CHEMICAL AND U-SERIES ISOTOPE VARIATIONS IN FELSIC TUFFS AT YUCCA MOUNTAIN, NEVADA, AS INDICATORS OF PAST WATER FLOW THROUGH THE UNSATURATED ZONE

Water percolating through the 500-m-thick unsaturated zone (UZ) at Yucca Mountain resulted in variable chemical modification and U-series radioactive disequilibrium in the 12.8-Ma felsic tuffs. Some block-bounding faults may be preferential pathways, as evidenced by bleaching, iron staining, and alteration to clays. In contrast, chemical and mineralogical alteration of the tuffs elsewhere is minimal.  U-series isotopes are useful for investigating Pleistocene water/rock interaction because water preferentially mobilizes U relative to less soluble Th and ²³⁴U relative to ²³⁸U via alpha-recoil-related processes, but rocks disturbed > 1 Ma will return to radioactive equilibrium. U, Th, ²³⁴U/²³⁸U, and ²³⁰Th/²³⁸U were determined in two sets of bulk-rock samples: (1) core samples from depths of 16 to 300 m in borehole USW SD-9 to evaluate changes in flow along a vertical profile in the UZ, and (2) tunnel-wall samples from a lateral transect across the Solitario Canyon fault at depths of 200 to 370 m to investigate the possibility of greater flow along major faults.  In addition, bulk-rock and leachable-ion geochemical analyses were conducted on the second set of samples.

Thirty bulk-rock samples forming the USW SD-9 vertical profile show systematic variations in U and Th concentrations reflecting lithostratigraphic compositional differences, but only small amounts of ²³⁸U-²³⁴U-²³⁰Th radioactive disequilibrium that are not associated with lithostratigraphic variations.  ²³⁴U/²³⁸U activity ratios (AR) of 0.94 to 1.01 and ²³⁰Th/²³⁸U AR of 0.93 to 1.10 reflect Pleistocene water/rock interaction in otherwise unaltered tuff.  Most analyses have ²³⁴U/²³⁰Th AR <1, which indicates U removal. 

Fifteen tunnel-wall samples show systematic chemical and mineralogical changes with distance from the Solitario Canyon fault; however, ²³⁴U/²³⁸U and ²³⁰Th/²³⁸U AR are not significantly different from those in the USW SD-9 profile. These analyses commonly have ²³⁴U/²³⁰Th AR >1 and ²³⁴U/²³⁸U<1, indicating that U mobility included both U gains and ²³⁴U losses.  The small amounts of radioactive disequilibrium in highly tectonized fault rock do not support a Pleistocene age for the observed rock alteration or the presence of substantially greater water fluxes at depth in this fault zone during Pleistocene time.