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

Vietnam Journal of Earth Sciences, 39(1), 14-26, DOI: 10.15625/0866-7187/39/1/9181 14 (VAST) Vietnam Academy of Science and Technology Vietnam Journal of Earth Sciences General characteristics of rare earth and radioactive elements in Dong Pao deposit, Lai Chau, Vietnam Nguyen Dinh Chau1*, Pieczonka Jadwiga1, Piestrzyński Adam1, Duong Van Hao3, Le Khanh Phon3, Jodłowski Paweł2 1Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Tech- nology (AGH UST), al. Mickiewicza 30, 30-059 Kraków, Poland 2Faculty of Physics and Applied Computer Science, AGH University of Science and Technology (AGH UST), al. Mickiewicza 30, 30-059 Kraków, Poland 3Faculty of Oil and Gas, Hanoi University of Mining and Geology (HUMG) Received 08 December 2016. Accepted 2 February 2017 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 proce- dures and measured using an alpha spectrometer coupled with silicon semiconductor detector and / liquid scintilla- tion 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. Keywords: REE, natural radionuclides, weathering zone, enrichment, surface and thermal waters, Dong Pao de- posit, Lai Chau Vietnam. ©2017 Vietnam Academy of Science and Technology 1. Introduction1 The rare earth elements (REE) play an *Corresponding author, Email: Nguyen.Chau@fis.agh.edu.pl increasingly important role in the world econ- omy. REE are implicated in many technolo- gies associated with energy, electronic, nano- materials, hybrid car components and others. Due to the production of spare parts of hybrid Nguyen Dinh Chau, et al./Vietnam Journal of Earth Sciences 39 (2017) 15 cars, wind turbines, battery alloys, magnets, aerospace, the future demand for REE is ex- pected to increase to above 25 times in com- parison with the current needs (Hoatson et al. 2011, Curtis 2011, Damascena et al., 2015). What is more in some countries the REE are even treated as economically and politically strategic commodities (Damascena et al., 2015). Vietnam is a country located in the Indo- china Peninsula with the marine coastline ex- tending over more than 1500 km making beach sands a resource of great REE potential (Hou 2005). Apart from that there are two very rich REE deposits in Vietnam, i.e. Nam Xe and Dong Pao. Both are located in Lai Chau province, Northwest Vietnam (Kušnir 2010; Pham Ngoc Can et al. 2011). These de- posits were discovered in the middle of the XX Century and are continuously investigated (Fromaget 1941; Fromaget & Saurin 1952; Dovzikov et al., 1965; Le Thac Xinh et al., 1988; Tran Trong Hoa et al., 2010; Pham Ngoc Can et al., 2011), but up to date some problems e.g. 3-4D models still require further examination. In the scope of the Bilateral Collaborative Project between Hanoi University of Mining and Geology (HUMG) and AGH University of Science and Technology (AGH UST) Kra- ków, Poland No. 01/2012/HD-HTQTSP, on November 2015 the AGH UST delegation vis- ited Dong Pao REE deposit and performed a geological survey to collect ore samples from both the upper weathered zone and the lower - drill core hard rocks from limestone and do- lomite sections, and water samples from streams, natural tap waters and thermal spring in the deposit area and its surrounding. The aim of this work is to determine the chemical, rare earth and radioactive element concentra- tions in the solid samples. The obtained re- sults together with the archival data serve as background to qualify the characteristic pa- rameters of the REE deposit in question. Water samples were collected and ana- lyzed to characterize the impact of the ores on the local environment, which are responsible for transportation of radioactive elements. 