Summary:
The Midwest Project includes the Midwest Main uranium deposit and the Midwest A uranium deposit. The uranium mineralization observed at the Midwest Main and Midwest A deposits is considered egress-style unconformity mineralization.
The Midwest property is located near the eastern margin of the Athabasca Basin region in northern Saskatchewan and overlies the Western Churchill Structural Province of the Canadian Shield. The sub-Athabasca bedrock geology of the area consists of Paleoproterozoic Wollaston Group metasediments and Archean orthogneiss, which are all part of the Wollaston-Mudjatik Transition Zone. The north-northeast-trending ductile to brittle structural trend that hosts the Midwest Main and Midwest A uranium deposits follows a steeply-dipping graphitic pelitic gneiss metasedimentary unit that is bounded by granitic gneisses and Hudsonian granite to the northwest and southeast, respectively.
These basement lithologies are unconformably overlain by the flat-lying, unmetamorphosed sandstones and conglomerates of the Athabasca Group. Extensions of basement fault zones, generally extending over 100 metres into the overlying sandstone, act as hosts for uranium mineralization and form the loci of the quartz dissolution and clay alteration zones that resulted in collapse of the property-scale conglomerate marker horizon.
The uranium mineralization encountered in the eastern Athabasca region is of the diagenetic-hydrothermal unconformity type. The location of this mineralization type is around the unconformity between the basal Athabasca Group and the underlying crystalline basement, particularly graphitic pelitic gneiss of the Wollaston Group (Hoeve & Sibbald, 1978); (Hoeve & Quirt, 1984); (Wallis, Saracoglu, Brummer, & Golightly, 1985); (Jefferson & Delaney, 2007); among others).
Sub-Athabasca Crystalline Metamorphic Basement
Major geological features include the contacts between the granitic gneiss/pegmatite units and the rheologically-softer graphitic pelitic gneisses. Brittle-ductile fault reactivation along this NE-trending anastomosing graphitic corridor, combined with several cross-cutting structures, is a key element to uranium precipitation in the Midwest Main area. The strongly folded, steeply-dipping, pelitic gneiss unit is composed of psammopelitic to pelitic gneiss. Porphyroblastic garnets, cordierite, and sulphides, are present in the pelitic gneiss, as well as variable amounts of graphite, often remobilized and sheared with a lustrous sheen. Many quartzo-feldspathic anatectic pegmatites are present. They conformably intrude the metasedimentary gneisses and contain chloritized biotite. Late shearing in the pelitic gneisses and contained breccias has occurred at the contacts with the pegmatites. Fault zones in the basement are often characterized by brecciation and strong hydrothermal alteration with clay mineral development. These fault zones generally extend into the sandstone above.
Athabasca Group Sandstone
The Athabasca Group sandstone, ranging from 180 to 210 metres in thickness in the Midwest property area, is comprised of Manitou Falls Formation sandstones and conglomerates of the MFb (Bird) Member. The upper 100 to 140 metres of sandstone is typically bleached to a buff colored, and is medium- to coarse-grained, quartz-rich, and cemented by quartz overgrowths, clay minerals (kaolin, illite), and/or hematite. Bleaching of the sandstone (removal of diagenetic hematite) is noted along much of the Midwest trend.
The lower portion of the sandstone column is more typically conglomeratic and contains less quartz cement. The conglomeratic beds contain quartz pebbles ranging from one to four centimetres in diameter, locally up to 30 centimetres.
Illitic clay-rich zones are commonly associated with areas of intense hydrothermal alteration and uranium mineralization. These zones are generally present in the basal 20 metres of the sandstone and associated with friable sand and conglomeratic beds.
Basement fault zones generally extend over 100 metres into the overlying sandstone, act as hosts for uranium mineralization, and form the loci of the quartz dissolution and clay alteration zones that resulted in collapse of the property-scale conglomerate marker horizon.
Quaternary Geology
The surficial sediments in the Midwest Project area consist of a thin layer of Quaternary till and glaciofluvial sand and gravel. Low relief drumlins and eskers are the dominant surficial feature in the area. The thickness of this overburden typically ranges from two to four metres in the project area but can be as thick as 15 metres.
Uranium Mineralization
The larger Midwest Main deposit straddles the unconformity; mostly in the sandstone with a lesser amount in the upper basement (Hoeve, 1984); (Hoeve & Quirt, 1984); (Wray, Ayres, & Ibrahim, 1985)). The deposit is lens to cigar–shaped, 600 metres long with pods of higher grade mineralization separated by lower grade mineralization. The width ranges from 10 metres to over 100 metres. The zone thickness ranges from five metres to ten metres. The Midwest Main Unconformity Zone occurs at depths ranging between 170 and 205 metres below surface. Perched mineralization occurs as discrete lenses located above the Unconformity Zone and up to 100 metres above the unconformity. The high-grade core is surrounded by lower-grade, more dispersed, fracture-controlled mineralization in both sandstone and, in minor amounts, in basement rocks. The high-grade mineralization forms a relatively flat-lying lensoid concentration, with a root extending down into the basement along a steeply-dipping fault. The fault is enclosed in an envelope, up to a few metres thick, of host-rock-altered clayey material that lacks diagnostic textures of either basement or sandstone. Host-rock alteration at Midwest Main is dominated by bleaching and quartz dissolution in the sandstone, illitic clay alteration, and development of grey zone chloritic alteration (Quirt D. H., 2012).
The Midwest Main unconformity mineralization is characterized by hydrogeological units based on measurements of mass (weight), density, porosity and permeability. Together, these units of measurements along with other physical properties help separate the geological domains into Hydrogeological Units (HGUs). The different HGUs defined in the Midwest deposit are divided into 3 main categories.
• M1: Non mineralized sandstones or conglomerates.
• M2: Uranium mineralized lithologies, referring to mineralization around the unconformity and perched lenses.
• M3: Altered Basement lithologies of varying degrees of mineralization.
The subcategories ranging from A:E; to classify the heterogenous variation of diagenetic and mineralogical characteristics that directly correlate to hydrogeological properties (effective porosity/permeability) as well as the potential uranium grade.
Midwest A
At Midwest A, mineralization is found directly at the unconformity contact, within conglomerates and coarse sandstones above the unconformity contact, and in minor amounts immediately below the unconformity in basement structures. Lithologies are similar to those present at Midwest Main. The mineralization located at the unconformity locally penetrates into the clay-altered basement units but is mostly in the overlying sandstone. The thicker zones of sandstone mineralization are dominantly in conglomerate units at the base of the Athabasca sandstone. The Midwest A deposit is approximately 450 metres long, 10 to 60 metres wide, and ranges up to 70 metres in thickness. It occurs at depths ranging between 150 and 235 metres below surface. Host-rock alteration at Midwest A is dominated by illitic clay alteration, bleaching and quartz dissolution in the sandstone, and development of grey zone chloritic alteration (Quirt D. H., 2012).