John A. Luczaj
(Current address: Department of Earth and Planetary Sciences / The Johns Hopkins University / 3400 North Charles Street / Baltimore, MD 21218)
In southwestern Kansas, diagenesis of the lower Krider Limestone was dominated by dolomitization and subsequent anhydrite precipitation and replacement. Typical dolomite crystals have a replacive, inclusion-rich core (50 - 75 micron) followed by a clear, colorless rim (up to ~20 micron thick). Unrecrystallized and partially recrystallized dolomite crystals typically have 3 CL zones, one of which exhibits an unusual greenish-colored polyhedral morphology (Zone 2). Qualitative analysis of induced fission-tracks suggests that most U resides in Zone 2, in an earlier dolomite (Zone 1) replaced by Zone 2, and in enriched detrital phosphatic grains.
Although some aqueous all-liquid inclusions are present, most fluid inclusions are aqueous and two-phase and contain a gas under pressure (4 - 30 bars) assumed to be methane. Inclusions are concentrated in the position of CL Zone 1 and are petrographically primary to recrystallization. They yield reliable Th's from 60 to 95oC (a few as high as ~150oC; mean is 85.4oC), indicating that the rocks were exposed to temperatures of at least 95oC. Th data suggest that some FIA's have undergone different degrees of reequilibration or were entrapped during variable temperature conditions. Te's of ~-57oC are consistent with a NaCl-CaCl2-MgCl2-H2O model composition for the inclusion fluid. A brine composition with an elevated Mg+2/Ca+2 ratio is consistent with evaporated sea water, however, the effect of a high SO4-2 concentration on the fluid inclusion eutectic temperatures in a complex system is not well known. The Tmice values ranged from -22.7 to -17.5oC with a mean of -21.2oC for all wells, which yields salinity values from 20.6 to 24.1 wt. % NaCl eq. (ave. 23.2 wt. % NaCl equivalent). There were no significant differences for Te or Tmice values between the all-liquid and the two-phase inclusions. All-liquid inclusions indicate some recrystallization of the dolomite below ~ 50oC. Stretching of all-liquid inclusions may be responsible for the two-phase inclusions in the lower Krider.
Regionally, the most U-rich (> ~7 ppm) part of the lower Krider is in an east-west oriented region of at least 450 km2 in the north central part of the Hugoton Embayment. Dolomite with the thickest, brightest, and most greenish cathodoluminescent polyhedral growth zones seems to be the most U-rich. Whole-rock U concentrations determined from well logs do not correlate directly with lithology and do not appear to be stratigraphically related to tidal flat or subaerial surfaces. TOC and U concentrations appear to have a vague correlation visually, but there is no significant correlation statistically. Measured U and Pb concentrations for 11 whole-rock dolostone analyses vary from 12.20 - 29.11 ppm and from 1.35 - 6.60 ppm, respectively. Isochron ages for 238U/204Pb vs. 206Pb/204Pb and 235U/204Pb vs. 207Pb/204Pb are 253 +/- 41 Ma and 256 +/- 19 Ma (2 sigma), respectively. A Pb-Pb isochron age of 390 +/- 110 Ma has been calculated for the same samples, but was not used in the interpretations.
A proposed model suggests that during the latest Permian (~ 256 +/- 19 Ma), regional reflux of highly saline, Mg+2-bearing brines associated with the regional precipitation of Permian evaporites (Sumner and/or Nippewalla Groups) precipitated U-bearing dolomite and recrystallized earlier dolomite in the lower Krider at low temperatures (< ~50oC). Some inclusions were thermally reequilibrated by stretching after the Permian possibly by either increased burial and a high geothermal gradient during the Cretaceous or by hot, gravity-driven fluid flow through the Rocky Mountain foreland during the Tertiary. U (and possibly other rare earth elements) may have been leached from various minerals in the Permian redbeds of the Sumner and Nippewalla Groups during brine reflux, or U may have been sourced from evaporated seawater. Local U incorporation in dolomite may have been influenced by local pore fluid factors. Methane incorporated in the inclusions at this time was generated locally from decay of original organic matter in the rocks. Massive natural gas deposits present in the Hugoton today migrated as a free phase since latest Permian time (~256 +/- 19 Ma).