Quantifying Hydrocarbon-Dominated Transport of Uranium and Vanadium

Hydrocarbons and co-existing basin brines are implicated in mobilizing and depositing metals, challenging previous models for the transport of metals in low-temperature sedimentary basins. This work evaluates uranium and vanadium transport by hydrocarbons and subsequent deposition in ore deposits located along the margins of the Bighorn Basin, USA, and the possible relationship with hydrocarbon migration from the Phosphoria Formation. To quantify volumes of oil and formation water responsible for depositing the total uranium and vanadium, we integrated geochemical analyses of co-existing Phosphoria Formation sourced oil and water with published solubility data, thermodynamic models, and numerical calculations. Oil contains significantly greater concentrations of vanadium (1.46−242 ppm) than uranium (0.02−0.86 ppb). Both metals were dominantly transported in oil, likely in porphyrin structures, rather than formation water. Compared to vanadium, uranium solubility in oil is temperature dependent; thus, different volumes of oil are needed to transport each metal to the deposits. The volume of oil required to transport vanadium (4−23 million barrels) matches well with the pore volume of the host formation for the ore deposits, the Madison Limestone, while ∼240 times that volume of oil (5.5 billion barrels) is required to transport and deposit uranium. The varying quantities of oil calculated could point to different mechanisms and timing of metal removal from the hydrocarbons. This geochemical approach to explain the transport of metals in basin fluids can be applied to other systems to understand ore genesis and identify regions for exploration in the Earth’s crust.