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Structural and biological control of the Cenozoic epithermal uranium concentrations from the Sierra Pe?a Blanca, Mexico |
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| Authors: | Samuel Angiboust Mostafa Fayek Ian M Power Alfredo Camacho Georges Calas Gordon Southam |
| Institution: | 1. Institut des Sciences de la Terre de Paris, UMR 7193 CNRS, Université Pierre et Marie Curie-Paris 6, 4 Place Jussieu, 75252, Paris, France 2. Department of Geological Sciences, University of Manitoba, 240 Wallace Building, 125 Dysart Road, Winnipeg, MB, R3T 2N2, Canada 3. Department of Earth Sciences, University of Western Ontario, London, ON, N6A 5B7, Canada 4. Institut de Minéralogie et de Physique des Milieux Condensés, Université Pierre et Marie Curie-Paris 6, UMR CNRS 7590, 4 Place Jussieu, 75252, Paris, France |
| Abstract: | Epithermal uranium deposits of the Sierra Pe?a Blanca are classic examples of volcanic-hosted deposits and have been used as natural analogs for radionuclide migration in volcanic settings. We present a new genetic model that incorporates both geochemical and tectonic features of these deposits, including one of the few documented cases of a geochemical signature of biogenic reducing conditions favoring uranium mineralization in an epithermal deposit. Four tectono-magmatic faulting events affected the volcanic pile. Uranium occurrences are associated with breccia zones at the intersection of fault systems. Periodic reactivation of these structures associated with Basin and Range and Rio Grande tectonic events resulted in the mobilization of U and other elements by meteoric fluids heated by geothermal activity. Focused along breccia zones, these fluids precipitated under reducing conditions several generations of pyrite and uraninite together with kaolinite. Oxygen isotopic data indicate a low formation temperature of uraninite, 45–55°C for the uraninite from the ore body and ～20°C for late uraninite hosted by the underlying conglomerate. There is geochemical evidence for biological activity being at the origin of these reducing conditions, as shown by low δ34S values (～?24.5‰) in pyrites and the presence of low δ13C (～?24‰) values in microbial patches intimately associated with uraninite. These data show that tectonic activity coupled with microbial activity can play a major role in the formation of epithermal uranium deposits in unusual near-surface environments. |
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