Geophysical Anomalies Associated with Uranium Mineralization from ...

JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.82, December 2013, pp.601-606

Geophysical Anomalies Associated with Uranium Mineralization from Beldih Mine, South Purulia Shear Zone, India ANIMESH MANDAL1, ARKOPROVO BISWAS1, SAURABH MITTAL1, WILLIAM K. MOHANTY1, SHASHI PRAKASH SHARMA1, DEBASHISH SENGUPTA1, JOYDIP SEN2 and A. K. BHATT3 1

Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur – 721302 2 Atomic Minerals Directorate for Exploration and Research, Jamshedpur – 831002, 3 Atomic Minerals Directorate for Exploration and Research, Jaipur – 302030 Email: [email protected]

Abstract: Beldih mine at the central part of the South Purulia Shear Zone (SPSZ) has been reported with low grade uranium-bearing formation within quartz-magnetite-apatite host in kaolinized formation. Therefore, the present integrated geophysical study with gravity, magnetic, radiometric, very low frequency electromagnetic (VLF) and gradient resistivity profiling methods around the known mineralized zones aimed at identifying the exact geophysical signatures and lateral extent of these uranium mineralization bands. The closely spaced gravity-magnetic contours over the low to high anomaly transition zones of Bouguer, reduced-to-pole magnetic, and trend surface separated residual gravity-magnetic anomaly maps indicate the possibility of high altered zone(s) along NW–SE direction at the central part of the study area. High current density plots of VLF method and the low resistive zones in gradient resistivity study depict the coincidence with low gravity, moderately high magnetic and low resistivity anomalies at the same locations. Moderate high radioactive zones have also been observed over these locations. This also suggests the existence of radioactive mineralization over this region. Along profile P2, drilled borehole data revealed the presence of uranium mineralization at a depth of ~100 m. The vertical projection of this mineralization band also identified as low gravity, low resistivity and high magnetic anomaly zone. Thus, the application of integrated geophysical techniques supported by geological information successfully recognized the nature of geophysical signatures associated with the uranium mineralization of this region. This enhances the scope of further integrated geophysical investigations in the unexplored regions of SPSZ. Keywords: SPSZ, Integrated geophysics, Uranium mineralization, Beldih mine, West Bengal. INTRODUCTION

Mineral exploration mainly start from the geological concept/understanding of the areas under investigations and this provides the guidance in selecting the geophysical methods for sub-surface investigation. The high density and radioactivity are the only physical properties permitting the direct detection of the uranium minerals (Ford et al. 2007). Again, this density and radioactive signal becomes weak for the mineralization at depth and with no surface exposure. Similar is the situation for the present study area (Beldih) (Fig.1a). The Atomic Mineral Directorate, Jamshedpur has carried out extensive geological and geochemical surveys across South Purulia Shear Zone (SPSZ) towards the north of Singhbhum Shear Zone (SSZ) and their results suggest the possibility of uranium mineralization in the area (Katti et al. 2010; Sen et al. 2010). The reconnoiter drilling in Beldih area has also confirmed the depth continuity (around 100

m) of uranium mineralization (Katti et al. 2010; Sen et al. 2010). This indicates that the possible radioactive mineralization is deep seated which may be guided by some structural features like, sub-surface faults, fractures or alteration zones. Integrated geophysical exploration program had been proved to be successful in identifying these sub-surface evidences of mineralization or associated structures/alteration zones (e.g., Mohanty et al. 2011; Chaturvedi et al. 2013; Patra et al. 2013). However, detailed geophysical studies have never been carried out for delineation of the alteration zones and rock types associated with the uranium mineralization of the region. Therefore, in the present study an integrated geophysical investigation using five methods namely, gravity, magnetic, radiometric, very low frequency (VLF) electromagnetic and resistivity has been carried out in and around the mineralization zones of Beldih mine area. The different anomalies are compared with each other and with the geologic maps to

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evaluate their effectiveness for displaying the uranium bands and to know the geophysical signature for the known uranium bands. GEOLOGY OF STUDY AREA

