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The hydrothermally altered vein type uranium mineralization has been reported at different places along the South Purulia Shear Zone (SPSZ) towards 50 km north of the Singhbhum Shear Zone. An integrated close grid gravity-magnetic study over nearly one km2 area between Kutni and Dandudih villages within this shear zone has been performed to delineate the detail structural features, depth continuation, and associated uranium mineralization zones. The observed Bouguer and calculated residual gravity anomaly map depicts an ESE-WNW (on the east) to E-W (on west) trending low gravity zone on the extreme northern side of the area. The total field magnetic anomaly map also depicts the presence of high anomaly patches which is in accordance with the field observation of the exposed iron oxides (e.g. magnetite and goethite) over the same part of the study area. The 2D inverse modeling across the northern low residual gravity anomaly zones reveals steeply dipping low density structures with dip due north, south or vertical at places. The NS widths of these zones vary from 50-300 m with maximum probable depth extent of ∼220 m. Previous researcher has also reported high radioactivity over the exposed quartz-magnetite-apatite bands on the northern part of the area. This band is exactly coinciding with the presently identified low gravity and high magnetic anomaly zones. These anomaly zones on the northern part of the study area are concluded to be hydrothermally altered uranium mineralization target zones. Thus, the results of this study will provide important guiding parameters for the future exploration programme over this region.
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JOURNAL GEOLOGICAL SOCIETY OF INDIA
Vol.85, March 2015, pp.299-304
0016-7622/2015-85-3-299/$ 1.00 © GEOL. SOC. INDIA
Integrated Gravity-Magnetic Study for Delineation of
Structural Guided Uranium Mineralization Zones at
Kutni, Purulia, West Bengal
ANIMESH MANDAL1,2*,WILLIAM K. MOHANTY1 and SHASHI PRAKASH SHARMA1
1Department of Geology and Geophysics, Indian Institute of Technology Kharagpur, Kharagpur – 721 302
2Presently, Theoretical Modeling Group, CSIR-National Geophysical Research Institute, Hyderabad –500 007
*Email: animeshphys@gmail.com
Abstract: The hydrothermally altered vein type uranium mineralization has been reported at different places along the
South Purulia Shear Zone (SPSZ) towards 50 km north of the Singhbhum Shear Zone. An integrated close grid gravity-
magnetic study over nearly one km2 area between Kutni and Dandudih villages within this shear zone has been performed
to delineate the detail structural features, depth continuation, and associated uranium mineralization zones. The observed
Bouguer and calculated residual gravity anomaly map depicts an ESE-WNW (on the east) to E-W (on west) trending
low gravity zone on the extreme northern side of the area. The total field magnetic anomaly map also depicts the
presence of high anomaly patches which is in accordance with the field observation of the exposed iron oxides (e.g.
magnetite and goethite) over the same part of the study area. The 2D inverse modeling across the northern low residual
gravity anomaly zones reveals steeply dipping low density structures with dip due north, south or vertical at places. The
NS widths of these zones vary from 50-300 m with maximum probable depth extent of ~220 m. Previous researcher has
also reported high radioactivity over the exposed quartz-magnetite-apatite bands on the northern part of the area. This
band is exactly coinciding with the presently identified low gravity and high magnetic anomaly zones. These anomaly
zones on the northern part of the study area are concluded to be hydrothermally altered uranium mineralization target
zones. Thus, the results of this study will provide important guiding parameters for the future exploration programme
over this region.
Keywords: Uranium, SPSZ, Gravity, Inverse modeling, Magnetic, West Bengal.
INTRODUCTION
Uranium is an important source of nuclear energy and
its importance will further increase in future with the decline
of worldwide fossil fuels. Several uranium producing
mines in India are located along the Singhbhum Shear Zone
(SSZ) of Eastern Indian Shield (Anand and Rajaram, 2006).
South Purulia Shear Zone (SPSZ) ~50 km north of
Singhbhum Shear Zone (SSZ) has been identified with
similar geometrical shape and both the tectonic belts have
characterized by intense shearing, brecciation and
hydrothermal alteration zones associated with mineralization
of uranium-copper-apatite-magnetite–REE (Basu, 1993;
Gupta and Basu, 2000; Acharyya et al. 2006; Vapnik et al.
