Case History

Integrated geological and geophysical studies for delineation of chromitedeposits: A case study from Tangarparha, Orissa, India

William K. Mohanty1, Animesh Mandal1, S. P. Sharma1, Saibal Gupta1, and Surajit Misra1

ABSTRACT

In Orissa, India, chromite deposits occur in a NE-SW trend-ing belt as discontinuous pods associated with tectonicallydeformed and metamorphosed ultramafic rocks. Geologicalmapping and detailed geophysical survey (including gravity,magnetic, electrical, and electromagnetic methods) for explor-ing chromite were conducted in a 5 km2 area at Tangarparha,located within the belt. Lithologies include sheared granite,quartzofeldspathic gneiss, and mafic/ultramafic rocks. The cal-culated Bouguer anomaly map shows a distinct positive anom-aly (up to 16 mGal) in the northern part of the area, indicatingthe existence of a very high density rock in the subsurface. Thetrend-surface analysis technique was applied to the gravityand magnetic data for regional-residual separation. The 2Dand 2.5D forward modelings of the residual gravity anomaly

suggest the presence of lithologies with densities higher thanmafic/ultramafic rocks in the subsurface. Chromite fragmentsrecovered from pits within the soil cover around the locationindicate that the very high density material is likely to be chro-mite. Correlation of magnetic and gravity anomalies further em-phasizes this possibility. The results of very low frequency(VLF) and DC-resistivity surveys reveal that the suspected chro-mite deposit is about 250–300 m long in a south-north direction,and 300–350 m wide in the east-west direction. The estimateddepth of the deposit varies from 35–100 m. VLF and DC-resistivity methods suggest that chromite occurs in the formof a small disseminated body within a mafic/ultramafic rock ma-trix. The ambiguity of interpretation is reduced by systematicintegration of complementary geophysical methods, comparedto that from any single geophysical technique.

INTRODUCTION

The state of Orissa in India (see inset, Figure 1, modified afterSaha, 1994) accounts for almost 98% of the chromite resources ofthe country (Mondal et al., 2006). These chromite deposits aremainly confined to a NE-SW trending belt (Deb and Chakraborty,1962) in the northern part of the state, and are concentrated withinmafic/ultramafic complexes, such as those at Sukinda and Nausahi(Figure 1). Most of the chromite deposits have already been iden-tified on the basis of surface exposure of mafic/ultramafic rocks thathost the chromite deposits (Mukerjee, 1992), and are currentlybeing mined. Apart from the established occurrences, there are sev-eral other places within the belt where mafic/ultramafic rocks are

exposed at the surface, such as at Tangarparha (the subject ofthe present study), that have not yet been explored for the presenceof chromite ore, although chromitite fragments are frequently re-covered from the soil cover in these areas. The main challengein exploration of this region is, therefore, to identify chromite de-posits in localities where geological conditions favor the occurrenceof chromite. However, the existence of a significant deposit has notyet been confirmed because of limited surface exposure. Presenceof chromite deposits in such areas can be established by applicationof a combination of geophysical techniques in consonance withgeological information. Such an integrated approach also yieldsmore precise information about the location and dimensions ofthe deposit. Since individual geophysical methods have some lim-

Manuscript received by the Editor 6 August 2010; revised manuscript received 15 April 2011; published online 7 November 2011.1Indian Institute of Technology, Kharagpur, Department of Geology and Geophysics, Kharagpur–721302, West Bengal, India. E-mail: wkmohanty@gg.iitkgp

.ernet.in; animeshphys@gmail.com; spsharma@gg.iitkgp.ernet.in; saibl@gg.iitkgp.ernet.in; misrasurajit@gmail.com.© 2011 Society of Exploration Geophysicists. All rights reserved.

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GEOPHYSICS. VOL. 76, NO. 5 (SEPTEMBER-OCTOBER 2011); P. B173–B185, 14 FIGS., 1 TABLE.10.1190/GEO2010-0255.1

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itations that affect their applicability at specific sites, such an inte-grated approach also helps to circumvent the problems associatedwith application of any single method (Aina and Olorunfemi,1996).Chromite is an early magmatic oxide mineral (Sahoo et al., 2009)

and in general is associated with ultramafic igneous rocks. It isusually a minor phase in these rocks, and is only in abundancein localized pockets within the ultramafic host. In these pockets,chromite forms a near-monomineralic rock called chromitite. Chro-mite exploration strategy, therefore, involves two steps: locating theultramafic host, and subsequently identifying domains of concen-trated chromite (chromitite) within the ultramafic rock itself. Thegeophysical signature of chromite deposits depends upon the phy-sical properties of chromite ore (for example density, magnetic sus-ceptibility, and resistivity). The density of chromite mineral variesfrom 4430 to 5090 kg/m3 (Deer et al., 1962) while that of mafic/ultramafic rocks ranges between 3000–3600 kg/m3. This densitycontrast is very high, and, therefore, gravity is the most appropriatemethod for the delineation of chromite deposits (Hammer et al.,1945), if the deposits are big and close to the surface. Small pits,5–10 m deep in the soil cover of the study area, contain fragments ofchromitite, indicating that a significant chromite deposit is possibly

present at shallow depth. Although the magnetic susceptibility ofchromite is higher than the associated rocks in this area, it variesover a wide range and is strongly dependent on the chemical com-position of chromite (Murthy and Gopalakrishna, 1982) and thegeological environment (Bhattacharya et al., 1969). Thus, magneticdata alone are ambiguous. The resistivities of the host mafic/ultramafic and granitic rocks are very high in comparison tochromite-rich lithologies. Therefore, DC-resistivity and very lowfrequency (VLF)-electromagnetic methods can also depict the pre-sence of conducting chromite-bearing zones (Bayrak, 2002) withinthe study area. Keeping these in mind, an integrated geophysicalsurvey was carried out in Tangarparha to pinpoint the location ofpossible chromite deposits.

