Late stage of weathering of uranium ores as a waste rock after historical silver mining, Joachimsthal, Czech Republic

Weathering of uranium ores and migration of radionuclides were studied in the area of 450 years old waste rock dump Geister II after silver mining in Joachimsthal. The dump shows significant contents of uranium series radionuclides and the site can be regarded as an analogue of modern uranium waste rock dumps from 20th Century. It is likely to provide valuable information about process of weathering and washing of radionuclides away from it due to its long-term exposure to oxidizing conditions. Samples of the dump horizons for laboratory scintillation gamma spectrometry were collected in digged well and also bores made with engine drilling. Alpha-autoradiography using LR 115, type 2 film detectors was employed to study of distribution of radioactive micro-objects in the profile. Electron microanalysis does not provide Ra concentration measurements, but localization of the active centres to the rims appointed indicates the possibility of radium bound to grain surfaces and layered silicates. A series of secondary uranyl phosphates samples from the site in a different stage of weathering was studied. XRD was employed for determining a mixture of uranyl phosphates. Mineral samples were studies using an optical microscopy with UV light. In several grains a transition zone between the fluorescent part and non-fluorescent part was visible. Those grains were studied by electron microanalysis in detail. It was determined that the corroded rims of the grains were depleted with Ca, U and P and enriched with Fe. Moessbauer spectroscopy was used for distinguishing of different types of Fe bond in studied samlpes and amount of phase with certain oxidation state in each sample. Goethit was determined in the mixture of secondary minerals. By means of alpha spectrometry was found out that the studied bassetite from the dump upper layer are not in a significant radioactive disequilibrium (U∼Ra), but it shows a high microporosity (Rn escape) in compare with unaltered meta-autunite. The performed research showed that uranium is redeposited from the waste rock to fossil soils under the dump. In the subsoil it is bound mainly to decomposed organic matter remainders and lesser amount is adsorbed on porous hydrated Fe3+ oxides, which play the role of an effective geochemical barrier. In the horizon of fossil soil the uranium concentration can reach up to 0.25 wt. %. Radium, however, is still bound in a horizon of active dump material and comes under a partial redeposition controlled by biological processes. It is possible to establish that the studied model mirrors the late stage of uranium migration. Leaching of uranium ore in the surface oxidizing conditions proceeded more than 450 years. Nearly total leaching of uranium from waste rock and its washing away from the dump occurred. It seems very probable, that a similar process will happen in the time horizon of several hundreds years in case of modern waste rock dumps after intensive uranium mining in other mining districts, as well. It is only a question, if there will be present effective geochemical barriers for its retain