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	The Sierra Peña Blanca of northern Mexico is a geologic terrain in the Basin and Range province of western North America.

Geology and Stratigraphy

The Sierra Peña Blanca of northern Mexico is a geologic terrain in the Basin and Range province of western North America with striking physical similarities to the proposed Yucca Mountain, Nevada high-level nuclear waste (HLW) geologic repository system. Both geologic terrains are large rotational fault blocks composed of silicic volcanic rocks of Tertiary age.

 

Map of geology and mines to the north of Chihuahua City. The Sierra Peña Blanca is at the top center of the map. Map source: Plano Geologica Minero, Chihuahua, Mexico, Edicion 1985, Sociedad Geologica Mexicana A.C., Delegacion Chihuahua. Scale is 1:500,000.

The Sierra Peña Blanca is a west-dipping horst block with a superimposed set of parallel northwest-striking normal (extensional) faults (Goodell, 1981). The regional structural province is bounded to the east by the Trans-Pecos Range and to the west by the Sierra Madre Occidental (George-Aniel et al., 1985).

Peña Blanca stratigraphy consists of a sequence of Cretaceous limestones and mudstones on which a series of Tertiary silicic volcanics has been deposited (Stege et al., 1981). The preserved total thickness of the volcanic units varies over the Sierra Peña Blanca area from 106 to 538 m, and rock ages range from 44 million years (Ma) to 35 Ma (Alba and Chavez, 1974). Host rocks for most of the uranium deposits in the district are the Escuadra, Nopal, and Coloradas Formations which are composed of variably welded tuffs with air-fall, ignimbrite, vitrophyre, lahar, and water-worked units.

(modified from Magonthier, 1984) Stratigraphic column of the Peña Blanca volcanic sequence after Magonthier (1984) and Cardenas-Flores (1985). Ages are from Alba and Chavez (1974). Tc = Corrales tuff, Tn1 = Coloradas tuff, Tn2 = Nopal rhyolite, Te1 = Escuadra tuff of the 44 Ma stage, and Te2 = Escuadra tuff of the 38.3 Ma stage.		View of the northern end of the Peña Blanca district looking southwest. The cliff former is the Mesa Formation - the youngest of the Tertiary age volcanics deposited in the region. The white-colored unit below the Mesa Formation is the Peña Blanca Formation -- a less resistant volcanic tuff.
View of the Nopal I mining camp looking to the north. Outcrops of the Escuadra Formation, a pinkish- to tan-colored rhyolitic unit are shown in the background.		View of the Nopal I mining camp looking to the southeast.  Questas formed by the Nopal Formation are shown in the background.

Uranium Deposits

The Sierra Peña Blanca hosts the Peña Blanca mining district -- one of the most important uranium ore reserves in Mexico. The Peña Blanca mining district is located about 50 km north of Chihuahua City. Many of the uranium deposits in the Peña Blanca district were drilled and/or developed by underground or open pit mining over a period of 20 years; however, there has been no mining activity at Peña Blanca since 1983. Combined, the Peña Blanca deposits include more than 2000 metric tons of U3O8 and constitute the bulk of Mexican uranium reserves (Goodell, 1981).
		View of the Margaritas mining area looking to the northwest. A mill for processing uranium ore from several deposits in the area (Margaritas, Nopal I, and Puerto I) was under construction near the Margaritas mining camp when mining activity ceased in the early 1980's.
Photos of uranium ore piles located near the Margaritas mining camp. Ore from several deposits including Margaritas, Nopal I, and Puerto I is stockpiled here.

The Nopal I Uranium Deposit

Among the several uranium deposits in the Peña Blanca district, the Nopal I deposit is the most studied analog relevant to an HLW repository at Yucca Mountain. Information on the geology, mineralogy, and geochemistry of Nopal I is available from previous research at the deposit (e.g., Cardenas-Flores, 1985; George-Aniel et al., 1985; Goodell, 1981; Ildefonse et al., 1990; Ildefonse et al., 1991; Leroy et al., 1987).

Analogous aspects of the Nopal I deposit include the silicic, tuffaceous host rocks, the semi-arid climate, the unsaturated hydrologic regime, and the presence of reduced and oxidized uranium mineralization.

View of the Nopal I deposit looking to the south. Uranium mineralization is exposed on excavated benches at the +00 and +10 m levels. The bench on which people are standing is the +10 m level.		Surface of the Nopal I deposit looking toward the southeast after clearing of rubble and debris.

The Nopal I deposit consists of a near vertical breccia pipe some 40 m in diameter which extends over a vertical interval of at least 100 m. Nopal I is hosted by tuffaceous rocks of the Nopal and Coloradas Formations, which are separated by a basal argillaceous zone, interpreted to be a vitrophyre. At depth the Nopal I orebody extends into the Pozos Formation which is largely a limestone conglomerate that formed on the surface of the Cretaceous limestone. The lowest level of the deposit is about 50 m above the local water table.

Geologic cross-sections of the Nopal I uranium deposit (after Pearcy et al., 1994)

The Nopal Formation is a rhyolitic tuff, densely welded and hematitic in unaltered zones. The Coloradas Formation is a densely welded, reddish-colored, lithic-crystal rhyolitic tuff. Removal of hematite and alteration of phenocrysts to clays and calcite are characteristic of Nopal and Coloradas tuffs altered by hydrothermal solutions associated with the ore body. Comparison of the chemical composition (major oxide abundances) between the Nopal and Coloradas Formations with that of the Topopah Spring Member of the Paintbrush Tuff Formation (the proposed repository horizon at Yucca Mountain) illustrates chemical similarity between the units.