2. Geological setting The Dong Pao Rare Earth deposit is situat- ed in the Northwest Vietnam, within the geographic coordinates of 10332’37’’- 10333’46’’E and 2218’84’’- 2219’13’’N, occupying an area of about 120 km2 (Le Khanh Phon et al., 2015). From the tectonic point of view this deposit belongs to the Son La-Lai Chau zone bounded in the east by the Red River fault and in the west by the Ma River suture zone (Figure 1). Within the Ma River suture zone there are a few intrusive units such as the Fan Si Pan Massif, Posen batholith and Muong Hum granite. The Fan Si Pan Massif is composed of granites and metagranites separated by narrow intercalations of Neoproterozoic metasediments, mainly mica schists and mar- bles. In this massif, the youngest igneous unit is the Yensun granite of Paleogene age (Żelaźniewicz et al., 2013). The Precambrian Posen batholith is characterized by the varia- bility in composition of mafic veins. This unit contains also migmatic patches and gneisses (Żelaźniewicz et al., 2013). The Muong Hum unit is characterized by the fine-grained alkali granite and syenite. This suit is poor in Sr, P, Ti, Ba, Ca but rich in Rb, Zr, Hf including REE up to 780 ppm. Based on geochemical characteristics, Żelaźniewicz and co-workers (2013) suggested that the Muong Hum granite was associated with either continental rifts or mantle plumes. In the investigated deposit there is the Dong Pao syenite, this formation occupies an area of 8.9 km2 trending NW-SE with length up to 5.5 km. The alkali syenite, porphyritic syenite are the main rocks of the Dong Pao granite, in which the orthoclase (63-92%), plagioclase (5-15%), quartz (2-20%), biotite, (2-6%) and pyroxene (0-8%) are the major minerals, and zircon, apatite and leucite are the minor ones. Vietnam Journal of Earth Sciences, 39(1), 14-26 16 Figure 1. Geological sketch of Dong Pao Rare Earth Deposit on the background of the tectonic sketch of the Indo- china block (part of the North-West Vietnam) Nguyen Dinh Chau, et al./Vietnam Journal of Earth Sciences 39 (2017) 17 A sedimentary sequence in the Dong Pao area is composed of Devonian siltstone- mudstone formation, Permian limestones, and bauxites, and iron-bearing formations covered conformably by Early and Middle Triassic fi- ne grained grey limestone. These rocks are overlain by the Late Triassic conglomerates and sandstones and in some places with Cre- taceous red beds (Tran Van Tri, 2011; Żelaźniewicz et al., 2013; Faure et al., 2014; Halpin et al., 2015). According to Nguyen Tien Du et al., 2011, there are 17 ore bodies in the studied area. The thickness of the ore bod- ies varies from tens to above 500 m and length of separate ore bodies vary from 300 to 1000 m. Subcrops of the ore bodies were sampled. The weathered zone is composed of altered fragments of barite-fluorite ores ce- mented with REE carbonates mixed with clay and Fe- Mn hydroxides. In the area, there are many streams flowing along the region relief, the main streams being Nam Hon and Nam Nu. The Nam Hon in the West flows along the trend of rock formations, and in the North, there is the Nam Nu stream crossing near per- pendicularly the barite and fluorite bodies in the region. At some places, there are karstic caves, especially there where carbonate for- mations occur. In the area, the water is re- charged from the surface reservoirs and is used by people for daily life and agriculture. 3. Sampling and analytical methods 3.1. Sampling After the Conference on the Earth Sciences held at HUMG on November 2015, the AGH- UST group made a geological reconnaissance and collected some ore samples P1, P2 in the weathered upper surface soil (Figure 2) and fragments of drill cores KF-133, LK-122 (Fig- ure 3) following the information by geologists working in place. The samples were collected and analyzed to compare results with the exist- ing data of the ore bodies numbered as follows: F4, F7, F9, F10, F14, F16, F17. The water samples were collected from the Nam Hon (F4), Nam Nu streams (P1), from the thermal spring located 200 m Southeast from the deposit (TM) and from the artificial concrete container built at the geological of- fice (F9). The tap water in the container is re- charged from rain water flowing from the mountain and lead by the bamboo pipes and finally reaches to the cement container; in