Geochemistry, geothermics and relationship to active tectonics of Gujarat and Rajasthan thermal discharges, India

Abstract Most thermal spring discharges of Rajasthan and Gujarat in northwestern India have been sampled and analysed for major and trace elements in both the liquid and associated gas phase, and for 18 O/ 16 O, D/H (in water), 3 He/ 4 He and 13 C/ 12 C in CO 2 (in gas) isotopic ratios. Most thermal springs in Rajasthan are tightly associated to the several regional NE–SW strike-slip faults bordering NE–SW ridges formed by Archaean rocks at the contact with Quaternary alluvial and aeolian sedimentary deposits of the Rajasthan desert. Their Ca–HCO 3 immature character and isotopic composition reveals: (1) meteoric origin, (2) relatively shallow circulation inside the crystalline Archaean formations, (3) very fast rise along faults, and (4) deep storage temperatures of the same order of magnitude as discharging temperatures (50–90°C). Thermal spring discharges in Gujarat are spread over a larger area than in Rajasthan and are associated both with the NNW–SSE fault systems bordering the Cambay basin and the ENE–WSW strike-slip fault systems in the Saurashtra province, west of the Cambay basin. Chemical and isotopic compositions of springs in both areas suggest a meteoric origin of deep thermal waters. They mix with fresh or fossil seawater entering the thermal paths of the spring systems through both the fault systems bordering the Cambay basin, as well as faults and fractures occurring inside the permeable Deccan Basalt Trap in the Saurashtra province. The associated gas phase, at all sampled sites, shows similar features: (1) it is dominated by the presence of atmospheric components (N 2 and Ar), (2) it has high crustal 4 He enrichment, (3) it shows crustal 3 He/ 4 He signature, (4) it has low CO 2 concentration, and (5) the only analysed sample for 13 C/ 12 C isotopic ratio in CO 2 suggests that CO 2 has a strong, isotopically light organic imprint. All these features and chemical geothermometer estimates of spring waters suggest that any active deep hydrothermal system at the base of the Cambay basin (about 2000–3000 m) has low-to-medium enthalpy characteristics, with maximum deep temperature in the storage zone of about 150°C. In a regional overview, both thermal emergences of Rajasthan and Gujarat could be controlled by the counter-clockwise rotation of India.