Uranium Exploration in the Athabasca Basin
The Athabasca Basin is an ancient sedimentary basin which hosts the World's most significant uranium mines and produces almost 30% per cent of the current world uranium production of 108 million pounds U3O8. Athabasca uranium deposits also have grades substantially higher than the world average grade of about 0.2% U3O8. The two dozen or so known uranium deposits within the Athabasca Basin have average grades of more than 3.0% U3O8. The two largest deposits, Cigar Lake and McArthur River have average grades of 20% and 24% U3O8 respectively. The near surface uranium mines in the Cluff Lake mine area produced more than 60 million pounds of uranium with gold as a by-product.
The Athabasca Basin is host to unconformity-associated type Uranium deposits. Mineralization occurs at, above or below the unconformity which separates the Proterozoic Athabasca Sandstone Group from the underlying metamorphosed gneissic rock. Uranium mineralization within the Athabasca Basin is primarily hosted by meta-sediments including pelites and calc-silicates and by sandstone of the Athabasca formation. The pelites are commonly graphitic (free carbon) which may have acted as a chemical reductant, fixing uranium from water circulated by large hydrothermal systems. Some Athabasca uranium deposits are associated with faults and these faults may cause displacements of the basal unconformity.
Deposits may range in shape from massive pitchblende cigar-shaped pods to discrete zones of pitchblende veins. Most of the important, known deposits occur within a few tens of metres of the unconformity and within the limits of detection of geophysical tools currently available (generally within 500m of the Athabasca surface).
Airborne surveys and ground geophysical magnetic, electromagnetic, seismic and gravity surveys can be utilized to identify pelitic belts, structures and graphitic conductors, all prospective for uranium deposits. Large plumes of alteration, including characteristic clays, occur above and around these uranium deposits. Geochemical surveying can be utilized to identify areas with such alteration and characteristic trace elements associated with uranium deposits. Since the uranium deposits typically have restricted geometry and are a difficult target to hit with a distant drill hole, it is important to develop a number of indicators in order to assign target priorities.
At present, the drilling of electromagnetic conductors, which originate from graphitic schists exhibiting strong electromagnetic geophysical signatures, is the preferred method of discovering high grade U3O8 at depth. The graphite is generally localized in faults which often extend from the basement up into or through the overlying Athabasca sandstone.
In the eastern portion of the Athabasca Basin (underlain by the basement zone called the Wollaston Domain), there are two types of uranium deposits found - one which has uranium, nickel, cobalt and arsenic (Polymetallic) and a second which is primarily uranium (Monometallic). The Polymetallic deposits have a high grade core which lies just below the unconformity and a lower grade envelope which can extend well up into the Athabasca sediments. The Monometallic deposits lie completely within the basement along faults within graphitic gneiss. The characteristics of the deposition of these deposits provide exploration tools in the search for other deposits - for Polymetallic deposits the presence of the other metals and large alteration zones which can extend to or near the Athabasca surface and for Monometallic deposits the presence of faults and graphite. Typically the alteration increases as the deposit is approached.
The three primary characteristics sought today by explorers in the Athabasca Basin are:
1. The target area of the Athabasca sandstone/basement contact.
2. There should be graphitic rocks.
3. There should be clay alteration in the upper basement or lower Athabasca sediments.
The Mechanism of Uranium Deposition
The moderately-sized Mesoproterozoic Athabasca Basin contains the preserved portion of the unmetamorphosed orthoquartzitic Athabasca Group which overlies a kaolinite-rich regolith on Paleoproterozoic and Archean basement. It is generally undeformed and hosts high grade unconformity-type uranium deposits. The clastic Athabasca Group formations have red-bed characteristics and contain dominant detrital quartz with a clay mineral matrix ± hematite ± goyazite, and minor heavy minerals (zircon, tourmaline, and Fe(-Ti) oxide). The matrix contains generally dominant kaolin (dickite, kaolinite) with lesser illite and occasional sudoite.
Uranium mineralization is related to prograde diagenesis within the Athabasca Basin. The spatial association of mineralization with the sub-Athabasca unconformity is attributed to diagenetic-hydrothermal interaction of basinal brines with relatively reduced basement fluids. Structurally-located sites of egress-type sandstone-basement interaction are characterized by faulting/fracturing and a hydrothermal mineral assemblage containing some or all of illite, Al-Mg sudoite, Mg-Fe/Mg-chlorite, hematite, euhedral quartz, tourmaline (dravite/magnesiofoitite), siderite, and pyrite, and varying quantities of uraninite/pitchblende and accessory Ni-Co-Fe arsenides, sulpharsenides, sulphides. Ore formation was not directly coupled to clay alteration.
Host-rock alteration affects sandstone, basement rocks, and regolith. Typical features include quartz dissolution (loss of coherence; significant volume loss; residual enrichment of clay and resistant heavy minerals; collapse of overlying sandstone), transformation/conversion and neoformation of clay minerals (illitization; chloritization), local tourmalinization and phosphoritization, and iron redox phenomena (hematitization; bleaching: hematite removal; pyrite; siderite).
Basement host-rock alteration also includes destruction of graphite and the formation of solid hydrocarbon 'buttons'. Alteration haloes are plume-shaped above the unconformity, extend for several hundred metres into the sandstone, and display lateral extensions.
Clay alteration is present along the length of the mineralized structure, but the alteration halo reaches its greatest dimensions associated with high-grade ore at the locus of sandstone-basement interaction, forming pipe-like features which rise through the sandstone. Below the unconformity, the alteration envelope is much smaller and funnel-shaped, converging downwards into the associated fault zone. Where the fault zone intersects the unconformity, the kaolinitic layer at the top of the regolith is illitized and chloritized.
The clay mineral alteration assemblage consists of illite, chlorite, and kaolinite in varying proportions, without smectites or mixed-layer clays. The broad alteration haloes grade from background kaolin + illite into illite-dominant, then sudoite-dominant zones, and into ore zone chlorite (± sudoite ± illite). The illite polytypes are 2M1 and 3T with elevated I002/I001 ratios. Sudoite dominates in the altered basement and 3T illite, with sudoite, dominates around ore, while trioctahedral chlorites occur in mineralization.