ORIGINAL ARTICLE Year : 2015  Volume : 38  Issue : 3  Page : 98101 Origin of thorium deposits in Kerala beach sands MR Iyer ExRadiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 400085, Maharashtra, India; ExProfessional, IAEA, Vienna, Austria, India Correspondence Address: The origin of the thorium deposits on the West Coast of India is not well established though a number of studies on the various aspects of these deposits have been carried out for more than 50 years. The deposits are believed to be washed down from highlands through water streams. It is proposed that atom ratios of various radioactive and stable products in the uranium and thorium series as a function of time could be effectively used for the purpose. Accordingly, the various ratios have been calculated as a function of the age in the time range of 10070,000 years. The stable products are included for the first time.Pb208/Th232 in Th232 series, Pb207/U235, Pb207/Pa231, and in U235 series and Pb206/U238, Pb206/U234, Pb206/Th230, and Pb206/Ra226 in U238 series, are presented. The ratios show significant variations in the time range studied. The ratio Pb206/Pb207 from the two uranium series is of interest and is examined. This ratio shows a variation of up to 100 times whereas, the natural isotopic ratio is around unity. It is suggested that an extensive measurement of these ratios and their correlation in samples from the hypothesized source in highlands, in river beds, and on the beach sand might give some idea of the transport path for these minerals. Further, it is suggested to measure the ratio of all the lead isotopes from the three series with Pb204 which is purely of primordial in origin which would also be useful to indicate the contribution from radiogenic sources. In order to undertake such a project which, of course, involves many disciplines large efforts in sampling, in modeling of possible transport paths and strategies for analysis are required.
INTRODUCTION The beach sand on the West Coast of India, particularly in Kerala, contains one of the rich deposits of thorium in the world. The country's nuclear power program is being built entirely dependent on these rich resources. Though a number of studies on the various aspects of these deposits have been carried out for more than 50 years, the exact origin of these is not well examined. THE SOURCE OF THE DEPOSITS I have been proposing for the last several years that there should be investigations on the source of the thorium deposits in the Kerala beach monazite area. [1] The deposits, as we all know is superficial and confined to narrow belts. There is a presumption that the sources of the thorium is in the highlands and has been deposited by the rivers or underwater streams, but no convincing proof for this is available. This appears to be a logical conclusion but not yet proved scientifically since the deposits could not have appeared from nowhere in a narrow belt in certain places. However, we do not see such rich thorium deposits existing in the Western Ghats to support this theory. Further, the river estuaries in the region do not show rich deposits of thorium. To the best of my knowledge, there are no correlation studies of uranium, thorium, and their daughter products in the rock samples in the hills, riverbeds, and in the beach deposits are available. Minor despots also occur in several places all along the coast and on the East Coast as well. Noticing high background around Kalpakklam area when the Environmental Survey Laboratory was initially being set up, the author had undertaken survey of the site and the sampling showed the presence of thorium bearing minerals for the first time in 1973. [2] On the other hand, there is a suspicion that the source could be the ocean bottom from where the mineral is washed ashore. [3] In that case, a study of the ocean depth should provide some clues. No investigations whatsoever exist to prove or disprove this contention also. It is proposed that a systematic study of the atom ratios of the various daughter products both radioactive and stable, in the uranium the thorium series could be used to investigate the source of the mineral and how it found its way to the beach sands. A computational evaluation of the ratios of the various daughter products was undertaken to explore if those can be used. COMPUTATION OF RATIOS The longlived and stable daughter products build up in U238, U235, and Th232 radioactive series were carried out starting from one microgram of natural uranium and taking into account the build up from both U238 and U234 in the U238 series. The natural isotopic abundances of U238 and U234 were taken into account. Similarly, the build of Pa231 and Pb207 from U235 series was carried out from the 0.712% natural abundance of U235. The thorium series has only the stable end product of Pb208 since all the in between products are of short halflife compared to the time scale considered here and build up was calculated from one microgram of thorium. What is presented here is a series of graphs of the ratios of the daughter products in the short timescale of geological age from 100 to 70,000 years to serve as baseline to see which ratios could be potentially used. Only the longlived products and stable products are considered. DISCUSSION OF THE RESULTS The daughter products in thorium series are mostly shortlived and hence only the ratio of stable Pb208 to Th232 is considered [Figure 1]. This is the only ratio of interest. There are many ratios possible within the U238 and U235 series. In the U235 series Pb207/U235 and Pb207/Pa231 are given in [Figure 2] and [Figure 3]. For the U238 series, the ratios of Pb206 with U238, U234, Th230, and Ra226 are shown in [Figure 4] [Figure 5] [Figure 6] [Figure 7]. Many of the ratios show a significant variation in the time interval considered.