2007). These similarities among the two shear zones increase
the possibility of the existence of uranium mineralization
along South Purulia Shear Zone. Direct geophysical
signature of uranium mineralization i.e. high radioactivity
has also been reported from places like Beldih, Raghunath-
pur, Kutni, Chirugora and Porapahar mostly along central
and eastern margin of SPSZ (Katti et al. 2010). Uranium
mineralization at Beldih mine area (nearby to the Kutni area)
has already been established by integrated geophysical
studies and bore hole data (Katti et al. 2010; AMD-BRNS
report, 2012; Mandal et al. 2013, 2014; Mittal et al. 2014).
The ferruginous kaolin rocks and quartz-magnetite-apatite
rocks have acted as the host for the mineralization of these
areas. Again, Dwivedi et al. (2008) identified some hydro-
uranium anomalous zones in and around the above areas
over South Purulia Shear Zone through geochemical study
of the water samples collected from the hand pump of these
areas. The present study area Kutni is one such region
where similar types of host rocks are observed (Katti et al.
2010). However, the surface signature of radioactivity
within the region of present study was very poor.
This indicates that radioactive mineralization may be deep
seated. This may be guided by some structural features
JOUR.GEOL.SOC.INDIA, VOL.85, MARCH 2015
300 ANIMESH MANDAL AND OTHERS
like, subsurface faults, fractures or shear zone. However,
till now no detail geophysical study has been carried in this
region in search of uranium mineralization. This is the first
time an attempt has been made to understand the sub-
surface geological and structural features (e.g. faults,
fractures, dip, and depth continuation etc.) as well as the
favorable locations for uranium mineralization in the study
area. Thus, the paper presents the integrated gravity-
magnetic investigation with sub-surface inverse model to
depict the depth and dip of the identified structures and
discusses the correlation with the established uranium
mineralization signatures of the nearby locations to increase
the confidence of the findings of this study. This will
ultimately provide a significant input to the future
exploration programme over this area.
GEOLOGY OF THE STUDY AREA
The present study area, Kutni lies in the eastern part of
the SPSZ (Figure 1a; modified after Dunn and Dey, 1942,
altered zones associated with magnetic material. These zones
can be identified with high resolution gravity-magnetic
study.
GEOPHYSICAL METHODOLOGY
Gravity
A total of ~200 gravity observations were carried out
using a W. Sodin gravimeter (sensitivity 0.01 mGal) along
12 NS traverses over the study area. The separation between
the traverses was nearly 75-100 m and the stations spacing
was varying from 30 m to 40 m depending on the
accessibility. The gravity measurements obtained from the
study area were tied to the nearest absolute gravity base
station value at Purulia railway station (Absolute gravity
value (gn) = 978796.73 mGal) (Qureshy et al. 1973). All
the raw gravity data were reduced to datum plane value
following the standard gravity data correction procedures
(Telford et al. 1990) to keep the effect of sub-surface mass
only. The free-air correction term was calculated using the
Fig.1. (a) General geological map of South Purulia Shear Zone (SPSZ), India (modified
after Dunn and Dey, 1942; Katti et al. 2010), and (b) Local geological map of
study area (Kutni) (AMD-BRNS report, 2012)
and Katti et al. 2010) and falls on Survey
of India toposheet no. 73 J/9. Most of the
area is covered by cultivated land with a
flat topography. Some litho-units like
quartzite or quartz reef or silicified apatite
rich rocks and iron oxide mainly magnetite
and goethite are exposed over the northern
part of the area following the WNW-ESE
foliation trend of SPSZ (Figure 1b; AMD-
BRNS report, 2012). The study area also
has some scanty outcrops of tuffaceous
phyllite, quartz-magnetite-apatite rock
(silicified apatite rich rock), quartzite,
quartz mica schist and altered metabasic
rocks (Figure 1b). The tuffaceous rock of
Singhbhum group forms the country rock
of the area (Katti et al. 2010). Both altered
ultramafic rocks and alkaline units are only
found in the southern part of Kutni. On the
northern segment of the study area, the rock
types consists of quartzite, mica schist and
quartz reef. The apatite bearing rock
intrudes altered tuffaceous country rock
on the northern side. The radioactive
mineralization is found within altered
quartz-magnetite-apatite rock bands
exposed as convex shaped on the northern
part of the area parallel to SPSZ (Katti et
al. 2010). Thus, the target of mineralization
zone is expected to be found within these
JOUR.GEOL.SOC.INDIA, VOL.85, MARCH 2015
INTEGRATED GRAVITY-MAGNETIC STUDY FOR DELINEATION OF URANIUM MINERALIZATION ZONE 301
standard free-air gradient of 0.3086 mGal/m. The average
crustal density value of 2.67g/cm3 has been utilized for
Bouguer correction. Theoretical gravity value was calculated
following the 1967 Geodetic Reference System (GRS67).
