Abstract

Recent reconnaissance mapping and sampling of some old uranium-copper prospects in the Eocene Uinta Formation have shown they have a unique metal assemblage of strongly anomalous Mo, Cd, Cu, Ag, Sb, As, U, and Re. The mineralization is spatially associated with the Duchesne fault zone and gilsonite veins of the western (Pariette and Ouray) vein systems. The Uinta Basin polymetallic deposits are zoned on a regional scale from more copper- and molybdenum-rich deposits west of the Green River to more uraniferous deposits on the east. Mineralization is hosted by cross-bedded or laminated, arkosic sandstone and is similar in character to both sediment-hosted stratiform copper deposits and tabular sandstone uranium deposits. Mineralization is interstitial, disseminated, and associated with bleaching, decalcification, carbonized plant fossils, and abundant iron-oxide nodules. The Uinta Basin mineralization is unique in that much of the total dollar value of the ore is frequently in molybdenum, running to over 1% Mo. The molybdenum occurs at the surface as a black, unknown Cu-Zn-Mo oxide mineral and ferrimolybdite.

A two-stage fluid model is proposed for the Uinta Basin, sediment-hosted, polymetallic deposits where an early reducing fluid is expelled from the saline facies of the Green River Formation followed by a later mineralizing fluid, generated by the red-bed strata of the Wasatch, Colton, and North Horn formations. These overpressured, sodic, diagenetic fluids were expelled from the deepest part of the basin (north), when they hydraulically fractured the overlying confining sedimentary rocks and allowed the flushing of large quantities of water upward along the pre-gilsonite structures and outward along sandstone aquifers. Early fluids reduced (bleached) the sandstone, turning the diagenetic iron-oxides to pyrite. Later mineralizing fluids were reduced by the carbonaceous detritus and pyrite in the sandstone, depositing metals, and forming the polymetallic mineralized zones.