Abstract

Copper (+ or - silver) ores occur along faults related to salt anticlines in red-bed sediments of the Paradox basin and in coal-bearing horizons of the Dakota Sandstone in southeastern Utah and southwestern Colorado. Deposits of this type occur at Salt Valley, Sinbad Valley, and Paradox Valley; the largest deposits are at Lisbon Valley. These deposits are related by geologic setting and ore and gangue mineralogy. Fluid inclusions, mineral chemistry, and C-O stable isotopes in calcite gangue associated with copper ores at Lisbon Valley suggest that mixing of two solutions caused precipitation of the ore. Fluid temperatures ranged from 72 degrees to 103 degrees C and salinities were 5 to 20 equiv wt percent NaCl during calcite deposition. Copper-bearing, saline, basinal fluids migrated upward along fault zones and deposited ore and gangue minerals upon mixing with shallower, reduced, Ba-rich ground water. Mineral precipitation was induced by a combination of dilution and reduction.

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