the paper, this water is defined as natural tap wa- ter. All the collected samples were shipped and analyzed at AGH-UST laboratories. Figure 2. The samples collected from the weathered (upper) zone Vietnam Journal of Earth Sciences, 39(1), 14-26 18 3.2. Analytical methods To determine the chemical and REE com- position, the solid samples were sent to Bu- reau Veritas Mineral Laboratories in Vancou- ver Canada, which possesses the Certificate of Analysis No KRA15000229.1. At the Veritas Mineral Laboratories, the chemical composi- tion of the solid samples is analyzed by laser ablation with induced plasma coupled with the mass spectrometer (LA ICP MS), the REE are analyzed using instrumental neutron activa- tion method (INAA). The LA ICP MS con- sists in the generation of the fine particles by a laser beam focused on the sample surface, the process known as Laser Ablation. Then the ablated particles are transported to the sec- ondary excitation source of the ICP-MS in- strument for digestion and ionization. The ex- cited ions in the plasma torch are subsequent- ly introduced to a mass spectrometer detector for elemental analysis. The LA ICP MS in- strument uses the laser wavelength of 193 nm with 14 J/cm2 of the power energy density for ablation, in ICP MS the power of 1350W for RF generation, the 14 L/min and 0.9 L/min argon gas for plasma torch and auxiliary re- spectively (Liu et al., 2008). The ICP-MS is calibrated using the NIST standard samples of well known chemical composition. The back- ground of the instrumental neutron activation method can be briefly described as follows: an analyzed sample is subjected to a thermal neu- tron flux and radioactive nuclides are pro- duced. As these radioactive nuclides decay, they emit gamma rays of characteristic ener- gies for each nuclide. Comparison of the in- tensity of these gamma rays with those emit- ted by a standard, one can estimate quantita- tively the concentrations of the various nu- clides. The sample for gamma measurement was ground until the grains were less than 2 mm in diameter, then it was dried in an oven at 120C for 24 hours to ensure that moisture was completely removed, then weighted accu- rately and packed in the aluminum cylindrical Marinelli beaker of 720 ml capacity and sealed to prevent escape of radon. The sealed samples were left for at least 22 days to reach secular equilibrium between the 222Rn and 226Ra. The activity concentrations of 40K, 226Ra and 232Th were determined using the gamma-ray spectrometer coupled with HPGe detector of the relative efficiency of 42% and resolution of 1.9 keV for 1332 keV line and calibrated using the IAEA reference materials RGU, RGTH, RGK as standard samples. The Figure 3. The core samples Nguyen Dinh Chau, et al./Vietnam Journal of Earth Sciences 39 (2017) 19 gamma lines 609.3 keV (46.1%), 1120.3 keV (15.0%) and 1764.5 keV (15.9%) from 214Bi were used to determine the activity concentra- tion of 226Ra, while that of 232Th were deter- mined from the gamma lines of 911.2 keV (29.0%) from 228Ac and 583.0 keV (30.9%) and 2614.4 keV (35.8%) from 208Tl. For 40K, its activity concentration was determined from its 1461 keV gamma line Jodłowski and Kalita (2010). The chemical composition of the water samples was analyzed using an ICP-AES PerkinElmer Optima 7300 DV spectrometer. The principal of the ICP-AES is similar to the LA ICP MS, but in the ICP-AES the laser ab- lation is not needed and instead of the atomic mass measurements, the wavelength of the atomic spectral lines is measured. The condi- tion parameters of ICP of both instruments ICP-AES and LA ICP MS are similar. The ICP-AES is calibrated with a multi-element standard solution of the Merckcompany. The uranium isotopes in the water samples were precipitated together with the manganese oxide, then the obtained sample was dissolved in HCl 9M and transmit through the Dowex resin exchange column. The uranium ions were rinsed from the resin column by adding the HCl 0.1M and again precipitated using the