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Chemical Equilibrium of the Dissolved Uranium in Groundwaters From a Spanish Uranium-ore Deposit

  • Antonio Garralon (a1), Paloma Gómez (a2), Maria Jesús Turrero (a3), Belén Buil (a4) and Lorenzo Sánchez (a5)...


The main objectives of this work are to determine the hydrogeochemical evolution of an uranium ore and identify the main water/rock interaction processes that control the dissolved uranium content. The Mina Fe uranium-ore deposit is the most important mine worked in Spain. Sageras area is located at the north part of the Mina Fe, over the same ore deposit. The difference remains at the fact that the uranium deposit was not mined in Sageras and was only perturbed by the exploration activities performed 20 years ago. The studied area is located 10 Km northeast of Ciudad Rodrigo (Salamanca) at an altitude over 650 m.a.s.l. The uranium mineralization is related to faults affecting the metasediments of the Upper Proterozoic to Lower Cambrian schist-graywacke complex (CEG), located in the Centro-Iberian Zone of the Hesperian Massif. The primary uranium minerals are uraninite and coffinite but numerous secondary uranium minerals have been formed as a result of the weathering processes: yellow gummites, autunite, metaautunite, torbernite, saleeite, uranotyle, ianthinite and uranopilite. The water flow at regional scale is controlled by the topography. Recharge takes place mainly in the surrounding mountains (Sierra Peña de Francia) and discharge at fluvial courses, mainly Agueda and Yeltes rivers, boundaries S-NW and NE of the area, respectively. However, at the site scale the mine is located relatively far away from the recharge zone and close to the rivers, which act as a discharge zone. Deep flows (lower than 100 m depth) should be upwards due to the river vicinity, with flow directions towards the W, NW or N. In Sageras-Mina Fe there are more than 100 boreholes drilled to investigate the mineral resources of the deposit. 35 boreholes were selected in order to analyze the chemical composition of groundwaters based on their depth and situation around the uranium ore. Groundwater samples come from 50 to 150 m depth. The waters are classified as calcium-bicarbonated type waters, with a redox potential that indicate that are slightly reduced (values vary between 50 to -350 mV). The TOC varies between <0.1 and 4.0 mgC/L and the dissolved uranium has a maximum value of 7,7 mg/L. According the analytical data of dissolved uranium, the mineral more close to equilibrium seems to be UO2(am). The tritium content in the groundwaters oscillate between 1.5 and 7.3 T.U. Considering that the mean value of tritium in rainwater from the studied area has a value of 4 T.U., can be concluded that the residence times of the groundwaters are relatively shorts, not longer than 50 years in the oldest case.



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Chemical Equilibrium of the Dissolved Uranium in Groundwaters From a Spanish Uranium-ore Deposit

  • Antonio Garralon (a1), Paloma Gómez (a2), Maria Jesús Turrero (a3), Belén Buil (a4) and Lorenzo Sánchez (a5)...


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