A508 Goldschmidt Conference Abstracts

Mineralogical controls of acid Variations in 238U/235U ratios in generation and metal leaching at natural uranium ore minerals from contrasting sedimentary exhalative sedimentary basins

sulphide deposits in the Yukon T. KURT KYSER*, D. CHIPLEY, A. VULETICH 1 Territory, Canada AND P. ALEXANDRE Department of Geological Sciences & Geological Y.T. JOHN KWONG Engineering, Queen’s University, Kingston, Ont. K7L CANMET Mining and Mineral Sciences Laboratories, Natural 3N6 Canada (*correspondence: [email protected]) Resources Canada, Ottawa, ON, K1A 0G1 ([email protected], [email protected], ([email protected]) [email protected])

The Anvil District and the Howard’s Pass area of Current investigations indicate that uranium isotopes respectively central and southeastern Yukon Territory, fractionate as a result of nuclear volume effects predicted by Canada, are renowned for their stratiform Zn-Pb deposits. Bigeleisen in his initial calculations of natural isotopic Both camps are located within the Selwyn Basin and the variations in minerals [1]. More recently, it has been suggested prevalent Zn-Pb deposits are all interpreted to be sedimentary that 238U/235U ratios will vary as function of uranium oxidation exhalative (SEDEX) in origin. The Anvil deposits are pyrite- state and will be the highest in reduced species [2]. rich and have undergone significant regional and contact Uraninite from sedimentary basins is susceptible to metamorphism. Recrystallization during metamorphism has recrystallization, reprecipitation and alteration to a variety of resulted in an increase in average grain size and the secondary uranium minerals that are often produced by conversion of the associated carbonates into calc-silicates and interaction with oxidizing fluids. This interaction should some of the pyrite into pyrrhotite. These have in turn effected therefore be reflected in the observed 238U/235U ratios. a reduction in acid buffering capacity and a partial The 238U/235U ratios of uranium minerals from sandstone- enhancement of sulphide reactivity in the host rocks, which hosted and calcrete-type uranium showings and deposits are not fully compensated for by a decrease in specific surface worldwide have been measured by multi-collector ICP-MS area derived from the coarser grain size. Consequently, acid (Neptune) and reported relative to uraninite from the mine drainage and metal leaching have plagued the mining McArthur River unconformity-related deposit in development both during active mine operation and site Saskatchewan, Canada. The total variation of the δ238U values decommissioning. In contrast, the Howard’s Pass deposits during measurement of the in-house standard is ±0.15 per mil, contain less pyrite (25%) and exhibit well-preserved whereas the total variation of δ238U values for uranium sedimentary structures. The abundance of carbonates within minerals is 1 per mil. As predicted, uraninites with the lowest and surrounding the sulphide deposits provides most of the apparent ages, and have suffered repeated recrystallization [3], country rocks with an inherent acid neutralization potential. have the highest δ238U values as a result of 235U loss during Limited metal leaching (mainly zinc and cadmium) in the interaction with oxidizing fluids. Additionally, more oxidized mineralized zones occurs as a result of galvanic interaction secondary minerals have lower δ238U values. In conjunction among the prevalent sulphide minerals. Natural acid rock with Pb isotopes and uraninite geochemistry, uranium drainage has been observed only in overlying black shale units fractionation could potentially be used in the study of uranium with little carbonates. Given the differences in detailed deposits and as an exploration tool. mineralogy and sulphide oxidation mechanisms, it would appear that the Howard’s Pass deposits could be exploited [1] Bigeleisen (1996) J. Am. Chem. Soc. 118, 3676-3680. with less significant environmental impacts than the Anvil [2] Schauble (2007) Geochim. Cosmochim Acta 71, 2170- deposits if proper planning and precautionary measures are 2189. [3] Alexandre and Kyser (2005) Can. Mineral. 43, taken. 1005-1017.