Bouguer anomaly map after the data reduction is shown in
Figure 2. The regional-residual separation of the gravity
anomaly was performed by trend-surface polynomial fitting
method (Unwin, 1978). After analyzing first to fifth order
polynomials, the best-fit first order polynomial is used as
the regional field. The residual gravity anomaly map of the
study area is shown in Fig.3.
Magnetic
The magnetic data were collected at each gravity
observation location of the study area using a Proton
Precession magnetometer (sensitivity 1 nT). The magnetic
data were subjected to diurnal and International geomagnetic
reference field (2011) (IGRF 11) model corrections to derive
the magnetic anomaly. The total field magnetic anomaly
contour map is shown in Fig.4.
RESULTS
The Bouguer (Fig.2) and residual gravity (Fig.3)
anomaly depicts one convex upward low gravity zone on
the northern side of the area in almost EW direction. This
region shows good correlation with the known surface
geology (especially with the exposed radioactive quartz-
magnetite-apatite bands) (Fig.1b). The north to NE part of
the study area mostly depicted high magnetic anomaly values
compared to the southern part (Fig.4). This indicates the
presence of magnetic material in the northern part of the
area. Low gravity anomaly zones along profiles K1-K2 of
Kutni area depict good correlation with the localized high
magnetic anomalies (Figs.5a-b). Surface geology also
reveals the presence of quartz-magnetite-apatite rocks within
low to medium grade hydrothermally altered metamorphites
of Singhbhum Group which is also characterized by
intense ferruginisation, kaolinisation, silicification and
mylonitisation (Fig.1b). These altered zones are reflected
by low gravity and high magnetic values in the respective
anomaly maps. However, the precise depth and width of
this region can be inferred after modeling these anomalies.
2D Residual Gravity Modeling
In the present study, two-dimensional (2D) compact
inverse modeling technique (Last and Kubik, 1983; Mandal,
2013; Mandal et al. 2014) has been applied along the residual
gravity profiles K1-K2 (Figure 3). This approach tries to fit
the observed anomaly with minimum area of source
Fig.2. Bouguer anomaly map of the study area (Fig.1b). Plus (+)
symbols are the gravity-magnetic survey locations.
Fig.3. Residual gravity anomaly map of the study area (Figure
1b) after separation of 1st order trend surface polynomial
from Bouguer anomaly. Plus (+) symbols are the gravity-
magnetic survey locations.
distribution. Thus, the approach has the ability to delineate
the mineral deposits and geological structures more
appropriately/compactly (Last and Kubik, 1983). The 2D
density constrained compact inverse modeling has been
performed using the MATLAB code developed by Mandal
(2013). The constrains on the density values are obtained
from the known surface geology. Most of the area is covered
by tuffaceous rocks (density ~ 2700-2800 kg/m3), metabasic
rocks (density >2800-3170 kg/m3), altered quartizite (density
~2650 kg/m3), and highly altered, silicified, kaolinized and
ferruginised regions (density ~2200-2600 kg/m3) in a
granitic basement of density ~2670 kg/m3 (Telford et al.
JOUR.GEOL.SOC.INDIA, VOL.85, MARCH 2015
302 ANIMESH MANDAL AND OTHERS
1990). Thus, a density contrast of 500 kg/m3 to –470 kg/m3
was applied with respect to the basement density of 2670
kg/m3 to carry out inverse modeling.
To apply the compact inversion approach the residual
anomaly values along the profiles have been interpolated
with a regular interval of 40 m using ‘cubic’ interpolation
method. The thickness of the cells along z direction was
increasing with depth with vertical layers at depths 1 m, 5
m, 10 m, 20 m, 40 m, 70 m, 100 m, 150 m, 220 m, 300 m,
and 400 m. The best fitted models are obtained by
minimizing the parameter (density) variation of last
successive iteration and shown in Fig.6a-b. The root-mean-
square (r.m.s.) misfits for the models are ~0.01 mGal or
less. The models (Fig.6a-b) depict steeply dipping structures
Fig.4. Total magnetic field anomaly map of the study area (Figure
1b). Plus (+) symbols are the gravity-magnetic survey
locations.
