Orientative lithogeochemistry of the archean and aphebian basement in the key lake uranium deposit region (Saskatchewan, Canada)

Abstract

More than half of the proven uranium reserves in the Athabasca Basin are contained within the Key Lake U-Ni deposit at the basin's southeastern edge. The area studied comprises several orientatively explored areas within an intrabasin zone approximately 7 km wide, which extends from Blanchard Lake in the west to Wilson Lake in the east (45 km WNW and 10 km NE from the Key Lake deposit, respectively). The uranium and nickel regional background of the Aphebian metasediments covered by the Athabasca Basin platform sediments has been studied on samples taken from vertical diamond drill holes (DDH) crosscutting elongated electro-conductive zones. Three clusters of DDHs, each located within specific lithostructural subunits of the ensialic Hudsonian mobile Cree Lake Zone, provided in total 502 samples of pelitic, mostly carbonaceous, metasediments and some of the commonly associated anatexites. Uranium concentrations follow a “close-to-lognormal’ distribution with a median value of 3.0 ppm; 18% of the samples contain above 10 ppm and only one percent above 100 ppm. Nickel yields again a “close-to-lognormal” distribution with a median value of 33 ppm, and only 6% of samples above 100 ppm. To compare the regional background obtained from the orientatively explored areas with the mineralized environment, the Aphebian metasediments and Hudsonian anatexites hosting the primary halo of the Key Lake deposit have been selected as the local background from the mineralized vicinity. Fifty-seven composite samples, each representing a 10-m vertical section of the 50° dipping peribasal Aphebian sequence, were collected from two deep DDH's drilled 200 m downdip of the Gaertner orebody. The U values within this 220 m thick sequence range from 1 to 10 ppm with a “close-to-lognormal” distribution and median of 3.2 ppm. Nickel yielded a median of 46 ppm, with 7% of samples above 100 ppm. This peri-basal sequence, grossly subdivided into three stratigraphic units displays a definite tendency to concentrate Th, K and U within the hanging wall; Ni, Co, As, Fe, S, Cr, V, Ti and Mg in the main graphite horizon; and Na, Ca and Mn within the footwall series. The graphite horizon of about 40 m thickness contains 2% organic carbon, 0.6% sulphur, a mean Ni value of 106 ppm, but a low average uranium content of 2.5 ppm. To compare local Aphebian background with the potential Archean source, 10 samples from the nearest (to the Key Lake deposit) margin of the Zimmer Lake Archean inlier were assayed. The set is characterized by foliated highly potassic and low Q-granitoids. Uranium ranges from 2.5 to 10 ppm with an average of 6.0 ppm. Ni shows a characteristically low average of 16 ppm. Several tentative conclusions can be drawn from the comparison of three main sets of data and from their internal variations: 1. (1) Archean rocks display a granitoid source whose original composition probably became masked by introduced K during the Hudsonian event. 2. (2) Local Aphebian rocks compared with local Archean indicate a tendency to be more mafic with enhanced concentrations of Mg, Fe, Ca, Ti, P, and S, and by lower Th. Such a bulk Aphebian composition is caused by the contribution of the graphitic lithology to the geochemical background. 3. (3) Most of metallic elements except U (eg. Ni, Co, As, V), which are also concentrated in the Key Lake deposit, reach higher concentrations within this graphitic lithology. There are no Archean rocks in the vicinity which might provide these Aphebian concentrations by simple sedimentation of the Archean detritus. 4. (4) Contrary to the widespread assumption that the unconformity-related type of uranium deposit displays “hot” Archean in the vicinity and/or Aphebian “U-protores”, the observations in the Key Lake area show that the orebodies and their primary haloes are overprinted with a sharp U-gradient over the low-U environment. Uranium and nickel-clarke values are common to both regional and orebody-hosting Aphebian environments of the Key Lake region. 5. (5) The high-grade Key Lake deposit (2.6% U 3O 8), future Western world leading uranium producer, is located within geochemically (U s.s.) non-anomalous lithologies.