ABSTRACT

Diagenesis of the lower Krider Member of the Nolans Limestone in the Hugoton embayment was dominated by dolomitization and subsequent anhydrite precipitation and replacement. Typical dolomite crystals have a replacive inclusion-rich core followed by a clear rim in transmitted light, and have three cathodoluminescent (CL) growth zones. Fission-track analysis suggests that most uranium resides in CL Zone 2, in an earlier dolomite zone replaced by Zone 2, and in enriched phosphatic minerals. Regionally, the most U-rich part of the lower Krider Member is an east-west oriented region in the north-central part of the Hugoton embayment. Uranium concentration does not correlate directly with lithology, location of tidal-flat deposits, or subaerial exposure surfaces. U and Pb concentrations for whole-rock dolostone analyses 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 dolomitization took place around 250 Ma and that the timing of U enrichment was 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°C. From fluid-inclusion data, petrographic and stratigraphic relationships, and U-Pb age dates from the dolomitic rocks, we suggest that during the Late Permian, reflux of low temperature (<50°C), Mg-bearing brines precipitated U-bearing dolomite and replaced earlier dolomite in the lower Krider Member and other units in the Chase Group. Some all-liquid fluid inclusions were later thermally reequilibrated by stretching after the Permian, possibly by either (1) later burial with a high geothermal gradient or by (2) hot, topography-driven fluid flow out of the Rocky Mountain foreland during the Tertiary.