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Radioactive Disequilibrium Studies in Uranium Series of Core Samples of Koppunuru Area, Guntur District, Andhra Pradesh, India

Srinivas Y*, Singh RV, Rahul Banerjee, Sharma PK and Verma MB

Atomic Minerals Directorate for Exploration and Research, Begumpet, Hyderabad

*Corresponding Author:
Srinivas Y
Atomic Minerals Directorate for Exploration and Research
Begumpet, Hyderabad
Tel: +918985738540; +914027776398
Fax: +914027764043
E-mail: [email protected]

Received date: December 15, 2016; Accepted date: January 19, 2017; Published date: January25, 2017

Citation: Srinivas Y, Singh RV, Rahul Banerjee, Sharma PK, Verma MB (2017) Radioactive Disequilibrium Studies in Uranium Series of Core Samples of Koppunuru Area, Guntur District, Andhra Pradesh, India. J Geol Geophys 6:277. doi: 10.4172/2381-8719.1000277

Copyright: © 2017 Srinivas Y, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

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Abstract

Disequilibrium studies were attempted on mineralised core samples (n=870) from Koppunuru uranium deposit located in south-western part of Palnad sub-basin, Guntur district, Andhra Pradesh, India. The area exposes Banganapalle quartzites unconformably deposited over altered biotite granite (basement). Uranium mineralisation in Koppunuru deposit is hosted by Banganapalle quartzites well above the unconformity, and grit/basement granite close to the unconformity contact. For disequilibrium studies, the core samples were broadly divided in two groups, (a) quartzite hosted (above unconformity) and (b) basement granite hosted mineralisation (below unconformity). Average disequilibrium factor of 41% has been recorded in favour of parent uranium in both types of core samples. It shows significant enrichment of uranium in the system as evident from 41% of disequilibrium in favour of parent uranium. This is probably due to significant migration of some of the daughter radio nuclides due to dissolution of minerals by groundwater action. Besides, the escape of radioactive radon might have accentuated the disequilibrium factor thus increasing the grade of uranium mineralization. The presence of fractures and faults in the study area are the probable conduits for radon migration/escape. Linear regression coefficient between uranium and radium is 0.98 indicates invariability of disequilibrium irrespective of grade.

Keywords

Uranium; Disequilibrium factor; Beta-gamma method; Gamma ray spectrometry; Guntur district; Andhra Pradesh

Introduction

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].

geology-geosciences-Geological-map

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].

