Basement-hosted Deposits Research Articles

Mineralogy and K–Ar geochronology of clay alteration associated with uranium mineralization in the Patterson Lake Corridor, Saskatchewan

The Patterson Lake Corridor (PLC) along the southwestern margin of the Athabasca Basin contains high-grade uranium deposits entirely within crystalline basement rocks. Visible–near infrared–shortwave infrared (VNIR–SWIR) spectroscopy measurements were collected on drill core samples from several locations in the PLC. The Triple R and Arrow deposits exhibit downhole spectral trends related to the crystallinity and thermal maturity of clays (illite and kaolinite) and mineralization. The K–Ar dates of silt-and-clay size fractions (10–6 μ m; 6–2 μ m; 2–0.6 μ m; 0.6–0.2 μ m; <0.2 μ m) from five clay-altered samples decrease with grain size, and span 1608 ± 17 Ma to 1060 ± 14 Ma for the Spitfire discovery ( n = 14) and 1342 ± 17 Ma to 289 ± 4.3 Ma for the Arrow deposit ( n = 4). Alteration assemblages are broadly similar to Athabasca Basin basement-hosted deposits, and K–Ar dates indicate that high-grade uranium mineralization in the PLC reflects remobilization and concentration of primary ores. Integration of geochronology, clay mineralogy and VNIR–SWIR spectral parameters identify fertile fluid conduits when expanded to property or corridor scales, and provide additional evidence that ore grades of the Athabasca Basin deposits reflect several stages of hydrothermal mineralization spanning c. 1000 Ma. Supplementary material: Supporting information document and datasets are available at https://doi.org/10.6084/m9.figshare.c.6033890 Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Three-dimensional gravity modelling applied to the exploration of uranium unconformity-related basement-hosted deposits: the Contact prospect case study, Kiggavik, northeast Thelon region (Nunavut, Canada)

In unconformity-related uranium deposits, mineralization is associated with hydrothermal clay-rich alteration haloes that decrease the density of the host rock. In the Kiggavik uranium project, located in the eastern Thelon Basin, Nunavut (Canada), basement-hosted shallow deposits were discovered by drilling geophysical anomalies in the 1970s. In 2014, gravity data were inverted for the first time using the Geosoft VOXI Earth ModellingTM system to generate three-dimensional (3D) models to assist exploration in the Contact prospect, the most recent discovery at Kiggavik. A 3D unconstrained inversion model was calculated before drilling, and a model constrained by petrophysical data was computed after drilling. The unconstrained inversion provided a first approximation of the geometry and depth of a low-density body and helped to collar the discovery holes of the Contact mineralization. The constrained inversion was computed using density values measured on 315 core samples collected from 21 drill holes completed between 2014 and 2015. The constrained modelling highlights three shallower and smaller low-density bodies that match the geological interpretation and refines the footprint of the gravity anomalies in relation to the current understanding of the deposit. The 3D inversion of gravity data is a valuable tool to guide geologists in exploration of shallow basement-hosted uranium deposits associated with alteration haloes and to assess the deposit gravity geometry.

