Migration of brines in the basement rocks of the Athabasca Basin through microfracture networks (P-Patch U deposit, Canada)
Résumé
The unconformity-type uranium deposits of the Athabasca Basin (Saskatchewan, Canada) are hosted near 21 the unconformity between a middle Proterozoic intracratonic sedimentary basin and an Archean to Paleo-22 Proterozoic metamorphic and plutonic basement. These deposits, which are considered to be the richest U 23 deposits in the world, are the result of massive basinal fluid migrations in the basement rocks.
24 This study shows that basinal brines have strongly penetrated into the basement not only through faults and 25 major pathways but also by way of dense networks of microfractures which favoured the percolation of 26 fluids down to considerable depths (hundred metres below the unconformity) and their chemical 27 modification (salinity increase) by interaction with basement lithologies. These processes are one of the 28 major causes of uranium mobility within the basement rocks and the formation of unconformity-type 29 mineralization.
30 Microfracture networks, which opened during the basinal brine stage (ca. 1600-1400 Ma) are interpreted as 31 sets of mode I cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after 32 healing. The stress field at that stage (σ 1 = N130-150 °E, subvertical) partly reopened the earlier microcrack 33 networks (σ 1 = N80-110 °E and N130-150 °E, subvertical) issued from the Trans-Hudson Orogeny late 34 retrograde metamorphic stage (ca. 1795-1720 Ma). The circulation of the two types of fluids (carbonic and 35 brines) occurs thus at two distinct events (Trans-Hudson Orogeny late retrograde metamorphism for 36 carbonic fluids and maximal burial diagenesis for brines) but the same main microfissure geometry was used 37 by the fluids. This demonstrates the existence of a similar stress field direction acting before and after the 38 basin formation. Moreover, the brine circulations in the basement acted in a wider volume than the clay-rich 39 alteration halo surrounding the U-ores, generally considered as the main envelope of fluid percolation 40 outside the fault systems. The data on the chemistry of the fluids and on the geometry of their migration at 41 various scales emphasise the fundamental role of the basement in the chemical evolution of highly ^saline 42 brines linked to unconformity-related uranium mineralization in the Athabasca Basin.
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