Abstract: Recognition of Oxidized Sulfide Minerals as Exploration Guide for Uranium
Richard L. Reynolds, Martin B. Goldhaber
The color difference between tan to red oxidized sandstone and gray reduced sandstone on either side of the redox interface--the locus of uranium concentration in roll-type deposits--has been used locally as an exploration guide within known uranium districts. Reduced sandstone in front of the roll is characterized in many deposits by the presence of sulfide minerals (particularly marcasite and pyrite) that occur as replacements of and overgrowths on iron-titanium oxide minerals and plant debris and as cement of detrital grains. Oxidation of the sulfides by oxygenated groundwater forms yellow to red ferric oxide and ferric hydroxide phases ("limonite") that replace the FeS2 minerals. Processes other than the oxidation of sulfides, however, can cause reddening o clastic rocks without the creation of a redox boundary, and so color alone is not a sufficient condition by which to judge the potential for uranium ore in frontier areas. Ferric oxides that form by the oxidation of sulfides can be distinguished from other forms of ferric oxide by reflection microscopic examination of polished grain mounts and polished thin sections. Diagnostic features of oxidized sulfides are (1) limonite pseudomorphs of sulfides, and (2) limonite containing internal textures that reflect characteristic textures of previously sulfidized detrital phases. Recognition of oxidized sulfides by reflection microscopy, then, can distinguish oxidized sandstones which might have a redox boundary downdip and therefore would be a favorable host for uranium concentration from those without such potential.
AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado