Athabasca basin unconformity-related uranium deposits have focused on major deposits and have not investigated sites with barren alteration systems that could clarify some of the critical factors controlling mineralization processes. A paragenetic study of the Wheeler River area reveals the presence of minerals that formed during the diagenetic, the main hydrothermal, which is subdivided into early, mid-, and late hydrothermal substages, and the late alteration stages. The diagenetic stage consists of early quartz overgrowths, siderite, rutile, hematite, and abundant dickite in the pore spaces of the Manitou Falls Formation. The early hydrothermal alteration substage is characterized by pervasive 1Mc muscovite alteration and minor goyazite clusters, which formed from oxidizing basinal fluids at temperatures around 240°C prior to 1550 Ma, based on Ar-Ar dates. The mid-hydrothermal alteration substage comprises dravite and sudoite in the basal 200 m of the Manitou Falls Formation, which are interpreted to have formed at temperatures around 175°C from fluids chemically distinct but isotopically similar to the basinal fluids involved during the early hydrothermal alteration substage. The late hydrothermal substage was observed only at zone K of the Wheeler River area and is characterized by the precipitation of clinochlore, copper sulfides, and florencite from reducing basement fluids emerging into the Manitou Falls Formation at temperatures around 230°C, creating a ~250-m-high bỹ 250-m-wide reducing halo. Oxidized uranium-bearing basinal fluids interacted with the Manitou Falls Formation during the early hydrothermal substage prior to the arrival of the reducing fluids during the mid-and late hydrothermal substages and this precluded uranium precipitation. The post-hydrothermal alteration stage is characterized by formation of kaolinite after late hydrothermal clinochlore near fractures by meteoric waters. A minimal amount of leachable radiogenic Pb, with a Pb-Pb model age of 1907 Ma that is older age than both the Athabasca basin and the main mineralization event of 1590 Ma, was encountered at zone K, indicating low probability of this area to host uranium mineralization. However, areas of possible unconformity-related uranium deposits were identified outside zone K wherein significant amounts leachable radiogenic Pb were observed.
Mineralium Deposita, 2011
The Athabasca Basin hosts many world-class unconformity-related uranium deposits. Recently, uranium reserves for the Eagle Point basement-hosted deposit have increased with the discovery of new mineralized zones within secondary structures. A paragenetic study of Eagle Point reveals the presence of three temporally distinct alteration stages: a pre-Athabasca alteration, a main alteration and mineralization comprised of three substages, and a post-main alteration and mineralization stage that culminated in remobilization of uraninite from primary to secondary structures. The pre-Athabasca alteration stage consists of minor amounts of clinochlore, followed by dolomite and calcite alteration in the hanging wall of major fault zones and kaolinitization of plagioclase and Kfeldspar caused by surface weathering. The main alteration and uranium mineralization stage is related to three temporally distinct substages, all of which were produced by isotopically similar fluids. A major early alteration substage characterized by muscovite alteration and by precipitation Ca-Sr-LREE-rich aluminum phosphatesulfate minerals, both from basinal fluids at temperatures around 240°C prior to 1,600 Ma. The mineralization substage involved uraninite and hematite precipitated in primary structures. The late alteration substage consists of dravite, uranophane-beta veins, calcite veins, and sudoite alteration from Mg-Ca-rich chemically modified basinal fluids with temperatures around 180°C. The post-main alteration and mineralization stage is characterized by remobilization of main stage uraninite from primary to secondary structures at a minimum age of ca. 535 Ma. U-Pb resetting events recorded on primary and remobilized uraninites are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb to a distance of at least 225 m above the mineralized zones.
Contributions to Mineralogy and Petrology, 1994
U-Pb geochronology of igneous zircon froln rhyolitic host rocks to the Archean Kidd Creek, Geco and Winston Lake massive sulfide deposits, in the Superior Province of Ontario, shows that volcanism, which accompanied mineralization, occupied a narrow time span (2717 + 2 Ma, 2720 _+ 2 Ma and 2723 _+ 2 Ma, respectively). Precise ages of hydrothermal monazite, allanite and rutile from alteration zones surrounding the above deposits indicate that these minerals crystallized 40---70 million years after volcanism. Monazite from Kidd Creek mine is 2659 __ 3 Ma old, in agreement with spatially associated 2664 __ 25 Ma old rutile. Monazite from a biotite schist at Geco mine gives a similar age of 2661 _+ ! Ma. However, monazite from a sericite schist, which hosts the ore at Geco mine, is 2675 _+ 2 Ma old. Abraded large monazite grains from three units in the Winston Lake deposit are coeval with biotite crystallization and record an age of 2677 _+ 2 Ma, approximately the same as monazite in the sericite schist at Geco. Data points from allanite fractions from both the Winston Lake and Geco deposits fall on a Pb-Pb isochron that gives an age of 2672 4-5 Ma. Rutile from Winston Lake gives a younger age of 2651 +6/-2 Ma and may date retrograde alteration of biotite to chlorite. The ca. 2676 Ma age of monazite from Winston Lake and in the sericite schist at Geco mine probably dates a regional metamorphic event that affected most of the southern Superior Province. The ca, 2660 Ma old monazite in the biotite schist at Geco mine and in the chlorite-sericite alteration at Kidd Creek may date later K-metasomatism caused by metamorphically derived fluids that were focussed along old fault structures. Such fluids were also responsible for local sulfide remobilization. Monazite and rutile are spatially associated with chlorite and sericire alterations at Kidd Creek. Their young ages indicate that these originally syngenetic mineral assemblages may have been significantly affected by regional metamorphisln. Formation of monazite at all three deposits studied was a result of significant REE remobilization during metamorphism. The discrete character of syn-metamorphic hydrothermal activity in different units of the same deposit, as well as its synchroneity among different, widely separated deposits, requires a mechanism for episodic injection of heat and fluid into the crust on a regional scale. These activities are broadly coeval with, and probably related to, plutonism within adjacent metasedimentary subprovinces and middle to lower crustal metamorphism in the Superior Province.
