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JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.86, October 2015, pp.489-499

A Comprehensive View from Geophysical Signatures over Chitradurga Schist Belt, Karnataka

1* 2 2 2 J.V. RAMA RAO , B. BALAKRISHNA , N.V.S MURTY , P. AJAYKUMAR , 3 3 3 M. V. RAMAKRISHNA RAO , R. S. ACHARYA and S. P. SANKARAM Geological Survey of India 1Geophysics Division, Eastern Region, Kolkata, 2Geophysics Division, Southern Region, Hyderabad, 3Retd. officials from Southern Region Hyderabad *Email: [email protected]; [email protected]

Abstract: Geology of Chitradurga Schist Belt (CSB) from Gadag to Srirangapatna, as inferred from the gravity and magnetic data analysis and supportive evidences from digitized Airborne Total count map is presented in this paper. Significant inferences have emerged from the integrated geophysical analysis coupled with geological information. Pattern, continuity and intensity of anomalies indicate that CSB can be viewed as three sectors; northern (Gadag), central (Chitdradurga) and southern (Nagamangala). Qualitatively geophysical signatures are diagnostic for the purpose of broad classification of the area into different geological domains such as granite-gneissic (gravity lows, fluctuating magnetic values and moderate radiometric counts), metavolcanic (gravity highs, magnetic highs and radiometric lows), metasedimentary (moderate gravity low, low magnetic values and moderate radiometric values) and granitic (gravity low, magnetic high and radiometric high) regions. Images prepared from gravity, magnetic and radiometric values indicated possibility of an additional arm of the schist belt from Huliyar to Hosdurga between the known Kibbanahalli and Chitradurga arms of CSB. From the gravity image it appears that the eastern margin of CSB may be encompassing Bukkapatna granite in the central sector of CSB. Gravity low closures and gravity high gradient between the granites of Chitradurga and those near Jagalur suggest different nature of geological continuity. Structural features, particularly the sheared contacts of metavolanics with metasedimentaries are demarcated based on gravity gradients. Based on these inferred features coupled with known geological controls of mineralization, a few target areas for gold / base metal exploration are identified. A major structural feature affecting the strike of arms of CSB may be the possible regions for emplacement of ultramafic bodies as observed near Sasival and Ghattishosalli. This in turn is significant for mineral exploration in Chitradurga Schist belt. Gravity and magnetic profiles drawn across CSB for every 15' latitude indicated crustal architecture beneath CSB. Significant rise is noticed in the background gravity field from the arms of CSB to the main schist belt suggesting that the Bababudan volcanics have generated from different depths compared to the Chitradurga volcanics. Further, long wavelength magnetic anomaly coinciding with the eastern margin of schist belt suggests the differences in nature of crust on either side. Frequency analysis of gravity and magnetic anomalies and the modeling indicated the depth persistence of geological units.

Keywords: Gravity, Magnetic, Radiometric responses, Chitradurga schist belt, Karnataka.

INTRODUCTION mineralization, and is being exploited since long Chitradurga Schist Belt (CSB) lies west of Closepet (Ramachandran et.al., 1997, 2000). It is widely studied by granite and is one of the longest schist belts of Dharwar earth scientists not only from the point of view of minerali- craton with a north-northwesterly trend extending from zation but also for its tectonic framework as its eastern Gadag in north to Srirangapatna in south (Karnataka, India). margin is a suture zone and is believed to be the dividing It attains a maximum width of 40 km near Chitradurga and line between the western and eastern Dharwar cratons (Kaila occupies over 6000 sq.km area with a strike length of 400 et al., 1979). km. It exposes the complete succession of cratonic rocks such as Sargur and Dharwar supracrustals, basement and GEOLOGY, STRATIGRAPHY, STRUCTURE intrusive gneisses and younger K-rich granitoids AND MINERALIZATION OF CSB (Ramakrishnan and Vaidyanadhan 2008). CSB is widely known for its mineral potential, particularly gold Chitradurga Schist Belt comprises of a thick pile of