Chemical compositions (values in weight percent)

The uranium mineralization at Nopal I is presently exposed on broad excavated benches at the +00 and +10 m levels, which together provide surface access to a horizontal section across the entire width of the deposit. Unmaintained shafts and adits also penetrate the deposit.
		Topographic map of the Nopal I uranium deposit showing area of excavated ground surface cleared of loose rock and debris, the area of U mineralization, and the outline of the +00 m adit below the +10 m level.

Adits at the +00 and -40 m levels provided access to the interior of the orebody. The breccia pipe contains high grade uranium mineralization in the form of uraninite, uranyl oxide hydrates, and uranyl silicates.

The Nopal I orebody is exposed along the wall separating the +00 and +10 m levels. The interior of the orebody is accessible at the +00 m level through an exploration tunnel.		Uranium ore from Nopal I showing oxidation of primary uraninite (black material) to yellow-colored uranyl minerals. Scale is in centimeters

	Yellow uranyl silicate minerals on the exposed surface of the Nopal I deposit. Scale is about 1 m across.

A variety of past and present processes at Nopal I are analogous to those that could affect radionuclide migration at Yucca Mountain. Processes governing the oxidation of uraninite, the resulting dispersion of uranium from the sites of original mineralization, and the formation of secondary uranyl silicates are analogous to processes that may affect spent nuclear fuel and elemental migration at the proposed repository.

References

Alba, L.A., and R. Chavez, 1974, K-Ar ages of volcanic rocks from the central Sierra Peña Blanca, Chihuahua, Mexico, Isochron West, Vol. 10, p. 21-23.

Broxton, D.E., R.G. Warren, R.C. Hagen, and G. Luedemann, 1986, Chemistry of diagenetically altered tuffs at a potential nuclear waste repository, Yucca Mountain, Nye County, Nevada, Lawrence Livermore National Laboratories (LANL), LA-10822-MS.

Cardenas-Flores, D.F., 1985, Volcanic stratigraphy and U-Mo mineralization of the Sierra de Peña Blanca district, Chihuahua, Mexico, in Uranium Deposits in Volcanic Rocks, IAEA, eds., International Atomic Energy Agency, IAEA-TC-490/31, p. 125-136.

George-Aniel, B., J. Leroy, and B. Poty, 1985, Uranium deposits of the Sierra Peña Blanca: three examples of mechanisms of ore deposit formation in a volcanic environment, in Uranium Deposits in Volcanic Rocks, IAEA, eds., International Atomic Energy Agency, IAEA-TC-490/31, p. 175-186.

Goodell, P.C., 1981, Geology of the Peña Blanca uranium deposits, Chihuahua, Mexico, in Uranium in Volcanic and Volcanoclastic Rocks - AAPG Studies in Geology No. 13, P.C. Goodell and A.C. Waters, eds., American Association of Petroleum Geologists, p. 275-291.

Ildefonse, P., J.P. Muller, B. Clozel, and G. Calas, 1991, Record of past contact between altered rocks and radioactive solutions through radiation-induced defects in kaolinite, in Scientific Basis for Nuclear Waste Management, T. Abrajano, Jr. and L.H. Johnson, eds., Materials Research Society, Symposium Proceedings 212, p. 749-755.

Ildefonse, P., J.-P. Muller, B. Clozel, and G. Calas, 1990, Study of two alteration systems as natural analogues for radionuclide release and migration, Engineering Geology, Vol. 29, p. 413-439.

Leroy, J.L., B. Aniel, and B. Poty, 1987, The Sierra Peña Blanca (Mexico) and the Meseta Los Frailes (Bolivia): The uranium concentration mechanisms in volcanic environment during hydrothermal processes, Uranium, Vol. 3, p. 211-234.

Magonthier, M.C., 1984, Les ignimbrites de la Sierra Madre Occidental et de la province uranifére de la Sierra Peña Blanca, Mexique, Mèm. Sci. Terre, Univ. P. et M. Curie, Paris, Vol. 84-17, 351 p.

Pearcy, E.C., J.D. Prikryl, W.M. Murphy, and B.W. Leslie, 1994, Alteration of uraninite from the Nopal I deposit, Peña Blanca district, Chihuahua, Mexico, compared to degradation of spent nuclear fuel in the proposed US high-level nuclear waste repository at Yucca Mountain, Nevada, Applied Geochemistry, Vol. 9, p. 713-732.

Stege, B., N.E. Pingitore, P.C. Goodell, and D.V. LeMone, 1981, Limestone bedrock as a barrier to uranium migration, Sierra Peña Blanca, Chihuahua, Mexico, in Uranium in Volcanic and Volcanoclastic Rocks - AAPG Studies in Geology No. 13, P.C. Goodell and A.C. Waters, eds., American Association of Petroleum Geologist, p. 265-274.

For more information about the Peña Blanca Natural Analog Project at SwRI or how you can contract with SwRI, please contact James D. Prikryl at jprikryl@swri.org, or call (210) 522-5667.
 

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September 16, 2009