Stratigraphy of the PB-1 Well, Nopal I Uranium Deposit, Sierra Peña Blanca, Chihuahua, Mexico

Abstract

The Nopal I site in the Peña Blanca uranium district has a number of geologic and hydrologic similarities to the proposed high-level radioactive waste repository at Yucca Mountain, making it a useful analogue to evaluate process models for radionuclide transport. The PB-1 well was drilled in 2003 at the Nopal I uranium deposit as part of a DOE-sponsored natural analogue study to constrain processes affecting radionuclide transport. The well penetrates through the Tertiary volcanic section down to Cretaceous limestone and intersects the regional aquifer system. The well, drilled along the margin of the Nopal I ore body, was continuously cored to a depth of 250 m, thus providing an opportunity to document the local stratigraphy. Detailed observations of these units were afforded through petrographic description and rock-property measurements of the core, together with geophysical logs of the well. The uppermost unit encountered in the PB-1 well is the Nopal Formation, a densely welded, crystal-rich, rhyolitic ash-flow tuff. This cored section is highly altered and devitrified, with kaolinite, quartz, chlorite, and montmorillonite replacing feldspars and much of the groundmass. Breccia zones within the tuff contain fracture fillings of hematite, limonite, goethite, jarosite, and opal. A zone of intense clay alteration encountered in the depth interval 17.45-22.30 m was interpreted to represent the basal vitrophyre of this unit. Underlying the Nopal Formation is the Coloradas Formation, which consists of a welded lithic-rich rhyolitic ash-flow tuff. The cored section of this unit has undergone devitrification and oxidation, and has a similar alteration mineralogy to that observed in the Nopal tuff. A sharp contact between the Coloradas tuff and the underlying Pozos Formation was observed at a depth of 136.38 m. The Pozos Formation consists of poorly sorted conglomerate containing clasts of subangular to subrounded fragments of volcanic rocks, limestone, and chert. Three thin (2-6 m) intervals of intercalated pumiceous tuffs were observed within this unit. The contact between the Pozos Formation and the underlying Cretaceous limestone basement was observed at a depth of 244.40 m. The water table is located at a depth of ~223 m. Several zones with elevated radioactivity in the PB-1 core are located above the current water table. These zones may be associated with changes in redox conditions that could have resulted in the precipitation of uraninite from downward flowing waters transporting U from the overlying Nopal deposit. All of the intersected units have low (typically submillidarcy) matrix permeability, thus fluid flow in this area is dominated by fracture flow. These stratigraphic and rock-property observations can be used to constrain flow and transport models for the Peña Blanca natural analogue.

Uranium-Series Constraints on Radionuclide Transport and Groundwater Flow at the Nopal I Uranium Deposit, Sierra Peña Blanca, Mexico

“…1-2). SZ intervals of the PB1, PB2, and PB3 wells, located immediately adjacent to the Nopal I ore deposit, occur mostly within Pozos conglomerate, with the lowest section of each well in Cretaceous limestone (6). SZ intervals for two wells located SE of the Nopal I deposit, PB4 and Pozos Ranch, are totally within Cretaceous limestone.…”

“…The mine adit is located at the +00 level of the deposit near PB1. UZ rocks near the mine adit consist of rhyolitic welded ash-flow tuffs (Nopal and Coloradas Formations) that host the ore deposit (6). Groundwater (Fig.…”

“…After tracer equilibration, uranium was purified using anion exchange columns and loaded onto single or triple Re filaments (7). Uranium isotopes ( 238 U, 235 U, 234 U) were measured multi-statically using a GV Sector 54 mass 6 spectrometer equipped with multiple Faraday cups and a Daly ion counting detector. Calibration between Daly and Faraday detectors was determined by switching the 235 U beam between these collectors, and instrumental mass fractionation was corrected by normalization to the natural 238 U/ 235 U ratio of 137.88.…”

Heterogeneous seepage at the Nopal I natural analogue site, Chihuahua, Mexico

“…Three wells were drilled in 2003 around the Nopal I deposit down to the water table to provide stratigraphic information, determine the water table depth, and permit sampling of the saturated zone water (Dobson et al, 2008). The water table is located at a depth of ~220 m below the +10 m bench, thus indicating that the Nopal I uranium orebody is located entirely within the vadose zone.…”

Spatial analysis techniques applied to uranium prospecting in Chihuahua State, Mexico

“…The exploration and exploitation of uranium ores in Chihuahua, Mexico, took place in the decades of [1970][1971][1972][1973][1974][1975][1976][1977][1978][1979][1980], by the enterprise Uranio de Mexico (URAMEX). Although there has not been commercial production of uranium in Mexico, in Chihuahua State there are several uranium outcrops and radioactive anomalies, which led Chihuahua to be known as the "uranium province" [1,2,3,4,5,6,7,8,9].…”