Fig.5. Comparisons of residual gravity anomaly and magnetic
anomaly variations along gravity-magnetic profiles.
(a) Profile K1, and (b) Profile K2.
Fig.6. Two-dimensional compact inverse residual gravity model
along gravity-magnetic profiles (Fig.3). (a) Profile K1, and
(b) Profile K2.
JOUR.GEOL.SOC.INDIA, VOL.85, MARCH 2015
INTEGRATED GRAVITY-MAGNETIC STUDY FOR DELINEATION OF URANIUM MINERALIZATION ZONE 303
with dip due north, south or vertical at places. The northward
low gravity convex shaped arc is identified as the lowest
density zone (with density ranges from 2200-2600 kg/m3)
due to its intense brecciation, mylonitization, hydrothermal
alteration and brittle to ductile nature (Fig.6a-b). The average
width and depth extent of this altered zone is larger in east
direction compared to that in west. The NS width and depth
extent is highest (nearly 300 m and 220 m, respectively)
near to the Kutni village (Figs. 1b, 3 and 6a).
DISCUSSION
The 2D sub-surface modeling and the gravity-magnetic
anomaly pattern over the study area depicts that some of
the altered zones are associated with magnetic materials.
These altered bands/zones are distributed on the northern
side of the study area in the form of convex upward shape
and almost parallel to the general foliation of the shear zone.
These zones are also coinciding with the exposed quartz-
magnetite-apatite rocks (Figs. 1b, 3, and 4). Mandal et al.
(2013, 2014) has demonstrated and proved that the
coincidence of low gravity and high/moderately high
magnetic anomaly is the signature of quartz-magnetite-
apatite hosted altered uranium mineralization zones at
Beldih mine area of this shear zone. A similar situation of
coincidence of low gravity and high magnetic anomaly along
the exposed quartz-magnetite-apatite rock band is reported
here. Along profile K1, two low gravity zones respectively
at ~180 m, and 310 m positions have shown good
correlations with magnetic highs at the same locations
(Fig.5a). The 2D model reveals that the depth of these low
density zones is extending up to nearly 220 m (Fig.6a).
Along profile K2, gravity lows at ~125 m, 690 m, and
875 m locations coincide with the localized magnetic highs
at the same locations (Fig.5b). The 2D model depicts three
low density zones with maximum possible depth of nearly
220 m and with dip vertical, due south, and due north
respectively (Fig.6b). The widths of these zones are nearly
100 m, 50 m and 50 m (Fig.6b). Katti et al. (2010) has also
reported relatively high radioactivity from the ferruginous
quartz-apatite breccias and magnetite rocks of this region.
Again, researchers have identified structural guided uranium
deposits in a region with high horizontal gravity gradients
at the adjacent to localized high magnetic anomalies (e.g.
Nazarov and Poluarshinov, 1971). Thus, the coincidence of
low gravity and high magnetic on the above regions along
the two profiles (K1-K2) indicates the presence of alteration
zones as well as the most possible regions for mineralization.
As these low density zones on the northern side of the study
area are also identified with some radioactivity and
coinciding with same type of host rocks (quartz-magnetite-
apatite) like in Beldih uranium mineralization (Katti et al.
2010), hence the zones are the most suitable locations for
uranium mineralization. These zones are mostly vertical in
nature but at some places dip changes towards north or south
(Fig.6a-b). The NS widths of these alteration zones vary
from ~50-300 m with maximum possible depth of ~220 m.
CONCLUSIONS
Present study area mostly covered by quartzite,
tuffaceous and metabasic rocks with limited exposure of
the altered quartz-magnetite-apatite rock intruding them.
Thus, gravity and magnetic surveys were performed
around Kutni village to investigate the sub-surface
structural features that could be associated with uranium
mineralization. Mainly, the E-W elongated convex shaped
low gravity and high magnetic bands on northern side of
the area has identified as the potential zones. The 2D gravity
models also depict low density bands mostly vertical and at
places dipping south to north. The NS widths of these low
density bands are varying from ~50-300 m with maximum
probable depth extent of ~220 m. The interpreted gravity
models show good correlation with local geology of the area.
The zones coinciding with low density (negative gravity
anomaly) and localized high magnetic anomaly values are
the most possible zone of uranium mineralization. Thus, the
results of this study can be used as guiding parameters for
the future exploration programme over this region.
Acknowledgements: The authors are 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) and the discussion with officials
from 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.
