Origin of the Caldas Novas dome (Brazil) and its thermal water circulation

Abstract The Caldas Novas dome, a 20 × 12 km elliptical structure, with its longer axis trending NNW-SSE, was previously described as a structural window into the south-central part of the Neoproterozoic Brasilia belt. The Brasilia belt is the central part of the Tocantins Province, a former orogen. Hot springs, broad gravimetric and magnetic features, and a radiometric anomaly over the dome are observed. The dome aligns with the well-known Alto Paranaiba Cretaceous Igneous Province. All these observations have raised questions in the past about a possible magmatic origin of the dome. In this work, existing regional gravimetric, seismologic, magnetic, radiometric, hydro-geochemical data, as well as new isotopic data, were integrated. From the magnetic data, it is clear that a NW-SE trending linear structure is the most significant geophysical feature of the analyzed dataset. We interpret that the NW-SE trending aeromagnetic lineaments may host basaltic dikes. The interpretation is based on the occurrence of basaltic rocks in the eastern portion of the Parana basin overlying the Brasilia belt, observed outcrops of basalt in the Ipora area, and the linear features identified here. Most characteristics identified in the magnetic data are also evident in the radiometrics, and a sharp anomaly in the eTh/K and eU/K ratio map is centered on the Caldas Novas dome. Weathering related depletion of potassium over the dome is the most probable explanation for the observed radiometric anomaly. From chemical and geological evidence, the groundwater in the area likely originates from meteoric processes. The thermal spring water near the dome is associated with deep circulation in fracture systems probably related to a broad shear/fault zone. This is supported by the Cretaceous dike swarm postulated in the region, possibly emplaced into an existing fracture system. We infer that a fracture system, which would continue beyond the limits of the aero-geophysical survey, most likely accounts for the groundwater circulation and heating.