Basin-scale production of hyperacidic brines is critical for the formation of high-grade and large-tonnage uranium deposits in sedimentary basins

Unconformity-related uranium deposits (URU deposits) in the Proterozoic Athabasca Basin (Canada) represent the largest and richest (with average grades ranging from 0.127 to 19.5 wt% U) U deposits in the world. Fluid inclusion studies and experimental data suggest that the ore-forming fluids of URU deposits are hyperacidic (pH < 3.5), oxidizing basinal brines carrying high concentrations of U (0.2 to 3700 ppm), which are several orders of magnitude higher than ordinary basinal and basement formation waters. While the oxidizing conditions of these fluids are well established, the mechanism responsible for the basin-scale generation of hyperacidic brines remains unknown. This paper aims to address this problem and to explain why the Athabasca Basin is exceptionally endowed with high-grade and large-tonnage uranium deposits. Based on petrographic and infrared hyperspectral logging data indicating basin-wide coexistence of kaolinite and illite in quartz-dominated sandstones, and fluid inclusion data showing high potassium (K) concentrations in diagenetic and ore-forming fluids within the Athabasca Basin, it is proposed that the production of hyperacidic brines resulted from a pervasive diagenetic reaction between K-rich brines and kaolinite: 3kaolinite [Al 2 Si 2 O 5 (OH) 4 ] + 2 K + ↔ 2illite [KAl 3 Si 3 O 10 (OH) 2 ] + 3H 2 O + 2H + . Geochemical modeling of fluid-rock reactions demonstrates that the basin-scale production of hyperacidic brines is achievable when a combination of three specific conditions is satisfied: 1) the presence of a thick package of compositionally mature sandstones, characterized by quartz-dominated framework grains with minimal (<1%) feldspar and the interstitial space filled with kaolinite; 2) the development of K-rich brines through seawater evaporation above the sandstone package; and 3) low fluid/rock ratios, which enable the reaction between kaolinite and K-rich brines to reach equilibrium and produce illite and H + . Recognizing the basin-scale development of hyperacidic brines and the specific conditions entailed offers insights into why the Athabasca Basin is exceptionally endowed with high-grade, large-tonnage U deposits. The conditions of hyperacidic brine production revealed in this study may be used for evaluating the U mineralization potential of other sedimentary basins.