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