Critical Geochemical and Mineralogical Factors for the Formation of Unconformity-Related Uranium Deposits: Comparison between Barren and Mineralized Systems in the Athabasca Basin, Canada

Abstract

The Spring Point barren alteration system and the Maurice Bay subeconomic uranium deposit, situated on the northwest rim of the Athabasca Basin, northern Saskatchewan, are hosted by the sandstones and conglomerates of the Fair Point Formation of the Athabasca Group. Macroscopic observations indicate two different styles of alteration for the two systems: chlorite-dominated, partly reduced alteration at Spring Point, and illite-dominated, strongly oxidized alteration at Maurice Bay. The paragenetic sequences developed for each area are similar, but the crystal chemistry of the alteration minerals differs markedly, indicating that the alteration fluids at Spring Point were richer in Mg and Ca than at Maurice Bay; this is consistent with a predominantly basement origin for the former. Stable isotope analyses of clay alteration minerals indicate that, in particular, the chlorite-related fluids from Spring Point have a basement signature (δ 18 O = 3.5‰ and δD = −13‰), whereas chlorite from Maurice Bay is more consistent with a mixture of basement and basinal fluids (δ 18 O = 5.2‰ and δD = −28‰). The absence of uranium-transporting basinal fluids at the critical time when reducing basement fluids were producing chlorite alteration is likely the single most important cause for the lack of uranium mineralization at Spring Point.