High Resolution Inversion of the Magnetization Vector in the ... - Geosoft

HIGH RESOLUTION INVERSION OF THE MAGNETIZATION VECTOR IN THE PONTES E LACERDA AREA Telma Aisengart1, Diego Barbosa1, Taronish Pithawala1, Janaína Gattermann Pereira1, Clara Porto1, José Luís Ando1, Mônia Rech1 1Geosoft Inc.

Introduction

Methodology

The Pontes e Lacerda city has been in the spotlight due to the gold discovery in the region and the wide distribution of gold nuggets images. This has attracted prospectors seeking gold and has motivated us to study the region. The study area is located in southwestern Mato Grosso state, near to the Bolivian border, and coincides with Sheet SD.21Y-C, 1:250.000, Pontes e Lacerda (fig. 1).

The usage of inversion applied to magnetic data is becoming an ordinary procedure. In areas with low latitude and/or in the presence of remanence, inverting the magnetization vector – MVI – has already shown to generate more reliable results (Aisengart, 2013; Barbosa & Pereira, 2013). The access to a higher computational power has allowed us to invert the whole area in high resolution (with a 150x150x75m cell). To avoid creating artifacts due to the relationship between the widely spaced lines (500m) and a low flight height (100m), the input to the inversion was the database created by sampling the grid with 10m cells, upward continued 150m, hence having one sample per cell.

This study was conducted using airborne geophysical data, provided by CPRM, acquired at a height of 100m relative to the terrain and flight lines separated by 500m. Located in a low magnetic latitude zone (-12°), a variety of magnetic anomalies with inverted polarity occur in the area, indicating the presence of remanence and/or demagnetization (fig.1).

Results

A

Fig.1 – On the left, location of the study area in red; on the right, the area in detail.

Geological Background • The study area is located southwestern Amazonic Craton, where it is formed by the junction of Paleo-Mesoproterozoic lands (1.8 to 1.4 Gyr): Jauru Terrain, Santa Helena Terrain, Rio Alegre Terrain, Paraguá Terrain and San Pablo Terrain. Jauru and Rio Alegre Domains predominate the area (Fernandes et al., 2005). • Jauru Domain comprehends part of the Jauru and Alto Jauru Terrain and the Santa Helena Terrain, or Pontes e Lacerda (Ruiz, 2005). • To the east occur the Aguas Claras Granodiorite and metavulcanic-sedimentary rocks (komatite, Metabasalt, Schist, Metatuff, Banded Iron Formation), with these last ones having a good magnetic response, giving origin to a few dipolar anomalies. • In central area, can be found the Santa Helena Intrusive Suite with low magnetic response. • To the west, the Jauru Domain is bounded by the Rio Alegre and Paragua Domains. • Bordering the Rio Alegre Terrain the contact is a normal ductile shear zone, well defined in the geophysical model. There you can also find the Metaconglomerates from Fortuna Formation (Fernandes et al., 2005), where the gold occurrences are located, and metamorphic rocks of the Pontes e Lacerda Group (Ruiz, 2005). • Bordering the Paragua Terrain, extreme southwest, Holocenic sediments covers the region, preventing the surficial characterization. The same occurs in the northern and southern borders of the Rio Alegre Domain (Saes, 1999).

C

B

Fig.2 – (A) Model resulting from the inversion (1083x750x44 cells) and with susceptibility value > 0.015 SI. (B) Elevation Slices of the model in -1000m, -6000m and -8500m. (C) Lithological map with susceptibility values >0.015 SI and Lithological map with magnetization vectors and vertical sections of the model.

Conclusions • Generation of a geophysical model in high resolution of a 1:250.000 Sheet, thanks to higher performance clusters; • The geophysical model obtained allowed the identification of the main geological and structural features in surface, besides possible sources in depth; • Bordering the Rio Alegre Terrain, where the contact is made by normal ductile shear zones, the model has shown magnetic anomalies aligned according to the NW-SE direction; • Despite the extensive Holocenic coverage in great part of the western area, the resulting geophysical model has shown a strong magnetic response in depth, indicating a possible presence of the basement corresponding to the Paragua and Rio Alegre Terrains.

Acknowledgements

A

To CPRM, for kindly giving us access to the airborne geophysical data, and to the Geosoft team, for their support.

Bibliography

B Fig.3 – A: Lithological map of the study area (CPRM, 2004); B: Tectonical map (modified from Fernandes et al., 2005).

• AISENGART, T. 2013. 3D Inversion of Magnetic Data at Low Magnetic Latitudes. Thirteenth International Congress of the Brazilian Geophysical Society, Rio de Janeiro. • BARBOSA, R.D & PEREIRA, J.G. 2013. Inversão 3D de dados Magnéticos na Região de Mara Rosa – Goiás, Brasil, utilizando Geosoft VOXI. Thirteenth International Congress of the Brazilian Geophysical Society, Rio de Janeiro. • CPRM, 2004. Mapa Geológico Estadual do Mato Grosso. Escala 1:1.000.000. Baixada do Geobank, fev/2016. • FERNANDES, C.J.; RUIZ, A. S.; KUYUMJIAN, R.M.; PINHO, F.E.C. 2005. Geologia e Controle Estrutural dos depósitos de ouro do Grupo Aguapeí - Região de Lavrinha, sudoeste do Cráton Amazônico. In: Revista Brasileira de Geociências, 35(1), p. 13-22. • RUIZ, A. S. 2005. Evolução Geológica do Sudoeste do Cráton Amazônico Região Limítrofe Brasil-Bolívia: Mato Grosso. 2005. 299 f. Tese de Doutorado - Programa de Pós Graduação em Geociências, Instituto de Geociências e Ciências Exatas, Universidade Estadual Paulista, Rio Claro. • SAES, G. S. 1999. Evolução Tectônica e Paleogeográfica do Aulacógeno Aguapeí (1.2 - 1.0 Ga) e dos Terrenos do seu embasamento na porção sul do Cráton Amazônico. 1999. 154 f. Tese de Doutorado - Programa de Pós Graduação em Geoquímica e Geotectônica, Instituto de Geociências, Universidade de São Paulo, São Paulo