The present study area situated at the major contact zone between North Singhbhum Mobile Belt (NSMB) in the south and the Chotanagpur Granite Gneissic Complex (CGGC) in the north (Gupta and Basu, 2000; Acharyya et al. 2006) in the east Indian shield at the central part of the ENE-WSW to E-W to ESE-WNW trending curvilinear lineament popularly known as South Purulia shear zone (see inset, Fig. 1a). Within the study area (Beldih) exposures of ultramafic rocks, carbonatite, metabasics, tuffaceous phylite, chloritemica schist, quartzite, and alkali granite and quartzmagnetite-apatite rocks are very frequently observed (Gupta and Basu, 2000; Acharyya et al. 2006; Vapnik et al. 2007; Sen et al. 2010) (Fig. 1a). The ferruginous kaolinite and quartz-magnetite-apatite rocks aligned in EW direction within the altered/intense brecciated ultramafic and alkali granite of the shear zone are acting as the hosts for uranium mineralization at Beldih (Katti et al. 2010; Sen et al. 2010) (Fig. 1a). The primary source of radioactivity of this region has been identified as adsorbed uranium on Fe-Mn hydroxide/oxide phases. The uranium mineralization along with pyrite, magnetite, apatite and siderite (Fe carbonate) is intercepted at vertical depth of 100–120 m in the borehole BLD3 of Beldih (Katti et al. 2010) (Fig. 1b). Based on this mineralogical association, the area in and around Beldih mine was chosen for the application of integrated geophysical study.

GEOPHYSICAL SURVEY Gravity-magnetic

Total of 58 gravity-magnetic observations were performed over the study area along four N-S profiles (Fig. 1a) with a station interval of 25 m using W. Sodin gravimeter (sensitivity 0.01 mGal) and a proton precession magnetometer (sensitivity 1 nT), respectively. The gravity measurements obtained from the study area were tied to the nearest absolute gravity base station at Purulia railway station (Absolute gravity value (gn) = 978796.73 mGal) (Qureshy et al. 1973). All the gravity-magnetic raw data were corrected for spatial and temporal noise by following standard data reduction procedure (i.e. Instrumental drift, free-air, and Bouguer correction for gravity; and diurnal, International Geomagnetic Reference Field-2011 (IGRF 11) model corrections for magnetic). In the present study, the reductionto-pole transformation of the magnetic anomalies is performed by analytical signal approach using Hilbert transform (Young, 2004). The Bouguer anomaly and reduced- to-pole total field magnetic anomaly map are shown in Fig. 2a and 2b, respectively. The regional-residual separation of Bouguer and magnetic anomalies were performed using third degree trend surface analysis method (Unwin, 1978). The obtained residual gravity and magnetic anomaly maps (Fig. 2c and 2d) show a good correlation with the surface geology (Fig. 2e and 2f). Radiometric

A radiometric survey was carried out using a portable a

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Fig.1a–b. (a) Local geological map of Beldih mine study area (modified after Katti et al. 2010) along with the gravity-magnetic, and gradient resistivity profiling, VLF and radiometric survey locations. The broad geology of North Singhbhum Mobile Belt (NSMB) and location of Beldih is shown in inset. (b) Longitudinal section through drill hole BLD3 (after Katti et al. 2010). JOUR.GEOL.SOC.INDIA, VOL.82, DEC. 2013

GEOPHYSICAL ANOMALIES ASSOCIATED WITH U-MINERALIZATION FROM BELDIH MINE, WEST BENGAL a

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pulsed Geiger–Muller and portable scintillation counter along two selected profiles (L1 and L3; Fig. 1a). For detailed radiometric survey over the high radiation zone, total radiation counts were measured at 10 m interval along the profiles. For each measurement, the sensor of Geiger–Muller counter was kept approximately 15 cm above the ground surface and operated for 100 seconds. The α-measurements were undertaken ~1 m above the ground surface and on the direct exposure to get maximum results. Each measurement was carried out for more than a minute and was repeated thrice. Very Low Frequency Electromagnetic

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The ABEM Wadi VLF instrument that utilizes only a magnetic field component was used for measurement. The strike of the formation around the mine pit is approximately in the E-W direction; hence a transmitter in E-W direction with frequency 19.8 kHz (located in Bombay, India) is selected and the VLF data are collected along three N-S profiles (L1–L3) with 10 m station interval (Fig. 1a). The apparent current density (J) is computed using the observed real VLF anomalies by application of digital linear filtering approach (Karous and Hjelt, 1983) and current density crosssections along the three profiles are presented in Fig. 3a. Gradient Resistivity Profiling

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Fig.2a–f. (a) Bouguer anomaly map of the study area, (b) Reducedto-pole total field magnetic anomaly map of the study area, (c) Residual gravity anomaly map of the study area, (d) Reduced-to-pole residual magnetic anomaly map of the study area, (e) Combine plot of residual gravity and geology, and (f) Combine plot of reduced-to-pole residual magnetic and geology map. Plus (+) symbols are the gravity-magnetic survey locations.

