Hydrothermal Phase Relations Among Uranyl Minerals at the Nopal I Analog Site
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Hydrothermal Phase Relations Among Uranyl Minerals at the Nopal I Analog Site William M. Murphy Geological and Environmental Sciences, California State University, Chico, CA, 95929 ABSTRACT Uranyl mineral paragenesis at Nopal I is an analog of spent fuel alteration at Yucca Mountain. Petrographic studies suggest a variety of possible hydrothermal conditions for uranium mineralization at Nopal I. Calculated equilibrium phase relations among uranyl minerals show uranophane stability over a broad range of realistic conditions and indicate that uranyl mineral variety reflects persistent chemical potential heterogeneity. INTRODUCTION Studies of the geochemical alteration of primary uraninite (an analog of spent nuclear fuel) and formation of secondary uranyl minerals at the Nopal I natural analog site contribute to understanding the source term in performance assessment models for the high level nuclear waste repository at Yucca Mountain [e.g., 1,2,3,4]. Field and petrographic studies yield a variety of interpretations of the conditions of uranium mineral formation at Nopal I. Characterizing the thermodynamic stability of uranyl minerals as a function of temperature aids evaluations of the strengths and limits of the analogy between Nopal I and the Yucca Mountain repository. Finch [5] examined phase relations among uranyl minerals at 25°C in the context of natural occurrences. Murphy [6] examined the significance of the retrograde temperature dependence of uranyl mineral solubilities in Yucca Mountain source term analyses. Murphy et al. [7] initiated studies of hydrothermal uranyl mineral phase relations at Nopal I. This article updates and expands these studies presenting original thermodynamic interpretations of uranyl mineral stabilities under hydrothermal conditions relevant to Nopal I and to Yucca Mountain. FIELD AND PETROGRAPHIC DATA FOR URANIUM MINERALIZATION Calas [8] concluded that deposition of uraninite at Nopal I coincided with hydrothermal alteration at low temperatures indicated by montmorillonite occurrences. Rising solutions encountered low permeability ignimbrite and deposited uraninite (pitchblende). Hydrothermal alteration of glass and feldspars released silica, which encased pitchblende and prevented its oxidation. Subsequent uranyl mineralization of underlying tuffs resulted from leaching pitchblende by meteoritic water [8]. Calas inferred that pyrite and vein calcite were derived from Cretaceous shale and limestone below the Tertiary volcanic strata. Goodell [9] characterized the uranium deposits at Nopal I as epithermal and precluded a magmatic-hydrothermal origin noting the absence of intrusive rocks. Uranium was derived from the alteration and leaching of volcanic glass by geothermal convective groundwater systems, and mineral deposition and rock alteration were instigated by reducing waters from organic- and pyrite?-rich Tertiary basin sediments [9]. George-Aniel et al. [10] divided mineralization at Nopal I in stages. Initially uraninite precipitated in microcracks inside volcanic
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