MID-PROTEROZOIC RIFTING AND ANTLER TECTONISM: TWO-STAGE METALLOGENESIS PRODUCED THE UNIQUE HIGH-GRADE UNCONFORMITY-TYPE URANIUM DEPOSITS OF THE ATHABASCA BASIN, CANADA

Despite prolonged world-wide search for analogues, Athabasca Basin unconformity-type uranium (U) deposits remain unequalled in grade. The reason for this may lie in its Paleozoic structural history and associated metallogenesis.

The Proterozoic Athabasca Basin U was emplaced when a Mid-Proterozoic basin-deepening event, part of the early stages of Columbia continental breakup, buried Early Proterozoic Athabasca Group sediments to depths where hydrothermal systems could operate, facilitated by mobilization of Athabasca Group evaporites.

Paleozoic U forms a large part of the Athabasca Basin unconformity-type U deposits. In the western 2/3 of the basin they may form the major part of the deposits, whether hosted in sub-Athabasca basement or within late Paleoproterozoic Athabasca Group sediments. Within the Athabasca Group most of the U is Paleozoic in age throughout. What sets the Athabasca Basin deposits apart from deposits in other Proterozoic basins is the higher grade, and the Paleozoic U metallogenetic event.

During the final Infra-Cambrian breakup of Rodinia disruption of the Athabasca Basin is reflected by Infra-Cambrian U ages in the deposits. The major reworking of the basin U occurred during the early Carboniferous (Tournaisian) Antler Orogen. This disrupted Devonian sediments, including the massive Prairie Evaporite, that were deposited directly on the Athabasca Group, overlying most, if not all of it. The northeast trending Peace River embayment developed in the earliest Carboniferous and lasted as a low into the Permian. Block faulting during the formation of the Peace River Embayment resulted in breaches of the underlying evaporites, and massive influx of concentrated brines into the Athabasca Basin and its basement. These brines scavenged U from Paleozoic and Athabasca Group sediments and underlying basement, removing some deposits, upgrading others, and creating new ones.

This basin development history underlines the significance of evaporites in the development of metal deposits, and that now-gone evaporites may be as significant as those still remaining. Subsequent basin history can be very important in the development and grade of ore deposits, even if the overlying basins, whose development triggered the last phases of metallogenesis, have since been removed by erosion.