The goal of this project was to analyze previously unrecognized connections between the movement of hydrocarbons and the movement of uranium and iron on the Colorado Plateau. Most approaches to research conventionally separate the metals from hydrocarbons, considering one or the other but not both. However, field evidence suggested that the movement and precipitation of metals – uranium and iron in particular – could both be linked to the geochemical changes caused by the movement of hydrocarbons through the rock. This pointed to an unrecognized connection between petroleum, iron, and uranium movement on the Colorado Plateau.

To test this idea, we collected and analyzed rock samples, visited mines, and mapped rock outcrops at five key field sites across the Colorado Plateau. At each of them, we documented geological relationships, mineralogical changes, and differences in rock chemistry that suggest that hydrocarbon, U, and Fe movements are strongly coupled. Mapping shows that deposits of uranium occur where the normally red rocks have been bleached white, partly by the reduction of iron from hematite to pyrite and partly by the removal of Fe from the rock (as indicated by whole-rock analyses of red and bleached sandstones). In rocks from various U deposits, we found chemical and mineralogical evidence that this bleaching of redbeds occurred relatively early in the paragenesis and was shortly followed by U mineralization (hematite, pyrite, and pitchblende inclusions under authigenic quartz overgrowths). We also found U-rich asphalt and blue-fluorescing fluid inclusions in rocks from the ore deposits, indicating hydrocarbon involvement in ore precipitation.

This evidence supports the interpretation that mineralization was a multi-step process involving first hydrocarbons and then more conventional metal-bearing fluids. Hydrocarbons bleached the rock relatively early in its history, reducing or removing iron and redepositing it in situ as pyrite or on the fringes of the bleached area as pyrite, hematite, or ferroan carbonate concretions. While individual deposit parageneses vary, in general the evidence indicates that U mineralization occurred around the same time as the hydrocarbon-induced bleaching or shortly thereafter. This is consistent with the known geochemistry of the Colorado Plateau rocks and of hydrocarbon reservoirs and U deposits in general. Reduction and removal of iron is a common side effect of hydrocarbon passage, which has been known for some years to cause bleaching in red rocks. Hydrocarbons, or rocks bleached by hydrocarbons, also have the right chemistry to trap uranium from solution by reducing it from its soluble U⁶⁺ to its insoluble U⁴⁺ state.

This new work indicates uranium movement and ore mineralization much earlier than previously thought, and indicates that the development of U deposits is closely related to the mobilization or reduction of Fe. While further interpretation is underway, this represents a significant revision to the previous metallogenetic model for the formation of uranium deposits in sandstone. It also helps to constrain the potential sources of uranium and timing of mineralization, two long-standing and unresolved questions in Colorado Plateau geological research, and provides insights that will aid future efforts to understand the geological and mineralization history of the region.

Last Modified: 11/03/2020
Modified by: Isabel F Barton