S.No BH NO. Mineralised zone Thickness x Ave.Grade (%eU3O8) Rock Type Depth of unconformity (m) n
from (m) to (m)
1 KPU/23 101.25 101.85 0.60 m of 0.040 Gritty Quartzite 107 8
104.75 106.45 1.70 m of 0.090 Gritty Quartzite
2 KPU/57 106.21 107.21 1.00 m of 0.319 Granite 104.3 9
3 KPU/68 53.15 53.75 0.60 m of 0.023 Gritty Quartzite 56 3
55.75 56.75 1.00 m of 0.031 Granite 2
4 KPU/76 63.15 64.35 1.20 m of 0.088 Gritty Quartzite 65.2 7
5 KPU/79 54.88 57.95 3.07 m of 0.130 Gritty Quartzite 58.5 23
60.05 61.27 1.22 m of 0.130 Granite 10
6 KPU/82 160 161.4 1.40 m of 0.041 Gritty Quartzite 162 3
163.8 164.8 1.00 m of 0.016 Granite 2
7 KPU/93 104.65 108.45 3.80 m of 0.130 Quartzite/Shale 119 11
114.95 116.35 1.40 m of 0.017 Gritty Quartzite
8 KPU/108 156.65 157.65 1.00 m of 0.043 Gritty Quartzite 161 5
158.95 159.25 0.30 m of 0.011 Gritty Quartzite
9 KPU/110 140.85 142.05 1.20 m of 0.021 Granite 140.7 14
148.63 149.79 1.16 m of 0.031 Granite
10 KPU/112 165.25 168.85 3.60 m of 0.051 Quartzite/Shale 169.45 28
Gritty Quartzite
11 KPU/118 140.85 142.45 1.60 m of 0.100 Gritty Quartzite 145.15 22
145 146 1.00 m of 0.019 Granite 9
12 KPU/123 159.1 163.1 4.00 m of 0.028 Gritty Quartzite 167.4 17
13 KPU/124 125.96 127.1 1.14 m of 0.224 Gritty Quartzite 127.8 16
129.9 131.38 1.48 m of 0.020 Granite 24
14 KPU/131 147.24 148.43 1.19 m of 0.017 Gritty Quartzite 149.6 23
148.72 150.03 1.31 m of 0.016 Granite 4
15 KPU/138 141.64 147.08 5.44 m of 0.322 Gritty Quartzite 156.4 93
147.9 152.81 4.91 m of 0.098 Gritty Quartzite
154.14 155.29 1.15 m of 0.025 Gritty Quartzite
155.4 158.11 2.67 m of 0.020 Granite 20
160.38 161.65 1.27 m of 0.018 Granite
16 KPU/144 128.38 130.04 1.66 m of 0.175 Gritty Quartzite 134.15 12
135.22 136.26 1.04 m of 0.024 Granite 38
139.04 140.27 1.23 m of 0.021 Granite
149.15 150.61 1.46 m of 0.032 Granite
17 KPU/147 128.05 129.25 1.20 m of 0.090 Gritty Quartzite 142.8 3
143.59 144.88 1.29 m of 0.022 Granite 13
18 KPU/152 135.95 138.05 2.10 m of 0.270 Gritty Quartzite 155.7 21
155.8 156.4 0.60 m of 0.026 Granite 10
160.75 161.8 1.05 m of 0.031 Granite
19 KPU/153 113 114.07 1.07 m of 0.019 Gritty Quartzite 151.2 13
132.75 133.75 1.00 m of 0.073 Gritty Quartzite
20 KPU/155 121.5 122.7 1.30 m of 0.074 Gritty Quartzite 138.5 6
21 KPU/156 154.4 155.75 1.35 m of 0.017 Gritty Quartzite 162.35 16
22 KPU/163 138.31 139.34 1.03 m of 0.067 Gritty Quartzite 143.8 28
142.15 143.66 1.51 m of 0.099 Gritty Quartzite
154.86 157 2.14 m of 0.152 Granite 37
23 KPU/166 146.22 147.42 1.20 m of 0.027 Gritty Quartzite 156.9 22
155.05 156.85 1.80 m of 0.043 Gritty Quartzite
156.9 159.3 2.40 m of 0.027 Granite 20
24 KPU/169 90.45 91.65 1.20 m of 0.048 Gritty Quartzite 98.8 12
25 KPU/172 40.03 41.11 1.08 m of 0.081 Quartzite 84.3 20
80.15 81.85 1.70 m of 0.038 Gritty Quartzite
86.65 86.55 0.90 m of 0.012 Granite 6
26 KPU/180 37.41 39.88 2.47 m of 0.139 Quartzite 91 28
79.38 80.42 1.04 m of 0.101 Gritty Quartzite
81.05 82.25 1.20 m of 0.067 Gritty Quartzite
27 KPU/181 106.55 108.55 1.20 m of 0.065 Gritty Quartzite 108.8 16
108.9 109.2 0.30 m of 0.200 Granite 1
28 KPU/184 115.65 116.67 1.02 m of 0.018 Gritty Shale 128.8 52
121.55 128.55 7.00 m of 0.038 Gritty Quartzite
139.55 141.45 1.90 m of 0.025 Granite 14
29 KPU/226 38.35 39.65 1.30 m of 0.031 Gritty Quartzite/Shale 40.65 7
30 KPU/230 25.45 27.35 1.90 m of 0.057 Conglomerate 27.9 10
31 KPU/242 79.05 80.95 1.90 m of 0.213 Shale/Quartzite 87.7 13
32 KPU/247 86.48 88.95 2.47 m of 0.105 Quartzite 111.55 45
104.2 106.6 2.50 m of 0.017 Grit/Conglomerate
109.15 110.45 1.30 m of 0.020 Grit/Conglomerate
112.15 112.75 0.60 m of 0.016 Granite 10
119 119.6 0.60 m of 0.023 Granite
33 KPU/252 69.1 70.6 1.60 m of 0.020 Quartzite 73.4 9
73.5 75.7 2.20 m of 0.018 Granite 7
84.3 85.3 1.00 m of 0.011 Granite
34 KPU/255 88.08 89.19 1.11 m of 0.039 Quartzite 100.9 28
94.35 96.25 1.90 m of 0.031 Conglomerate
Total 870

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)Uβ-CUγ [25] (1)

Where Uβ= β activity of uranium in sample.

Uγ = γ activity of uranium in sample.

C=ratio of Raβ to Uβ 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.

Results and Discussion

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.

