General characteristics of rare earth and radioactive elements in Dong Pao deposit, Lai Chau, Vietnam

Abstract

One of the important rare earth deposits is the Dong Pao localized in Lai Chau province, West-North of Vietnam. Generally, the deposit is composed of two parts, the lower and the upper. The lower part is composed of dolomite, limestone and intrusive rocks, while the upper part of the profile is represented by a weathered zone containing soil and fragments of mixed barite-fluorite ores. The concentrations of natural radionuclides, chemical compositions of ores, including rare earth elements (REE) in solid samples, were determined by gamma spectrometer equipped with a HPGe detector, laser ablation inductively coupled plasma mass spectrometry (LA ICPMS) and activation method, respectively. In the samples taken from the ore bodies within the weathered zone the REE concentration is about 10 wt.% and both ²³⁸U and ²³²Th amount to 0.01 wt.%, while in the samples from the hard part of ore the REE and ²³⁸U as well as ²³²Th contents amount only to 0.3 wt.% and 0.001 wt.%, respectively. So the enrichment of the REE and natural radioactive elements in the deposit is a consequence of the weathering processes. The water samples were taken from the streams, natural tap and thermal water intakes localized in the studied deposit and surrounding region. The ²³⁸U, ²³⁴U, ²²⁸Ra, ²²⁶Ra concentrations in the water samples were prepared by the adequate radiochemical procedures and measured using an alpha spectrometer coupled with silicon semiconductor detector and a/b liquid scintillation counter. In the stream water, the concentrations of both ²²⁶Ra and ²²⁸Ra vary from 100 to above 300 mBq/L, while in the natural tap and thermal waters they amount to tens mBq/L. The concentration of ²³⁸U, ²³⁴U in the thermal water is 80 and 110 mBq/L respectively, while in the surface water concentrations of uranium isotopes are below 30 mBq/L.

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