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Gradient profiling has been carried out by placing current electrodes at 400 m apart and measuring the potential in the central one third portion of the profile (Furness, 1993; Yadav and Singh, 2007). In the present study, gradient resistivity profiling measurements are performed using a very sensitive resistivitymeter along three selected profiles over nearly north-south electrodes spreading (GRP1–3, Fig. 1a). These profiling were carried out using a fixed current electrode separation of 400 m with respect to the centre point. In all the gradient profiles measurements the potential electrodes are kept 10 m apart, and subsequently, moving them at regular intervals of 10 m. Apparent resistivity is computed for each position of the potential electrodes and assigned a value corresponding to the center of the potential electrodes. RESULTS Gravity

Bouguer and residual gravity anomaly maps (Fig. 2a and 2c) show some low gravity anomaly patches along profiles P1, P2 and P3 correlating well with the geology of the region (Fig. 2e). The profile P1 on the western side of mine mostly passes through the wet cultivated land. The ultramafic ± carbonatite rock bands are sporadically exposed over the

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Fig.3a–b. (a) Current density sections as computed from the VLF real anomaly along profiles L1, L2, and L3, (b) Combine plot of current density sections and geology map, the W–E continuation of the conducting zone is shown within black dashed line.

study area. It is very much possible that the southernmost part of the profile P1 also have such high density rocks under the thin soil cover. This may give rise to this high gravity over these regions. Again, kaolinization may present below the cultivated land area causing low gravity. The region along profile P2 mostly covered by mine dump but the borehole (BLD3) depicts the kaolin zone under the mine dump. This kaolinized altered zone causes the gravity low along this profile. This low gravity zone further continues toward east of P2 along the northern side of profile P3 which mostly passes through the cultivated land. Thus, the low gravity regions are correlated with the kaolinized/altered quartz-magnetite-apatite band of the study area (Fig. 2e and 2f). However, the high gravity values are associated with mafic-ultramafic rocks since the broken fragments of the rocks are observed in the mine dump (Figs. 1a and 2c). Magnetic

The reduced-to-pole total field magnetic anomaly map

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(Fig. 2b) depicts high to low magnetic anomaly transition from SW–NE direction. This indicates the existence of a highly altered zone along NW–SE direction passing near to GRP2 at the central region of the study area. The ultramafics and carbonatite, and altered zones are observed as magnetic high (Figs. 1a and 2d). The moderately high anomaly patches and high to low transitions zones along profiles P1 and P2 depict good correlation with the low gravity anomaly patches (Fig. 2c, 2d, 2e and 2f). These high to low transition zones are probably the altered zones containing magnetic minerals and provide a good clue to sub-surface information. Very Low Frequency Electromagnetic

The south to north profile L1 on the west of mine depicts two conductive zones at 150–300 m and 400–500 m locations (Fig. 3a–b). Along L2 on east of the mine two narrow and localized conducting zones at ~85 m and ~250 m locations as well as a broad high current density zone between 125–200 m are observed (Fig. 3a–b). Further east

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Fig.4a–c. Comparison of residual gravity-magnetic anomaly, apparent resistivity and radioactive gamma activity along different profiles. (a) gravity-magnetic profile P1, GRP1, and L1 (b) gravity-magnetic profile P2, and GRP2, (c) gravity-magnetic profile P3, GRP3, and L3. JOUR.GEOL.SOC.INDIA, VOL.82, DEC. 2013

GEOPHYSICAL ANOMALIES ASSOCIATED WITH U-MINERALIZATION FROM BELDIH MINE, WEST BENGAL

along L3 three high current density zones are observed between 50–150 m, 250–400 m, and 450–500 m locations (Fig. 3a–b). High current density corresponds to conducting features (possibly fracture, kaolinized zones or altered zones in this area where it coincides with low gravity anomaly) and low current density corresponds to resistive structures (granite or ultramafic and carbonatite rocks in this area). Resistivity

The calculated apparent resistivity values for gradient resistivity profile 1 (GRP 1) (Fig. 4a) vary nearly from 80 Ohm.m to 1000 Ohm.m. A conductive valley (of resistivity ~200 Ohm.m) at around 250 m positions between two high resistive (~400 Ohm.m) humps is observed along this profile. In the next profile line (GRP 2) (Fig. 4b); the apparent resistivity value varies in the range of 10–200 Ohm.m. A sharp drop of apparent resistivity (~10 Ohm.m) at around 170 m location indicates the presence of shallow surface conducting features. However, another low and broad resistive zone (apparent resistivity ~20–30 Ohm.m) at 220– 275 m location represent the deeper surface conducting features. The third profile (GRP3) (Fig. 4c) at 160 m east of GRP2, mostly passes through the low resistive zones compared to the profiles on the western side. A broad low resistive zone (apparent resistivity ~80 Ohm.m) at 100–250 m location is depicted between two high resistive zones of this profile. Radiometric The profile L1 on the west of mine starts from south and covers a distance of 600 m. There are three high gamma activity zones throughout the profile around 150 m, 250– 300 m, and 400 m locations (Fig. 4a). The corresponding gamma activity values are nearly 20, 25 and 15 µR/h, respectively. The profile L3 on the eastern side of the mine starts from north and covers a distance of 615 m. Some high gamma activity zones corresponding to its background has been observed along this profile at 80–100 m (~6–8 unit high), 290–300 m (~2 units high), and 350 m (~2-3 units high) (Fig. 4c). The gamma activity reaches a highest value of ~15 µR/h at 80–100 m location. These high surface radiation and gamma activity zones are the radioactive zones and may be associated with the possible uranium mineralization zones of the area. DISCUSSION

