Integrated geophysical investigation of a possible new alkaline occurrence in SE Brazil M.S.M. Mantovani * , A. Rugenski, L.A. Diogo, W. Shukowsky Instituto de Astronomia, Geofı ´sica e Cie ˆncias Atmosfe ´ricas, Universidade de Sa ˜o Paulo, Rua do Mata ˜o 1226, Sa ˜o Paulo 05508 900, SP, Brasil Received 29 June 2004; accepted 30 May 2005 Abstract Airborne geophysical data by the Brazilian Geological Survey (CPRM) reveal the presence of a magnetic anomaly with no geological expression at the surface but with characteristics similar to those associated with alkaline complexes that outcrop in the same area. This observation motivated an attempt to characterize the anomaly source using various geophysical methods. This article presents satellite images for different spectral bands, as well as the results of a seismic survey, gravity and ground magnetic surveys along a profile crossing the anomaly, and density and susceptibility measurements of different lithologies in the area. The obtained information was used to constrain 2 1 / 2 -dimensional simultaneous gravity and magnetics and three-dimensional magnetics only models. The modeling results converge to indicate the presence of a body of approximately 3.3!10 9 metric tons, whose top is approximately 40 m from the surface and that has 1 km of maximum depth. q 2005 Elsevier Ltd. All rights reserved. Keywords: Alkaline occurrence; Gravity; Magnetic; Registro anomaly; Remote sensing Resumen O levantamento aerogeofı ´sico SP-RJ do Servic ¸o Geolo ´gico do Brasil (CPRM) evidencia uma anomalia magne ´tica, sem correspondentes geolo ´gicos em superfı ´cie, mas com caracterı ´sticas semelhantes a `quelas produzidas por complexos alcalinos que afloram na mesma a ´rea. Esta observac ¸a ˜o induziu a utilizac ¸a ˜o de diferentes metodologias geofı ´sicas para caracterizar sua fonte. Sa ˜o apresentadas imagens de sate ´lite para diferentes bandas espectrais, resultados de sondagens sı ´smicas, levantamento gravime ´trico e correspondente magne ´tico em superfı ´cie ao longo de um perfil que corta a anomalia, e medidas de densidade e susceptibilidade das diferentes litologias da a ´rea. Todas as informac ¸o ˜es obtidas foram utilizadas como vı ´nculos para a modelagem direta simulta ˆnea dos dados gravime ´tricos e magne ´ticos de superfı ´cie, com geometria 2 1 / 2 D, e modelagem com geometria 3D dos dados magne ´ticos do levantamento ae ´reo. Os resultados das modelagens convergem para um corpo de cerca 3.3!10 9 toneladas, pro ´ximo da superfı ´cie (w40 m) e com profundidade ma ´xima de 1 km. q 2005 Elsevier Ltd. All rights reserved. 1. Introduction In subarea 3 of the aeromagnetic survey of the Brazilian Geological Survey (CPRM), which corresponds to 47.71428W and 24.54018S, a magnetic anomaly of normal polarization has been noticed, but no surface geological equivalent has been observed. This anomaly is located in the Alkaline province of the Ponta Grossa Arch (Almeida, 1983), in the vicinity of the Juquia ´, Jacupiranga, Pariquera-Ac ¸u, and Canane ´ia alkaline complexes (Fig. 1). With the exception of the Juquia ´ Complex, which presents reverse magnetic polarization (Slavec et al., 2001), and the Jacupiranga Complex, whose complexity prevents defining its direction of polarization, these alkaline complexes present normal polarization (Ferreira and Algarte, 1979). The emplacement of these complexes occurred between 130 and 82 Ma, according to K–Ar estimated ages (Gomes and Cordani, 1965; Amaral et al., 1967; Born, 1971; Amaral, 1978; Ferreira and Algarte, 1979; Morbidelli et al., 1995). Most of these alkaline rocks have similar potassium contents, unlike the Juquia ´ and Jacupiranga complexes, which are associated with carbonatites (Gomes et al., 1996). The other complexes contain original and some new minerals formed by the assimilation of the host rock, which is essentially composed of carbonates, Journal of South American Earth Sciences 20 (2005) 259–266 www.elsevier.com/locate/jsames 0895-9811/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2005.05.011 * Corresponding author. Tel.: C55 11 30914762; fax: C55 11 30914755. E-mail addresses: marta@iag.usp.br (M.S.M. Mantovani), andrerug@iag. usp.br (A. Rugenski), liliana@iag.usp.br (L.A. Diogo), wladimir@iag.usp.br (W. Shukowsky).