Multiple Gold Mineralizing Styles in the Northern Pataz District, Peru

Gold production from the northern Pataz district in the eastern Andean Cordillera of Peru has been sourced mainly from mesothermal quartz-carbonate-sulfide veins hosted by the Mississippian Pataz batholith. Gold is also found in basement-hosted veins underlying the batholith, in the Vijus-Santa Filomena area of the district. Both are located within a central horst; similar vein mineralogy and proximal phengitic white mica alteration are common to both. However, comb-textured quartz, the chemical compositions of bulk ore and sulfide minerals, and the presence of barite and siderite veins suggest that the basement-hosted veins formed at a shallower crustal level. Similar expressions of hydrothermal alteration associated with anomalous gold, As, Sb, and Tl are also present in the adjacent Lavasen graben, where alteration is intimately associated with volcanic processes that deposited the Mississippian Lavasen Volcanics. K-Ar and 40 Ar- 39 Ar ages for hydrothermal illite from all three locations range between Mississippian and Late Triassic but are consistent with a single Mississippian hydrothermal event, if the data record a minimum age for original illite formation. The geologic setting, mineralization styles, and chemical data suggest a range of crustal depths, ranging from mesothermal batholith-hosted veins through shallow to intermediate depths for the Vijus-Santa Filomena area to a near-surface epithermal setting for the Misquichilca area. Telescoping of this 10- to 13-km crustal range into a 3-km topographic section of the Andes is attributed to syn- and postmineralization uplift and erosion. Sulfide-rich high-grade ore shoots and moderately saline fluid inclusions in the batholith-hosted veins are inconsistent with the original orogenic gold model and suggest a magmatic source component for the ore fluid, consistent with stable isotope (O, H, C, and S) compositions of quartz, illite, carbonates, and sulfides. The isotopic data suggest a mixed magmatic-meteoric ore fluid in the basement-hosted deposits of the Vijus-Santa Filomena area and the volcanic-hosted Misquichilca area. Both the Pataz batholith and the Esperanza Subvolcanic Complex are of the same Mississippian age as the hydrothermal alteration and mineralization. The Esperanza Subvolcanic Complex, comagmatic with the Lavasen Volcanics, contains cognate mineral clots from which a subjacent magma chamber can be inferred. It exhibits potassic, calc-silicate, and argillic alteration, and evidence for the evolution of an Fe-rich volatile phase. The Lavasen-Esperanza magma suite is ferroan and weakly alkaline, with A-type affinities. These features provide a conceptual genetic link with hydrothermal alteration associated with gold mineralization, including Fe (±As) sulfides, phengitic white mica, celadonite, Fe-rich carbonates, and less common Fe oxides. An oxidized intrusion-related model is proposed for gold and hydrothermal alteration in the northern Pataz district.

Tectonic Deformation and Fluid Flow: Implications for the Formation of Unconformity-Related Uranium Deposits

Numerical experiments were carried out to investigate the fluid flow in typical Proterozoic basins that host unconformity-related uranium deposits. We have constructed a simplified conceptual model by integrating important features shared by the Athabasca, Thelon, and Kombolgie Basins. Based on this conceptual model, various numerical scenarios were designed to explore the interaction between fluid flow, heat transport, topographic relief, and tectonic deformation. Our modeling suggests that thermally driven free convection may develop in the thick sandstone sequence at a geothermal gradient of between 25° and 35°C/km during periods of tectonic quiescence, with a maximum velocity of about 1.0 m/yr. However, reactivation of preexisting basement structures and the generation of new faults suppressed free convection and led to deformation-dominated fluid flow or mixed convection, depending on strain rates. During compressive deformation, reduced brines in the basement may flow out along fractured zones and encounter uranium-bearing fluids in the clastic sequence to form sandstone-hosted deposits. In contrast, basement-hosted deposits are likely to form during extension, when oxidized basinal brines flow into faulted structures to interact with reduced minerals or fluids in the basement. Maximum flow rates are estimated to be up to ~7 and ~2.5 m/yr in the relatively high permeability fracture zone for compressive and extensional deformation, respectively, given a strain rate of 10 −13 s −1 and a geothermal gradient of 30°C/km. In addition, the rate of pressure accumulation and dissipation is different in various geologic units depending on their hydrological properties and strain rates.

Evolution of a Carboniferous carbonate-hosted sphalerite breccia deposit, Isle of Man

A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitised Carboniferous limestones overlying Ordovician–Silurian metasedimentary rocks on the Isle of Man. Although base metal sulphide deposits have been mined historically on the island, they are nearly all quartz vein deposits in the metamorphic basement. This study investigates the origin of the unusual sphalerite breccia and its relationship to basement-hosted deposits, through a combination of petrographic, cathodoluminescence, fluid inclusion, stable isotope and hydrogeologic modelling techniques. Breccia mineralisation comprises four stages, marked by episodes of structural deformation and abrupt changes in fluid temperature and chemistry. In stage I, high-temperature (T h > 300°C), high-salinity (20–45 wt.% equiv. NaCl) fluid of likely basement origin deposited a discontinuous quartz vein. This vein was subsequently dismembered during a major brecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively. Fluid inclusions in these minerals record temperatures of ~105–180°C and salinities of ~15–20 wt.% equiv. NaCl. The δ34S values of sphalerite (6.5–6.9‰ Vienna-Canyon Diablo troilite) are nearly identical to those of ore sulphides from mines in the Lower Palaeozoic metamorphic rocks. The δ18O values for quartz and dolomite indicate two main fluid sources in the breccia’s hydrothermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition in the breccia is compatible with the introduction and cooling of a hot, basement-derived fluid that interacted with local sedimentary brines.

