Uranium Ore in the Mariano Lake and Lake Valley Cores and the Genesis of Coffinite in the Grants Uranium Region, New Mexico - 1988

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Hansley reported in 1988 that petrographic and geochemical research on uranium-bearing sandstones in the Upper Jurassic Morrison Formation in the Grants uranium region, northwestern New Mexico, indicate that compactional fluids, which migrated from fine-grained fluvial and lacustrine units into adjacent sandstone intervals during early diagenesis, were instrumental in primary ore formation. These pore fluids carried uranium and other elements derived from the alteration of abundant volcanic material in the Brushy Basin Member of the Morrison Formation. Where these fluids reacted with concentrations of organic matter in sandstones of the Westwater Canyon Member, uranium was adsorbed, reduced, and concentrated into
primary ore deposits.

Primary (carbonaceous) ore has undergone a significant amount of diagenesis due to the bacterial and thermal maturation of amorphous organic matter. Progressive diagenesis decreased the organic carbon:uranium ratio in primary ore, transforming it to various mixtures of microbotryoidal ore and coffinite. Coffinite formed where silica activity was high due to dissolution of aluminosilicate minerals by organic-acid-bearing solutions. Organic acids were responsible for a wide variety of water-rock reactions, which were catalyzed by a warm (at least 100°C) deep-basin fluid that migrated updip through permeable ore-bearing sandstones in the mid-Tertiary.

Regularly interstratified illite-smectite, chlorite, adularia, quartz, and ankerite precipitated at this time. From the late Tertiary to the present(?) time, downdip migration of meteoric waters from outcrops along the southern margin of the San Juan basin caused widespread oxidation of organic matter and redistribution of uranium into orebodies along a regional oxidation-reduction front.

Recognition that diagenesis can transform organic-rich ore into organic-poor ore and that organic acids may facilitate low-temperature formation of coffinite has important implications for interpreting the genesis of sediment-hosted ore deposits. Many organic-poor uranium deposits in the Westwater Canyon
Member of the Morrison Formation in the Grants uranium region and in the Salt Wash Member of the Morrison Formation on the northern part of the Colorado Plateau may have formerly been organic-rich (primary) ore.