JOUR.GEOL.SOC.INDIA, VOL.85, MARCH 2015
304 ANIMESH MANDAL AND OTHERS
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(Received: 29 May 2014; Revised form accepted: 12 September 2014)
... In the north of SPSZ, Chottangpur granite gneiss complex (CGGC) is present and approximately 50 km away in the south, Singhbhum Shear Zone (SSZ) lies. Since both the SPSZ and SSZ tectonic belts have analogous properties like intense shearing and hydrothermal alteration zones that comprise shear controlled hydrothermal type Uranium deposits with considerable amount of magnetite (Mahadevan, 1988;Anand and Rajaram, 2006) and other deposits of copper-apatite-magnetite-REE (Mandal et al., 2015;Basu, 1993;Gupta and Basu, 2000;Acharyya et al., 2006), so to uncover the potential Uranium deposits, one must focus on identifying such shear zones in the subsurface. ...
... In mineral exploration, the contrast in physical properties between the host rock and the desired anomaly makes electrical resistivity tomography (ERT) and gravity methods significant for the delineation and the quantitative investigation of the target. The delineation of lateral extent and depth of hydrothermal uranium mineralization zones hosted by ferruginous kaolin rocks and quartz-magnetite-apatite rocks (Mandal et al., 2015) near Kutni village of South Purulia Shear Zone is done by means of electrical resistivity tomography and gravity methods. ...
... We have already mentioned that the region encompasses hydrogeological uranium deposits. These deposits are found accumulated with clay and Fe\ \Mn oxides, which are one of the most suitable conditions for gravity and resistivity investigations (Mandal et al., 2015). Since no method comes without any limitations. ...
... The first case is taken after a residual gravity anomaly data over a uranium mine at Beldih, Purulia, West-Bengal, India. The residual gravity anomaly data is taken from Profile 26 (after Mandal et al., 2015). The data is digitized from the figure at equal intervals. ...
Chapter
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In general, “tide” refers to both the body tide and ocean tide of the Earth. While ocean tide is more widely recognized in public than body tide, Earth tide refers to body tide in solid Earth geosciences. In fact, body tide is the largest periodic perturbation in the solid Earth. A brief sketch of ocean tide is included in this chapter, however, the main concern here is “body tide.” To predict the displacement, gravity, deflection of vertical associated with Earth’s body tide at any specified time epoch at certain place has been an important task in gravity prospecting and gravimetric geodesy. “Love numbers,” which are the three kinds of ratios between the response of real Earth and that of an ideal Earth under tidal force, and are quite conveniently used in body tidal predictions. Accurate Love numbers can only be determined through a numerical computation with the Earth’s physical property model. Another information required for body tide prediction are the positions of the Moon and the Sun. Because tidal force rapidly decreases with distance (1/r³), other planets, even Jupiter, induce negligible amounts of tide (relatively less than 10⁻⁵). Therefore it is of utmost importance to have the accurate ephemerides computation algorithm about the two largest tide raising celestial bodies; the Moon and the Sun. In this chapter the method to predict body tide of the Earth is described with emphasis on the two algorithms; (1) to compute Love numbers, and (2) to predict body tidal perturbation using Love numbers and lunar/solar ephemerides. And these are actually the main stems of K2-Tide; an Earth tide prediction computer program. Brief explanations about ocean tide model and load Love number as well as fluid Love number are attached at the end of this chapter.
... The first case is taken after a residual gravity anomaly data over a uranium mine at Beldih, Purulia, West-Bengal, India. The residual gravity anomaly data is taken from Profile 26 (after Mandal et al., 2015). The data is digitized from the figure at equal intervals. ...
Chapter
Full-text available
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Chapter
The interpretation and resolution of residual gravity anomaly due to multiple bodies (viz. sphere, horizontal and vertical cylinder) and a combination of these were interpreted in this work. Residual gravity anomalies due to some multiple synthetic models were interpreted applying a global optimization method. In the case of a single structure, an increase in depth from the top of the subsurface structure decreases the resolution whereas an increase of amplitude increases the anomaly for spherical structure. It is the same for the horizontal cylinder but a vertical cylinder, the width of the anomaly changes with increasing depth. In the case of multiple structures, the amplitude increases or decreases with a change in the value of amplitude coefficient keeping fixed depth and vice versa for change in the amplitude with a change in the depth value. However, for multiple vertical cylinder-like structures, it is difficult to interpret the anomalies while changing the values of amplitude and depth. Moreover, a combination of these three structures often leads to the non-interpretation of one structure which may not show any anomalies associated with it. The efficiency of multiple structures for the delineation of residual gravity anomaly data is developed in this study using different synthetic models and field measurements from Beldih mine and South Purulia Shear zone associated with uranium mineralization.