U redox fronts and kaolinisation in basement-hosted unconformity-related U ores of the Athabasca Basin (Canada): late U remobilisation by meteoric fluids

Proterozoic basement-hosted unconformity-related uranium deposits of the Athabasca Basin (Saskatchewan, Canada) were affected by significant uranium redistribution along oxidation–reduction redox fronts related to cold and late meteoric fluid infiltration. These redox fronts exhibit the same mineralogical and geochemical features as the well-studied uranium roll-front deposits in siliclastic rocks. The primary hydrothermal uranium mineralisation (1.6–1.3 Ga) of basement-hosted deposits is strongly reworked to new disseminated ores comprising three distinctly coloured zones: a white-green zone corresponding to the previous clay-rich alteration halo contemporaneous with hydrothermal ores, a uranium front corresponding to the uranium deposition zone of the redox front (brownish zone, rich in goethite) and a hematite-rich red zone marking the front progression. The three zones directly reflect the mineralogical zonation related to uranium oxides (pitchblende), sulphides, iron minerals (hematite and goethite) and alumino-phosphate-sulphate (APS) minerals. The zoning can be explained by processes of dissolution–precipitation along a redox interface and was produced by the infiltration of cold (<50°C) meteoric fluids to the hydrothermally altered areas. U, Fe, Ca, Pb, S, REE, V, Y, W, Mo and Se were the main mobile elements in this process, and their distribution within the three zones was, for most of them, directly dependent on their redox potential. The elements concentrated in the redox fronts were sourced by the alteration of previously crystallised hydrothermal minerals, such as uranium oxides and light rare earth element (LREE)-rich APS. The uranium oxides from the redox front are characterised by LREE-enriched patterns, which differ from those of unconformity-related ores and clearly demonstrate their distinct conditions of formation. Uranium redox front formation is thought to be linked to fluid circulation episodes initiated during the 400–300 Ma period during uplift and erosion of the Athabasca Basin when it was near the Equator and to have been still active during the last million years. A major kaolinisation event was caused by changes in the fluid circulation regime, reworking the primary uranium redox fronts and causing the redistribution of elements originally concentrated in the uranium-enriched meteoric-related redox fronts.

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

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 K-feldspar 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 phosphate-sulfate 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.

The Millennium Uranium Deposit, Athabasca Basin, Saskatchewan, Canada: An Atypical Basement-Hosted Unconformity-Related Uranium Deposit

The Millennium uranium basement-hosted unconformity-related deposit is one of the most recent discoveries of basement-hosted deposits, and may hold as much as 46.8 million pounds (18,000 metric tonnes U) at an average grade of 4.53 percent U3O8. The alteration zone associated with the deposit is atypical, as it consists predominantly of muscovite rather than the chlorite and muscovite commonly found in the other basement-hosted deposits of the Athabasca basin. The preore stage consists of early minor clinochlore alteration at temperatures of about 275°C, interpreted to result from retrograde metamorphism of the basement rocks prior to the deposition of the Athabasca basin. This clinochlore event is followed by an extensive 1Mc muscovite alteration of the basement rocks by Na-K-Fe-rich basinal brines at temperatures of about 250°C, creating up to 25 percent void space in the basement rocks. Subsequently, a weak chamoisite alteration resulted from Fe-Mg-Ca-rich fluids with temperatures of about 300°C that evolved from reactions between the basinal brines and the basement rocks. Ore stage uraninite has an extrapolated chemical age of 1590 Ma and is contemporaneous with synore 1Mc muscovite and iron oxide deposited at temperatures near 250°C from fluids with similar δ 18O but lower δ D values to preore basinal fluids. The low δ D values indicate postdepositional exchange of H isotopes with Cenozoic fluids. Postore alteration consists of dravite and sudoite precipitated from Mg-Ca rich fluids, from the same evolved basinal brines that precipitated preore chamoisite, at temperatures of about 185°C, filling voids created during the preore stage. Our results indicate that the pre- and synore minerals were formed dominantly by basinal fluids, with no contribution from basement fluids, which differ from some other basement-hosted deposits in the Athabasca basin where basement-derived fluid influx was more significant. Several U-Pb and Ar-Ar resetting events recorded on preore muscovite and on uraninite are coincident with fluid flow induced by distal orogenies, remobilizing radiogenic Pb from the deposit. The radiogenic Pb remobilization events were not widespread as the presence of radiogenic Pb is restricted to basement rocks proximal to the deposit and along the unconformity.