Organic matter diagenesis as the key to a unifying theory for the genesis of tabular uranium-vanadium deposits in the Morrison Formation, Colorado Plateau

Abstract

Interstitial, epigenetic amorphous organic matter is intimately associated with uranium in the Grants uranium region on the southern part of the Colorado Plateau in northwestern NewMexico and is considered essential to genetic models for these deposits. In contrast, uranium minerals are intimately associated with authigenic vanadium chlorite and vanadium oxides in amorphous organic matter-poor ores of the Slick Rock and Henry Mountains mining districts on the northern part of the plateau, and therefore, in some genetic models amorphous organic matter is not considered crucial to the formation of these deposits. Differences in organic matter content can be explained by recognizing that amorphous organic matter-poor deposits have been subjected to more advanced stages of diagenesis than amorphous organic matter-rich deposits, during which organic compounds were broken down, solubilized, and removed during various diagenetic stages (e.g., sulfate reduction, methanogenesis, and organic acid production). Most diagenetic alterations (e.g., dissolution of framework grains and cements and precipitation of coarse-grained coffinite, vanadium clays, and vanadium oxides) characteristic of amorphous organic matter-poor deposits occurred when temperatures exceeded 80øC--a temperature typical of the organic acid stage of diagenesis.Two factors combined to assure that a high percentage of the amorphous organic matter was removed from the Morrison Formation during diagenesis: the fact that the organic matter was humic (and therefore highly oxygenated), and that the juxtaposition of authigenic smectite and amorphous organic matter on grain rims facilitated clay mineral-organic catalysis reactions leading to the breakdown of large organic molecules into smaller, soluble molecules.Evidence that amorphous organic matter was involved in the genesis of organic matterpoor, as well as organic matter-rich, deposits includes the presence of leached iron-titanium oxides and strongly etched garnets (both attributed to leaching by organic acids), the gradation from organic matter-rich to organic matter-poor deposits in the Grants uranium region, the inverse correlation between the amount of amorphous organic matter and the intensity of diagenetic alteration, the ubiquitous occurrence of amorphous organic matter inclusions in coffinitc, and the similarity in geometry of tabular uranium-vanadium ores and Holocene humate deposits. By recognizing that amorphous organic matter was crucial to the initial concentration of uranium (even in those deposits that presently contain little amorphous organic matter) and by recognizing the effects of the various stages of diagenesis on the ore, one genetic model may be applied to all of the sandstone-hosted, tabular-type uranium-vanadium deposits in the Morrison Formation on the Colorado Plateau.

Speciation of uranium in surface-modified, hydrothermally treated, (UO{sub 2}){sup 2+}-exchanged smectite clays

“…These reduced ions were then precipitated as oxides, hydroxides or precursor clay minerals. [53] Our results support this proposed mechanism. Thus a mechanism to reduce the uranyl ion under environmentally relevant conditions has significant potential to limit transpoft of this potentially dangerous material.…”

Vanadium: Global (bio)geochemistry

“…Cycles of reduction and oxidation fueled by organic matter likely transformed the source minerals over time into the diverse mineral assemblages that currently exist (Hansley and Spirakis, 1992;Spirakis, 1996). Minerals in the Colorado Plateau encompass V in +3, +4 and +5 valence states and reflect a wide range of reduction potentials and pH values during formation (Evans and Garrels, 1958 and Botinelly, 1959;Roach and Thompson, 1959).…”

Dinoflagellate cyst biostratigraphy, palynofacies, depositional environment and sequence stratigraphy of the Agardhfjellet Formation (Upper Jurassic–Lower Cretaceous) in central Spitsbergen (Arctic Norway)

“…They attributed the eastward thickness increase of the shale as being a result of tectonic movements during the Tertiary. Considering the occurrence of AOM in tectonic decollement zones (Hansley & Spirakis, 1992), this could explain the higher proportion of plant debris in Knorringfjellet, but not the higher abundances and diversities among the dinoflagellate cysts.…”