0016-7622/2015-86-4-489/$ 1.00 © GEOL. SOC. INDIA 490 J. V. RAMA RAO AND OTHERS sediments and volcanics. Gist of geological account and various other aspects of CSB are listed in chronological order for easy reference (Sehsadri et al., 1981). Succession of cratonic rocks, such as, Sargurs and Dharwar supracrustals, basement and intrusive gneisses and younger granitoids are exposed in this curvilinear and long belt. Sargur Group is well represented in Ghatti- hosahalli, J.C. Pura, Mayasandra, Sasival and Nagamangala bands. Bababudan Group is seen in Kibbanahalli-Doddaguni, Mayakonda, Brahmasagara, Honakere, Chitadurga-Gadag, Javanahalli and Yadiyur- Karighatta areas. Chitradurga Group covers most of the belt consisting of Vanivilas and Ranibennur sub-groups comprising of metavolcanics and metasedimentaries (Fig.1). In the center of the basin, a thick pile of greywacke turbidites overlie both the shallow water and deep water sequences (Ramakrishnan and Vaidyanadhan, 2008). Younger granites as seen near Chitradurga, Hosadurga, Jampalnaikankote, and Bukkapatna in Chitradurga area and Doni in Gadag area intrude into adjoining gneisses and schistose rocks. Intrusive granites into the peninsular gneisses and schists, become main features of this area (Ramakrishnan and Vaidyanadhan, 2008). CSB displays structural complexity with plunging synclines, anticlines, basement domes and shear zones (Drury, 1983; Chadwick, 2003). Polymict conglomerates, marine sequences, shallow water sequences and overlying greywackes are other important components of CSB. Geochemistry, depositional environment and tectonic setting of CSB are well documented (Rao, 1995). Major faults and fold forms of CSB are mapped and stratigraphic sequence is given by Mukhopadhyay et.al., (1981) and Ghosh Roy and Ramakrishnan (1985). CSB shows prominent antiformal closures having gneisses and/ or granite at their core at Chitradurga, Kandavadi, Dodaguni and Sirankatte. Synformal folds are noticed near Hiriyur, Kibbanahalli and Yadiyur-Karighatta (Ramakrishnan and Vaidyanathan, 2008). Gadag portion is re-interpreted as a structural duplex with many tectonic slices (Chadwick et al. Fig.1. Simplified geological map of CSB depicting the disposition 2003). Several tectonic features and tectonised contacts of schistose rocks and intrusive granites between Dharwar supracrustals and granite-gneissic rocks are brought out based on interpretation of Landsat images CSB houses a number of economic minerals such as gold, (Drury, 1983). copper, lead, zinc, antimony and pyrite in basic volcanics, A prominent shear zone at the eastern margin of CSB iron and manganese in cherts and phyllites of the western marks the transitional boundary between western and eastern margin, limestone and dolomite in Javanahalli belt, barytes Dharwar cratons. A detailed structural study of this crustal in fuchsite quartzite, asbestos. corundum, vermiculite, scale shear zone and its significance on amalgamation of soapstone and magnesite in the contacts of ultramafic zones two different crustal blocks and the status of Bukkapatna and feldspar, quartz and mica in pegmatite and quartz veins granite is reported by Sengupta and Roy (2012). Prominent within the peninsular gneissic complex (Seshadri et al., and mineralized shear zone of about 30 km length is mapped 1981). Gadag part of CSB, is known for gold mining activity between the metavolcanics and metasedimentaries in the since ancient times, where gold mineralization is confined central part of CSB from Jagalur to Gonur (Khan, 2001). to three potential sectors (Jayaprakash, 2005). Main lode

