Origin of Uranium Mineralization at Coles Hill Virginia (USA) and its Natural Attenuation within an Oxidizing Rock-Soil-Ground Water System

Development of a scientific basis for management of uranium bearing wastes and contaminants requires information from natural geologic systems. The following study of the Coles Hill uranium deposit and associated weathered zone constrains processes leading to the natural attenuation of uranium within an oxidizing, fluid rich environment typical of the eastern US. At the Coles Hill deposit fracture hosted, primary U(IV) bearing mineral assemblages formed during hydrothermal activity associated with Mesozoic faulting. The most abundant ore assemblage consists of coffinite and apatite, but uraninite-zeolite and uraninite-calcite assemblages are also present. Within the shallow bedrock there is a uranium redox transition where alteration of U(IV) minerals has produced secondary uranium minerals. Geochemical data suggests that the volume of rock containing this U(IV)/U(VI) transition is acting as a closed system with respect to uranium mass transport during oxidation. The dominant mechanism of uranium fixation within the oxidizing zone is the precipitation of Ba-U(VI) phosphates (meta-autunite group). Speciation and mineral stability calculations indicate that ground waters from the Coles Hill weathered zone are saturated with respect to Ba-meta-autunite and that this mineral is capable of buffering dissolved uranium concentrations to values lower than 20 parts per billion. U(VI) phosphates of the meta-autunite group are not stable in the vadose zone (soil pH ~ 4.5) at the Coles Hill site. In this zone uranium is associated with (Ba, Ca, Sr) aluminum phosphate of the crandallite group as well as with phosphate sorbed to iron oxy-hydroxide mineral coatings. Uranium leached from the vadose zone is reprecipitated as new meta-autunite minerals below the water table due to higher pH conditions of ~6.0 and relatively high activity ratios of dissolved phosphate to carbonate (e.g. log [H2PO4-/HCO3-] > -3). It is estimated that the U(VI) phosphates responsible for the natural attenuation of uranium at this site persist within the weathering zone for hundreds of thousands of years. Thus, the Coles Hill deposit represents an excellent natural laboratory for the study of uranium attenuation with potential applications for the design and implementation of cost effective remediation and containment strategies, such as soil amendments techniques and in-situ reactive barriers technologies.

Name thesis:degree_name

Ph. D.

Level thesis:degree_level

doctoral

Discipline thesis:degree_discipline

Geological Sciences

Department dc:contributor.department

Geological Sciences

Grantor dc:publisher

Virginia Tech

Year dc:date.issued

2001

Author dc:creator

• Jerden, James L.

Chair dc:contributor.committeechair

• Sinha, A. Krishna

Committee members dc:contributor.committeemember

• Hickmott, Donald
• Zelazny, Lucian W.
• Beard, James S.
• Henika, William S.
• Rimstidt, J. Donald

• Autunite
• Uranium Transport
• Uranium Deposit
• Natural Attenuation
• Crandallite

Statement dc:rights

• In Copyright

Licence dc:rights.uri

• http://rightsstatements.org/vocab/InC/1.0/

Dc Identifier Other

etd-10032001-141240

OAI identifier oai:identifier

oai:vtechworks.lib.vt.edu:10919/29173