Final report of the Peña Blanca natural analogue project

Abstract

Sulfides formerly existed throughout the stratigraphic section penetrated by borehole PB1. The shallowest surviving sulfides include pyrite associated with uraninite (not intersected by PB1) and protected by silicification in the Nopal Formation and sulfides primarily within lithic inclusions in the Coloradas Formation. Sulfides are common only below a depth of about 180 m in the Pozos conglomerate and Cretaceous limestone. Pseudomorphous iron oxide-oxyhydroxide replacements of sulfides are present in the Nopal and Coloradas Formations. The downward transition from rare to common sulfide preservation at the 180-m depth in the Pozos conglomerate may mark the stable position of a former water table. The depth also corresponds to a downward transition to lowerpermeability rocks. Both factors could have restricted the interaction of rock sulfides with oxygenated water in what is now the deep unsaturated zone. URANIUM SERIES GEOCHEMISTRY OF NOPAL I GROUNDWATERS Groundwaters at Nopal I and nearby regional wells were analyzed for uranium-series disequilibria. Unsaturated-zone seepage waters were collected between 2000 and late 2006. The waters were analyzed for chemical and isotopic suites including total uranium, 234 U/ 238 U, total thorium, 230 Th, 226 Ra, and 239 Pu. Seepage waters from the front of the +00 m adit, where the ore body is located, generally have 234 U/ 238 U activity ratios near unity (0.9 to 1.5) and uranium concentrations between 0.7 and 422 ppb. These attributes result from dissolution of rock with a preponderance of uranium in secular equilibrium, as well as longer water-rock interaction times and interaction of water with uranium-rich rock. Analyses of high-grade uranium ore from Nopal I confirm that uranium-series radionuclides high in the decay chain are close to secular equilibrium. Waters from the rear of the adit, away from the ore body, have higher 234 U/ 238 U activity ratios ranging from 2.6 to 5.2 and mostly lower uranium concentrations (0.1 to 14.9 ppb). These differences reflect preferential dissolution of recoil-related 234 U and uranium concentrations affected by varying fluid flux and/or water-rock interaction times. Seepage-water samples from the middle of the adit have 234 U/ 238 U activity ratios and uranium concentrations overlapping the values for the front and back of the adit. Stable-isotope data for seepage samples show evidence of evaporation during the dry season that would have increased the concentrations of solutions. The effects on uranium-isotopic systematics are described below. Uranium concentrations and isotopic ratios were measured for shallow saturated-zone waters from Nopal I and regional wells. For most wells, samples were collected from 2003 to 2006. Uranium concentrations in the Nopal wells have decreased over time as the local effects of drilling contamination have diminished. During the last year of sampling (2005-2006), concentrations were between 23 and 61 ppb in PB1 and between 370 and 1077 ppb in PB3. Concentrations in the smaller sample set for PB2 are ...