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 A COMPREHENSIVE VIEW FROM GEOPHYSICAL SIGNATURES OVER CHITRADURGA SCHIST BELT, KARNATAKA 491 zones are at the contact of metavolcanics with greywacke- (Total Count) map (Rama Rao et al., 2002) argillite suite of rocks. In Chitradurga sector, gold Gokaran et al. (2004) proposed geo-electric model based mineralization is associated with quartz sulphide veins and/ on broadband magneto telluric data collected along east west or stringers, localized along shear zones and sulphide traverse over the granite-greenstone terrain of Dharwar disseminations in the cherts. Gold occurrences in eastern craton and has suggested a suture along Chitradurga-Gadag part of CSB are mainly associated with Hiriyur Formation. schist belt, formed by thrusting of western Dharwar craton Generally, there are three types of occurrences viz. (1) BIF- beneath the eastern Dharwar craton with an easterly dip of associated gold prospects, (2) auriferous-quartz-sulphide 20°-30°. This proposed thrust is interpreted to be older to lode deposits and (3) gold lode deposits associated with the formation of schists and controlled the Chitradurga- granodiorite-tonalite bodies (Krishna Rao, 2001). Possibility Gadag schists. of copper-nickel and PGE mineralization in mafic-ultramafic complexes of Chitradurga schist belt is suggested (Devaraju Airborne Radiometric Maps et al., 2009). For the purpose of supportive evidences, the radiometric (Total count) maps in the analog format (GSI-BRGM, 1971- 72) were digitized for every gravity and magnetic station PREVIOUS GEOPHYSICAL WORK and the data was processed for generation of image and Geophysical studies in parts of Chitradurga belt contours and presented in Fig.2. The radiometric data was conducted earlier gave rise to broad inferences. Aero collected with a flight interval of 500m and at an altitude of geophysical surveys were carried out on 1:30,000 scale 120m. covering the entire CSB with magnetic, EM and radiometric Since the total count values are more influenced by (Total Count) surveys (GSI-BRGM, 1971-72). These aero- the surface geology, the airborne radiometric maps are geophysical maps were interpreted selectively for different expected to greatly help in geological mapping of CSB in parts of CSB for identifying target areas of mineralization general. To designate characteristic values over the broad (Ramachandran et al., 1997, 2000). Gravity surveys in part geological domains viz. granite-gneiss, granite, metabasalt of CSB gave an idea about granite-greenstone association and greywacke-argillite suite of rocks, four ranges of and the thickness of schist belt in central part radiometric values are chosen based on the earlier work. (Subrahmanyam and Verma, 1982). Geophysical mapping Low values of total counts < 400 (cps) were observed over with gravity and magnetic (TF) methods in 1:50,000 scale metavolcanics of Gadag, Doni, Magala, Nandibevuru, G R covering parts of 35 topo sheets of degree sheet Nos. 48 M, Halli, Kennedlu, Paramanahalli, J G Halli, Bukkapatna N and 57 A, B, C, D, forms the basis of this work. A few of (west), Kibbanahlli, Yediyur and Nagamangala. Both the the inferences pertaining to Chitradurga area were presented granite-gneisses at the contacts of CSB and meta- (Rama Rao, 2002). sedimentaries of Kadkol-Bagewadi, Turuvanur and Hiriyur Earlier ground geophysical work is limited to parts of exhibited medium range counts from 400 to 600 (cps). CSB on a regional scale and inferences drawn are broad in Granites within the CSB near Doni, Belhatti, Harpanahalli, nature. Based on aeromagnetic and gravity anomalies in Jaglur, Chitradurga, J N Kote, Hosdurga and Bukkapatna central part of CSB, Naqvi (1973) has identified two showed total count values of more than 600 (cps). generations of folds. Analysis of gravity anomalies yielded the nature of sub- Present Data Acquisition, Data Standards and Processing surface geology. Other significant inferences are in Methods estimating the depth persistence of volcanics (0.8 to 2.5 km) During the period 1999-2004, to understand the nature and thickness of granite body to be of the order of 7.2 km. of geology and structure, entire Chitradurga schist belt and Gravity interpretation of Dharwar craton by Subrahmanyam the adjoining terrain was covered with gravity and magnetic and Verma (1982) has given an insight into greenstone- (TF) surveys on 1:50,000 scale covering about 15000 sq.km gneiss-granite association. The 30 mGal positive anomaly area with 7175 stations spread over 35 topo sheets. Gravity over CSB was accounted for 13 km of schist belt over central and magnetic surveys along all the available tracks and part. The aeromagnetic total field anomaly maps confirmed cart tracks were carried out at 1 km station interval. Thus the division of Dharwar craton along CSB based on average station density achieved was about one station per heterogeneity of magnetic anomalies (Anand and Rajaram, 2 sq km area. 2002). Northern part of CSB was analysed in detail based Gravimeter (CG-3, Scintrex) and magnetometer on gravity and magnetic mapping and airborne radiometric (Envimag- Scintrex) were deployed with resolutions of 0.005

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 492 J. V. RAMA RAO AND OTHERS

mGal and 1 nT respectively. Including leveling surveys, the GADAG accuracy of surveys achieved was with a repeatability of Doni 0.1 mGal and 10 nT of gravity and magnetic anomalies. Kadkol Mundargi Measurements at a number of stations were repeated for the

