Mineralogy and Geochemistry of the Host-Rock Alterations Associated with the Shea Creek Unconformity-Type Uranium Deposits (Athabasca Basin, Saskatchewan, Canada). Part 1. Spatial Variation of Illite Properties

Abstract

Unconformity-related uranium deposits, which represent a significant high-grade uranium resource, are systematically surrounded by a host-rock alteration halo enriched in clay minerals. Illite is often the major clay mineral component of the halo and it displays a variable crystal structure. New data are provided on the crystal structure and the chemistry of illite encountered within and outside of the alteration halo surrounding the Shea Creek deposit. Two illite populations were distinguished using textural and structural criteria: samples rich in the tv-1M polytype display thin (sub-micrometer) and ‘hairy’ shapes, while samples richer in the cv-1M polytype contain illites with rigid lath-like shapes several micrometers wide. In barren ‘regional’ sandstone, the trends with depth of the textural and microstructural properties of illite particles are: (1) an increase of particle size, (2) an evolution to a more isometric form, and (3) a dominance of the cv-1M polytype over the tv-1M polytype. These trends record diagenetic processes under conditions of deep burial and differ from those observed in altered sandstone around the uranium mineralization. The altered sandstone is characterized by enrichment in the tv-1M polytype near the unconformity and/or brittle structural features. This tv-1M illitization took place in response to structurally-controlled infiltration of basement rocks by diagenetic brines which were further recycled after interaction into the overlying basin. Variations of the illite structural and textural properties may result from nucleation/growth kinetics and may be indicative of a change in the flow regime, and/or a change of saturation state of the fluid vs. illite. The tv-1M illite may be favored in environments characterized by a high fluid/rock ratio and a high supersaturation state of the fluids in proximity to mineralization.

Nanoscale relationships between uranium and carbonaceous material in alteration halos around unconformity-related uranium deposits of the Kiggavik camp, Paleoproterozoic Thelon Basin, Nunavut, Canada

“…During the alteration at Kiggavik, quartz was dissolved and all primary metamorphic silicate minerals (feldspars, muscovite, biotite, chlorite) were replaced by mainly illite and sudoite (Al-Mg di-trioctahedral chlorite) in various proportions. This alteration paragenesis is similar to that of many other Proterozoic unconformity-associated U deposits in Canada and Australia (Beaufort et al 2005;Laverret et al 2006;Hoeve and Quirt 1984;Polito et al 2004;Riegler et al 2014). …”

The thermal stability of radiation-induced defects in illite

“…A selection of four pure illite specimens has been made in a larger data set of about 60 samples from illite-and sudoiterich alteration zones developed in the Woodburn Lake Group, an Archean volcano-sedimentary sequence hosting unconformity-related uranium mineralization, in the Kiggavik area, Nunavut, Canada (Riegler 2013;Riegler et al 2014), and in the Athabasca sandstones of the Shea Creek mining prospect, Saskatchewan, Canada (Laverret et al 2006). The age of alteration related to the ore formations is constrained within 1.2-1.4 Ga in both uranium deposits according to illite K/Ar geochronological data (Miller et al 1989;Laverret et al 2010).…”

“…It is formed of a tetrahedral-octahedral-tetrahedral (TOT)-layered structure, with a closed interlayer space occupied by potassium ions and an elevated charge of about 0.9 per half unit cell. In several sedimentary contexts, it can be used as a witness of past alteration conditions that may be also favorable to uranium concentration (Patrier et al 2003;Beaufort et al 2005;Laverret et al 2006). Despite the multiple geochemical and mineralogical investigation tools currently used to study U-deposits (Plant et al 1999), the recognition of past circulations of U-bearing fluids at the origin of ores remains a difficult task.…”

Textural and paleo-fluid flow control on diagenesis in the Paleoproterozoic Franceville Basin, South Eastern, Gabon

“…Illite in the FA sandstone is more abundant in the pore space without any signs or traces of transformation of previously formed clay matrix. Throughout the FA Formation, XRD results of randomly oriented preparations indicate that 1M t is the dominant illite, indicating that the formation of this polytype is kinetically favoured (Jahren and Aagaard, 1989), and also suggest their precipitation in an environment characterized by high fluid-rock ratio (Meunier and Velde, 2004;Laverret et al, 2006). The occurrence of 2M 1 polytypes in both fine and coarse size fractions possibly suggests new generations of illite or could be regarded as a result of higher diagenetic stage and/or contributions from detrital micas which are present throughout the formation.…”