2 D29. Investigations in Economic Geology

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Skagit 5

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

Paper number: 2-10
Program: Technical Sessions
Time: 10:15 AM – 10:30 AM

Authors

Abstract

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.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-297976

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.