2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 151-7
Presentation Time: 3:20 PM


HILL, Scott J., Geology and Geophysics, University of Utah, 115 S. 1460 E., Rm. 383, Salt Lake City, UT 84112-0102, PETERSEN, Erich U., Geology and Geophysics, University of Utah, 115 S. 1460 E. Rm. 383, Salt Lake City, UT 84112 and CHAN, Marjorie A., Department of Geology and Geophysics, University of Utah, 115 S 1460 E, Room 383 FASB, Salt Lake City, UT 84112, techsource@msn.com

Core logging and petrographic analysis of braided stream intervals in the Salt Wash Member of the Jurassic Morrison Formation from the Sahara Mine reveal thin, interbedded shales containing altered volcanic ash, humic matter and highly uraniferous, immature coal. Adjacent sands contain relict humic matter and terrestrial plant fragments, both uraniferous, in repeating, horizontal striations called ”horsetail ore”.

Framboidal and cubic pyrite are enriched in horsetail ore to 50% of rock volume, replacing organic matrix. Coffinite and uraninite intermix with sulfides. Rauvite occurs with relict humic matter. Detrital magnetite is absent or etched in ore zones, removing iron and leaving a titanium-rich lattice-work. Brown bottom shales have a reduced green halo bordering channel sands. Calcite cement is depleted in horsetail ore but highly enriched in adjacent sands with up to 50% authigenic calcite. Orange vanadium oxide rinds occur above and below dense horsetails.

Core magnetic susceptibility measurements show anomalous lows around horsetails. Uranium in humic matter is slightly enriched in U235. Sulfur isotope data for organic origins and vitrinite reflectance and fluid inclusion work for maximum aquifer temperature are pending.

These findings point to humic-acid rich source fluid from swamps leaching uranium and other metals from volcanic ash. The source fluid fell into adjacent sand aquifers, creating acidic and reducing conditions and trapping humic-acid molecules in fine-grained clastics.

Later, compacting evaporites below the Salt Wash released sulfate-rich mineralizing fluid into the aquifer, setting up a two-fluid interface. Ca++ and SO4 - - diffused into the source fluid. Ca++ flocculated humic-acid. Anaerobic bacteria reduced sulfate, precipitating sulfides and coffinite, slowly oxidizing the source fluid and self-limiting the process.

Subsequent burial heat warmed humate, caused humate carboxylation, and released CO2 and metal ions. Uraninite and rauvite precipitated. Carbonate cement built up and vanadium oxide rinds formed as Ca++, CO2, and V dispersed away from sites of humate alteration.

Close affinities with humate-rich uranium deposits (i.e. Grants District, New Mexico) are seen with possible mobile uranium and down-dip roll front deposits expanding exploration scenarios.