Summary:
The McClean Lake uranium deposits lie near the eastern margin of the Athabasca Basin in the Churchill Structural Province of the Canadian Shield. The bedrock geology of the area consists of Precambrian gneisses unconformably overlain by flat lying, unmetamorphosed sandstones and conglomerates of the Athabasca Group. The Precambrian basement complex is composed of an overlying Aphebian aged supracrustal metasedimentary unit infolded into the older Archean gneisses. The younger Helikian aged, Athabasca sandstone was deposited onto this basement complex. The basement surface is marked by a paleoweathered zone with lateritic characteristics referred to as regolith.
McClean Lake consists of nine known ore deposits, which are unconformity-related deposits of the unconformity-hosted variety: JEB; Sue A, B, C, D and E; and the McClean North and South trends which contain some ten small uranium deposits, or pods, as well as Sue F deposit (formerly referred to as Caribou).
Uranium mineralization is hosted in hematite-altered clay-rich zones containing massive layers of illite. In the McClean North trend, the illite forms a mushroom–shaped envelope tilted to the north. Uranium occurs as fine-grained coffinite veinlets and nodules of pitchblende, and as masses of pitchblende/uraninite.
Within the McClean Lake area, the basement geology under the Athabasca sandstones is characterized by a dome and basin setting in which large Archean granitoid domes alternate with Aphebian metasedimentary rocks. The McClean North and South deposits are situated between two Archean basement domes and are aligned along two trends within a linear belt of graphitic gneisses. These east-northeast trending gneisses may represent a splay off the west extension of the Tent Seal fault that forms the north contact of the Collins Bay dome with Aphebian intermediate to felsic gneiss, calc-silicates, and quartzites. The Sue uranium deposits lie on a north-trending segment of the graphitic gneisses at the west contact with the Collins Bay dome, approximately three kilometres to the east.
The McClean North and South mineralized trends strike N70qE to EW and are approximately 500 m apart. Uranium deposits occur along the trends as 11 elongated pods straddling the Athabasca sandstone- basement contact. The uranium mineralization is hosted in altered sandstone and basement rocks and is surrounded by a clay alteration halo that includes chlorite and hematite. The illite clay alteration extends upwards along fractures in the sandstones for tens of metres where it is capped by silicified sandstones (Kilborn, 1990). In the basement footwall of the mineralization, alteration consists of bleaching, chloritization, argillization, and hematization.
The hanging wall sandstones are typically 150 m to 160 m thick and are covered by 1 m to 10 m of glacial overburden. Beneath the sandstones, the regolith varies from 15 m to 45 m thick, but it is invariably destroyed in the zones of uranium mineralization. Uranium mineralization in the North trend pods occurs over vertical widths of typically 10 m to 20 m. In cross section, the pods are flat, lenticular to oval shaped bodies with thicknesses from 7 m to 15 m. The higher grade portions of the pods undulate from 13 m above to 12 m below the sandstone–basement contact which is, on average, 160 m below the surface at approximately the 275 m elevation.
At the Sue deposits, combinations of normal and reverse faults that parallel the eastdipping foliation in the graphitic gneisses have resulted in basement relief of 10 m to 20 m. Reverse faulting stepped the unconformity down to the west. The Sue A and B deposits occur along the western flank of a basement horst which has eight to ten metres of relief. Northeasterly and northwesterly striking faults offset and modify the major north-south structural controls, creating conditions which limit, or significantly control, the extent of mineralization along the trend.
The Sue trend lies on the west flank of the Collins Bay granitoid dome and is hosted by a north-south segment of a regionally extensive, steeply dipping thin band of graphitic gneiss within the Wollaston Domain. The Sue D deposit lies north of Sue E and south of Sue C/A open pit along the principal Sue trend consisting of a north-south, multiple shear structure and graphite unit. The Sue trend, from Sue E to the north of Sue B, is 2.5 km long. Further north, the favourable graphitic gneisses follow the Collins Bay granite contact and swing west in the “Sue nose area” and thence continue west-southwest, extending approximately two kilometers to the Caribou deposit site that is located approximately two kilometres northwest of Sue D.
The Sue D deposit, as defined by a 0.1% U3O8 grade envelope, is approximately 118 m long by 20 m wide and 10 m to 30 m thick. Uranium mineralization is hosted by faulted/fractured, brecciated and altered graphitic paragneiss below the Athabasca Sandstone-Aphebian basement angular unconformity. The sandstone-basement contact dips west as a result of a series of down-throws along reverse faults paralleling the deposit strike.
The Sue D uranium mineralization dips moderately to steeply east as a series of lenses lying at depths of 85 m to 200 m. In the north, an upper zone of low to moderate grade uranium mineralization is approximately 20 m thick and extends from just above the unconformity to depth and narrows to the south. A deeper or lower, higher grade zone occurs in anastomosing north-trending faults approximately 50 m to 200 m below the unconformity and is 10 m to 30 m thick and up to 16 m wide. Cross cutting structures control mineralization at fault intersections and also displace or thicken the zone by fault repetition. The upper and lower zones merge in the north end of the deposit but diverge to the south.
The uranium mineralizaton is strongly fault-controlled at Sue D. Five fault systems have been described by Wilson et al. (1994) as follows:
1. N10ºE to N15ºE reverse faults, with vertical movement up to 30 m, displacing the sandstone–basement contact and defining the 30 m to 40 m wide Sue D mineralized zone and controlling mineralization. The structures dip 60ºE to 70ºE consistent with the mineralization lenses.
2. N30ºE reverse faults dipping 60ºW to 70ºW. These cut the main structures and divide the zones into upper and lower.
3. N70ºE mineralized subvertical cross faults with dextral offsets. These displace the deposit but also thicken mineralization at the intersections with the northtrending main faults. These faults terminate the deposit to the north and south (Wilson et al., 1994).
4. N125ºE unmineralized, subvertical sinistral cross faults cut the mineralization in the centre of the deposit.
5. North-trending faults that dip 20ºE to 30ºE and cut off the main north-trending mineralized structures at depth in the south portion of the deposit.
RPA independently interpreted structures from the spatial distribution of uranium in drill hole assays. Three cross faults, two with sinistral movement and one with dextral, have clearly displaced uranium mineralization in the deposit laterally and vertically in the order of 5 m to 20 m.