neodymium chloride. The precipitate was placed onto the plastic filter of 0.1 m porosi- ty and measured using an alpha spectrometer coupled with silicon PIPs detector. To control the chemical yield and determine the 238U and 234U concentrations, the trace quality amount of 232U solution was added into the studied water sample at the beginning of the chemical procedure (Nguyen et al., 2010). The radium isotopes were precipitated from the water sample together with barium as the sulfate compound, then the obtained precipitate was cleared up from the other messing isotopes by dissolution it in the EDTA solution. The radium was again precip- itated though decreasing pH sample to 4.5 by adding the acetic acid. The precipitate was washed using distilled water and centrifugal machine, then placed into the special glass vi- al of 22 mL and mixed with the gel scintilla- tion cocktail of 12 ml. The sample was meas- ured using the Wallac 1414 / Liquid Scin- tillation counter (Nguyen et al., 1998). The quality of the used methods was tested by comparing many measurements done by sev- eral international organizations. 4. Results and discussions Table 1 summarizes the measured oxides of the main metals and some trace elements for the collected solid samples. Table 2 pre- sents the ranges and average concentrations of some oxides of the selected ore bodies report- ed by Nguyen Tien Du et al., 2011. Compar- ing the data in both Table 1 and Table 2, we can see that the data presented by this work are contained in the ranges of the data given by Nguyen Tien Du et al., 2011, but all the values of the oxides analyzed by us are com- parable with the minimum values in the rang- es of the adequate oxides. The calcium, sili- con, and iron are principal components of the carbonate rocks. Based on the data of this work, the analyzed oxides in the borehole cores and weathered zone (soil) in the forms of vertical bars are presented in the Figure 4a, 4b, and 4c. The Ca, Mg, Fe and alkali metals (Na, K, Cs) are very vulnerable to weathering processes resulting their concentrations in soil are far lower than that in hard rock by several times, while Mn, Ti, Sc, Co, and Be are very resistant and enriched to several folds in the weathered zone. According to Rösler and Lange (1972), the weathering degree can be appreciated using the weathering index de- fined by the formula: OKONaMgOCaOOAlSiO OHOKONaMgOCaOVi 22322 222   (1) The oxides in formula (1) are expressed in mole units, and the weathering index for stud- ied samples is equal to 0.187, 0.076, 0.731 and 0.700 for P1, P2, LK122 and KF-133 re- spectively. Vietnam Journal of Earth Sciences, 39(1), 14-26 20 Table 1. Analyzed concentrations of the metal oxides and trace elements in the collected samples sample SiO2 Al2O3 Fe2O3 MgO CaO Na2O K2O TiO2 P2O5 MnO Cr2O3 Ni Sc Ba Be Co Cs % % % % % % % % % % % ppm ppm ppm ppm ppm ppm P1 6.76 0.16 0.98 0.01 14.58 0.01 0.01 0.04 0.23 0.71 0.012 20 4 50000 2 2.2 0.1 P2 8.23 0.23 1.94 0.01 6.27 0.01 0.01 0.07 0.39 1.13 0.011 20 3 50000 2 4.5 0.1 KF-133 1.66 0.81 2.75 0.31 49.41 0.01 0.32 0.03 0.01 0.17 0.002 20 1 50000 1 1 0.7 LK-122 2.37 0.07 1.99 0.16 51.52 0.01 0.02 0.02 0.85 0.16 0.002 20 1 16778 1 1 0.1 Table 2. Ranges and average values of concentrations of the metal oxides (%) in the ore bodies (Nguyen Tien Du et al., 2011) Ore body SiO2 Al2O3 Fe2O3 P2O5 CaO TiO2 PbO ZnO F4 5.34-52.8 (22.77) 7.08-70.30 (29.56) 1.22-12.01 (4.42) 0.11-1.12 (0.38) 0.06-50.83 (6.86) 0.04-1.01 (0.33) 0.08-0.97 (0.34) 0.02-0.21 (0.06) F7 7.08-70.30 (29.56) 0.35-22.64 (8.30) 0.98-20.64 (4.82) 0.02-1.78 (0.13) 0.00-41.43 (6.39) 0.04-1.14 (0.37) 0.04-0.90 (0.24) 0.02-0.34 (0.10) F9 2.02-58.53 (15.70) 0.35-27.79 (5.48) 1.08-20.94 (3.04) 0.02-1.30 (0.12) 0.04-41.39 (5.50) 0.02-2.11 (0.22) 0.06-0.82 (0.29) 0.01-0.26 (0.06) F10 1.92-62.22 (24.84) 0.40-39.27 (8.41) 1.56-12.91 (5.79) 0.02-0.63 (0.12) 0.04-53.47 (4.48) 0.05-1.12 (0.50) 0.04-1.39 (0.37) 0.02-0.35 (0.11) F14 5.88-42.62 (24.00) 1.56-12.96 (6.09) 1.19-30.96 (12.65) 