GEOLOGY OF THE STUDY AREA

The study area (Tangarparha) (Figure 1) is a small but vital part ofa major tectonic contact within the Indian Shield. To the north of thearea is the Singhbhum craton. The Singhbhum Province is a part ofthis craton that comprises a basement of Archaean granites andgneisses along with meta-volcanosedimentary cover sequences(Mondal, 2009) (Figure 1). Further south of the present study area,high grade granulite facies rocks of Eastern Ghats Mobile Belt

(EGMB) show a general NE-SW trend. The con-tact between the Singhbhum Province and theEGMB is highly tectonized; most of the chromiteoccurs within this zone.A major part of the present study area, parti-

cularly in the south, is covered with laterite.Based on the available outcrops, the major litho-types present in the area are identified as shearedgranite, quartzofeldspathic gneiss (QFG), andmafic/ultramafic rocks. Mafic/ultramafic rocksoccur as pods that are aligned approximately par-allel to a mylonitic shear zone that cuts throughthe QFG unit (Figure 2). The sheared granite iscoarse-grained but foliated, with prevailing pinkfeldspar and occasional garnet. The QFG ismineralogically similar to the sheared granite,but has a distinct segregation banding. Apartfrom granitic gneisses, the QFG unit also in-cludes rare quartzites. The mafic/ultramaficrocks include two components — mafic (doleri-tic) dykes, and coarser-grained cumulates com-prising olivine norite and olivine gabbro.Cumulus layers of ultramafic rocks (harzburgiteand dunite) occur in the vicinity of the study areaand may underlie the norite and gabbro. The ma-fic/ultramafic unit is closely associated with theQFG; the disposition of this unit in the map (i.e.,Figure 2) suggests that they form part of a once-continuous layer that is now boudinaged and dis-tended along the mylonitic shear zone.Cumulate textures within the mafic/ultramafic

body indicate that they must have once been partof a magma chamber of significant size, in whichcrustal assimilation and magma mixing pro-cesses were in progress during magma convec-tion. The cumulate layers indicate that theprocesses that contribute to chromitite formation,

Figure 1. Geological map of Singhbhum (modified after Saha, 1994), Tangarparha isthe study area (see Figure 2).

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i.e., the presence of a mafic magma, evidence for magma mixing,and cumulus layers are all present within the distended mafic pods.These distended pods are geologically the most prospective host ofthe chromite.

GEOPHYSICAL METHODOLOGY

Keeping in mind the geology and the physical properties of therocks of the study area, gravity, magnetic, VLF, and DC-resistivitymethods were applied to detect the possible occurrence of chromite.The methodology adopted in each case is described below, and thegeophysical survey locations are shown in Figure 3.

Gravity study

The Differential Global Positioning System (DGPS) and gravitysurveys were started from the northern portion of the study area onthe basis of available accessibility, along west-east traverses withstations at intervals of 25 m (Figure 3). A total of 346 gravity sta-tions were covered during the survey.Gravity measurements were carried out employing a W. Sodin

Gravimeter (instrument constant ¼ 0.1082 mGal∕Div:). All grav-ity measurements in the Tangarparha area weretied to the absolute local gravity base stationestablished at the Industrial Development Cor-poration Limited (IDCOL) guest house withrespect to the known absolute value of Kapilas-pur Road railway station (Absolute reading ¼978706.46 mGal) (Qureshy et al., 1973). Allthe raw data were subjected to standard correc-tion procedures (instrumental drift, free-air,Bouguer, and terrain) to restrict the effect ofthe distribution of density to the subsurfacematter only. A theoretical gravity value was cal-culated using the Geodetic Reference System1967 (GRS67). To compute the Bouguer correc-tion, the average crustal density was taken as2670 kg∕m3 (Hinze, 2003; Lawal and Akaolisa,2006). Terrain correction was applied to the datausing the line mass integral method (Yen et al.,1994). The Bouguer anomaly map after the datareduction is shown in Figure 4.

Regional-residual separation of gravity data

The Bouguer anomaly data is a result of thecombined effect of the widespread deep massdistribution (regional) and shallow near surfacebodies (residual). Separation of the regional com-ponent from the Bouguer anomaly is a crucialstep in gravity interpretation and subsurfacestudy, and can be accomplished using varioustechniques, e.g., graphical smoothing (Telfordet al., 1990), second vertical derivative (Hender-son and Zietz, 1949), trend-surface analysis(Agocs, 1951; Merriam and Harbaugh, 1964;Beltrao et al., 1991; Roach et al., 1993), filtering(Griffin, 1949; Zurflueh, 1967; Spector andGrant, 1970; Pawlowski and Hansen, 1990), orusing 3D inversion algorithm (Li and Oldenburg,1998; Malehmir et al., 2009). Each of these

