GRC Transactions, Vol. 38, 2014 High Heat Generating Granites of Siwana, Rajasthan D. Chandrasekharam1, C. Varun2, G. Garg1, H. K. Singh1, and G. Trupti1 1Department of Earth Sciences, Indian Institute of Technology Bombay, India 2GeoSyndicate Power Pvt. Ltd., Mumbai, India Keywords This structure appears to have been formed due to Caldera Siwana Ring Complex, Malani igneous suit, granites, EGS, subsidence and intrusion of radial dikes around intrusive granite thermal springs, Rajasthan (Bhushan and Mohanty, 1988). However, presence of stishovite in the soils round the ring complex invoked a meteoritic impact Abstract Siwana Ring Complex falling within the Malani Igneous Suit in Rajasthan, India hosts large volumes of high heat generating granites with high concentration of radioactive elements and the heat generated by these plutons vary from 4 to 41 μw/m3. Thermal waters issuing through these granites are chloride rich due to long period of circulation within these granites. Such granites are po- tential candidates for future EGS projects in India and elsewhere. Introduction Neoproterozoic felsic magmatism in western India (Rajast- han) is represented by a large felsic igneous suit of rocks known as the Malani Igneous Suit (MIS) that spread over an area of 55000 km2 and is the largest felsic magmatic event in the world (Kochhar, 1989, Bhushan, 1991, 2000). The suits of rocks are distributed around Tosham, Jhunjhunu, Siwana, Jalore, Barmer, Pali, Jaisalmer in Hariyana and Rajasthan (Fig. 1). The rock types include granites and its volcanic equivalents such as rhyolites, trachytes and rhyodacites. All these rocks are characterized by high content of radioactive elements. As a consequence, the entire region registered high heat flow values due to the heat generated by the decay of the radioactive U, Th and K. We describe here the geology, structure and the heat generation capacity of the rocks of granitic composition of Siwana ring complex. Siwana Ring Complex The Siwana Ring Complex (SRC) is the most spectacular feature in Thar Desert of Rajasthan that can be seen from Landsat Figure 1. Geological map of Rajasthan showing the distribution of Malani imagery and Google earth map. SRC occupies an area of 800 sq.km Igneous Complex suite of rocks. The box refers to the Siwana ring complex is located in Barmer district of Rajasthan and is located 120 km shown in figure 2 (modified after Bakliwal and Ramasamy, 1987; Roy and SW of Jodhpur (Fig. 2). Rao, 2000; Roy and Jakhar, 2002; Minissale, 2003). 625 Chandrasekharam, et al. for the origin of this ring complex (Tripati et al., 2010) while Pascoe (1960) suggested that the circular Siwana structure could be the root of a volcano.The rock types in the ring complex in- clude rhyolite, granite, trachyte, basalt, gabbro and dolerite in the decreasing order of abundance. The contact between the rhyolites and the granites is seen along the inner periphery while the contact between these two units along the outer periphery is seen only at Mokalsar, Indrana, Ludara and Mawri (Fig. 2). Nearly forty five flows were recorded between the periphery of the complex and the central axis of the projected caldera (Bhushan and Chittora, 1999).The felsic rocks exhibit flow structure, while basalt displays vesicular and amygdular structure. Columnar jointing is observed occasionally. The granite is coarse grained and occurs in pink and grey colours. The alkali feldspars in the granites are altered and give cloudy appearance. The grain size varies considerable and is not uniform. In thin sections, a hypidiomorphic granophyric texture is very commonly seen. The presence of sanidine and bipiramidal quartz indicates a high temperature origin for the granites in the complex (Garg, 2012). The granites around the complex rise to 925 m above the ground and a large part of the granite mass is covered by alluvium (Fig.2). Radioactivity of the Granites The characteristic feature of the granites of Siwana and other plutons within the Malani Igneous Complex is the abundance of Figure 2. The Siwana Ring Complex. Location of the thermal springs are radioactive elements like uranium, thorium and potassium. The shown in Red star (modified after Bhushan and Mohannty, 1988; Singh and Vallinayagam, 2009. concentration of these elements in the granites and the radioactive heat production (RHP) by these granites of Siwana are shown in Table 1. Radioactive element content in Siwana granites (Data source: 1-5 Table 1. Singh et al., 2014; 6-9: Singh and Vallinayagam 2013) These values are much above the average values of granites 5 S U Th K RHP Heat Flow μW/m3 of granites (Rybach, 1976).This heat apparently heating No (ppm) (ppm) (wt%) Th/U (µWm-3) (mWm-2) the circulating fluids within these rocks giving rise to the thermal springs located within these granites (Fig. 2). 