ROLE OF POST-DIAGENETIC REDUCTION (BY HYDROCARBONS?) IN THE FORMATION OF PARADOX BASIN (COLORADO PLATEAU) U-V DEPOSITS

The Colorado Plateau hosts stratabound, largely tabular U-V deposits in the Chinle (Triassic) and Morrison (Jurassic) formations. Proposed origins for metal trapping include: 1) precipitation on indigenous organic matter (humate, coal); 2) precipitation at an interface between reducing and oxidizing fluids (e.g. Sanford, 1992, Econ Geol). New geologic and petrographic evidence along with published observations in the oil-rich Paradox Basin suggests that a hydrocarbon-bearing fluid may have chemically prepared the host or been present during metallization.

Petrographic study of samples from multiple districts demonstrates complex parageneses sharing key characteristics. Quartz overgrowths cover early hematite fringes. Later U-V minerals (uraninite, coffinite, montroseite), pyrite and other sulfides are locally found beneath overgrowths but are typically interstitial. Roscoelite and other V-silicates replace authigenic quartz. In addition to identifiable fossil plant material, amorphous, variably fluid, sulfur-rich organic matter (bitumen? humate?) fills pores and occurs as fluid inclusions; it localizes the reduced phases (as previously described). These relationships and discordant bleaching of strata show that the host rocks were originally red beds. Local hematite concentrations fringing these zones may reflect related redistribution of Fe. In some areas, U occurs near the intersection of bitumen-bearing sandstone host strata with red beds (e.g., Big Indian, UT) even though the host contains widespread plant material.

As proposed by others, these observations imply involvement of two fluids, either sequentially or simultaneously. Our evidence is that one fluid contained hydrocarbons and reduced the red beds, leaving pyrite ± bitumen. Absolute timing is unclear. If sequential, reduction created a redox trap for U from an oxidizing fluid. This seems likely for Morrison-hosted deposits, where sinuous orebodies and the presence of both hematite and uraninite under quartz overgrowths are inconsistent with a simple mixing interface. Most Chinle deposits appear to be similar, but localization of large, high-grade ores at Big Indian over a bitumen-bearing bleached sand argues for mixing. Plant material was locally important, but overall evidence points to a critical role for two fluids.