methods is associated with certain ambiguities and subjectivitieswhich produce nonunique results. The interpretation, therefore, re-quires a decision on which methods provide the most satisfactoryresults, based on predefined criteria, such as the ability of the re-sidual surface generated by each method to successfully modelstructures consistent with surface geology and observed densityvalues (Gupta and Ramani, 1980). In the present study, trend-surface analysis (Unwin, 1978) was preferred over filtering to se-parate the regional component from Bouguer anomaly as the regio-nal surface obtained from the former shows reasonable smoothingof the original Bouguer anomaly.Inspections with first- to fifth-order polynomials show that the

third-order polynomial is more realistic for the regional surface(Figure 5). The residual anomaly (Figure 6) is obtained after remov-ing the third-order regional surface from the Bouguer anomaly data.Exposures of QFG/sheared granite correspond to the lowest gravityanomalies; lateritic outcrops show marginally higher values. Thehighest gravity anomaly values are associated with surface expo-sures of mafic/ultramafic rocks. Thus, the residual anomaly mapshows good correlation with the surface geology.

Figure 2. Geological map of the present study area (Tangarparha).

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Magnetic study

A total of 312 magnetic stations were established along the grav-ity profiles (Figure 3). The number of magnetic stations was limiteddue to the interference caused by the presence of high electric-transmission power lines. The total magnetic field of the observa-tion points were measured using a proton precession magnetometer(sensitivity ¼ 1 nT). The diurnal corrected total magnetic field vari-ation is shown in Figure 7. The International Geomagnetic Refer-ence Field 2010 (IGRF 2010) model was used as the regional field.

After employing these corrections and trend-surface analysis, theresidual magnetic anomaly contour map was plotted using the mini-mum curvature interpolation method (Figure 8a). Unlike gravityanomalies, magnetic anomalies do not appear vertically abovethe anomalous body and are asymmetric in nature, due to the in-clination and declination of the earth’s magnetic field at low lati-tudes (Roest and Pilkington, 1993). These distortions in magneticanomalies can be removed by the reduction-to-pole (RTP) method,using various approaches (Baranov, 1957; Spector and Grant, 1970;

Nabighian, 1972; Roy and Aina, 1986; Silva,1986; Aina, 1991). In the present study, an ana-lytical signal approach using Hilbert transform(Young, 2004) is used for RTP and the residualmagnetic anomaly reduced to pole is shown inFigure 8b.

VLF study

AVLF electromagnetic survey was performedalong E-W and N-S profiles with a 10-m stationinterval (Figure 3) corresponding to gravity highusing ABEM WADI equipment. Since this in-strument is able to measure at one frequency ata time, measurements along various profiles aremade at a particular frequency. Measurementsalong E-W profiles are made using the Viziana-garam (18.2 kHz) transmitter, which is located ina southerly direction from the study area. Further,measurements along N-S profiles are made usingthe Bombay (19.8 kHz) transmitter, which is lo-cated in a westerly direction from the study area.The apparent current density (J) is computedusing the real and imaginary VLF anomaliesby application of digital linear filtering approach(Karous and Hjelt, 1983). Since apparent currentdensity cross sections using real and imaginaryanomalies resemble similar features, only appar-ent current density sections using real anomaliesare presented.

Electrical resistivity study

Resistivity soundings using the Schlumbergerarray were performed at selected locations (Fig-ure 3) corresponding to the gravity highs to de-pict the actual resistivity distribution in thesubsurface. A DC-resistivity meter with a vari-able power supply up to 450 V in the interval of15 volt was utilized for the survey. Individualvalue of potential difference and current flowwith the accuracy of 0.1 mV and 0.1 mA is re-corded to compute the apparent resistivity.Further, self-potential is canceled precisely foraccurate measurement of the potential difference.Apparent resistivity is computed from the mea-sured data and interpreted using 1D very fastsimulated annealing (VFSA) global inversiontechnique (Sharma and Kaikkonen, 1999). Inter-preted resistivities (Table 1) obtained at differentlocations are used to construct geo-electrical

Figure 3. Study area and observation locations of gravity, magnetic, VLF, and DC-resistivity surveys.

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cross sections and are correlated with geological structures presentin the area.

RESULTS AND DISCUSSION

Gravity

The Bouguer and residual anomaly maps show low-intensityanomalies in most of the study area. However, they show a distincthigh, up to 16 mGal and 8 mGal, respectively, in the northern part ofthe study area (Figures 4 and 6). A relatively high value is also ob-served in the central portion (around 20° 55.2 0 N and 85° 40.2 0 E)

of the residual anomaly map, although this high is not as prominentas the one in the northern part.

Modeling of residual gravity anomaly

The subsurface structure can be studied in detail by modeling theresidual gravity anomaly. In our present study, 2D and 2.5D forwardmodeling approaches were employed along the two profile lines(AA′ and BB′ in Figure 6) across the first high positive anomalyregion with different density and depth combinations. The 2D mod-eling was done using the GRAVMAG software (Burger et al.,2006), which calculates the gravity effect for solids of infinite strike

Figure 4. Bouguer anomaly map with contour interval of 0.5 mGal.The ‘þ’ represent the gravity observation points and the ‘O’ representthe base station location.

Figure 5. Regional gravity anomaly map with contour interval of0.1 mGal.