0.04-4.02 (1.34) 0.11-43.60 (7.65) 0.05-0.81 (0,33) 0.12-1.15 (0.36) 0.03-0.45 (0.15) F16 1.29-44.91 (17.19) 0.33-21.19 (5.94) 3.45-22.13 (10.95) 0.20-5.16 (0.92) 0.00-32.47 (2.60) 0.05-1.10 (0.49) 0.09-1.22 (0.46) 0.05-0.66 (0.26) F17 20.95-59.06 (46.01) 6.75-15.65 (12.93) 4.66-16.34 (9.24) 0.37-1.77 (0.70) 0.00-21.15 (1.33) 0.27-0.58 (0.40) 0.14-1.30 (0.33) 0.04-0.17 (0.07) In Vietnam, there is the tropical climate, and in the region the intensive vegetable farm- ing resulting in the surface water being rich on CO2, ammonium and the occurrence of a lot of deep fractures in the rocks. The alkaline el- ements are leached from the rocks by a chem- ical reaction as follows: RSiO2 + CO2 + H2O = RCO3 + SiO2 + H2O where R are Ca, Mg, Fe, Na2, K2, Cs2 The REE concentrations determined for the collected samples and the REE average concentrations of the ore bodies reported by Nguyen Tien Du et al., 2011 are summarized in Table 3 and Table 4 respectively. Both the data and LREE/HREE in the Nguyen Tien Du and co-workers report (2011) are compa- rable with that of the samples from the upper weathered zone, which can be connected with the fact that the samples presented in the Nguyen Tien Du et al., (2011)’s report were taken from the weathered zone. The REE concentration in the weathered zone reaches to 10 wt.%, so the studied depos- it can be classified to one of the richest REE deposits in the World (Hoatson et al., 2011). The patterns of the REE and radionuclides in the weathered and hard rocks are similar (Fig- ure 5), but their contents in the weathered zone are enriched to near forty fold in com- parison to that in the hard rock (Table 4). The ratio (LREE/HREE) in the studied samples ranged from 56 for the hard rock to 121 for weathered one. According to Hornig- Kjargaard, 1998 and Żelaźniewicz et al., 2013, the mentioned phenomena can be ex- plained that at the beginning the rare elements were within the intrusive formations Muong Hum and/or Dong Pao syenite (cf. Figure 1), then they underwent enrichment due to ther- mal metamorphic processes occurring in the carbonatite formations, so the REE ores were formed as lenses or dykes of various shapes and dimensions. Finely the REE were en- riched by the surface chemical weathering processes. Nguyen Dinh Chau, et al./Vietnam Journal of Earth Sciences 39 (2017) 21 Figure 4. Concentrations of the el- ements in the weathered zone (P1,P2) and hard rocks (KF-133, LK-122l) Vietnam Journal of Earth Sciences, 39(1), 14-26 22 Table 3. Measured concentrations of the rare earth and radioactive elements (ppm) and weathering index (Vi) for the collected samples Elements P1 P2 KF-133 LK-122 La 39240 31100 798 823 Ce 44600 36000 1340 1240 Pr 3540 2990 122 104 Nd 9830 8390 381 326 Sm 903 783 43.6 39.0 Eu 212 183 10.1 9.7 Gd 583 494 26.8 24.8 Tb 37.6 33.1 2.6 2.2 Dy 122 112 11.0 9.0 Ho 13.3 12.2 1.4 1.3 Er 30.0 25.5 3.2 2.9 Tm 3.6 3.3 0.5 0.4 Yb 19.2 17.4 2.7 2.7 Lu 1.9 1.8 0.4 0.3 REE 99100 80100 2750 2580 238U 104.4 115 11,9 11,2 232Th 104 83.3 5,2 8,8 40K (%) 0,11 0.8 0.05 0,07 Vi 0.187 0.076 0.700 0.731 Table 4. The average concentrations of the REE (ppm) in the ore bodies (Nguyen Tien Du et al., 2011) Element F4 F7 F19 F10 F14 F16 F17 La 9001 18925 14932 13735 8374 9017 7982 Ce 11615 24371 19166 17686 10988 11821 10313 Pr 1094 2286 1812 1677 1023 1098 967 Nd 3186 6612 5237 4846 2954 3156 2808 Sm 327 671 543 492 315 337 289 Eu 169 334 267 246 158 169 141 Gd 159 322 358 238 150 16 141 Tb 18 36 29 27 17 18 16 Dy 48 97 79 72 47 51 43 Ho 7 15 12 11 7 8 7 Er 22 31 25 24 14 15 14 Tm 2 5 4 3 3 3 2 Yb 7 14 11 10 7 7 6 Lu 1 2 2 2 1 1 1 LREE 25392 53199 41957 38682 23812 25598 22500 HREE 264 522 520 387 246 119 230 LREE/ HREE 96 102 81 100 97 215 98 Figure 5. Patterns of the REE and radionuclides in the weathered and hard zones The leaching and removal of intrusive for- mations by water activity and chemical pro- cesses led to the accumulation of igneous apa- tite, oxides, sulfides, and silicates. These phe- nomena were accompanied by