PH CONTROL ON URANIUM DEPOSITION IN VANADIUM-BEARING ROLL-FRONT DEPOSITS, WYOMING

Two mineralogically, chemically, and texturally distinct types of arkose-hosted uranium deposits have been recognized in the Powder River Basin, WY. Type 1 deposits include uraninite and coffinite in clays forming after alkali feldspars and in chlorite forming after biotite. Pyrite occurs as abundant framboidal aggregates. Calcite and phases with vanadium as a major constituent are rare to absent in Type 1 deposits. Type 2 deposits include abundant carnotite/tyuyamite forming after uraninite and coffinite along fractures in alkali feldspar and along virtually all grain boundaries. V-O and V-Fe-O phases without uranium are major consitutents and locally form intergranular cements. Textural relations confirm that uranium vanadate mineralization postdates formation of V-Fe-O phases at least locally, and calcite cement is locally replaced by V-Fe-O cement. Pyrite is common and occurs as isolated euhedral crystals; framboidal pyrite is extremely rare. Calcite is common and locally forms intergranular cement. Uranium mineralization is largely absent from calcite-cemented portions of the rock, and is conspicuously absent in clay-rich regions in Type 2 deposits. Both deposit types show the spatial distribution of zones down the hydrologic gradient that is typical of roll-front deposits: an oxidized zone (reddish yellow color, near depletion in organic matter and pyrite, and abundant ferri-hydroxides), a relatively narrow uranium-enriched redox front, and a reduced zone (gray color, organic material and pyrite present).

Uranium occurs primarily in the hexavalent state in Type 2 deposits, and in tetravalent state in Type 1 deposits. Vanadium is largely restricted to rare carnotite-group minerals in Type 1 deposits, indicating that it occurs only in the pentavalent state. Vanadium in the V-O cement in Type 2 deposits (tentatively identified as doloresite by XRD) is most probably tetravalent. Oxidation state of vanadium is strongly and inversely proportional to pH. Increasing pH of ground water associated with interaction with calcite may oxidize V⁺⁴ to V⁺⁵; this change would move the system into the stability field of carnotite group vanadate phases. Uranium in Type 2 deposits may be precipitated due to oxidation of vanadium at constant Eh in these deposits.