Atlantic Geology, 2008
The Copper Lake area of mainland Nova Scotia is one of several vein-controlled mineralized (Cu-Au-Co) systems associated with widespread carbonate and iron-oxide alteration proximal to the east-trending Cobequid-Chedabucto Fault System. Although this mineralization has been known for decades, its metallogenic affinity remains poorly defined, and in recent years an IOCG (iron oxide-copper-gold) model has been suggested. In order to determine the age of mineralization and provide an important time constraint for developing a metallogenic model, direct dating of the mineralization and associated alteration was undertaken. At Copper Lake, mineralization occurs in a set of sulphide-carbonate fissure veins hosted by fine-grained metasedimentary rocks of the Middle Devonian Guysborough Group. Dating of the sulphide-alteration (pyrite) and phyllic-alteration (muscovite) stages of the ore system utilized the Re-Os and 40 Ar/ 39 Ar methods, respectively. The two different chronometers yield ages of about 320 Ma and provide an absolute age for the mineralization. As part of this study additional geochronological data were obtained for detrital zircon (U-Pb age of 1634 ± 11.2 Ma) from the host sedimentary rocks, as well as timing of thermal events at ca. 370-380 Ma, 350 Ma and < 300 Ma based on whole rock 40 Ar/ 39 Ar and chemical Th-Pb dating of host rocks and monazite. The Th-Pb dating of monazite indicates that rare-earth element mobility accompanied mineralization. Collectively, the data indicate that the area experienced multiple thermal events, but hydrothermal activity related to mineralization is constrained to about 320 Ma and is tentatively interpreted to relate to structural focusing of fluids that may have been driven by a mid-crustal level mafic heat source. The mineralizing event coincides with regional Alleghanian deformation in this part of the Appalachian orogen and thus reflects larger-scale tectonothermal processes. RÉSUMÉ Le secteur du lac Copper, dans la partie continentale de la Nouvelle-Écosse, constitue l'un de plusieurs systèmes minéralisés (Cu-Au-Co) régis par des filons associés à une altération étendue en oxyde de fer et en carbonates proximale du système de failles orienté vers l'est de Cobequid-Chedabucto. Même si l'on connaît cette minéralisation depuis des décennies, son affinité métallogénique demeure mal définie et des chercheurs ont avancé ces dernières années un modèle OFCO (oxyde de fer-cuivre-or). On a réalisé une datation directe de la minéralisation et de l'altération connexe pour déterminer l'âge de la minéralisation et établir une délimitation chronologique importante pour l'établissement d'un modèle métallogénique. Dans le secteur du lac Copper, la minéralisation est présente dans un ensemble de filons de fissures remplies de sulfures-carbonates à l'intérieur de roches métasédimentaires à grains fins du groupe du Dévonien moyen de Guysborough. Les méthodes Re-Os et 40 Ar/ 39 Ar, respectivement, ont permis la datation des stades de la sulfuration (pyrite) et de l'altération phylliteuse (muscovite). Les deux différentes méthodes chronométriques ont fourni des âges d'environ 320 Ma et confèrent un âge absolu à la minéralisation. On a obtenu, dans le cadre de cette étude, des données géochronologiques supplémentaires de zircon détritique (datation U-Pb de 1634 ± 11,2 Ma) des roches sédimentaires hôtes ainsi que le moment des événements thermiques, situés à environ 370-380 Ma, 350 Ma et < 300 Ma d'après une datation de la roche totale 40 Ar/ 39 Ar et une datation chimique Th-Pb des roches hôtes et de la monazite. La datation Th-Pb de la monazite révèle qu'une mobilité des métaux des terres rares a accompagné la minéralisation. Les données signalent collectivement que le secteur a connu plusieurs événements thermiques, mais que l'activité hydrothermale apparentée à la minéralisation est limitée à environ 320 Ma; on l'interprète provisoirement comme une activité liée à une concentration structurale des fluides qui pourrait avoir été provoquée par une source de chaleur mafique mi-crustale. L'épisode de minéralisation coïncide avec la déformation alléghanienne régionale dans cette partie de l'orogène des Appalaches et correspond en conséquence à des processus tectonothermaux à plus grande échelle.
Economic …, 2005
Unconformity-type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks. Mineralization may extend up to 400 m into the underlying basement rocks. Whereas sandstone-hosted unconformity-type deposits have been well studied, deposits hosted primarily in the basement have not. This study examines the deposits at Rabbit Lake, Dawn Lake, and McArthur River, in the Athabasca basin of Canada, which are hosted by the metamorphic Archean and Early Paleoproterozoic rocks forming the basement to younger Late Paleoproterozoic sandstones. Alteration is similar in the three deposits and is characterized by three distinct paragenetic stages: (1) preore alteration involving illitization of plagioclase and amphibole, followed by chloritization of biotite and illite, which formed at ca. 230°C; (2) ore-stage alteration, characterized by uraninite and coarse-grained illite, which formed at ca. 240°C; (3) postore alteration comprising spherulitic dravite, vein chlorite, quartz, calcite, and Fe, Cu, Co, and Pb sulfides, which formed at ca. 135°C. Fluid circulation associated with emplacement of later Mackenzie dikes initiated partial recrystallization of uraninite. A later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca. 50°C.