Abstract

Much controversy has arisen over both the depositional and diagenetic environments of the lacustrine sediments and the chemical evolution of waters within those sediments that make up the upper Oligocene Creede Formation. Previous studies of the neighboring Creede Ag-Pb-Zn district proposed that saline and isotopically heavy fluids from the Creede Formation were involved in ore deposition. Examination of rhythmically laminated limestones from two Continental Scientific Drilling Program drill cores (CCM-1 and CCM-2) provided a unique opportunity to study carbonate sediments of a caldera lake. The upper parts of CCM-2 (50–148 m depth) are characterized by laminations of low-Mg calcite containing brine shrimp fecal pellets (with an aragonite precursor), organic matter (including bacterial mat–like laminae), calcite pseudomorphs after gypsum, and cements of bladed low-Mg calcite and tabular clinoptilolite. The degree of recrystallization and crystal coarsening is greater in the lower part of CCM-2 (172–309.4 m depth) and in CCM-1 (8.8–155.2 m depth).

These observations suggest that ancient Lake Creede was an evaporative, partially meromictic, saline to hypersaline, near-neutral, Na-Mg-SO4-Cl or Cl-SO4 lake. SO2−4 and H+ were supplied to the lake by the oxidation of volcanic SO2(gas), which buffered the alkalinity of the lake. Brine shrimp in the epilimnion are consistent with salinities greater than seawater and with warm surface temperatures. The lack of bioturbation in rhythmically laminated carbonate-siliciclastic couplets indicates either meromictic conditions or saline conditions or both. Bacterial mat-like laminae suggest that lake waters were either relatively shallow or clear. Displacively grown gypsum crystals indicate times when evaporative ground-water conditions dominated (e.g., during low lake levels), and their absence may indicate changes in water chemistry and lake level. Sulfate reduction during burial resulted in the conversion of gypsum and iron oxides to calcite and pyrite. Maximum estimates for the duration of the lake range from 132 323 to 76 340 yr and are based on rhythmites in CCM-2. Results from this study support the hypothesis of saline and isotopically heavy waters evolving within the Creede Formation prior to ore deposition.

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