Origin of U-mineralizing brines in the Athabasca Basin, Canada (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.
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.
Stabilization of early-formed dolomite: a tale of divergence from two Mississippian dolomites
Sedimentary Geology, 2000
Several large hydrocarbon accumulations in Alberta, Canada are hosted in dolomitized successions of stacked, thin sabkha-capped cycles of Visean age. Porosity is micro-intercrystalline and occurs in dolomitized restricted subtidal and intertidal muds that have their fine primary fabric preserved. Two such fields are here considered, that illustrate divergent and contrasting modes of dolomite stabilization despite initial similarities in facies and textures. The dolomite in the upper Debolt Formation of the Dunvegan Field (NW Alberta) forms planar-e or microsucrosic fabrics with crystals in the 1-20 mm range. The dolomite is non-ferroan, Ca-rich (average of 58 mol% CaCO 3 ), and poorly ordered. Its stable isotopic signatures range from Ϫ0.12 to ϩ3.4‰ VPDB for d 18 O (mean ϩ1.3‰) and ϩ0.9 to ϩ4.3‰ VPDB for d 13 C (mean ϩ2.6‰). The average radiogenic 87 Sr/ 86 Sr ratio for this dolomite is 0.7077. Both sets of values are consistent with dolomite precipitation from Mississippian marine or modified marine (evaporated) seawater. These parameters are strongly reminiscent of Holocene protodolomites and hence suggestive of a sabkha dolomitization process (shallow seepage reflux or evaporitive pumping). This dolomite with its high associated porosity (average of 15%, and up to 38%), relatively unaltered mineralogical and chemical signatures, both preserved despite 4 km of burial depth, suggests a very unique set of relatively non-reactive physico-chemical conditions during burial (likely a closed system). In contrast, the dolomite from the Mount Head Formation of the Shell Waterton Field (SW Alberta) has undergone measurable neomorphic alteration in several stages in deeper burial environments (open system). Such alteration has affected its crystal size (range Ͻ10-100 mm), and isotopic chemistry (d 18 O ranges between Ϫ1.5 in the least altered dolomite and Ϫ13.2‰ VPDB; d 13 C ranges from ϩ3.9 to Ϫ1.5‰ VPDB; and 87 Sr/ 86 Sr ranges from 0.7078 to 0.7090). The dolomite has retained a degree of non-stoichiometry with an average Ca content of ϳ55 mol% CaCO 3 . The Mount Head dolomites clearly indicate ongoing reactivity between the rocks and basinal fluids during burial, and are in this respect representative of the norm for dolostones in the Western Canada Sedimentary Basin. ᭧
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
Geochimica Et Cosmochimica Acta, 2011
Analyses of halogen concentration and stable chlorine isotope composition of fluid inclusions from hydrothermal quartz and carbonate veins spatially and temporally associated with giant unconformity-related uranium deposits from the Paleoproterozoic Athabasca Basin (Canada) were performed in order to determine the origin of chloride in the ore-forming brines. Microthermometric analyses show that samples contain variable amounts of a NaCl-rich brine (Cl concentration between 120,000 and 180,000 ppm) and a CaCl 2 -rich brine (Cl concentration between 160,000 and 220,000 ppm). Molar Cl/Br ratios of fluid inclusion leachates range from 100to100 to 100to900, with most values between 150 and 350. Cl/Br ratios below 650 (seawater value) indicate that the high salinities were acquired by evaporation of seawater. Most d 37 Cl values are between À0.6& and 0& (seawater value) which is also compatible with a common evaporated seawater origin for both NaCl-and CaCl 2 -rich brines.