Article
In the eastern part of India, South Purulia Shear zone (SPSZ) forms an elementary feature of Tamar-Porapahar lineament that trends in approximately E-W direction. In terms of Uranium mineralization prospects, SPSZ contributes a significant fraction when it comes to exploration. For a detailed investigation of subsurface characteristics, a quasi 3D resistivity survey using multi-electrode Schlumberger array configuration was performed near Kutni village in South Purulia Shear Zone (SPSZ). For this approach, we carried out measurements at various parallel profiles perpendicular to the strike in the area, to achieve the spatial resistivity distribution beneath the subsurface. Interpretation of the electrical resistivity tomography (ERT) results shows the presence of a conducting body at a depth of approximately 25-30 m that dips in the north direction and has a continuous extent in the E-W direction. Such low resistivity regions in 2D resistivity sections, when compared with gravity results, the outcome was in concurrence with low-density zones. These low resistivity and low gravity anomaly zones may correspond to the hydrous Uranium altered regions. Two more multi-electrode Schlumberger profiles were taken parallel to the strike direction near the exposed outcrop that showed the presence of anomalous zones. The integrated approach of geophysical methods used in this paper improves the likelihood of target location and helps in successfully deciphering the dimension of the geological features present beneath the surface.
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This study is an attempt to correlate VLF-EM data with the radiometric measurements to decipher the subsurface structure and to locate uranium mineralization in the shear zone. The study area is around Beldih mine which is an open cast apatite mine located on the South Purulia Shear Zone. VLF method has been applied to map the structure and the presence of radioactive minerals has been delineated by the detection of high α and γ counts with respect to the background radiations. High radiation counts and high surface γ activity are found just above the higher apparent current-density zones in all the profiles studied, at various locations, indicating uranium and/or thorium mineralization as well as good correlation between these techniques.
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The Beldih open cast mine of the South Purulia Shear Zone in Eastern India is well known for apatite deposits associated with Nb–rare-earth-element–uranium mineralization within steeply dipping, altered ferruginous kaolinite and quartz–magnetite– apatite rocks with E–W strikes at the contact of altered mafic–ultramafic and granite/ quartzite rocks. A detailed geophysical study using gravity, magnetic, and gradient resistivity profiling surveys has been carried out over �1 km2 area surrounding the Beldih mine to investigate further the dip, depth, lateral extension, and associated geophysical signatures of the uranium mineralization in the environs of South Purulia Shear Zone. The high-to-low transition zone on the northern part and high-to-low anomaly patches on the southeastern and southwestern parts of the Bouguer, reducedto- pole magnetic, and trend-surface-separated residual gravity–magnetic anomaly maps indicate the possibility of highly altered zone(s) on the northern, southeastern, and southwestern parts of the Beldih mine. The gradient resistivity survey on either side of the mine has also revealed the correlation of low-resistivity anomalies with low-gravity and moderately high magnetic anomalies. In particular, the anomalies and modeled subsurface features along profile P6 perfectly match with subsurface geology and uranium mineralization at depth. Two-dimensional and three-dimensional residual gravity models along P6 depict the presence of highly altered vertical sheet of low-density material up to a depth of �200 m. The drilling results along the same profile confirm the continuation of uranium mineralization zone for the low-density material. This not only validates the findings of the gravity model but also establishes the geophysical signatures for uranium mineralization as low-gravity, moderate-tohigh magnetic, and low-resistivity values in this region. This study enhances the scope of further integrated geophysical investigations along the South Purulia Shear Zone to delineate suitable target areas for uranium exploration.
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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.
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Aeromagnetic data on a part of the Singhbhum uranium province, India, within the framework of `Operation Hard Rock' have been analyzed. The aeromagnetic anomaly map, its analytic signal amplitude, the Euler solutions and apparent susceptibility map helped in identifying the nature and depth of the magnetic sources in the study region. The Singhbhum Shear Zone is clearly delineated. The location of the mined uranium deposits coincide with the shallow magnetic sources. The present study also identified distinctive magnetic sources between the Dalma Volcanics and the Chotanagpur Granitic Gneissic Complex that suggest the presence of a sub-surface shear. The magnetic sources in this newly identified shear zone lie at a depth of 200 m and depict a similar magnetic signature and susceptibility as those of the Singhbhum Shear Zone where uranium is being mined. This shear could be the subsurface contact between high-grade metamorphic rocks (amphibolite facies) to the north and the greenschist facies low-grade metamorphics to the south. This shear zone, also characterized by radiometric anomalies, possibly indicates a zone of concealed uranium deposit that can be explored in the future. The present analysis shows the importance of aeromagnetic surveys and its utility in exploration for concealed mineral deposits.