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length and polygonal cross section using the solution of Hubbert(1948) and Talwani et al. (1959). As the sheared granite to the eastand the quartzofeldspathic gneiss to the west of the high anomalyregion are both composed of quartzofeldspathic material, theirdensity was taken to be identical (2670 kg∕m3). The density ofthe mafic/ultramafic rocks is assumed to range between 3000 and

3570 kg∕m3 (gabbro/norite to dunite); higher density of the rockcan be achieved by the presence of chromite (44305090 kg∕m3). A clay layer (1770 kg∕m3) of variable thicknessis assumed on the surface. Subsurface forward modeling alongthe two profiles was done keeping these geological constraintsin mind. Based on the lateral extent of these bodies estimated from2D modeling, 2.5D models are constructed using the approach ofRamarao and Murthy (1989), with a given density contrast andstrike length for the different bodies. Constraints on the depth extent

Figure 7. Total magnetic field map of the study area. The ‘þ’ re-present the magnetic observation points and the ‘O’ represent thebase station location.

Figure 6. Residual gravity anomaly map with contour interval of0.6 mGal. AA′ and BB′ are the two profile lines.

Figure 8. Residual magnetic anomaly map with contour interval of250 nT. The ‘þ’ sign represent the magnetic observation points andthe ‘O’ represent the base station location. (a) Without RTP, and(b) after RTP.

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of the chromite body are obtained from the resistivity study (∼35 m,discussed below in resistivity section).During modeling, density of the host rock was systematically var-

ied from that of a pure mafic/ultramafic rock (3000–3570 kg∕m3),to higher densities (mafic/ultramafic rock + a high proportion ofchromite), keeping the background rock density fixed at2670 kg∕m3. A good fit with the observed anomaly is achievedonly by using densities in excess of 4650 kg∕m3, implying a highproportion of chromite (density > 4650 kg∕m3) within the ultrama-fic host (density ≤ 3570 kg∕m3). The best fit 2D model for profileAA′ (Figure 9a) is obtained using a layer of density 4670 kg∕m3

containing a small pod of 4870 kg∕m3, with a layer of density3570 kg∕m3 below them. In the 2.5D model, the best fit alongAA′ is obtained using a pod of 100 m strike length and density of5000 kg∕m3 embedded within a 300 m layer of density4870 kg∕m3 (Figure 9b). Along BB′, a 400 m layer of density4870 kg∕m3 shows a good fit (Figure 10b). Along both profiles,the ore body is underlain by an ultramafic layer with strike length1500 m and density 3570 kg∕m3. The high density layer has irre-gular depth along the profiles. Since chromite is the only locallypresent mineral in these rocks that has an appropriately high densityto explain the fit, the models over the high residual anomaly regionstrongly suggest the existence of chromite-bearing rocks in thesubsurface at shallow depths (see Figures 9a, 9b, 10a, and 10b).This is also consistent with the chromite fragments collected fromthe pits within the soil cover at latitude 20° 55.52 0 N, longitude85° 40.22 0 E. The rms misfits of the best fitting 2D models(Figures 9a and 10a) are 1.16 mGal and 1.40 mGal, respec-tively and that for 2.5D best fit models (Figures 9b and 10b) are1.51 mGal and 1.71 mGal, respectively, for the above mentionedprofiles.

Magnetic

The dipolar nature of the magnetic field results in a high-lowanomaly pair as observed in the total field map and residual mag-netic anomaly map of the study area (Figures 7 and 8a, respec-tively). After applying RTP, the anomaly map depicts themonopolar anomaly produced by the causative body (Figure 8b).The location of the magnetic high (20° 55.57 0 N, 85° 40.27 0 E, Fig-ure 8b) corresponds precisely with the location of the gravity highanomaly zone (Figure 6) in a low anomaly background (i.e., QFGand sheared granite). The high magnetic anomaly is compatiblewith the occurrence of chromite.

VLF

Apparent current density is computed using the Karous and Hjelt(1983) filtering approach, and is presented in Figure 11a and 11b.High current density corresponds to conducting structures (possiblychromite in this area where it coincides with high gravity anomaly)and low current density corresponds to resistive structures (graniteor mafic/ultramafic rocks in this area). The profiles P50 and P52 (inFigure 11a) both traverse from south to north with a separation of175 m and show good correlation with each other. A high currentdensity is depicted at the 300 m location on both profiles. Further, atthe 600 m location, a moderately conducting structure is depictedfrom current density on both profiles. Current density contoursshow alternate bands of resistive and conducting structures in boththe profiles. The conducting features along both the profiles be-

tween the 300–400 m, 500–600 m, and 700–1000 m locations de-pict synformal (i.e., bowl-shaped) structures (P50 and P52,Figure 11a). The conducting domains within the synforms couldbe associated with chromite mineralization as this region is also as-sociated with high gravity and is located in a geologically favorablezone (associated with ultramafic rocks). Profile P58 (Figure 11a)traverses the area covered by thick laterite in a south to north direc-tion. It can be clearly seen that the subsurface is reasonably homo-geneous except at the 1000 m location, which possibly depicts thecontact between laterite and granitic rock and may be associatedwith groundwater saturation. Though several conductive and resis-tive features are depicted along the profiles P59 and P60 (Fig-ure 11a), these areas are homogeneous and covered with laterite.The sporadic VLF anomalies are unlikely to be geologically signif-icant, and could be associated either with groundwater in the area,or a result of power line disturbance.Profile P53 (Figure 11b) traverses a distance of 600 m from west

to east direction. Conducting features depicted between 200 and500 m depict zones of possible chromite mineralization (P53, Fig-ure 11b). High gravity anomaly is also observed at these very lo-calities, which strongly indicates the presence of chromite. Thewidth of the conducting features gradually decreases as we movefurther south, parallel to profile P53 (along profiles P54 andP55). The width of the conducting features also decrease towardthe north of the profile P53 along profile P57, but appear to bewidening at depth. These two profiles clearly depict that the con-ducting band of P53 gradually separates out into two bands in P57with a NE-SW strike direction. There is very good correlation in theinitial part of the profiles P56 and P57.

