Diagenesis of the lower Permian Krider member, southwest Kansas, USA: Fluid-inclusion, U-Pb, and fission-track evidence for reflux dolomitization during latest Permian time

Diagenesis of the lower Krider Member of the NaIans Limestone in the Hugoto n embayment was dominated by dolomitization and subsequent anhydrite precip itation and replacement. Topical dolomite crystals have a replacive inclusi on-rich core followed by a clear rim in transmitted light, and have three c athodoluminescent (CL) growth zones. Fission-track analysis suggests that m ost uranium resides in CL Zone 2, in an earlier dolomite zone replaced by Z one 2, and in enriched phosphatic minerals. Regionally, the most U-rich par t of the lower Krider Member is an east-west oriented region in the. north central part of the Hugoton embayment, Uranium concentration does not corre late directly with lithology, location of tidal-flat deposits, or subaerial exposure surfaces. U and Pb concentrations for wholerock dolostone analyse s range from 12 to 29 ppm and from 1.4 to 6.7 ppm, respectively. Concordant U-Pb and Pb-Pb isochrons and a Total Pb/U concordia plot indicate that dol omitization took place around 250 Ma and that the timing of U enrichment wa s younger than the Early Permian depositional age. Fluid-inclusion analyses suggest that the U-rich dolomitization took place at low temperatures in the presence of a Na-Ca-Mg-Cl brine and was followed by a later event that heated the rocks to at least 95 degrees C, From flui d-inclusion data, petrographic and stratigraphic relationships, and U-Pb ag e dates from the dolomitic racks, we suggest that during the Late Permian, reflux of low temperature (<similar to 50 degrees C), Mg-bearing brines pre cipitated U-bearing dolomite and replaced earlier dolomite in the lower Kri der Member and other units in the Chase Group. Some all-liquid fluid inclus ions were later thermally reequilibrated by stretching after the Permian, p ossibly by either (1) later burial with a high geothermal gradient or by (2 ) hot, topography-driven fluid how out of the Rocky Mountain foreland durin g the Tertiary.