Graphite does not function as a direct reducing agent for unconformity-related uranium mineralization – Evidence from the graphitic metapelite-hosted Gryphon uranium deposit, northern Saskatchewan, Canada

It is generally agreed that the unconformity-related uranium (URU) deposits in the Athabasca Basin (Canada) resulted from reactions between basin-derived, oxidizing, U⁶⁺-bearing fluids and reducing agents carried by basement-derived fluids or lithologies enriched in reducing elements. Based on the observation that most URU deposits are spatially associated with graphite-rich zones in the basement, it is believed that either the graphite itself or hydrocarbons derived from it acted as major reducing agents for U mineralization. Several previous studies downplayed graphite as a direct reducing agent for U mineralization because few or no direct contacts between uraninite and graphite have been observed. However, it remains skeptical whether or not this is due to insufficient observation, and what happened to the graphite if it was not directly involved in U precipitation. In this study, graphitic metapelite samples were collected from the Gryphon URU deposit for detailed petrographic and Raman spectroscopic study. Two types of graphite were identified, an earlier phase (Gr1) that is parallel and deformed together with the schistosity, and a later phase (Gr2) that is post-deformation, both of which are pre-U mineralization. Thorough microscopic examination confirmed that there is no direct contact between uraninite and graphite in the thin sections. Raman spectroscopic studies of the graphite indicate that the structural order of both Gr1 and Gr2 decreases toward the orebody. These observations indicate that graphite was not a direct reducing agent causing the precipitation of uraninite. Hydrocarbons (mainly CH₄) produced from in situ fluid-graphite reaction was not responsible for the uraninite precipitation either, because such a process would also have produced textures showing replacement of graphite by uraninite. Instead, the graphite initially present in the host rocks that are now occupied by the orebodies was likely dissolved by the hydrothermal fluids ahead of the precipitation of uraninite. The actual reducing agents causing the precipitation of the bulk of uraninite in the orebodies are likely CH₄ derived from ex situ fluid-graphite reaction below the deposit.