Linkage between economic growth of any country and energy requirement is well known, and hence, sustainable energy resources are essential. During the past three decades the world was able to cope with an increasing energy demand by relying more on fossil fuel [1-3]. However, the progressively dwindling reserves of fossil fuel and a deeprooted concern about global warming, arising out of CO₂ emission due to the excessive use of fossil fuel, have now developed a growing interest in nuclear energy as an alternative green source [4,5]. Uranium is one of the main nuclear fuels, continuous supply of which is needed for sustainable development in the energy field. In India, an extensive exploration programme is being carried out in different geological domains to establish new uranium resources and reserves to overcome the demand and supply gap in nuclear energy sector. The uranium series disequilibrium in radioactive ore poses a critical problem for proper assessment of the resource in most of the cases. However, it is observed that the magnitude and frequency of radioactive disequilibria is generally ignored which leads to underestimation or overestimation of ore reserve [6,7]. Thus, to overcome uranium ore deposit evaluation related constraints, the disequilibrium studies are significant both in field and laboratory counting measurements.

In the uranium series, the system is considered to be in radioactive equilibrium when all the daughter products decay at the same rate in which they are produced from the parent isotope in an ideally closed system [6,8,9]. Thus, at radioactive equilibrium, each of the daughter products would be present in a constant proportion to the parent isotope. However, such equilibrium is rarely observed in uranium ore bodies [7,9]. Various studies have been undertaken on the nature and significance of disequilibrium conditions in uraniferous deposits [10], which have indicated that different geologic and physicochemical processes influence the system. The addition or removal of any isotope in the disintegration series in a radioactive mineral causes disequilibrium in the proportions of the parent isotope to its daughter products [9]. The present paper deals with the evaluation of disequilibrium pattern in unconformity proximal and fracture controlled types of uranium mineralisation in Koppunuru area, Guntur district, Andhra Pradesh, India using beta-gamma and gamma-ray spectrometry techniques on mineralised borehole core samples.

Geological Setting

Koppunuru area is located in the south-western part of the Neoproterozoic Palnad Sub-basin, where Kurnool sediments are deposited unconformably over the Archaean to Palaeoproterozoic basement granite and gneisses [11,12]. Basement granites are exposed as an inlier over an area of 5 km x 2.5 km to the south-east of Koppunuru, and along the up-thrown block of regional WNW-ESE trending fault to the south of Koppunuru (Figure 1). These are dissected by a number of ENE–WSW, NE–SW, NW–SE and a few N–S trending lineaments represented by dolerite dykes, fractures and faults [13]. The Kurnool sediments are mainly represented by Banganapalle, Narji and Paniam formations comprising quartzites, shales and limestones. The Banganapalle Formation (10-173m thick), the oldest sedimentary lithounit in the study area, and comprises quartzite and intercalated grey shale sequence with basal conglomerate/grit [12].

Figure 1: Geological map of Koppunuru area, Guntur district, Andhra Pradesh along with studied borehole locations.

Proterozoic unconformity-related uranium deposit in Koppunuru and adjoining areas is predominantly (~85%) hosted by Banganapalle quartzite and grit. At places the mineralisation also transgresses below the unconformity contact in basement granites along the fracture planes [7,14-19]. It is observed that the mineralisation follows a predominant N-S to NNE–SSW trend sympathetic to faults and fractures in the area. Pitchblende and coffinite are identified as primary uranium ore minerals in Koppunuru deposit, while uranophane, phosphuranylite, metazeunerite and U-Ti complex occur as secondary uranium minerals [15,17].

Sampling and Analytical Techniques

A total of 870 mineralised core samples from 34 boreholes of Koppunuru area have been collected for disequilibrium studies, which includes 620 quartzite/grit and 250 granite samples. Drilling was done using different capacity mechanical coring rigs (Rock Drill-30, Drill Max-400 and Trolley mounted Rock Drill-300). Uranium mineralisation has been intercepted along the studied boreholes at different depths ranging from 26.0 m to 170.0 m. Different mineralised bands show grade and thickness ranging from 0.01% to 0.322% eU₃O₈ and 0.6 m to 7.0 m respectively. Details of mineralized intercepts, lithounit, depth of unconformity, and number of samples collected is shown in Table 1. The mineralised core samples were crushed to -200 mesh to homogenize. 50 g and 140 g of sample were taken after conning and quartering for determination of U₃O₈ (%) by β/γ method, and equivalent U₃O₈ (% eU₃O₈), radium equivalent U₃O₈ (%Ra eU₃O₈), % ThO₂ and % K using High Energy Gamma Ray Spectrometry (HEGS) respectively [6,20]. The representative samples were transferred to airtight plastic containers and kept for about a month for attainment of radioactive equilibrium between radon daughters and parent radium in the uranium series [21,22]. Besides, U3O8 contents in the samples were also estimated by simultaneous measurement of total beta and total gamma radiations using a LND 73201 beta tube, and 1.75” x 2” NaI(Tl) scintillation detector, respectively in the samples [23,24].

