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
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.
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
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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
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
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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.
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.
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
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).
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.
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
JOUR.GEOL.SOC.INDIA, VOL.85, MARCH 2015
304 ANIMESH MANDAL AND OTHERS
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