DC-resistivity

On the basis of interpreted model parameters (resistivities andthicknesses) for various soundings (Table 1), three geo-electricalsections (Figure 12a–c) were derived in the area where the gravityhigh has been observed. The resistivity of mafic/ultramafic rock andgranite (or quartzofeldspathic gneiss) is indistinguishable, with avalue of more than 1000 Ωm. The high gravity anomaly zonehas a resistivity value of 50–60 Ωm, which is attributed to chromite.A very low resistivity value of around 10 Ωm corresponds to analluvial clay material.Section 1 is prepared using soundings S4, S2, and S3, and covers

a distance of about 400 m in a S-N direction (Figure 12a). Section 2(Figure 12b), prepared using soundings S5, S6, and S7, is parallel toSection 1 but is further to the west. Both sections show good cor-relation. A thin alluvial clay layer of variable thickness (2–10 m) isinterpreted at the top of both sections. This is underlain by a layerhaving resistivity 50–60 Ωm, which is interpreted to correspond tochromite. The thickness of this chromite-bearing layer is less in Sec-tion 2 compared to Section 1; a maximum thickness of 32 m is in-terpreted at the S2 sounding location. The thickness of the chromite-bearing layer reduces towards the north. It is about 10 m at S3, 8 mat S6, and 5 m at the S7 sounding location. Along Section 1, a thirdlayer is also inferred at sounding location S3 (Figure 12a), whoseresistivity lies in the range of clay; this layer is possibly a fine-grained sheared rock at the contact zone between the chromite for-mation and the sheared granite. The resistivity of this contact layerdepends upon its grain size and the chromite proportion in thesheared matrix. Interestingly, a similar low resistive layer has beeninterpreted below this chromite-bearing layer at sounding location

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Figure 9. Forward modeling of residual gravity anomaly along AA′ profile. (a) 2D modeling and (b) 2.5D modeling.

Figure 10. Forward modeling of residual gravity anomaly along BB′ profile. (a) 2D modeling and (b) 2.5D modeling.

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Figure 11. Current density computed from theVLF real anomaly along profiles P50–P60.(a) N-S VLF profiles (19.8 kHz), and (b) E-WVLF profiles (18.2 kHz).

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S7 (Figure 12b). In both the sections, this chromite-bearing layerand its sheared contact formation are underlain by a highly resistivelayer which could be either granite or mafic/ultramafic rock,or both.

Using S5, S2, S8, and S9 soundings, a third section (Section 3)has been prepared in a SW to NE direction, covering a distance ofabout 750 m (Figure 12c). The surface clay layer is thick at eitherend of this section, but thin in the central part of the profile. This isunderlain by the chromite-bearing layer at sounding location S2, butnot at locations S5 and S8 (Figure 12c). At location S8, the surfaceclay layer is directly underlain by the same sheared rock materialobserved in Section 1 (location S3) and Section 2 (location S7). Thelarge current flow observed at location S8 is attributed to higherchromite content in the sheared rock compared to sounding loca-tions S3 and S7. The lowest layer in Section 3 is interpreted to re-present granitic basement.Figure 13 represents the fits between the observed and 1D mod-

eled resistivity data at sounding location S2, corresponding to thehigh gravity anomaly. It also shows current flow for a unit appliedvoltage at various current electrode separations. The magnitude ofcurrent flow increases and decreases alternately with depth, indicat-ing the disseminated nature of the subsurface conducting structure.An induced polarization effect has also been observed in the studyarea as the potential remains above zero for some time after switch-ing off the current during resistivity sounding measurements. Thisis a signature of two different kinds of current flow in the sub-surface — electronic (in metallic body) and ionic (in fluid-containing rocks), consistent with the presence of disseminatedmetallic minerals. During resistivity survey in the present studyarea, induced polarization was observed at soundings S2, S3, S6,S7, and S8. This qualitatively suggests the presence of disseminatedconductors. In contrast, a pure (massive) chromite formation wouldhave generated steady current and potential. This is also in agree-ment with the VLF current density cross-sections from the sus-pected target.

Figure 12. Subsurface 1D modeling of the interpreted resistivity inthe form of geo-electrical sections, (a) Section 1 (S-N), (b) Section 2(S-N), and (c) Section 3 (SW-NE).

Figure 13. Fittings between the observed (•) and model data (———)after inversion of data from resistivity sounding S2 and current flowpattern at various current electrode separations.

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INTEGRATED INTERPRETATION

The results of the gravity, VLF, and DC-resistivity studies havebeen integrated and show convergence in a locality near the north-ern part of the area.