Table 1: Details of mineralised zones in boreholes and host rock of Koppunuru area, Guntur district,Andhra Pradesh.

Estimation of uranium

The concentration of U₃O₈ in the sample was estimated by simultaneous measurement of total beta and total gamma radiations by beta gamma method using equation:

U₃O₈= (1+C)U_β-CU_γ [25] (1)

Where U_{β= β} activity of uranium in sample.

U_{γ = γ} activity of uranium in sample.

C=ratio of Ra_β to U_β in standard.

The detection limit is 90 ppm with ± 10% error. For accuracy an IAEA reference standard RGU-1 (U₃O₈ value 460 ppm and Ra(eU₃O₈) value 470 ppm) was also analysed (n=5). The U₃O₈ value obtained for RGU-1 by this method was 453 ± 24 ppm.

Estimation of Ra (eU₃O₈), ThO₂ and K

Ra( eU₃O₈ ), ThO₂ and K concentrations in the samples were estimated by using gamma ray spectrometry. A 5” x 4” NaI(Tl) scintillation detector was used for the analysis. The detector was coupled to a dMCA-pro-digital- Multi Channel Analyser (Terjet, Germany). The dMCA directly digitizes signals from the radiation detector and stores them in the format desired by the inbuilt software (winTMCA32). For the estimation of Ra(eU₃O₈), the 1.76 MeV gamma ray energy was measured from the Bi-214 as the daughter of radium series always remains in equilibrium with radium. The estimation of ThO₂ was done by measuring the 2.62 MeV gamma ray energy from Tl-208 and the 1.46 MeV of gamma ray energy was measured for the estimation of % K. Prior to this, energy calibration was done using standard gamma ray sources ¹³⁷Cs 662 KeV and ⁶⁰Co 1173KeV and 1332 KeV energies. The stripping and sensitivity factors were calculated using standard reference material RGU-1, RGTh-1 and RGK-1supplied by IAEA, Vienna. An In-house (developed at Atomic Minerals Directorate for Exploration and Research, Hyderabad) equilibrium U₃O₈ standard was also used for sensitivity calculations. The samples and standards were taken in the plastic containers of the same volume and size to maintain a same counting geometry to minimize the geometrical error. The containers were sealed carefully to avoid the escape of radon gas from the samples. The counting of samples was done in a Low Background room which is ~4 ft below the ground level and walls of the room are made of quartz, with a thickness of 0.9 m. The energy spectra from each sample were obtained by placing the sample on the top of detector.

The Ra(eU₃O₈) concentration was calculated by dividing the net peak area of the characteristic gamma ray energy of 1.76 MeV to the sensitivity of radium [26]. Sensitivity of Ra(eU₃O₈) with a counting time of 200 s is 6.5 counts/ppm for 140 g of sample weight and detection limit is 2 ppm (error <10%). ThO₂ concentration was calculated by dividing the net peak area of the characteristic gamma ray energy of 2.62 MeV to the sensitivity of thorium and similarly the concentration of % K is calculated by dividing the net peak area of characteristic gamma ray energy of 1.46 MeV to the sensitivity of potassium. Sensitivity of ThO₂ with a counting time of 200 s is 2.1 counts/ppm for 140 g of sample weight and detection limit is 5 ppm (error <10%). The net peak area of gamma ray was obtained by subtracting background counts and stripping of the higher energy contribution.

The uranium concentration, Ra(eU₃O₈) concentration of 870 core samples from 34 boreholes of Koppunuru deposit with disequilibrium factor (DF), number of samples of both above and below unconformity, mineralised depth ranges, and minimum, maximum and average values of U₃O₈, Ra(eU₃O₈) and ThO₂ of the studied core samples are given in Table 2.

Table 2: Details of U₃O₈, Ra(eU₃O₈ ) and disequilibrium factor (DF) of borehole core samples of Koppunuru area, Guntur district, Andhra Pradesh.