S. No. BH No. No of sample U3O8 (ppm) Ra(eU3O8) ppm Av. DF No. of samples
ThO2
(n) Min Max Av. Min. Max. Av. (ppm) Above u/c below u/c
1 KPU/23 8 107 2875 774 73 2028 624 21 1.17 8 0
2 KPU/57 9 91 1344 293 57 536 158 19 1.64 0 9
3 KPU/68 5 139 420 255 84 372 181 26 1.47 3 2
4 KPU/76 7 210 3600 1333 132 2270 830 57 1.47 7 0
5 KPU/79 33 122 13529 1749 91 8185 1236 26 1.32 23 10
6 KPU/82 5 140 521 285 104 413 221 17 1.34 3 2
7 KPU/93 11 99 715 330 83 592 267 5 1.27 11 0
8 KPU/108 5 94 448 203 69 294 139 21 1.44 5 0
9 KPU/110 14 113 1848 488 111 996 303 32 1.37 0 14
10 KPU/112 28 95 2730 516 64 2483 375 22 1.35 28 0
11 KPU/118 31 95 1099 291 75 927 251 20 1.28 22 9
12 KPU/123 17 90 1247 352 59 941 265 34 1.4 17 0
13 KPU/124 40 92 7040 656 55 6034 530 22 1.29 16 24
14 KPU/131 27 90 1095 207 68 820 163 26 1.28 23 4
15 KPU/138 113 93 8933 1489 80 7689 1215 25 1.26 93 20
16 KPU/144 50 105 7298 724 78 5081 533 63 1.33 12 38
17 KPU/147 16 101 9851 1179 96 6881 795 78 1.33 3 13
18 KPU/152 31 111 2104 422 98 1541 335 12 1.23 21 10
19 KPU/153 13 123 4264 621 97 3104 470 6 1.27 13 0
20 KPU/155 6 117 3055 1154 85 1908 781 12 1.37 6 0
21 KPU/156 16 122 559 226 94 302 160 11 1.44 16 0
22 KPU/163 65 91 7863 841 72 4635 594 27 1.33 28 37
23 KPU/166 42 95 6700 558 61 5098 402 27 1.44 22 20
24 KPU/169 12 229 3165 1048 187 2188 712 18 1.36 12 0
25 KPU/172 26 102 3241 802 81 2297 566 9 1.4 20 6
26 KPU/180 28 110 13851 2004 28 6885 1333 15 1.88 28 0
27 KPU/181 17 105 4973 1506 15 3491 1170 33 1.62 16 1
28 KPU/184 66 114 6066 509 63 2240 307 15 1.5 52 14
29 KPU/226 7 95 681 316 100 373 224 13 1.32 7 0
30 KPU/230 10 118 3623 632 84 1968 507 8 1.43 10 0
31 KPU/242 13 101 9558 3071 86 5652 2065 6 1.29 13 0
32 KPU/247 55 115 3923 544 82 2909 394 16 1.59 45 10
33 KPU/252 16 90 953 259 46 358 129 32 2 9 7
34 KPU/255 28 90 907 358 74 679 245 28 1.46 28 0
  Total 870                    
  Average 112 4120 765 80 2711 544 24 1.41 620 250

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.

geology-geosciences-Linear-regression

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

geology-geosciences-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.

S. No. BH. No. No. of samples DF
1 KPU/57 9 1.64
2 KPU/68 2 1.06
3 KPU/79 10 1.2
4 KPU/82 2 1.57
5 KPU/110 14 1.37
6 KPU/118 9 1.36
7 KPU/124 24 1.29
8 KPU/131 4 1.28
9 KPU/138 20 1.46
10 KPU/144 38 1.32
11 KPU/147 13 1.33
12 KPU/152 10 1.36
13 KPU/163 37 1.34
14 KPU/166 20 1.49
15 KPU/172 6 1.62
16 KPU/181 1 1.03
17 KPU/184 14 1.45
18 KPU/247 10 1.86
19 KPU/252 7 1.85
    Total=250 Average=1.41

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

S. No. BH. No. No. of samples DF
1 KPU/23 8 1.17
2 KPU/68 3 1.75
3 KPU/76 7 1.47
4 KPU/79 23 1.38
5 KPU/82 3 1.18
6 KPU/93 11 1.27
7 KPU/108 5 1.44
8 KPU/112 28 1.35
9 KPU/118 22 1.25
10 KPU/123 17 1.4
11 KPU/124 16 1.3
12 KPU/131 23 1.28
13 KPU/138 93 1.24
14 KPU/144 12 1.36
15 KPU/147 3 1.36
16 KPU/152 21 1.2
17 KPU/153 13 1.27
18 KPU/255 6 1.37
19 KPU/256 16 1.44
20 KPU/163 28 1.3
21 KPU/166 22 1.39
22 KPU/169 12 1.36
23 KPU/172 20 1.33
24 KPU/180 28 1.88
25 KPU/181 16 1.66
26 KPU/184 52 1.51
27 KPU/226 7 1.32
28 KPU/230 10 1.43
29 KPU/242 13 1.29
30 KPU/247 45 1.51
31 KPU/252 9 1.79
32 KPU/255 28 1.46
    Total = 620 Average=1.40

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).

>
Range eU3O8(ppm) No of Samples Avg. U3O8(ppm) DF=U/Ra
90-200 412 162 1.47
201-400 204 332 1.29
401-600 49 613 1.32
601-800 33 953 1.39
801-1000 20 1264 1.45
1001-1500 53 1711 1.4
1501-2000 28 2275 1.33
2001-2500 21 3067 1.42
2501-3000 9 3480 1.3
3001-4000 18 4572 1.35
4001-8500 22 7817 1.39
 

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

Conclusion

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.

Acknowledgment

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.

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