Anomaly zones

A very good correlation between the subsurface residual gravitymodeling (along BB′ profile), VLF current density plot (along P53),and resistivity Section 3 is observed for the most prominent anom-aly zone around the region between latitudes 20° 55.5 0 Nand

20° 55.6 0 N and longitudes 85° 40.2 0 E 85° 40.3 0 E within thestudy area. The position of S5, S2, and S8 sounding locations ofresistivity Section 3 are marked in the corresponding VLF currentdensity profile (P53) and subsurface residual gravity modelingalong BB′ profile (Figure 14). It is to be noted that gravity sectionBB′ and electrical section 3 are as spatially close as permissible inthe terrain. However, the VLF section P53 is oriented E-W and notparallel to the other two. This occurs because the VLF profile se-lection is constrained by the availability of the transmitter in the

southern direction (in this case Vizianagaram). This is the best avail-able VLF profile section that covers the suspected ore body, andmakes the most acute angle with gravity section BB′ and electricalsection 3.Gravity modeling along BB′ profile depicts the absence of pos-

sible chromite-bearing rock at S5 and S8 sounding locations wherethe conducting layer is also absent. Thus, the integrated studyclearly suggests that chromite exists in this anomaly region andits strike length is approximately 300–350 m in an east-west direc-tion with contact surfaces that dip toward each other, to define asynformal shape. Possible depth of the target body at S2 locationis ∼35 m; this is also in agreement with the gravity modeling, butthe depth varies along the profile. The gravity modeling (2D and2.5D, Figure 10a and 10b) suggests that the depth of the targetreaches a maximum of 100 m to the southwest of sounding locationS2. The VLF profile P53 also shows a conducting body extendingto a depth of ∼100 m. The gravity profile along the south-northdirection (Figure 9a and 9b) also reveals excellent correlation withthe resistivity results along Section 1 (Figure 12a). The width of thecausative body on the south-north direction is approximately 250–

Figure 14. Integrated interpretation of residualgravity model (along profile BB′), VLF currentdensity (along profile P53) and resistivity sec-tion 3. The S5, S2, and S8 sounding locationare shown in Gravity and VLF results. Whilethe horizontal scale is comparable the verticalscale varies in each case. Note the coincidenceof the zone of high gravity anomaly with the cor-responding VLF current density and resistivitycross sections.

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300 m, but again its depth increases from 35 m at sounding locationS2 to 150 m further south, based on the resistivity Section 1 andgravity modeling along AA′. Induced polarization effect and thealternate conducting bands along the VLF current density profilesconfirm the presence of a disseminated conducting body. In gravitymodeling, the very high density (4670–4870 kg/m3) pods within amatrix with density 3570 kg∕m3 suggest the presence of a chro-mite-rich layer overlying a mafic/ultramafic rock with compara-tively lower chromite content. The chromite-rich body hasdensity 46704870 kg∕m3, which is less than the density of purechromite (5090 kg∕m3, Deer et al., 1962) and therefore gravitymodeling also indicates that the body is disseminated in nature.It may also be noted that the density of the mafic/ultramafic rockmatrix cannot exceed 3330 kg∕m3 unless a very dense mineral suchas chromite is present within the rock. The model density of3570 kg∕m3 indicates the presence of small amounts of chromitewithin the underlying mafic/ultramafic rock as well.A small gravity high (2–3 mGal) is also seen at the location

20° 55.2 0 N and 85° 40.2 0 E (Figure 6). The VLF anomaly also re-veals a conducting feature at this location which corresponds to the300 m location on the VLF profile P50 and P52 (Figure 11a). Re-sistivity soundings at S1 and S14 also suggest the possibility of thepresence of chromite-bearing rocks at this location (Table 1). Thegravity anomaly having 2–3 mGal magnitude may result either be-cause of lower chromite concentration in the suspected body, or be-cause the body itself is located at a deeper level.

CONCLUSIONS

The study area does not have surface exposures of chromite, butthe geology strongly indicates the possibility of its presence withinthe mafic/ultramafic rock unit. Therefore, geophysical methods can

be favorably employed to identify the possible chromite accumula-tions within the mafic/ultramafic rock. From this integrated geophy-sical study it is clear that the first anomaly zone is a potentiallychromite-bearing zone. The depth of the suspected deposit in thiszone extends from 35–100 m with a strike length of 300 to 350 m inE-W and 250–300 m in a S-N direction. The depth and density ofthe target body is not uniform throughout its strike length. There-fore, chromite may be encountered at different depths at differentplaces with different chromite content within the anomaly regions.Subsurface occurrence of chromite is strongly indicated from the

chromitite fragments recovered from exploratory pits in the soilcover of the region. The presence of chromite is also suspectedbased on the chromite-producing mines located close to the studyarea. Thus, although every geophysical method has its own inherentambiguity in data interpretation, the results of several geophysicaltechniques in conjunction are consistent with the presence of a chro-mite-rich layer in the area. This study demonstrates that for theexploration of minerals such as chromite, that have sporadic occur-rence but a distinct geophysical signature, integrated geophysicalmethods are the most appropriate and useful. In the present case,gravity and electrical resistivity sounding are concluded to bethe most suitable methods to identify and delineate the chromitemineralization zone. However, VLF and magnetic methods arecheaper and less time-consuming, and can be used as first orderguides before conducting more elaborate gravity and electrical re-sistivity surveys.

ACKNOWLEDGMENTS

We are extremely grateful to the Industrial Development Cor-poration Limited (IDCOL), Govt. of Orissa for the financial supportto carry out this study. Initial discussion and access to previous data

Table 1. Summary of the interpreted model parameters for various soundings.