Belhatti purpose of data validation. Bouguer gravity anomalies were Magala computed by reducing the data to IGSN-71 Gravity bases HirehadagalliNandibevuru and using 1980 International Gravity formula and assuming the average crustal density to be 2670 Kg/m3. Magnetic Harpanahalli anomalies were computed by applying IGRF corrections as Sokke per the epoch of surveys. Data was processed in Geosoft Anabur for generation of images, Power spectrum and modeling. Jagalur Surfer and Grapher softwares were used for the shaded relief

Bharmasagara maps and generation of profiles. Challakere G.R.Halli During the collection of gravity and magnetic data, a CHITRADURGA number of representative rock samples were collected for J.N.Kote the purpose of density and susceptibility measurements Holalkere (Rama Rao et al., 2003, 2003a, 2004 and 2005) Hiriyur Representative properties of rock samples in Table 1 Sirankatte J.G.Halli have been used for the purpose of mapping and modeling Hosdurga Sira of gravity data. Generally, granites, granite gneisses and greywacke-argillite suite of rocks exhibited low density Bukkapatna Huliyar values in comparison to meta-basalts, quartzite, BIF, amphibolite, gabbro and pyroxenite rocks. Altered products Sasival such as chlorite schist and sericite schist have shown reduced

Kibbanahalli density values. Overlapping susceptibility values were measured on all the formations excepting BIFs which 625 C.S.Pura 575 showed very high ranges. Ultramafic rocks (talc schist) have 525 shown high density and moderate susceptibility values. 475 Yediyur 425 Bouguer Gravity anomalies: Bouguer gravity anomaly 375 Nagamangala values show variation of over 40 mGals from the gneisses TC in the western margin to meta-volcanics of CSB. As there is a distinct density contrast amongst the geological formations; 3 MANDYA meta-volcanics (2870 Kg/m ) to granite gneisses (2610 Kg/ 3 3 Srirangapatna m ) to granites (2590 Kg/m ) and metasedimentaries (2460 Kg/m3), gravity surveys are effective in geological mapping. MYSORE Thus the granites, greywacke-argillite suite of rocks Scale 1:2000000 generally occupying the antiforms within the schist belt are 25000 0 25000 expected to reflect as major gravity lows in comparison to (meters) metavolcanics occupying the synforms of schist belt. It is WGS 84 / UTM zone 43N also observed that there is regional rise in gravity values over gneisses from west to east of CSB, particularly in the Sl. Rock Type Observed Total central part, indicating the changes in the nature of crust on No count values either side of CSB. 1 Metavolcanics < 400 (cps) Bouguer gravity shaded image presented in Fig.3, brings 2 Metasedimentaries 400 to 500 (cps) out the disposition of the schist belt and its arms in the form 3 Gneisses 500 to 600 (cps) of shades of gravity highs. This image clearly depicts the 4 Granite >600 (cps) schist belt as three sectors; Gadag (northern), Chitradurga Fig.2. Radiometric total count image, Chitradurga schist belt, (central) and Nagamangala (southern) with clear Karnataka. discontinuities in between them. Northern sector has

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 A COMPREHENSIVE VIEW FROM GEOPHYSICAL SIGNATURES OVER CHITRADURGA SCHIST BELT, KARNATAKA 493