In the present integrated geophysical investigation; regions with low gravity and resistivity, high magnetic, high VLF current density and high gamma activity value depict good correlations with each other and with the surface JOUR.GEOL.SOC.INDIA, VOL.82, DEC. 2013

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geology (Figs. 1a, 2e–f, and 4a–c). The coincidence of high to low transition of gravity-magnetic anomalies, and low resistivity values indicate the existence of alteration zones over the study area. However, the best correlation between these anomalies and identified radioactive band has been observed along P2, GRP2, L2 (VLF) profile and borehole BLD3 (Fig. 4b). Two zones are identified; zone1 with low gravity, low to high magnetic, low resistive/high conductive zones at around 125–200 m (Figs. 3a and 4b) and another with low gravity, high magnetic, low resistive/high conductive at around 225–275 m locations (Figs. 3a and 4b). From drill core data (Fig. 1b) it is clear that these two zones are coinciding with the altered and brecciated ultramafic, kaolinized zone along with quartz-magnetiteapatite hosted radioactive zones under the overburden mine dump at a vertical depth of around 100–120 m (Fig. 1b). The zone of low gravity, low to high magnetic and low resistivity/high conductivity is perfectly matched with drill core data of BLD3 at a depth of ~100 m where low gravity for hydrothermal alterations, high magnetic due to host characteristics (quartz-magnetite-apatite), and low resistivity/high conductivity resulting from development of pyrite minerals along with uranium mineralization. These types of correlation with low gravity, high magnetic, low resistive/high conductive zones are also observed on GRP1, L1, and P1 on the western side of the mine over three zones (Fig. 4a). However, in zone 1 of the same profile high resistivity is observed at 125 m but on either side of that resistive zone high conducting zone is showing in VLF profile (L1, Fig. 3a). On the extreme east, the same correlations are observed between gravity, magnetic, resistivity and radioactivity variations except in zone 2 where gravity is high (Fig. 4c). High radioactivity has also been observed over the above mentioned zones of the two profiles L1 and L3 (Fig. 4a and 4c). This also strongly suggests the presence of radioactive mineralization over the region. The combine plot of VLF current density pseudosections on the geology map also depicts the W–E continuation of the conductive band (Fig. 3b). Thus, the study indicating the continuation of the observed uranium band at borehole BLD3 on both the sides of the mine in the direction of WNW–ESE and this trend is conformable to the regional strike of the shear foliation. CONCLUSIONS

It can be concluded from the correlation with the known uranium bands at borehole 3 (BLD3) that the combination of gravity, resistivity, radiometric and VLF methods is most suitable for delineation of sub-surface hydrothermal alteration zones. The zones with low gravity anomaly, low

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resistivity/high current density (or conductivity) along with high magnetic anomaly are the most favourable target for uranium mineralization of this area. However, magnetic method can also reveal the sub-surface alteration zones and structural pattern when the mineralization is associated with magnetic material. On the other hand surface radioactivity study also aids in the identification of shallow radioactive zones. The quantitative modeling of the geophysical data sets based on this study will further help to understand the sub-surface structures and mineralization nature of this study area. Thus, the convergence of geophysical signatures of appropriate geophysical techniques increases the reliability of the interpretation and provides input for more detail exploration programme in the region.

Acknowledgements: This is a joint collaborative work of the first three authors and they are Ph.D. students of IIT Kharagpur. We gratefully acknowledge the financial assistance provided by the Board of Research in Nuclear Sciences (BRNS) under Department of Atomic Energy, Government of India (Project No. 2007/36/85-BRNS) in collaboration with Atomic Minerals Directorate (AMD) for Exploration and Research, Eastern Region (ER) Jamshedpur. We are grateful to the Director of AMD for giving us an opportunity to work as collaborators for fulfilling the objectives of this project. We are also thankful to the Editor and Managing Editor for their prompt and efficient editorial handling, and two anonymous reviewers for their constructive suggestions.

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(Received: 3 June 2013; Revised form accepted: 25 September 2013)

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