The linear regression equation between radium concentration and uranium concentration has been found from the regression plot:

Y(U₃O₈)=1.384 X (Ra(eU₃O₈)) +5.862 with R²=0.976 (2)

The linear regression plot of studied samples has indicated a correlation coefficient of 0.976 (Figure 2). This plot indicates the association of daughter product with significant enrichment of parent and good correlation among them. The average ThO₂ concentrations of these borehole core samples range from 5 ppm to 78 ppm. The split details of number of boreholes, the concentration ratio of ThO₂/ U₃O₈ in different boreholes is shown in Figure 3. This histogram is showing statistics of the distribution pattern of boreholes in different ratio ranges of ThO₂/ U₃O₈ viz. maximum 10 boreholes fall between 0.01-0.02 range while only 1 borehole falls under 0.13-0.14 range category.

Figure 2: Linear regression plot between Ra (eU₃O₈) and U₃O₈ of borehole core samples.

Figure 3: Histogram showing ThO₂/U₃O₈ ranges of borehole core samples of Koppunuru area, Guntur district.

The disequilibrium factor (DF) in the sample is calculated by the following formula

DF=U₃O₈ in the sample/Ra (eU₃O₈) in the sample (3)

The disequilibrium is towards the parent uranium if the value of DF is more than one (DF>1). This is favourable for the prospector as it shows enrichment of uranium resulting in positive corrections in the final ore reserve estimation based on total gamma ray logging data. In contrast, if the value of DF is less than one (DF<1), then disequilibrium is towards the daughter radium and is a non-desirable condition for uranium prospecting. It signifies partial removal of uranium from the system leading to a lowering of final ore reserve estimates based on total gamma ray logging data. All the boreholes have shown average DF significantly greater than 1 and the disequilibrium factors for studied core samples are listed in Table 2, which shows an average value of 1.41. This suggests enrichment of uranium either due to remobilization and deposition of uranium at the present locale or leaching of daughter products of the uranium series leading to an increase in concentration of parent uranium. These features are further supported by the presence of fractures, faults, felsic and mafic intrusive signifying pre- and post-depositional reactivation in the area providing a hydrothermal gradient for remobilization [17,19]. In addition, the presence of higher hydrouranium content (<10 ppb) away from the ore deposit suggests a possible role of groundwater on radioelement migration and fixation at suitable locales [27].

The studied mineralised core samples are broadly classified in two groups i.e. granite (below uniformity) and cover rock of Banganapalle quartzite/grit (above the unconformity). The disequilibrium factor is separately calculated for both types of samples. Average disequilibrium factor for the granite and quartzite/grit samples is 1.41 (Table 3) and 1.40 (Table 4) respectively. Thus, the study distinctly indicates close similarity in disequilibrium factor for both groups of samples, irrespective of the different lithic compositions and geologic position.

Table 3: Disequilibrium factor of Granites borehole core samples (below unconfirmity), Koppunuru area, Guntur District, Andhra Pradesh.

Table 4: Disequilibrium factor of Banganapalle quartzite /grit borehole core samples (above unconformity), Koppunuru area, Guntur district, Andhra Pradesh.

Impact on the ore reserve estimation

The presence of disequilibrium in uranium series between parent and daughter Radium-226 implies that ore grades of mineralised zones demarcated based on total gamma ray logging results needs to be corrected. Usage of Disequilibrium factor will lead to an upward revision in the total estimated resources. The disequilibrium correction factor calculated for core samples of different boreholes ranges from 1.29 to 1.47 of eU₃O₈ (Table 5).

Table 5: Disequilibrium factor core samples with different eU3O8ranges of Koppunuruarea, Guntur district, Andhra Pradesh.

The disequilibrium studies on the mineralised samples from the boreholes of the Koppunuru deposit has indicated:

1. Presence of strong disequilibrium in favour of parent uranium, with average value of 1.41.

2. DF is nearly same in the mineralisation hosted by both basement granites and Banganapalle quartzite.

3. Presence of disequilibrium in the mineralised zone implies an upward correction in the ore grades, calculated based on total gamma log, by a factor of 1.41. Thus, an increase in grade and tonnage of the total estimated resources in Koppunuru deposit is indicated.

The authors are grateful to Shri. Parihar PS, Director, AMD, Hyderabad for giving permission to publish this paper. They also extend sincere thanks to Shri. Nanda LK, Additional Director (OP-III) and Dr. A.K. Chaturvedi, Additional Director (R&D), AMD, Hyderabad for their suggestions and encouragement.