Abstract

The Permian Cutler Formation in Lisbon Valley, San Juan County, Utah, is composed predominantly of fluvial arkosic sandstones, siltstones, shales, and mudstones that were deposited by meandering streams that flowed across a flood plain and tidal flat. Two types of channel deposits are recognized from their sedimentary structures: meandering and distributary. The flood plain was occasionally transgressed by a shallow sea from the west, resulting in the deposition of several thin limestones and marine sandstones. The marine sandstones were deposited as longshore bars. Wind transported sand along the shoreline of the shallow sea, forming a coastal dune field. Marine and eolian sandstones are more common in the upper part of the Cutler Formation in the southern part of the area, whereas in the central and northern part of the area the formation is predominantly fluvial. Crossbed orientation indicates that streams flowed S. 67° W. on the average, whereas marine currents moved sediment S. 36° E. and N. 24° W., and wind transported sand S. 80° E.

The uranium in the Cutler Formation is found in the central and northern part of the area, in the upper part of the formation, in fluvial sandstone bodies that were deposited predominantly in a distributary environment. No uranium is known in the marine or eolian sandstones. Petrographically, the uranium-bearing sandstones are identical to other Cutler fluvial sandstones except that they contain less calcite cement and more clay and are slightly coarser grained. The diagenetic sequence indicates that uranium and vanadium were introduced late in the sequence, after oxidation had formed hematite and before the formation of calcite cement. Ore formation has modified the host sandstones very little.

The uranium and vanadium minerals are finely disseminated and thus difficult to identify but seem to include some uraninite, coffinite, uranophane, and carnotite. Much of the uranium is associated with iron oxide grain coatings and matrix. The uranium and vanadium occur both together and independently. Both calcium and iron are depleted, and barium is concentrated in the ore zone. No significant organic carbon was found in the ore zones, and small amounts of selenium are concentrated at the base of the ore.

Formation of these orebodies has occurred without any obvious reductant. Perhaps sorption of uranyl by hematite was the concentration mechanism. The time of formation is not known; evidence is present for both a Permian age and a Triassic or younger age.