Sounding no Sounding direction

Resistivity (Ωm) Thickness (m)

ρ1 ρ2 ρ3 ρ4 ρ5 h1 h2 h3 h4

S1 N-S 32.3 555 — — — 8.6 — — —S2 E-W 11.0 60.0 670 — — 1.0 32.3 — —S3 E-W 34.9 58.6 20.2 872 — 2.3 10.9 8.6 —S4 E-W 17.0 21.9 7.4 603 — 2.1 3.0 5.1 —S5 E-W 14.4 9998 — — — 10.6 — —S6 E-W 11.0 37.4 119 — — 1.4 7.9 — —S7 E-W 22.8 3.0 75.7 14.6 9898 1.4 0.3 5.5 12.3

S8 E-W 37.9 11.8 20.8 9988 — 0.6 3.2 8.2 —S9 E-W 50.0 11.9 9994 — — 0.4 10.7 — —S10 E-W 5.3 9992 — — — 5.1 — — —S11 E-W 25.2 2.0 753 — — 2.8 0.3 — —S12 E-W 12.0 4.1 14.3 9995 — 0.9 0.7 8.1 —S13 E-W 10.7 22.7 9993 — — 1.2 13.3 — —S14 E-W 35.0 2486 32.6 93506 — 8.0 5.7 16.7 —S15 E-W 142 10.0 203 — — 1.7 6.9 — —S16 E-W 100 2.2 767 13.7 12904 2.3 0.7 3.3 9.3

B184 Mohanty et al.

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facilitated by IDCOL (Bhubaneswar) and Orissa Mining Corpora-tion, Ltd (Bhubaneswar) greatly helped us to isolate the methodol-ogy of geophysical investigation. Special thanks are also due toChairman-cum-Managing Director, IDCOL, Chairman OMCL,and Mr. C.S. Mishra, IDCOL for their active interest and constantsupport during the study. We would also like to express our greatappreciation of the comments of the Associate Editor, anonymousreviewers, and Dr. Neda Bundalo, whose painstaking reviews haveled to substantial modification of our ideas, and a drastic improve-ment of the manuscript.

REFERENCES

Agocs, W. B., 1951, Least-squares residual anomaly determination: Geo-physics, 16, 686–696, doi: 10.1190/1.1437720.

Aina, A., 1991, Transformation of magnetic intensity anomalies over theChad Basin into gravity field: Nigerian Association of Petroleum Explora-tionists Bulletin, 6, 89–93.

Aina, A., and M. O. Olorunfemi, 1996, Comparative results for electricalresistivity, magnetic, Gravity and VLF methods over a marble lens: Ex-ploration Geophysics, 27, 217–222.

Baranov, V., 1957, A new method for interpretation of aeromagnetic anoma-lies: Pseudogravimetric anomalies: Geophysics, 22, 359–383, doi: 10.1190/1.1438369.

Bayrak, M., 2002, Exploration of chrome ore in Southwestern Turkey byVLF-EM: Journal of the Balkan Geophysical Society, 5, no. 2, 35–46.

Beltrao, J. F., J. B. C. Silva, and J. C. Costa, 1991, Robust polynomial fittingmethod for regional gravity estimation: Geophysics, 56, 80–89, doi: 10.1190/1.1442960.

Bhattacharya, B. B., K. Mallick, and A. Roy, 1969, Gravity prospecting forchromite at Sukinda and Sukrangi, Cuttack District, Orissa (India):Geoexploration, 7, 201–240, doi: 10.1016/0016-7142(69)90028-3.

Burger, H. R., A. F. Sheehan, and C. H. Jones, 2006, Introduction to appliedgeophysics: W. W. Norton & Company.

Deb, S., and K. L. Chakraborty, 1962, Origin of chromite deposits associatedwith the ultrabasic rocks of the eastern part of the Indian peninsula: Pro-ceedings of the National Institute of Sciences of India Part A, Physicalsciences, 27A, no. 5, 508–519.

Deer, W. A., R. A. Howie, and I. Zussman, 1962, Rock-forming minerals,non-silicates 5: Longman.

Griffin, W. P., 1949, Residual gravity in theory and practice: Geophysics, 14,39–56, doi: 10.1190/1.1437506.

Gupta, V. K., and N. Ramani, 1980, Some aspects of regional-residual se-paration of gravity anomalies in a Precambrian terrain: Geophysics, 45,1412–1426, doi: 10.1190/1.1441130.

Hammer, S., L. L. Nettleton, and W. K. Hastings, 1945, Gravimeter pro-specting for chromite in Cuba: Geophysics, 10, 34–49, doi: 10.1190/1.1437146.

Henderson, R. G., and I. Zietz, 1949, The computation of second verticalderivatives of geomagnetic fields: Geophysics, 14, 508–516, doi: 10.1190/1.1437558.

Hinze, W. J., 2003, Bouguer reduction density: Why 2.67?: Geophysics, 68,1559–1560, doi: 10.1190/1.1620629.

Hubbert, M. K., 1948, Line-integral method of computing the gravimetriceffects of two dimensional masses: Geophysics, 13, 215–225, doi: 10.1190/1.1437395.

Karous, M., and S. E. Hjelt, 1983, Linear filtering of VLF dip-angle mea-surements: Geophysical Prospecting, 31, 782–794, doi: 10.1111/gpr.1983.31.issue-5.

Lawal, K. M., and C. Z. Akaolisa, 2006, Bouguer correction density deter-mined from fractal analysis using data from part of the Nigerian BasementComplex: Nigerian Journal of Physics, 18, no. 2, 251–254.

Li, Y., and D. W. Oldenburg, 1998, Separation of regional and residual mag-netic field data: Geophysics, 63, 431–439, doi: 10.1190/1.1444343.

