HR: 08:00h
AN: GS22A-01 INVITED     [Abstracts]
TI: The Nopal 1 Uranium Deposit: an Overview
AU: * Calas, G
EM: calas@impmc.jussieu.fr
AF: Institut of Mineralogy and Physics of Condensed Matter, 140 rue de Lourmel, Paris, 75015, France
AU: Allard, T
EM: Allard@impmc.jussieu.fr
AF: Institut of Mineralogy and Physics of Condensed Matter, 140 rue de Lourmel, Paris, 75015, France
AU: Galoisy, L
EM: galoisy@impmc.jussieu.fr
AF: Institut of Mineralogy and Physics of Condensed Matter, 140 rue de Lourmel, Paris, 75015, France
AB: The Nopal 1 natural analogue is located in the Pena Blanca uranium district, about 50 kms north of Chihuahua City, Mexico. The deposit is hosted in tertiary ignimbritic ash-flow tuffs, dated at 44 Ma (Nopal and Colorados formations), and overlying the Pozos conglomerate formation and a sequence of Cretaceous carbonate rocks. The deposit is exposed at the ground surface and consists of a near vertical zone extending over about 100 m with a diameter of 40 m. An interesting characteristic is that the primary mineralization has been exposed above the water table, as a result of the uplift of the Sierra Pena Blanca, and subsequently oxidized with a remobilization of hexavalent uranium. The primary mineralization has been explained by various genetic models. It is associated to an extensive hydrothermal alteration of the volcanic tuffs, locally associated to pyrite and preserved by an intense silicification. Several kaolinite parageneses occur in fissure fillings and feldspar pseudomorphs, within the mineralized breccia pipe and the barren surrounding rhyolitic tuffs. Smectites are mainly developed in the underlying weakly welded tuffs. Several radiation-induced defect centers have been found in these kaolinites providing a unique picture of the dynamics of uranium mobilization (see Allard et al., this session). Another evidence of this mobilization is given by the spectroscopy of uranium-bearing opals, which show characteristic fluorescence spectra of uranyl groups sorbed at the surface of silica. By comparison with the other uranium deposits of the Sierra Pena Blanca and the nearby Sierra de Gomez, the Nopal 1 deposit is original, as it is one of the few deposits hving retained a reduced uranium mineralization.
DE: 3617 Alteration and weathering processes (1039)
DE: 3665 Mineral occurrences and deposits
DE: 3904 Defects
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 8404 Volcanoclastic deposits
SC: Geochemical Society [GS]
MN: 2007 Joint Assembly