Ferrous Iron (Fe+2) Released From Iron-Rich Chlorite as a Reductant for Unconformity-Related Uranium Mineralization: Insights From Reactive Fluid Flow Modeling

Abstract

Debate continues over the reducing mechanisms for the formation of unconformity-related uranium (URU) deposits. This paper evaluates, for the first time, the potential of iron-rich chlorite as a reductant for uranium mineralization using reactive fluid flow modeling method. Our results confirm that Fe²⁺, released from the breakdown of iron-rich chlorite, can reduce aqueous hexavalent uranium to precipitate economically significant URU deposits similar in size and grade to those formed with CH₄ as the reducing agent. The resulting uranium mineralization tends to occur in the basement and below the downwelling parts of overlying basinal fluid circulation cells, where oxidizing basinal fluid percolates across the unconformity and reacts with upward flowing reducing basement brine. Therefore, the basinal fluid circulation pattern controlled by the permeability of the sandstone aquifer is critical in determining the formation and distribution of URU deposits. When the sandstone layer is more permeable, the simulated uranium deposits become larger in size, and vice versa. If the sandstone permeability is <5 × 10⁻¹⁴ m², no obvious uranium deposits can be formed. In contrast, the permeability of fault zones does not have a significant effect on uranium mineralization, although it does affect fluid flow behaviors within the fault zone itself. We also demonstrate that fault zones do not appear to be a prerequisite for the formation of URU deposits when Fe⁺² serves as a reductant, which highlights important exploration implications. Uranium exploration should, in addition to continuing to target graphitic fault zones, also consider areas where faults and/or graphite units do not exist.