Table 1. Physical properties of representative rock samples Sl. Rock Type No of Density Mag Susceptibility No. Samples Kg/m3 x 10-6Cgs units Range Characteristic Range Characteristic value value 1 Granite 77 2550-2690 2590 25-165 55 2 Granite gneiss 50 2530-2730 2610 20-160 65 3 Meta basalt 156 2680-3190 2870 35-110 82 4 Quartzite 30 2580-2960 2720 30-70 65 5 BIF/Chert 63 2700-3440 2910 30-5800 1515 6 Greywacke-Argillite 45 2350-2670 2460 14-149 65 7 Chlorite/Sericite schist 44 2440-2750 2610 16-86 38 8 Amphibolite 130 2760-3220 2940 25-108 64 9 Talc chlorite schist 43 2630-2940 2800 22-211 70 10 Hornblende/Pyroxenite 5 3020-3280 3090 30-63 47 11 Phyllite 11 2.36-2.62 2550 35-610 148 12 Gabbro 18 2800-3040 2930 20-482 108 13 Dyke 16 2750-3020 2910 40-262 130 predominantly metavolcanics as seen from Gadag to Granites and granite gneisses show fluctuating anomalies Nandibevuru. CSB is widest in the central part (Hosdurga- as seen in Fig.4. Metavolcanics, within the CSB, reflect as Hiriyur), with equal distribution of linear patches of magnetic high zones indicating that basaltic lava has cooled metavolcanics (gravity highs) and metasedimentaries slowly and the rocks are rich in magnetite minerals having (gravity lows). A narrow patch of gravity high characterizes high susceptibilities (Grant, 1984-85). Ultramafic intrusions the southern part. General trend of CSB is NW-SE (northern are reflected as moderate magnetic anomalies. In the central and central parts) to near N-S (southern part). Southern part part of CSB where metasedimentaries are predominant, the is devoid of any metasedimentaries. area is characterized by low magnetic field variations. BIF’s A significant observation from the gravity image is the at the contacts of formations showed high intensity gravity ‘high’ between Huliyar and Hosdurga, almost parallel anomalies, thus indirectly helped in identifying the to the known Kibbanahalli arm of CSB probably indicates formational contacts. The division of CSB in to three sectors an additional arm of schist belt with intermittent surface on the basis of gravity anomalies is corroborated well with expression. magnetic domains.

Intrusive granites: By virtue of low density of granites Bouguer Gravity Anomaly Axes Map and Inferences with respect to the gneisses and meta-volcanics, distinct To understand the regional structure of CSB and its fold gravity lows are observed at several places within CSB as pattern, the gravity high and gravity low axes are drawn on near Doni, Chitradurga-Harpanahalli, Jagalur, J.N.Kote and the shaded Bouguer gravity image of CSB presented in Fig.5. Bukkapatna. Of these, granites near Doni in northern sector This exercise yielded the continuity of geological features and J N Kote in central sector are reflected as circular gravity based on the strike continuity of alternating gravity highs closures and Bukkapatna granite as a two-dimensional and gravity lows. Four sets of gravity high and gravity low gravity low zone indicating the narrow disposition and axes are identified passing through different formations. It intrusive nature. Strike continuity of gravity low recorded is observed that the high density metavolcanics are the from Chitradurga suggests the disposition of Chitradurga ‘synforms’ and are the topographic lows depicted as ‘gravity granite in NW-SE direction up to Harpanahalli through high axis’ in comparison to granites and metasedimentaries Bilichodu. Gravity gradients separating this low zone from which are the ‘antiforms’ generally occupying the the one recorded near Jagalur indicates the separate nature topographic highs and possibly have the gneisses as their for Jagalur granite. This inference is at variance with the basement reflected through ‘gravity low axis’. The various consideration of granites of Chitradurga – Jagalur as a single geological features inferred are described from west to east entity in the existing surface geological map. of CSB. The gravity high axis-1 (gh-1) has no surface expression Magnetic (TF) anomalies of CSB: Magnetic anomalies with very limited exposures and is inferred as an additional generally reflect high intensity anomalies through out CSB. arm of CSB connecting to the main belt at Huliyar and

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 494 J. V. RAMA RAO AND OTHERS

77°00' ° 3 75°30' 75°45' 76°00' 76°15' 76°30' 76°45'

' 1 GADAG -65

5 Doni -68

5 5 Mundargi -71

1 Kadkol 1 -73

' Belhatti -75

Northern Sector -77 0 ° Magala

' 1 -79 HirehadagalliNandibevuru -81

4 5 Harpanahalli -83

5 4 -85

' 1 Sokke -87 Anabur

1 ' -89

° 3 2 Jagalur

° -92

' 1 -95 Bharmasagara -98

1 ' Challakere

4 5 G.R.Halli -103

4 CHITRADURGA

' 1 3 J.N.Kote BG mGal

4 0 Holalkere

0 1

1 Hiriyur Sirankatte 3

J.G.Halli 1

3 5 Hosdurga

4 ° Sira

1 Central Sector Bukkapatna

' Huliyar

3 4

1 Sasival

Kibbanahalli 3

3 1: Eastern margin of schist belt

' 1 2. Boundary of Chitradurga granite C.S.Pura 3. Inferred shear zone

4. Inferred arm of schist belt 3

3 Yediyur

' 1 Southern Sector 1

1 ' Nagamangala

' 1 Scale 1:2000000

' 25000 0 25000

3 MANDYA

2 (meters)

