CHLORITE ALTERATION OF PRE-ORE PYRITE AT THE MCARTHUR RIVER URANIUM DEPOSIT, ATHABASCA BASIN: POSSIBLE IMPLICATIONS TO ORE DEPOSITION

Introduction

The McArthur River uranium mine in the Athabasca Basin, Canada, hosts the largest high-grade uranium deposit in the world, accounting for 12% of global uranium production in 2015. McArthur River is a Proterozoic unconformity-related uranium deposit, with ore bodies located in the P2 fault zone at the unconformity between conglomerate and metamorphic basement rock, and up to 120 m below the unconformity in basement rock. Oxidized basinal brines are considered the most likely medium for transporting dissolved UO₂²⁺ to the unconformity [1]. The reductant responsible for precipitating UO₂ from solution is unknown [2], but there is a clear spatial association between the ore zones and the P2 reverse fault.

Discussion of Results

Core logging and sample collection were conducted during the 2015 field season. Petrography and SEM investigations were used to establish the paragenesis and alteration styles in altered basement rocks. FE-SEM, EMP, and XRD investigations revealed the presence of di-trioctahedral sudoite in metamorphic basement samples from the P2 fault zone near the unconformity.

Here, a new model for the reduction of uranium at McArthur River is proposed. Our observations indicate that the P2 reverse fault is characterized by the abundant presence of fault-hosted pre-ore pyrite veins. The formation of these veins resulted in the illitization of feldspar in the basement rocks. Oxidizing Na-Mg-Ca chloride basinal brine, forming abundant secondary fluid inclusions, penetrated into the basement along the P2 fault and oxidized the pre-ore pyrite veins, resulting in the reduction of the brine. Pyrite veins and illite were replaced by Fe³⁺-bearing sudoite and fluorapatite. It is hypothesized that the reduction of the basinal brine by the pre-ore pyrite was responsible for the reduction of UO₂²⁺ and the formation of the world-class McArthur River unconformity-related deposit.

[1] Hoeve and Sibbald (1978) Econ. Geol. 73, 1450-1473 . [2] Mercadier et al. (2010) Lithos 115, 121-136.