replacement, decomposition, oxidation of the primary igne- Nguyen Dinh Chau, et al./Vietnam Journal of Earth Sciences 39 (2017) 23 ous minerals and crystallization of the second- ary minerals. In consequence, the overlying weathered zone is enriched in insoluble phos- phates, clays, iron and manganese bearing ox- ides containing REE, U, Th, Nb, Ta, Zr, Ti, V, Cr, Ba and Sr (Hoatson et al., 2011). In the studied deposit, the REE minerals are parisite, bastnäsite, apatite, barite, fluorite and Celestine were observed. The picture of BSE (Back Scat- tered Electron) and the spectrum of energy dif- fraction scattered (EDS) of the parisite mineral are shown in the Figure 6 and Figure 7 respec- tively. The contribution of the REE in the pa- risite is (in wt.%) 14.89 for La, 23.36 for Ce, 2.19 for Pr, 6.59 for Nd and near 2 for remain- ing heavy REE. Figure 6. BSE picture The mineralization of the stream and natu- ral tap waters is equal to above two hundred mg/dm3, the magnesium in tap and thermal waters are twice comparing to the stream ones (Table 5). The radium isotopes concentration in the stream water range from 100 to 300 mBq/dm3, while in the thermal and natural tap waters amount only to several tens mBq/dm3. The phenomenon is quite different for urani- um isotopes, their concentration in the thermal water (groundwater) are equal to 110 and 78 mBq/dm3 for 238U and 234U respectively. Vietnam Journal of Earth Sciences, 39(1), 14-26 24 This level is a few times higher than that in the stream and natural tap waters, which range near a few mBq/dm3. Though the uranium and radium concentration in natural tap water are lower than permit maximum contaminant level for drinking water (180 mBq/dm3 for 238U and 185 mBq/dm3 for total radium isotopes (226Ra + 228Ra)), but significantly higher than the con- centrations of these isotopes in Red River and tap waters (WHO 2006; Nguyen et al., 2016). Such high concentration of the uranium and ra- dium in the water can be the diagnostic feature of the region, where the rock formations are rich in the natural radioactive elements. Figure 7. EDS spectrum of parisite Table 5. Measured concentrations of main ions (mg/dm3) and natural radioactive isotopes (mBq/dm3) in the water samples Water sample F3 P1 F9 TM Na+ 3.4 2.7 2.3 10.6 K+ 1.6 0.7 4.2 6.9 Ca2+ 38.0 54.8 37.0 99.8 Mg2+ 3.5 5.2 9.5 14.4 Sr2+ 0.9 0.8 2.5 8.9 Cl- 5.1 4.2 3.4 4.2 SO42- 3.0 2.3 6.3 184 HCO3- 137 187 159 185 TDS 206 271 256 560 pH 7.8 8.0 8.0 7.9 226Ra 123 336 29 12 228Ra 105 309 33 18 234U 14 38 28 78 238U 21 16 30 110 Hydro-chemical type of water HCO3-Ca HCO3-Ca HCO3-Ca-Mg HCO3-SO4-Ca Nguyen Dinh Chau, et al./Vietnam Journal of Earth Sciences 39 (2017) 25 5. Conclusions The Dong Pao deposit is rich in light rare earth elements such as La, Ce, Pr and Nd, their total concentration approximates 10 wt.% in the weathered zone. The mass contents of uranium and thorium in the studied REE deposit are comparable reaching about 0.01 wt.% and 0.001 wt.% in the upper weathered and lower limestone and dolomite zones respectively. The pattern of U, Th and REE in the ore bodies occurring in the weathered zone is sim- ilar to that in the lower zone, but in the weath- ered zone the contents of these elements are higher by a few orders of magnitude in com- parison with the lower dolomite and limestone zone. The LREE/HREE ratio suggests that the mineralization processes in the Dong Pao de- posit could be described as follow: the REE, uranium and thorium elements firstly came from the Mantle together with the Dong Pao alkali syenite and porphyritic syenite; next, the ores were precipitated from hydrothermal fluids and finally enriched by the surface weathering processes. The radium and uranium concentration in the surface and natural tap waters are a conse- quence of the leaching processes of the ele- ments from the weathered rocks and can be a geological indicator in the prospecting of de- posits rich in the natural radionuclides. 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