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The South Purulia Shear Zone (SPSZ) is a part of the conspicuous -E-W to -ESE-WNW trending Tamar-Porapahar lineament. This borders the Meso-Proterozoic greenschist facies lithopackage of the Singhbhum Group (SG) comprising metasedimentaries, felsic volcanics, mafites/ ultramafites, granitoids, and alkaline/carbonatite suite of rocks bounded by the Chhotanagpur Gneissic Complex (CGC) attaining amphibolite to granulite facies in the north. CGC presumably had certain loci of crustal weakness where pre-rift crustal sagging led to the deposition of supracrustal rocks. Intracratonic rifting had then initiated in those pre-existing zones of crustal weakness and troughs of the resident crust. The intracratonic basin was covered by huge volume of the felsic volcanics represented by layered tuffaceous volcaniclastic, felsic ignimbrites, crystal-vitric tuff and tuffaceous phyllite. Meta-greywacke has been considered as epiclastic deposit derived from felsic volcanics. The nature of welding, absence of phenocrysts typical of lava flows, parallel aligned lenticular pumice fragments exhibiting pumice-lenticle foliation, fiamme and eutaxitic fabric and petrochemistry could imply that the felsic ignimbrite was close to welded pumiceous ash-flow tuff having dacitic to rhyodacitic composition. Mafic-ultramafic suites of rocks in close proximity to felsic volcanics suggest a bimodal character of magmatic episode in the area. Bands and lenses of tourmalinite disposed along the contact of the CGC and the SG, argued to be of volcanogenic sedimentary origin, possibly demarcates the rupture locales of a rift. The alkali-feldspar granite (± richterite, ± aegirine-augite), syenite and carbonatites suggest tapping of deeper fractures in the rifted basin. This was coupled with the presence of granite plutonism in the setting. Geochemical signatures imply that felsic volcanics are chiefly of dacitic composition with strong LREE enrichment and narrow range of fractionation while the metabasic rock chemistry scatter in basalt to basaltic andesite field. Trace element data further corroborate 'volcanic are granite', 'within plate granite' and 'continental arc granite' composition for the felsic volcanics. The basin was then subscribed to compressional regime where ductile shear zones developed syn to post kinematic to F1 with the northern block (CGC) thrusting over the southern block (SG) on northerly dipping planes. Thus, an entire sequence of rifting, tourmalinite emplacement, felsic volcanism, mafic extrusion, granite plutonism, syenite/ carbonatite emplacement and sedimentation of pelites/epiclastics during and after opening of the basin in an intracratonic setting is demonstrated. Subsequently, development of the shear zone (SPSZ) took place over the area at the close of the basin.
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Alkali syenite, apatite-magnetite rock, carbonatite and phlogopite-amphibolite occurring as members of the Alkaline-Carbonatite Complex, belonging to the Precambrian Singhbhum Group of rocks, are found to occur at different centres along WNW-ESE-trending South Purulia Shear Zone. The alkaline-carbonatite rock occurrences are found to be located at the intersection of NW-SE lineament with the WNW-ESE-trending one close to this shear zone. Major- and trace-element composition of the syenitic rocks displays considerable variation. Prospective apatite deposits have been proved at Chirugora, Kutnia, Mednitanr and Beldih along the shear zone. Significant concentration of REE, Nb, Sr and Zr has been recorded in the different rock types of the Complex studied at different centres along the shear zone. -from Author
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Presents a criterion for the inversion of gravity data by minimizing the volume of the causative body, thus maximizing its compactness. The anomalous density distribution is obtained using an iterative technique which is numerically stable and rapidly convergent. The advantage of this approach is that desirable geologic characteristics are automatically incorporated into the model with a minimum of subjective judgements on the part of the interpreter. The practical effectiveness of the method is demonstrated by the inversion of published gravity data. The results obtained are compared with existing models and with available drilling information. -from Authors