Abstract

The Jackpile-Paguate uranium deposit of northwestern New Mexico occurs in the fluvial Jackpile sandstone of the Jurassic Morrison Formation. The uranium ore is present as matrix impregnations of uranium and epigenetic carbonaceous material in a series of stacked, tabular lenses within the arkosic sands. Structure, isopach, and grade thickness maps for one lens define a horizontal mineralized zone with continuity over approximately one-half of a square mile, substantially broader than the sedimentary structures of host fluvial sediments.Sand-sized clasts containing variable proportions of quartz, kaolin, montmorillonite, and mixed layer illite-montmorillonite comprise approximately 20 percent of the sands. These clasts are believed to have been volcanic glasses which altered after deposition, together with the fine volcanic detritus of the underlying Bushy Basin Member, to supply uranium and produce a chemical environment for the formation of the deposits. Mineral assemblages associated with the deposits and believed to have formed with the ore lenses include quartz overgrowths, albitized and skeletalized feldspars, and illite-montmorillonite rims on clasts. Feldspar-destructive alteration, apparently related to the post-Jurassic unconformity and the Cretaceous marginal marine Dakota Formation which overlies the Jackpile sandstone, appears to have destroyed both the ore lenses and some of their associated mineral assemblages. The ore deposits, therefore, probably formed prior to the deposition of the Cretaceous rocks during the early postdepositional alteration of the volcanic-rich fluvial sediments.

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