1 17.8 165 5.32 9.27 16.5 204.8 2 19.6 102 4.98 5.2 12.6 165.6 Thermal Springs 3 11.8 44.7 4.77 3.79 6.6 105.7 4 6.88 31.3 4.86 4.55 4.4 83.9 The surface temperature of the thermal springs issuing through these granites varies from 30 to 40 °C. In fact certain bore well 5 7.9 28.2 3.47 3.57 4.3 83.1 drilled for domestic use also yield water with this temperature. It 6 46.71 152.97 4.1 3.27 23.56 269.6 has been reported that these thermal springs are mixing with the 7 86.11 148.1 4.05 1.72 33.52 367.5 local groundwater at shallow levels before emerging to the surface (Singh et al., 2014). High chloride content in the thermal springs 8 24.7 105.44 3.95 4.27 14.39 180.1 (1200 to 2800 ppm, Garg, 2012) indicate prolonged interaction 9 98.71 216.8 3.5 2.2 41.68 446.9 with the granites there by getting enriched in chloride due to dis- solution of biotite and hornblende which are the main source of chloride in rocks of granitic composition (Savage et al., 1985; Conclusions Chandrasekharam and Antu, 1995). In fact many thermal systems that circulate within the granite have high chloride content due The granites exposed in the Siwana Ring Complex of Rajast- to long residence time and is the heat source being radiogenic han, like other granites exposed in the Indian subcontinent, contain (Chandrasekharam and Chandrasekhar, 2007, 2008, 2009, 2010, high content of radioactive elements and thus generate heat greater Chandrasekhar and Chandrasekharam, 2008; Singh and Chan- than generated by normal granites. The radioactive heat production drasekharam, 2010; Lashin et al., 2014; Chandrasekharam et by these granites is about 4 to 41 μW/m3. The wet geothermal al., 2014). These investigations clearly demonstrate that natural systems located along major rift zones (Cambay, Narmada Rift, geothermal systems in granites are common in several provinces Godavari rift) are driven by such high heat generating granites. and detailed geological, geophysical and geochemical investiga- In certain sites, like Bakreswar, West Bengal and Tattapani in the tions on such provinces will help in enhancing our understanding Narmada rift zone, the thermal gases contain high helium content about EGS systems. varying from 1 to 2 % v/v (Ghose and Chatterjee, 1980; Minissale et 626 Chandrasekharam, et al. Chandrasekharam, D., Lashin, A., Al Arifi, N. Singh, H.K. 2014: Meeting al., 2000) indicating the thermal reservoir within such granites The future energy demand of Saudi Arabia through high heat generating gran- reservoir temperatures estimated using cation geothermometers in ite. International Journal of Earth Sciences and Engineering, 7 (1), 1-4. the thermal waters circulating in such granites vary from 160 to Garg, G.C. 2012: Radiogenic Granites and Their Bearing on the Thermal 265 °C with reservoir depths varying from 2 to 3 km (Minissale et Spring, NW Rajasthan, M Tech thesis IIT Bombay, 1-78. al., 2000; Chandrasekharam and Chandrasekhar, 2010). The total Ghose, D. and Chatterjee, S.D. 1980: Genesis ofthe abundance of helium surface exposure of such high heat generating granites in India formation in natural gas emanating from thermal springs. Proceed.Indian is 150 000 sq.km while the area occupied by MIS felsic rocks is Sci. Acad. 46, 81-83. 55000 sq km and that of SRC is 800 sq km. It is estimated that the Kochhar, N. 1989: High heat producing granites of the Malani Igneous Suite, granites of India have the capacity to generate energy equivalent Northern Peninsular India. Indian Minerals, 43(3-4), 339-346. to 3.133 x 1022 BTU (Chandrasekharam and Chandrasekhar, Lashin, A., Chandrasekharam, D. Al Arifi, N., Al Bassam, A. and Varun, C. 2010). As new data on the radioactive element concentration in 2014: Geothermal energy resources of Wadi Al-Lith basin, Saudi Arabia. Indian granites are flooding the literature, the future of granites in J. African Earth Sci. (under review). supporting electricity demand of this country is looking brighter. Minissale, A., Vaselli, O., Chandrasekharam, D., Magro, G., Tassi, F. and In fact new uranium mines are being opened based on the high Casiglia, A. 2000: Origin and evolution of “ intracratonic” thermal concentration of radioactivity in Cuddppah district in Andhra fluids from central-western Peninsular India. Earth. Planet. Sci. Lett., Pradesh (Dr Achar, Atomic Minerals Division, India, Personal 181, 377-394. communication). Perhaps in the next decade, India may become Minissale, A., Chandrasekharam, D., Vaselli, O., Magro, G., Tassi, F., Passini, one of the hot spots for EGS systems in the entire South East Asia. G.L. and Bhrambat, A. 2003: Geochemistry, geothermics and relationship to active tectonics of Gujarat and Rajasthan thermal dischsrges, India. J. Vol. Geoher. Res., v.