The viability of surficial geochemical methods for deep unconformity-related U exploration in the Athabasca Basin, Canada

Abstract

Outer Ring and Halliday Lake deep (~670-800 m) unconformity-related U prospects are located in the eastern portion of the Athabasca Basin. Petrographic, lithogeochemical and short-wavelength-infrared (SWIR) spectroscopic studies were conducted to understand the nature of geochemical dispersion in the basin. Aqua regia partial digestion lithogeochemical results indicate that REEs and HFSEs vary with stratigraphy, are likely hosted in restite phases and thus are not useable pathfinder elements. Aqua regia and weak acid leach (WAL) results indicate that As, Ca, Co, Cr, Cu, Fe, Mn, Mo and Ni are viable pathfinder elements whose enrichments can be traced from the unconformity to the surface. These pathfinders are associated with faulting, fracturing and alteration and appear to be related to graphitic metapelitic and calc-silicate gneisses. Minimal radiogenic Pb is present in sub-economic U mineralization indicating that it has formed recently and that it may have been remobilized from high grade U mineralization. WAL U-Pb isotope results indicate that radiogenic Pb has been dispersed through the entire thickness of the Athabasca Group sediments throughout the basin history. Soil samples from which the <2 µm soil clay fraction was separated and tree cores were collected over the Halliday Lake prospect. X-ray diffraction indicates the clay fractions are host to plagioclase, which is derived beyond the basin edge. Elements such as K, Mg, Co, Cr, Cu, Mn, and Ni appear to be related to this basement component. However, these elements are also enriched in association with anomalous bedrock lithogeochemistry and alteration implying a mixture of the basement and sandstone signatures. Similar elements are enriched in the tree cores in an area associated with thin overburden, anomalous alteration and lithogeochemistry suggesting that the trees roots may be directly tapping the underlying bedrock. Radiogenic 207Pb/206Pb ratios from the clay fraction are spatially associated with drill holes, which intercepted sub-economic U mineralization, while common 207Pb/206Pb ratios are associated with barren drill holes. The tree core 207Pb/206Pb ratios are generally less radiogenic however some are consistent with the clay fraction ratios. Geological, geophysical and surficial geochemical data indicate that the Halliday Lake prospect may be host to unconformity-related U mineralization.