Geochemical, isotopic, and geochronlologic constraints on the formation of the Eagle Point basement-hosted uranium deposit, Athabasca Basin, Saskatchewan, Canada and recent remobilization of primary uraninite in secondary structures (original) (raw)
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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 …, 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.
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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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
Origin of U-mineralizing brines in the Athabasca Basin, Canada
2009
CITATIONS 2 READS 97 7 authors, including: Some of the authors of this publication are also working on these related projects: The role of nanoparticles in geochemistry of gold View project erratum to Onanga Mavotchy et al., CRG, 2016 View project Abstract
Canadian Mineralogist, 2016
The P2 fault, a 13 km-long steeply dipping reverse fault, is the main structural control of the McArthur River uranium deposit in the eastern Athabasca Basin, northern Saskatchewan, Canada. Three types of tourmaline were observed in the metasedimentary basement rocks along the P2 fault: early oxy-schörl [(Na 0.47 A 0.37 Ca 0.16)(Fe 2þ 1.30 Al 0.91 Mg 0.72 Ti 0.07) Al 6 (Si 5.79 Al 0.21 O 18)(BO 3) 3 OH 3 (O 0.63 OH 0.29 F 0.08), where A ¼ vacancy] of metamorphic-anatectic origin, hydrothermal oxydravite [(Na 0.57 Ca 0.23 A 0.18 K 0.02)(Mg 1.93 Fe 2þ 0.62 Al 0.29 Ti 0.15)Al 6 (Si 5.93 Al 0.07 O 18)(BO 3) 3 OH 3 (O 0.57 OH 0.23 F 0.20)], and magnesiofoitite [(A 0.77 Na 0.20 Ca 0.02 K 0.01)(Mg 1.99 Al 0.92 Fe 3þ 0.07)Al 6 (Si 6 O 18)(BO 3) 3 (OH 3)(OH 0.71 O 0.25 F 0.04)]. Oxy-schörl formed in granitic pegmatites, a partial melt product of the metasediments during peak metamorphism. Oxy-dravite formed from hydrothermal fluids after the peak metamorphism but before deposition of the Athabasca sandstones, whereas magnesio-foitite is a product of later, low-temperature hydrothermal activity. Both oxy-schörl and oxy-dravite are coarse-grained (from 500 lm up to 1 cm), whereas magnesio-foitite occurs as radial aggregates of fine, prismatic crystals (,15 lm in width). Magnesiofoitite crystallized together with sudoite, illite, and ''APS'' minerals (alunite supergroup LREE-rich aluminum phosphate-sulfate minerals) along the entire studied length (~7 km) of the P2 fault and is abundant in proximity to the Zone 2 ore body of the McArthur River deposit. In the ore zone, the assemblage occurs with uraninite and is partially overprinted by late, remobilized uraninite and sudoite. Therefore, magnesio-foitite is likely contemporaneous with the main stage of uranium mineralization. It is characterized by a high vacancy at its X-site (0.70-0.85 apfu) and high Al at its Y-site (0.70-1.12 apfu), suggesting that magnesio-foitite likely replaced pre-existing high-Al phases, such as kaolin and sudoite. The occurrence of magnesio-foitite along the entire P2 fault, in both areas of mineralization and apparently barren areas, suggests chemically similar fluids travelled along the entire P2 fault, but only produced ore in localized areas.
Scientific Reports
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. Uranium (U) is a trace element with an average crustal abundance of 1.7 ppm (0.5 ppm for oceanic crust and 2.7 ppm for upper continental crust) 1. The minimum grade for economic exploitation of U is ~0.03 wt.% U 2 (for sandstone-hosted deposits), which is about 170 times the average crustal value. However, most 'unconformity-related' U deposits associated with Proterozoic sedimentary basins have average grades of >0.3 wt.% U, with many >2 wt.% U 2,3. Several giant U deposits in the Athabasca Basin in northern Canada have average grades above 10 wt.% U, including the McArthur River (14.87 wt.% U 3 O 8-345.2 million pounds of U 3 O 8 4), Cigar Lake (17.84 wt.% U 3 O 8-234.9 million pounds of U 3 O 8 4), Arrow high-grade core (18.84 wt.% U 3 O 8-164.9 million pounds of U 3 O 8 5) and Phoenix (19.1 3 wt.% U 3 O 8-71.3 million pounds of U 3 O 8 6) deposits, which are enriched by up to 100,000 times the crustal value. These deposits and many others like them in other Proterozoic basins have been the subject of intense study by large numbers of researchers, and different models have been proposed 7-9 , but it is still not well understood why these basins, particularly the Athabasca Basin, are so richly endowed in U. The ore-forming fluids of the unconformity-related U deposits have been shown to be brines with high concentrations of U (up to 600 ppm U) based on analysis of fluid inclusions from the U deposits in the Athabasca Basin 10-12. Halogen and noble gas geochemistry 13-15 and B isotope signatures in tourmaline associated with the U mineralization 16 suggest that the ore-forming brines are of seawater evaporation origin, although an alternative origin from dissolution of evaporites also has been proposed 17. The source of the U has been controversial, with opinion divided over whether the U was mainly derived from detrital minerals in the basin 8,18-22 , or mainly from the underlying basement rocks 9,10,23-27. The argument for a basin source of U is based mainly on the oxidizing nature of the sediments (as indicated by the development of red beds), which is favorable for U dissolution and
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, 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.
Applied Geochemistry, 1997
Abatract--The Cigar Lake unconformity-type U deposit is one of the largest and highest grade U deposits in the Proterozoic Athabasca Basin, northern Saskatchewan, Canada. Cigar Lake has recently been the focus of an international, 3-a, collaborative program in which this U deposit was studied as a natural analogue for a spent nuclear fuel repository• The deposit is located near the eastern margin of the Athabasca Basin, 430 m below the surface, at the intersection between Hudsonian-age faults and the unconformity between Athabasca group sandstones and Aphebian metasediments. Three stages of U mineralization have been identified based on cross-cutting relationships and textures observed in thin section and back-scattered electron (BSE) images, O isotope data and chemical compositions. All stages of U mineralization have been variably altered to Ca-rich, U-hydrate minerals or uranyl oxide hydrate minerals and coft:inite.