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In this paper we present the results of an investigation on the electric crustal structures depicted by a magnetotelluric (MT) transect cutting through the easternmost sector of the Alta Floresta Gold Province, in the SW Amazon Craton, Brazil. The MT dataset was acquired along a 200 km NNE-SSW transect encompassing 35 broadband and 12 long-period stations, covering a period range of approximately 10 −4 to 10 4  s. We adopted a data processing scheme based on robust estimators over the electromagnetic cross-spectra for combined single and remote reference stations in order to estimate horizontal and vertical transfer functions. The dataset was inverted using a 3-D MT based minimum structure algorithm, considering the data dimensionality, directionality analysis and the irregular station distribution. A set of inversions considering different model parameters have been performed to achieve a model with robust main features and to test the stability of the obtained solutions. A resolution study was also proposed in order to evaluate the average depth of model sensitivity. The electrical structures depicted can improve the understanding of the ancient tectonic features encrypted at the cratonic stable crust, contributing to elucidate the mechanisms that might have controlled its ore genesis. Conductors in the mid-crust correlate with shallow mafic enclave-rich granitic and foliated volcanic facies outcrops, linking the geoelectric anomalies to a possible mid-crustal source correlated to the Statherian magmatism, which is believed to be the main pulse related to gold metallogenesis at this province. [ABSTRACT FROM AUTHOR]

The interaction between the São Francisco Craton and its surrounding belts during the Gondwana formation produced an intricated structural framework with superposition of tectonic events. This work aims to infer the subsurface geoelectric structures along a cryptic suture between the Brasília and Ribeira belts in order to verify tectonic limits between crustal blocks. A three-dimensional data inversion using the ModEM code was applied in 83 magnetotelluric sites of broad band and long period with remote reference. The processing was done by the robust method and with analysis of dimensionality and directionality through the phase tensor and induction vectors, respectively. Finally, an inversion model was generated, defined in a homogeneous mesh (103 × 103 x 45) and 1000 Ω. m of 2.3 × 2.3 km (x, y) even including the ocean. The full impedance tensor data were inverted and after 224 iterations an adjustment of 1.98 was achieved. The modelled MT North and South profiles show distinct geoelectric frameworks at NW and SE segments. To the NW thin-skinned tectonics is evidenced by low angle dipping conductors below the tectonic domains of the Brasília Belt and the interference zone with the Ribeira Belt. To the SE of the MT profiles the MT model shows two resistive blocks and a continuous conductor that follows a detachment fault below the Cenozoic Taubaté Basin. We interpret the limit between the different geoelectric profile segments as the suture between Brasília and Ribeira Belts. The limit matches the first Ribeira Thrust in the North MT profile or the Buquira shear zone in the South MT profile. The Precambrian geometry of resistive blocks and conductors is modified in the Amparo Antiform by reverse and normal faults with vertical displacement reaching thousands of meters. Deep crustal and mantle geoelectric structures were also evidenced by our 3D MT modelling: a) high conductive domains with resistivity around 400 Ω. m in the middle and lower crust; b) deep vertical conductors (DVC) with roots at the base of the lithosphere. [ABSTRACT FROM AUTHOR]

A magnetotelluric (MT) survey was conducted on the Caldas Novas geothermal reservoir located in the state of Goia´s, Central Brazil. The region of Caldas Novas is a popular tourist spot because of the occurrence of hot water springs. The purpose of the first MT survey in this area is to provide more information on the geoelectrical structure of this important geothermal reservoir. The MT method is a frequency domain technique that utilizes naturally occurring magnetic and electric signals as source to obtain a resistivity map of the subsurface. Since temperature and permeability are some of the factors controlling electrical resistivity, MT is widely utilized for surveying geothermal areas such as Caldas Novas. Data were acquired along two profiles crossing the Serra de Caldas (Caldas Mountains) with a total of 25 MT stations. Frequencies of acquisition were in the range from 0.008 to 176 Hz. Spacing between stations were usually around 5 km. Apparent resistivity and phase data from the transverse electric (TE) and transverse magnetic (TM) modes were computed for both profiles. Very high (100,000 Ω m) apparent resistivity values in the TM mode indicate distortion, possibly caused by 3-D resistive structures. In this work, we focus on Profile 2, which was acquired aligned at N40°E, approximately following the direction of weakness N50°E. Two-dimensional (2-D) inversion suggests a basin-like model with very high-resistivity block structures associated with concentric faulting below the Caldas Mountains inside a resistive basin that extends to depths of approximately 25–30 km. This model is in good agreement with gravity data and the available geological information in the area and can help delineate areas of new geothermal reservoirs. The high resistivity associated with the depression in the MT model can be interpreted as caused by the low-permeability quartzites that form the Caldas Mountains. Although the survey configuration does not allow for a full three-dimensional (3-D) interpretation, a 3-D resistivity model was constructed and the MT responses computed. The 3-D synthetic data explains the behavior of both the TE and TM modes in this faulted resistive environment. [Copyright &y& Elsevier]