Thermodynamic constraints on the mineralogical and fluid composition evolution in a clastic sedimentary basin: the Athabasca Basin (Saskatchewan, Canada)

To evaluate the critical parameters controlling the mineralogical evolution of a dominantly elastic sedimentary basin, the Athabasca Basin (Saskatchewan, Canada), activity diagrams have been constructed with respect to the mineralogy, thermo-barometry, and fluid composition documented from previous studies. Relative to standard diagrams, which exist for ideal clay minerals, the present diagrams are the first to include all of the significant minerals present in the sediments, accounting specifically for the occurrence of dravite, sudoite, and two types of illite having identical chemical compositions, but different crystal morphologies and structures. The occurrence of the different mineralogical assemblages is mainly controlled by the K+/H+ and Mg2+/(H+)(2) activity ratios, and the silica and B(OH)3 activities in addition to temperature and pressure. Variations in the activities of the major brine components (Cl-, Na+, Ca2+) and the oxygen fugacity (above the hematite-magnetite buffer) have little effect on the mineral stabilities. The mineralogical evolution of the Athabasca Group sandstone results from mass transport processes involving a basement-derived fluid, richer in K, Mg, B, and/or pH, and possibly undersaturated in silica. The zoned distribution of the minerals in the uranium-mineralized Shea Creek area can be explained by a progressive interaction of the basement-derived fluid with the diagenetic brine and minerals of the Athabasca Group sandstone. During peak diagenesis, the brine in the basal Athabasca Group was buffered by the assemblage 1Mc illite-kaolinite. During the syn-ore stage, the estimated values of log(K+/H+), log(Mg2+/(H+)2) and B(OH)3 activity decrease from 3.9, 6.4, and 0.005 in breccia zones immediately above the orebody, down to 2.7-3.6, 4.7-6.1, and 0.004 in the inner alteration halo, and down to 2.7, 4.9, and 0.003 in the outer alteration halo, respectively.