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Title: Stratigraphy of the PB-1 well, Nopal I uranium deposit, Sierra Pena Blanca, Chihuahua, Mexico

Abstract

The Nopal I site in the Pena 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 encounteredmore » 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 {approx}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 Pena Blanca natural analogue.« less

Authors:
; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
Sponsoring Org.:
Earth Sciences Division
OSTI Identifier:
944983
Report Number(s):
LBNL-914E
Journal ID: ISSN 0020-6814; IGREAP; TRN: US0900747
DOE Contract Number:  
DE-AC02-05CH11231
Resource Type:
Journal Article
Journal Name:
International Geology Review
Additional Journal Information:
Journal Name: International Geology Review; Journal ID: ISSN 0020-6814
Country of Publication:
United States
Language:
English
Subject:
54; 58; FLUID FLOW; HIGH-LEVEL RADIOACTIVE WASTES; LIMESTONE; MONTMORILLONITE; NATURAL ANALOGUE; RADIOACTIVITY; RADIOISOTOPES; STRATIGRAPHY; TRANSPORT; TUFF; URANIUM; URANIUM DEPOSITS; VOLCANIC ROCKS; WATER TABLES; YUCCA MOUNTAIN

Citation Formats

Dobson, P, Fayek, M, Goodell, P, Ghezzehei, T, Melchor, F, Murrell, M, Oliver, R, Reyes-Cortes, I A, de la Garza, R, and Simmons, A. Stratigraphy of the PB-1 well, Nopal I uranium deposit, Sierra Pena Blanca, Chihuahua, Mexico. United States: N. p., 2008. Web. doi:10.2747/0020-6814.50.11.959.
Dobson, P, Fayek, M, Goodell, P, Ghezzehei, T, Melchor, F, Murrell, M, Oliver, R, Reyes-Cortes, I A, de la Garza, R, & Simmons, A. Stratigraphy of the PB-1 well, Nopal I uranium deposit, Sierra Pena Blanca, Chihuahua, Mexico. United States. doi:10.2747/0020-6814.50.11.959.
Dobson, P, Fayek, M, Goodell, P, Ghezzehei, T, Melchor, F, Murrell, M, Oliver, R, Reyes-Cortes, I A, de la Garza, R, and Simmons, A. Fri . "Stratigraphy of the PB-1 well, Nopal I uranium deposit, Sierra Pena Blanca, Chihuahua, Mexico". United States. doi:10.2747/0020-6814.50.11.959. https://www.osti.gov/servlets/purl/944983.
@article{osti_944983,
title = {Stratigraphy of the PB-1 well, Nopal I uranium deposit, Sierra Pena Blanca, Chihuahua, Mexico},
author = {Dobson, P and Fayek, M and Goodell, P and Ghezzehei, T and Melchor, F and Murrell, M and Oliver, R and Reyes-Cortes, I A and de la Garza, R and Simmons, A},
abstractNote = {The Nopal I site in the Pena 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 {approx}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 Pena Blanca natural analogue.},
doi = {10.2747/0020-6814.50.11.959},
journal = {International Geology Review},
issn = {0020-6814},
number = ,
volume = ,
place = {United States},
year = {2008},
month = {8}
}