' 1 WGS 84 / UTM zone 43N Srirangapatna

5 MYSORE

' 1 75°30' 75°45' 76°00' 76°15' 76°30' 76° 45' 77°00'

Fig.3. Bouguer Gravity Image of CSB

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 A COMPREHENSIVE VIEW FROM GEOPHYSICAL SIGNATURES OVER CHITRADURGA SCHIST BELT, KARNATAKA 495

' 5 extends up to east of Hosdurga granite. It is more or less

0 75°30' 75°45' 76° 76°15' 76°30' 76°45' 77° °

3 3

' 1 GADAG parallel to Kibbanahalli arm of CSB. It is flanked on either side by gravity low; gl-1 over granites of Holalkere –

1 ' Doni

1 ° C.N.Halli and Sirankatte gneissic dome. Kibbanahalli arm 5

' 1 Kadkol Mundargi and Chidtradurga arm of CSB consisting of metavolcanics Belhatti

1 upto Brahmasagara where they occur as relict patches is

° 5

Magala ° 1 marked as gh-2. Depending on the regional gravity picture HirehadagalliNandibevuru it can be inferred that this feature represents arms of

5 Harpanahalli

5 CSB consisting of volcanics generated from deeper levels.

' 1 Sokke This high axis is flanked by major gravity low axis; gl-2 Anabur

0 starting from granites of Harapanahalli – Bilichodu and

3 Jagalur

' 1 Chitradurga and passing through metasedimentaries Bharmasagara Challakere consisting of greywacke-argillite suite of rocks of Hiriyur

G.R.Halli 4

1 ° and granite gneisses of southern part of CSB. Metavolcanics

5 4 CHITRADURGA

' 1 J.N.Kote of Gadag area through Nandibevuru in northern CSB and

Holalkere 1 those at Sokke – G R Halli – west of Bukkaptna and Yediur

° 4

° 1 Hiriyur form the gravity high axis gh-3. The regional gravity picture Sirankatte J.G.Halli suggests that these volcanics are generated from relatively

Hosdurga 3

4 ° Sira shallower levels in comparison to Bababudan volcanics.

1 Bukkapatna Though there is no continuity, it can be inferred that the Huliyar

3 fourth low axis, gl-4 connects the plutonic rocks of Doni 0

' 1 Sasival and metasedimentaries of Bagewadi and narrow patch of gneisses and passing through intrusive granites of J N Kote

Kibbanahalli 1

1 ° and Bukkapatna. To the east of this low zone is the narrow

' 1 MA C.S.Pura patches of volcanics near the eastern margin of CSB and

50 1 possibly the J G Halli belt form the gravity high axis gh-4.

° 3

3 ° 1 -50 Yediyur Gneisses of eastern margin of CSB is the last mapped low -150 Nagamangala axis; gl-4.

-250 1

4 ° -350 Thus entire CSB can be visualized as alternating

' 1 nT sequence of gravity lows and gravity highs trending in NW-

2 SE and N-S directions; the fold pattern can be visualized

MANDYA °

' 1 Srirangapatna based on the continuity of these features. In drawing these inferences, some missing links are inferred.

' MYSORE 1

5 2

1 °

° 1

2 5 1 75°30' 75°45' 76° 76°15' 76°30' 76°45' 77° ' Eastern Margin of CSB 25000 0 25000 50000

(meters) Eastern margin of CSB is significant in the context of WGS 84 / UTM zone 43N division of Dharwar craton into two blocks, the crustal scale Fig.4. Magnetic map shows predominantly high zones in Gadag and Nandibevuru areas in the northern sector where extent from Gadag to Mandya and the difference in the metavolcanics are predominant. Magnetic lows populate metamorphic grade of supracrustal belts of Chitradurga and in the central sector where CSB is widest and Javanahalli on its either side (Sengupta, 2012). Further this metasedimentaries are predominant. All along the eastern shear zone has also affected the linear Bukkapatna granite. margin, high intensity bipolar anomalies are observed. From the variations in the crustal thicknesses, deduced from Close examination of magnetic anomalies between Hiriyur DSS studies, on the either side of eastern margin of CSB, and Bukkapatna confirms the contacts between J G Halli Kaila (1979) has proposed the division of Dharwar craton belt and Bukkapatna granite and corroborate with the along this zone. Similarly, the heterogeneity of aeromagnetic inferences drawn from gravity data. J G Halli belt show anomalies in this zone is significant (Anand, 2002). In the magnetic high zones and Bukkapatna granite is characterized by sparse variations. The trend and nature of light of the above, the responses from the ground gravity the anomalies indicate that the eastern margin of CSB and magnetic surveys hold lot of importance. By following should be encompassing J G Halli belt and Bukkaptna the high gravity gradients from north to south, the eastern granite. margin of CSB can easily be visualized from Gadag in

