Jean‐Dominique Meunier, Patrick Landais, Marc Monthioux, Jean‐Jacques Braun1
1Groupement Scientifique C.N.R.S. CRECU, 54 Vandoeuvre-les-Nancy (France)
Tóm tắt
The Cottonwood Wash U-V tabular deposits are hosted by the Salt-Wash Member of the Jurassic Morrison Formation. Petrological study of ore and barren host-rocks and analysis of organic matter were conducted to test redox genetic models. Barren sandstones are mainly cemented by quartz, calcite and kaolinite. Tabular ore is cemented by U-bearing vanadiferous clays. Pitchblende and coffinite are present in some coalified logs. U and V mineralization occurred during diagenesis after interpénétration of detrital quartz grains. Two different generations of pyrite are observed. Accumulation and preservation of organic matter composed of conifers fragments were possible because of favorable paleogeographical and paleoclimatological conditions. Chemical composition of organic matter is interpreted as an oxidation event that occurred during diagenesis. This oxidation was related to the circulation of uranium-bearing, oxidizing solutions during burial. The fact that diagenetic oxidation is not strong enough to destroy organic matter supports the slow-rate oxidation identified in the genesis of secondary pyrite and suggests that the oxidizing capacity of the mineralizing fluid was lower than the reducing capacity of the hosted rocks. When these solutions encountered reduced organic-rich zones, U and V precipitated. The results indicate that Colorado Plateau tabular deposits formed from low oxygen waters compared to the solutions which formed typical roll front deposits.
