Uranium-vanadium deposits in the Salt Wash Member of the Morrison Formation in Montezuma Canyon, San Juan County, Utah, exhibit zoning that is interpreted as a result of metal transport by diffusion. The concentric zones consist of a brown nonmineralized core, an olive-gray mineralized shell, and a gray nonmineralized outer zone. The brown zone is iron-stained, porous sandstone commonly containing abundant carbonaceous material. The curved mineralized layer completely encloses the brown zone, and is composed of oxidized uranium-vanadium minerals that impregnate sandstone. The gray zone is light-gray sandstone tightly cemented with calcite and commonly freckled with limonitic specks.Formation of the deposits most likely took place during Late Cretaceous or early Tertiary time when the ore-bearing sandstones were deeply buried and saturated with connate waters. The concentration of organic material in the brown zone suggests that this zone was once saturated with a reducing solution containing soluble organic compounds like alcohols and aldehydes derived from the organic material. The gray zone, on the other hand, was probably saturated with an oxidizing solution containing uranium and vanadium. Where these two solutions were in contact, oxidation-reduction reactions took place that caused the precipitation of low-valent uranium and vanadium minerals.The ellipsoidal shape of the ore layer and the lack of any exit or entrance for flowing solutions indicates that the dissolved metals moved through the gray zone by diffusion and were precipitated at the periphery of the reducing zone. Recent weathering and oxidation have altered the primary low-valent minerals to high-valent forms without noticeable leaching of the ore metals.
