Graphite-bearing and graphite-depleted basement rocks in the Dufferin Lake Zone, south-central Athabasca Basin, Saskatchewan (original) (raw)
Related papers
The Canadian Mineralogist, 2016
Unconformity-type uranium deposits from the Athabasca Basin area are considered to be the result of mixing between oxidized basinal brines and basement-derived reduced fluids/gases, and/or reduced basement rocks. Graphite and its breakdown products are suggested to be responsible for uranium mineralization by acting as a reductant that could trigger deposition of uranium. In addition, the presence of well-connected graphite and sulfides within graphitic pelitic rocks is considered to be potentially indicative of basement structures, as the minerals are often concentrated along the structures, which hence become electromagnetic (EM) conductors. Thus, exploration for uranium deposits is often focused on the search for EM conductors. Variably graphitic pelitic schists with a steeply dipping fabric underlie the sedimentary rocks of the basin in the Dufferin Lake zone, south-central Athabasca Basin (Saskatchewan, Canada). Up dip, and just below the unconformity with the Athabasca Group, the pelitic schists are replaced by texturally similar rocks within ''graphite-depleted zones''. These zones consist of chlorite and hematite alteration (Red/Green Zone; RGZ) and a bleached zone immediately adjacent to the unconformity. Both zones are characterized by a lower concentration of carbon and sulfur, with the bleached zone showing higher concentrations of uranium and boron, the latter corresponding to a high dravite content. The major elements composition of the graphite-bearing pelitic schists and altered equivalents (RGZ) are similar. Raman analyses indicate that well-ordered carbon species (graphite to semi-graphite) are present in the pelitic schists, with both types more common within shear zones. In contrast, only rare low-ordered carbon species (carbonaceous matter) were detected in the graphite-depleted samples within the RGZ. This variation is interpreted to be the result of graphite consumption by oxidizing fluids migrating downward from the Athabasca Group. This graphite consumption may have resulted in the production of a mobile reductant (gas or fluid), which may have played a subsequent role in the deposition of uranium mineralization.
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, 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.
2015
Geochemical surveys of surficial media (soil, water, and gas) have been conducted to evaluate and prioritize methods of detecting the presence of deeply-buried unconformity-related U deposits. The study selected two sites: the Phoenix and Millennium deposits in the eastern Athabasca Basin, Saskatchewan. The Phoenix deposits lie at a depth of ca. 400 m along the unconformity between Athabasca sandstones and the basement rocks, and the Millennium deposit at a depth of ca. 750 m along a major shear deformation zone in the basement. Humus and B-horizon soil samples show elevated metal contents including U directly above the ore bodies and WS Shear Zone at the Phoenix deposits, and broad areas over shear zones at the Millennium property. The elevated values of metals in the soil samples were reproduced in subsequent years of sampling in both properties. Laboratory leach experiments on humus using a variety of acids indicate that the elevated contents of metals are tightly held in organics, not adsorbed on the surface of clays or organic matter. Examination of sandstone geochemistry over the Phoenix deposits shows a chimney-like distribution of elevated metal contents from the deposits to the upper sandstones. The uppermost sandstones contain elevated metal contents, including U. Principal component analysis reveals high scores of elements associated with U, such as rare earth elements and Pb in the basal Read Formation and the uppermost Dunlop Member of the Manitou Falls Formation. The evidence suggests that metals were dispersed in the sandstones during hydrothermal alteration related to ore-formation but were recently dispersed into the surface media. The proposed interpretation is consistent with low 206 Pb and 207 Pb in humus samples and high contents of 222 Rn in ground waters. With a half-life of 3.8 days, 222 Rn cannot be transported from the deeply-seated ore to the surface in several days, and likely originated from U and/or 226 Ra (direct parent of 222 Rn) present in upper sandstones and soil. The concentrations of He are extremely high in groundwater close to the surface projection of the Millennium ore body and higher at deeper levels. The data appears to suggest upward diffusion of He from the U ore, but the distribution of high He in two study sites suggests its dispersion both vertically and laterally with groundwater flow. In summary, deeply-buried U deposits produce geochemical anomalies in surface media, but the expression of anomalies and media vary at different sites in response to local glacio-fluvial history, soil development and hydrological conditions.
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
The Canadian Mineralogist, 2020
ABSTRACTA large data set comprising near-total digestion analyses of whole rock samples from the Athabasca Basin, Saskatchewan, Canada (based principally on the Geological Survey of Canada open file 7495), containing more than 20,000 analyses, was used to define the average chemical composition of Athabasca Group sandstones and of unconformity-related uranium deposits hosted by the basin.The chemical composition of unaltered and un-mineralized Athabasca Group sandstones is dominated by Al (median Al2O3 of 1.14 wt.%), Fe (median Fe2O3 of 0.24 wt.%), and K (median K2O of 0.11 wt.%; Si was not measured), corresponding mostly to the presence of kaolin, illite, and hematite, in addition to the most-abundant quartz. The median concentration of U in the barren sandstones is 1 ppm, with 5 ppm Th, 3 ppm Pb, and 56 ppm ΣREE. Other trace elements present in significant amounts are Zr (median of 100 ppm), Sr (median of 69 ppm), and B (median of 43 ppm), corresponding to the presence of zircon, ...
Geofluids
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 ...