Impact of incipient weathering on uranium migration in granitic waste rock piles from former U mines (Limousin, France)

Abstract

International audienceThis study investigates the impact of incipient weathering on uranium (U) migration at the surface of granitic waste rock piles from former U mines in Limousin (France) operated half a century ago. We used a multi-scale mineralogical and geochemical approach to evaluate the impact of incipient weathering on uranium mobility in a chaotic waste rock pile. The surficial part of five former mines located in the “La Crouzille” and “La Creuse” mining districts was investigated. Four groups of samples were defined based on field observations and mineralogy, granites, lamprophyres, hydrothermally altered rocks and weathering products. Petrology, mineralogy and geochemistry investigations show the importance of mineralogical and geochemical transformations during incipient weathering over a few decades. The Chemical Index of Alteration (CIA) is successfully used to assess the weathering state of samples in the waste rock pile chaotic deposit. The mean CIA value is around 60 for unweathered/incipiently weathered granites whereas highly weathered samples have CIA around 70. This means that no significant major element loss occurs in the granitic rocks suffering incipient weathering. Various micron-size secondary U minerals consist in a large diversity of oxidized uranyl minerals (uranophane, uranyl sulfate, curite, autunite and other uranyl phosphates), with dominant uranyl phosphates, indicating local U mobility in the waste rock pile. Autunites micromorphology indicates different degrees of weathering of the samples in which it crystallizes. U is primarily concentrated in the clay fraction (< 2 μm), associated with secondary minerals from granite alteration (clay minerals such as smectite and vermiculite, Fe-oxides and phosphate minerals) and, to a limited extent, relict primary minerals. These weathering products contribute to U(VI) stabilization under oxidizing conditions

Uranium speciation in weathered granitic waste rock piles: an XAFS investigation

“…For a better overview of the different compartments of the piles, the surface of the piles as well as soil pits were collected. 9 The main U(IV) bearing magmatic or hydrothermal minerals are uraninite, zircon or monazite. Secondary neoformations of U(VI) rich minerals related to weathering are uranyl phosphates, and clay minerals and Fe oxyhydroxides with U sorbed.…”

“…A minor portion of the waste rocks corresponds to low-grade ore below the economic cut off value ranging from 100 to 300 ppm. 9 The waste piles consist of decimetric to metric blocks submitted to weathering. Compared to massive granite, the waste rock piles have a much higher specic surface area, allowing them to break down more easily into granitic arena, leading to the formation of technosoils that may accumulate mobile U(VI).…”

“…Compared to massive granite, the waste rock piles have a much higher specic surface area, allowing them to break down more easily into granitic arena, leading to the formation of technosoils that may accumulate mobile U(VI). [9][10][11][12][13] Weathered granitic material generally produces phyllosilicates, Fe oxyhydroxides or uranyl phosphates that are efficient traps for U sequestration with sorption or neoformation processes. [14][15][16][17][18][19] The two major problems posed by these waste rocks lie in determining how oxidation related to weathering is affecting the piles based on the U valence of the minerals and, determining what the dominant speciation is.…”

“…Furthermore, these sets correlate with categories of samples determined on eld and conrmed by geochemical and mineralogical analyses. 9,12 There are, however, also some peculiarities in these correlations pertaining to the extended geometries around the U. This disorder also reduces the precision of the metrical results from curve-tting in describing the nuances of the U environments.…”

Release, Migration, Sorption, and (Re)Precipitation of U during Peraluminous Granite Alteration under Oxidizing Conditions in Central Portugal

“…In general, it occurs in granites, pegmatites, and associated quartz veins. Uraninite is unstable in acid and oxidizing conditions such as those found in acid hydrothermal and meteoric fluids; it is easily dissolved, and is probably the most important source of dissolved U in groundwater emanating from weathered granite terrains [6][7][8][9][10][11][12]. Other minerals from magmatic rocks (e.g., thorite, huttonite, thorianite, monazite, titanite, xenotime, allanite, zircon) host uranium, and their alteration by acid hydrothermal or meteoric fluids is also a source of dissolved U in hydrothermal fluids or surface waters and groundwater; many uranium deposits are derived from them [10,[13][14][15][16].…”