Paragenesis and Geochronology of the Nopal I Uranium Deposit, Mexico - 1999

Uranium (Nuclear Minerals) / Exploration, Mining, Marketing, & Acquisitions

Fayek, and 9 co-authors, reported in 1999 that uranium deposits can, by analogy, provide important information on the long-term performance of radioactive waste forms and radioactive waste repositories. Their complex mineralogy and variable elemental and isotopic compositions can provide important information, provided that analyses are obtained on the scale of several micrometers. Here, the authors present a structural model of the Nopal I deposit as well as petrography at the nanoscale coupled with preliminary U-Th-Pb ages and oxygen isotopic compositions of uranium-rich minerals obtained by Secondary Ion Mass Spectrometry (SIMS). This multi-technique approach promises to provide "natural system" data on the corrosion rate of
uraninite, the natural analogue of spent nuclear fuel.

Previous depositional models of the Nopal I deposit [4, 141, which were based on chemical Pb ages and U-Th ages suggested that deposit formation and alteration occurred as follows: (a) initial uraninite precipitation 8±5Ma, (b) alteration to U⁶⁺ phases such as schoepite and weeksite/boltwoodite, (c) a major oxidation event 3.2-3.4 Ma precipitating uranophane, and (d) precipitation of uraniferous Fe-oxyhydroxides (<300 ka), and opal and
calcite (54 ka).

The authors concluded that the localization of the alteration, mineralization, and brecciation of the Nopal I deposit. Weathering resulted in the formation of secondary U⁶⁺ minerals along fractures, the precipitation of clay minerals, and the development of oxidation zones. Subsequent fault movement along the southwestern margin of the deposit increased brecciation and enhanced permeability, resulting in increased oxidation. U-Th-Pb isotopic ages show that initial uraninite precipitation occurred at 32*8 Ma. A major oxidation event characterized by the precipitation of widespread uranophane did occur at ~3.1 Ma. This event was followed by a reducing event that precipitated a second generation of uraninite at ~1.6 Ma. Subsequently, the deposit was subjected to continued oxidation resulting in the precipitation of schoepite (~85 ka) and weeksite/boltwoodite (~41 ka).

The relationship between uraninite from the Pozos conglomerate and the ~1.6 Ma colloform uraninite from the main ore body is unclear. However, association between the young uraninite and anatase suggests that the Pozos conglomerate formed a natural barrier that sequestered uranium before it reached the water table.