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 496 J. V. RAMA RAO AND OTHERS

profiles are drawn at 15' interval from west to east across CSB starting from 12° 30' N to 15°15' N. These profiles presented in Fig.6 has indicated that there is a general rise in the gravity values from west to east and arms of CSB have originated from deeper levels (Bababudan volcanics) in comparison to the main CSB (Chitradurga volcanics). It is also observed that there are no Bababudan volcanics in

BG GRAVITY AND MAGNETIC PROFILES TF mGal ACROSS CHITRADURGA SCHIST BELT, KARNATAKA nT

-60 600

-70 400 200 -80 0 -90 15°15' -200 -100-60 -400200 -64 0 -68 75.5 76 76.5 77 -72 -200 -76 15°00' -80 -400800 600 -84 400 200 -88 14°45' 0 -92-75 -200100 -80 0 -85 -100 -90 -200 -95 -300 -60 14°30' -100 -400-40 -70 -80

-80 -120 -160 -90 -200 -100 14°15' -70 -240100

-80 0 -100 -90 -200 -100 14°00' -300 -60-110 -400200

-70 0 -80 75.5 76 76.5 77 -200 -90 -400 -100 13°45' -110-60 400-600 -70 200 -80 0 -90 -200 -100 -400 -110-68 13°30' -600100 -72 0 -76 -100 -80 -200 -84 13°15' -300 -88-76 -40-400

-80 -80 -120 Fig.5. Gravity axes map, Chitradurga schist belt, Karnataka. -84 -160 13°00' -72-88 -200-40 northern sector to east of Hiriyur in central sector -76 -80 -120 -80 encompassing the J N Kote granite and the linear volcanic -160 -84 12°45' -200 patches to its east. By extending the same trend, logically -80-88 -24040 -84 0 -40 the eastern margin of CSB in the southern sector should -88 -80 -92 encompass the Bukkapatna granite and J G Halli belt and 12°30' -120 -96 -160 volcanic patches near Yediur and Nagamangala. This extends 75.5 76 76.5 77 up to Srirangapatna. However, the zone between east of Hiriyur to Bukkapatna is at variance with the geologically Fig.6. Gravity (Red line) and Magnetic (Blue line) profiles are mapped eastern margin as both Bukkapatna granite and J G generated from the gridded data of gravity and magnetic Halli belt are not shown as parts of CSB based on the anomalies and are drawn for every 15’ Latitude. As these geological evidences. Alternatively, if this logic has to be profiles cut across CSB the responses give the regional character and the disposition of schist patches and intrusive followed northwards, even J N Kote granite has to be shown granites. There is a distinct change in the regional character outside CSB. These inferences are drawn purely on the basis of gravity values from the arms of CSB to main schist belt. of gravity image and not being influenced by geological A bipolar magnetic anomaly at the eastern margin is observations. suggesting a deep fracture and divergent nature of crustal To ascertain further, gravity and magnetic anomaly blocks.

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 A COMPREHENSIVE VIEW FROM GEOPHYSICAL SIGNATURES OVER CHITRADURGA SCHIST BELT, KARNATAKA 497 the southern sector and are only restricted to the arms of anomalies help in deducing these zone as there could be a CSB. Importantly, a diagnostic magnetic high amplitude fall in the density values and an increase in total count values. bipolar anomaly is consistently seen in the region of eastern After studying the signatures over the known mineral zones margin of CSB indicating the change in the nature of crust some additional areas are identified as future targets. Two around this place. This anomaly further confirmed that the such blocks are presented in Fig.7A&B eastern margin of CSB should be encompassing the J G Halli belt. Nature of Crust in CSB and Modeling To understand the nature of crust and for estimating the Mineralized shear zones: Mineralization in CSB is thickness of intrusives such as granites and the schsit patches, confined to the areas between central shear zone and eastern gravity anomalies are analysed by computing the power shear zone. The contacts of meta-volcanics with meta- spectrum and presented in Fig.8A. This information and the sedimentaries hold promise for ore-bearing zones. Other than density values measured are used for modeling the CSB at the central shear zone which is mineralized, it is possible three places in the central sector where the schist belt is that there could be extensions of this zone and some parallel widest and number of parallel patches are seen. Density shear zones in the central sector. Gravity and radiometric values assumed for modeling are for granite-gneisses - 2670

