High-heat-producing granites of East Dharwar Craton around Gugi, Karnataka, and their possible influence on the evolution of Rajapur thermal springs, Deccan Volcanic Province, India
© Singh et al.; licensee Springer 2014
Received: 23 September 2013
Accepted: 30 January 2014
Published: 17 May 2014
The thermal springs of Rajapur situated along the west coast of Maharashtra and parts of high-heat-generating granites of Gugi in Karnataka (India) seem to be genetically related. The present investigation involves quantification of the heat generated by the Gugi Granites using the the U, Th and K contents in the rock samples and probing their possible influence on the evolution of the Rajapur springs, based on the geochemistry of the thermal waters, published Bouguer gravity anomaly data, and the overall geological setup.
Fourteen water samples from Rajapur including thermal and groundwater samples were analysed for major ions and five rock samples from the gugi area were analysed for U, Th and K.
Rajapur thermal spring is of Na-HCO3 type, while other thermal springs along the west coast are either Na-Cl type or Na-Cl-CO3 type. The stable isotope data of the thermal waters signifies mixing with the ground water. The gravity anomaly data supports the extension of the Gugi Granites below the Deccan Volcanics of the study area.
The present investigation suggests that the Gugi Granites could be the main source of heat for these thermal springs, even though the springs issue through the Deccan volcanic flows.
The present investigation deals with the possible influence of high-heat-producing granites of Gugi on the propagation of the Rajapur thermal springs. With the help of gravity anomaly data (GSI in 2006), an attempt has been made to find the extent of these granites below the volcanic flows at the thermal springs site.
Geology of Rajapur thermal spring site
Geology of the Gugi Area
A detailed geological map was prepared based on the field observations around Gugi. The Archaean Granite is exposed with Proterozoic sediments, which are mostly carbonates of the Bhima Group. Both are in contact with an E-W trending reverse fault, steeply dipping towards the north, known as ‘Gugi-Karalagere fault’, extending up to Karalagere in the west with a maximum width of 500 m. This fault, cutting across the carbonate and granitoid rocks and horizontal beds of sedimentary formation of the Neoproterozoic Bhima group, forms steep dips in the fault zone. Lamination, brecciation, and asymmetric folds are clearly observed in the carbonate rocks.
Occasionally, small blocks of granite are exposed around Gugi. Grey and purple shale, limestone, and basement conglomerate of the Bhima Basin are exposed along the fault zone. Dark-colored, coarse-grained blocks of peninsular gneiss are also observed in several places. Grey soil is observed because of the presence of phosphate in the Bhima Basin sediments.
Sample collection and water analysis
Physical and chemical parameters of water samples from the study area
Estimation of radioactive heat
U, Th (in ppm), and K (%) values in selected rock samples
Heat flow (mW/m2)
Gugi is located in the Eastern Dharwar Craton (EDC) where the heat flow values range between 25 and 51 mW/m2 (Senthil Kumar and Reddy ), common to the all Archean provinces (Jaupart and Mareschal ). In the Gugi granites, the observed concentration of U is at 1.1 to 6.4 ppm, Th at 1.0 to 66.8 ppm, and K at 0.2 to 5.9 ppm, and the average heat flow value is 42.5 mW/m2/m (Table 2). However, in the Deccan basalt province, the average concentration of U is at 0.64 to 1.79 ppm, Th at 0.54 to 1.32 ppm, and K at 0.17 to 0.34% (Paul et al. ). These values indicate the RHP value to be in the range of 0.27 to 0.59 μW/m3 which is much less than that of the Gugi granites, signifying the role of Gugi granites in the propagation of the Rajapur thermal springs.
Results and discussion
To understand the circulation of the water through the Gugi granite, we conducted water-rock interaction experiment for selected granite samples. The granite samples were crushed to <1 mm. The water-rock interaction experiment was carried out in a glass chamber with fluid/solid ratio of 10:1 at 100°C. Rainwater was utilized in the experiment as the interacting fluid. Water-rock interaction experiment with samples derived from the granites shows wide range of geochemical variation. Most of them fall in the Na-SO4 field (Singh et al. ), while the thermal springs and the surface waters fall in the Ca-HCO3 field (Figure 5). It indicates circulation of meteoric water within the granites (host of the thermal reservoir) and emerging through the Deccan Basalt flows, after mixing with the near-surface groundwater and thus becoming rich in Ca-HCO3 component. It further strengthens our earlier view (Chandrasekharam and Chandrasekhar ) that the granites are the main source of heat for the thermal springs in several geothermal provinces of India.
The reservoir temperature calculated using the above equations individually gives two different values (281°C and 70°C respectively) since the reaction involved K-Na equilibrates at high temperatures while K/√Mg equilibrates at low temperatures. Further, the reaction involving K and Mg equilibrates is faster, and their temperatures estimated from surface geothermal waters give very low reservoir temperatures. Reactions involving K and Na do not adjust quickly to the physical environment at shallow depths. So in this case, cation geothermometry overestimates the reservoir temperature.
Cation composition geothermometer
With the cation composition geothermometer (CCG), estimated reservoir temperature is 157°C.
The Rajapur thermal waters are of Na-Ca-HCO3 type, and the stable isotope analysis of thermal and river waters shows a close relationship, which indicates mixing of the geothermal water with the near-surface groundwater. The thermal water is rich in Na+, K+, Mg++, and HCO3–. Water-rock interaction experimental result shows the circulation of the meteoritic water through granites and emerging as the Rajapur thermal springs. Higher total dissolved solids in thermal water can be ascribed to high-temperature water-rock interaction at great depth. Due to the basalt-water interaction at Rajapur, the Na to K and Mg to Na ratio and excessive Ca become less sensitive to temperature variation. The concentration of silica showed a great variation (70 to 400 ppm; Chandrasekharam et al. ; Ramanathan: Geochemistry of the thermal springs located along the West Coast, unpublished work). Reservoir temperature estimation based on CCG is here considered as reliable, which gave a reservoir temperature of 157°C. Also, the surface and groundwater from Rajapur show very low concentration of other major ions in comparison to the thermal water in the area (Minissale et al. ), whereas the surface and groundwater from Gugi have higher concentrations of major ions with high total dissolved solids. There is no indication of an interaction of seawater with the thermal spring water as is the case of other west coast springs. Thus, the geochemistry of thermal water confirms the location of thermal reservoir to the Precambrian granites underlying the Deccan Basalt.
Radioactive heat production data of granites of Gugi show high values on an average of 42.6 mW/m2, which are similar to many other potentially viable areas around the world. This is, hence, considered as the greatest possible heat source of the reservoir of the Rajapur thermal springs.
The gravity profile (Figure 7) from Gugi to Rajapur shows a disparity in gravity anomaly, perhaps due to the variation in thickness of the Deccan Flood Basalt. The gravity anomaly in Rajapur is 10 mGal, which is lower than others observed in the Gugi area. If we apply complete Bouguer correction in the Rajapur area for the 600-m-thick basalt layer, we find a comparatively similar gravity anomaly as in the Gugi area. Also, the existence of low density rock like granite underlying the DFB cannot be ruled out. With the help of the gravity anomaly, it can be predicted that the main heat source for the Rajapur thermal springs is the granitic batholith, which extends from Gugi to the Rajapur. Finally looking at all these aspects, it can be postulated that the thermal waters from Rajapur area, coming from underneath Dharwar Granitic Batholith and underlying the basaltic terrain, interact with the near-surface groundwater on their way to the surface, to emerge as thermal springs.
Authors are thankful to Prof. S. Vishwanathan for his valuable suggestions to improve the quality of the manuscript.
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