Malehmir, A., H. Thunehed, and A. Tryggvason, 2009, The paleoprotero-zoic Kristineberg mining area, northern Sweden: Results from integrated3D geophysical and geologic modeling, and implications for targeting oredeposits: Geophysics, 74, B9–B22, doi: 10.1190/1.3008053.

Merriam, D. F., and J. W. Harbaugh, 1964, Trend-surface analysis of regio-nal and residual components of geologic structure in Kansas: KansasGeol. Survey Spec. Dist. Publ., 11, 1–27.

Mondal, S. K., 2009, Chromite and PGE deposits of Mesoarchaeanultramafic-mafic suites within the greenstone belts of the SinghbhumCraton, India: implications for mantle heterogeneity and tectonic Setting:Journal Geological Society of India, 73, 36–51.

Mondal, S. K., E. M. Ripley, C. Li, and R. Frei, 2006, The genesis ofArchaean chromitites from the Nuasahi and Sukinda massifs in theSinghbhum Craton, India: Precambrian Research, 148, 45–66, doi: 10.1016/j.precamres.2006.04.001.

Mukerjee, S. K., 1992, Mineral exploration in the twentieth century: IndianJournal of History of Science, 27, no. 4, 461–475.

Murthy, I. V. R., and G. Gopalakrishna, 1982, Remanence hysteresis proper-ties of chromites: Proceedings of the Indian Academy of Sciences (Earthand Planetary Sciences), 91, no. 2, 159–166.

Nabighian, M. N., 1972, The analytic signal of two-dimensional magneticbodies with polygonal cross-section: Its properties and use for automatedanomaly interpretation: Geophysics, 37, 507–517, doi: 10.1190/1.1440276.

Pawlowski, R. S., and R. O. Hansen, 1990, Gravity anomaly separation byWiener filtering: Geophysics, 55, 539–548, doi: 10.1190/1.1442865.

Qureshy, M. N., D. V. Subba Rao, S. C. Bhatia, P. S. Aravamadhu, andC. Subrahmanyam, 1973, Gravity bases established in India byN.G.R.I. Part IV: Geophysical Research Bulletin, 11, no. 2, 136–152.

Ramarao, P., and I. V. R. Murthy, 1989, Two FORTRAN 77 function sub-programs to calculate gravity anomalies of bodies of finite and infinitestrike length with the density contrast differing with depth: Computerand Geosciences, 15, no. 8, 1265–1277.

Roach, M. J., D. E. Leaman, and R. G. Richardson, 1993, A comparison ofregional-residual separation techniques for gravity surveys: ExplorationGeophysics, 24, 779–784.

Roest, W. R., and M. Pilkington, 1993, Identifying remanent magnetizationeffects in magnetic data: Geophysics, 58, 653–659, doi: 10.1190/1.1443449.

Roy, A., and A. O. Aina, 1986, Some new magnetic Transformations: Geo-physical Prospecting, 34, 1219–1232, doi: 10.1111/gpr.1986.34.issue-8.

Saha, A. K., 1994, Crustal evolution of Singhbhum North Orissa, EasternIndia: Memoirs of the Geological Society of India, 27, 341.

Sahoo, R. K., J. K. Mohanty, S. K. Das, and A. K. Paul, 2009, Chromites ofIndia: Their textural and mineralogical characteristics: International Sym-posium on Magmatic Ore Deposits.

Sharma, S. P., and P. Kaikkonen, 1999, Appraisal of equivalence and sup-pression problems in 1D EM and DC measurements using global optimi-zation and joint inversion: Geophysical Prospecting, 47, 219–249, doi: 10.1046/j.1365-2478.1999.00121.x.

Silva, J. B. C., 1986, Reduction to the pole as an inverse problem and itsapplication to low-latitude anomalies: Geophysics, 51, 369–382, doi: 10.1190/1.1442096.

Spector, A., and F. S. Grant, 1970, Statistical methods for interpreting aero-magnetic data: Geophysics, 35, 293–302, doi: 10.1190/1.1440092.

Talwani, M., J. W. Worzel, and M. Landsman, 1959, Rapid gravity compu-tation of two-dimensional bodies with application to the Mendocino sub-marine fracture zone: Journal of Geophysical Research, 64, 49–59, doi: 10.1029/JZ064i001p00049.

Telford, W. M., L. P. Geldart, and R. E. Sheriff, 1990, Applied geophysics:Cambridge University Press.

Unwin, D. J., 1978, An introduction to trend surface analysis: Concepts andTechniques in Modern Geography no. 5, Geo Abstracts, University ofEast Anglia

World Data Centre for Geomagnetism, Kyoto, 2010, International Geomag-netic Reference Field. Data set accessed 25 July 2010 at http://wdc.kugi.kyoto-u.ac.jp/igrf/point/index.html.

Yen, H. Y., Y. H. Yeh, and C. H. Chen, 1994, Gravity terrain correctionof Taiwan: TAO: Terrestrial, atmospheric, and oceanic sciences, 5,no. 1, 1–10.

Young, C. T., 2004, Basic magnetic processing and display in MATLAB,http://www.geo.mtu.edu/profile/CTYOUNG.HTM, accessed 28 March2011.

Zurflueh, E. G., 1967, Application of two-dimensional linear wavelengthfiltering: Geophysics, 32, 1015–1035, doi: 10.1190/1.1439905.

Chromite exploration at Tangarparha B185

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