Bagewadi sector

Turavanur sector Sasival-Hosdurga-Siregere sector Fig.7. (A) Integrated analysis of gravity anomalies superposed on to radiometric total count image brought out two possible zones as target zones for possible mineralization as known signatures of Kadkol area are seen in this area. Bagewadi sector is characterized by gravity low and moderate radiometric values. It is flanked by gravity highs and radiometric lows. The contacts of metavolcanics and metasedimentaries hold promise for mineralization. (B) This area is well known for mineralization in G R Halli area through which central shear zone passes through. Two parallel shear zones are identified characterized by gravity lows and moderate radiometric values, which could be potential zones for mineralization. (C) A NNW-SSE fracture zone is inferred from the gravity contours extending from Sasival north west of Tiptur to Siregere west of Chitradurga of about 90 km length mainly affecting the arms of CSB. It is observed that wherever there is discontinuity in the arms of CSB and change in the strike direction along this zone, ultramafic rocks have intruded as noticed near Sasival and east of Hosdurga and Ghattihoshalli thus indicating that the emplacement of ultramafic rocks is guided by this zone. These small ultramafic complexes may hold promise for hosting mineralization.

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 498 J. V. RAMA RAO AND OTHERS

RADIALLY AVERAGED POWER SPECTRUM BOUGUER GRAVITY ANOMALIES 10 10 ln(Power)

0 0 ln(Power)

-10 -10 0.0 0.1 0.2 0.3 0.4 0.5

DEPTH ESTIMATE B

20 20 Depth(K_unit)

10 10 Depth(K_unit)

0 0.0 0.1 0.2 0.3 0.4 0.5 0 Wavenumber(1/K_unit) Fig.8. (A) Frequency analysis of Bouguer gravity anomalies indicated three density interfaces coinciding with the possible depth persistence of granites and schist belts to be of the order of 2 to 4 km and crustal level changes around 16 km. C kg/m3, granites - 2600 kg/m3, schist belts - 2850 kg/m3 and lower crust with 2900 kg/m3.

CONCLUSIONS

Gravity and magnetic mapping of CSB and integration with the available radiometric maps and geology has revealed significant inputs into the known aspects of Geology. Identification of additional arm of CSB from Huliyar to Hosdurga, disposition of Chitradurga granite D up to Harpanahalli, eastern margin of schist belt, possible shear zones between central shear zone and eastern margin Figs. 8 B, C & D. Gravity anomaly profiles drawn in E-W direction and a major fracture zone affecting the disposition of arms across CSB at 14°30' N, 13°45' N and 13°15' N show of schist belt and its role in the emplacement of ultramafic different back ground levels for arms of CSB and the main rocks need geological validation. Frequency analysis of CSB, suggesting that these volcanics have originated from different levels. Modeling of first profile indicates depth gravity anomalies and modelling indicated depth persistence persistence of Chitradurga granite to be 6 km and Jagalur of intrusives into CSB and the regional charcter of gravity granite to be 4km and Volcanic patches to be of the order field revealed an abrupt change of background values of 2 to 4 km. Second profile indicates depth persistence of between arms of CSB and the main schist belt. Bababudan volcanics to be 2 km and Chitradurga volcanics up to be 8 to 10 km. Third profile shows that volcanic patches Acknowledgements: Authors wish to thank the Geo- are narrow and have depth persistence of 3 to 5 km. logical Survey of India for the opportunity to map the CSB and its adjoining terrain. Many colleagues have contributed the kind permission of Additional Director General, GSI, in during and after the data collection and processing in the publishing the work. Reviewers comments have helped in form of stimulated discussions. First author places on record improving the presentation.

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015 A COMPREHENSIVE VIEW FROM GEOPHYSICAL SIGNATURES OVER CHITRADURGA SCHIST BELT, KARNATAKA 499

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(Received: 4 March 2014; Revised form accepted: 11 August 2014)

JOUR.GEOL.SOC.INDIA, VOL.86,OCT.2015