{Figure 1}{Figure 2}{Figure 3}{Figure 4}{Figure 5}{Figure 6}{Figure 7} Combining the two series the stable Pb206 to Pb207 shown in [Figure 8] ratio gives a different trend and shows a variation of nearly 100 times in the time range considered. This ratio is just one in the natural lead and hence the local addition from radiogenic source will be indicated in its departure form one. The ratio of Pb206, Pb207 with Pb208 is not given since the latter belong to a different series and need to be tied up with the U/Th ratio with which the calculations are started.{Figure 8} It is suggested that the ratios of Pb206, Pb207, Pb208 with Pb204 might also help. This has the merit that only Pb204 is of primordial origin while others are radiogenic and possibly primordial also. The ratios with Pb204 could serve as a normalizing factor. The natural isotopic ratio of lead isotopes are shown below: [INLINE:1] Thus, the ratio with Pb204 could indicate the extra contribution from the natural series but lacks information on age and need to be used along with the other ratios of the lead isotopes from U and Th decay chains to indicate the time dependent migration pattern. Use of stable daughter product ratios is a new suggestion and is not studied earlier so far as I know, though the author has successfully employed stable fission products ratio method for applications in nuclear safeguarding of longterm repositories. With the availability of analytical methods which can establish these ratios, they offer a possible tool for studying radionuclide migration pattern. Earlier studies of ratios of daughter products in natural radioactive series in various natural matrices were carried out extensively by Joshi and Ganguly. [4] However, they have not considered the stable products. The correlation of these ratios in the samples from the hypothesized source in highlands, in river beds, and on the beach sand might give some idea of the transport path for these minerals. SUGGESTIONS FOR FUTURE WORK It is well known that in addition to variation with age the actual ratios would be subject to various geochemical processes of fractionation, leaching, etc., Thus for using these ratios proposed in this work, suitable modeling has also to be proposed. The particle size of the thorium deposits in beach sand is reported to be of the 7 μm mean diameters but some of them are much larger. If the transport is through water streams in dissolved form, one need to hypothesize how they ended up in fairly large size particles. Or was the dissolved matter coated on the in situ particles? Can one think of transport of the large particles as such through the rivers? This can be perhaps resolved experimentally by studying the profile of the radioactivity across the diameter of the particles. If the transport is in the form of larger clusters of the mineral, it is possible to get some clues. In order to lessen the effects of daughter product deposition/separation from the parent during its transport (speciation) one need to take the biggest of the particles and separate them and carry out the analysis of daughter products ratio in those fractions. Proper sampling techniques need to be adapted for examining the ratios. The variation of isotopic ratios as a depth profile in monazite deposits should also be studied. Studies could be done of the deposits in various spatial grids along the coast quantitatively and qualitatively to that in the overlooking highlands to see if any correlation exists. Such a study may help to account for the high deposits in some locations and smaller or no deposits in other locations. An analysis of uranium to thorium ratio in the rock samples and on the beach would also be useful, keeping in mind that generally thorium is less labile than uranium. The thorium to uranium ratio in monazite sand is in the order of 25, However, the ratio is much less in the rock samples. In order to undertake such a project which, of course, involves many disciplines, large efforts in sampling, in modeling of possible transport, and strategies for analysis are required. The marine origin of the deposits should also be considered. The outcome of the project would throw light on geochemical processes as well. Before embarking on a major project, it is suggested that some feasibility studies be carried out in order to define scope and objectives. What is attempted in the present work is the only study of the variation of the ratios with respect to age in the hope that these along with geochemical modeling would prove to be useful. For the first time, stable products are also included. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest. References


