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. The Morro do Padre niobium Deposit, in the Late-Cretaceous Catal˜ao 2 alkaline-carbonatite complex, central Brazil, consists of stockworks of nelsonite and carbonatite dykes intruded into Precambrian phyllites, quartzites, and amphibolites. A gamma-ray spectrometry and magnetic susceptibility petrophysical survey was conducted on the cores of 73 drill holes in fresh-rock, producing a total of 1295 geophysical samples. Nelsonite, the host rock of the niobium mineralization in the Morro do Padre Deposit, has a characteristic geophysical signature, with higher gamma-ray spectrometry counting rates and magnetic susceptibility values, compared to other rock types. The studied nelsonites may be divided into N1 and N2 types. N2 nelsonite is richer in K, U and Th than N1. Carbonatites are divided into magnetic (C1) and nonmagnetic (C2) varieties. The nonmagnetic carbonatites can be subdivided into C2a and C2b. The C2a carbonatite is richer in K, U and Th than C2b, which is consistent with the presence of apatite and/or monazite in the former. The geophysical 3Dmodeling has shown that the main mineralized body is elongated in the E-W direction. It is about 100 m wide and 300 m long with a maximum depth of approximately 850 m reached by drilling. Keywords: 3D ore modeling, niobium ore, applied geophysics, alkaline rocks, nelsonite. RESUMO. O deposito de niobio do Morro do Padre no complexo carbonatitico alcalino de Catalao 2 do Cretaceo Superior, regiao central do Brasil, consiste em stockworks de nelsonito e diques de carbonatito intrudidos em filitos pre-cambrianos, quartzitos e anfibolitos. A pesquisa petrofisica de gamaespectrometria e de susceptibilidade magnetica foi realizada em testemunhos de 73 furos de sondagem em rocha fresca, produzindo um total de 1.295 amostras. Nelsonito, a rocha hospedeira da mineralizacao de niobio no deposito Morro do Padre, tem uma assinatura geofisica caracteristica, com maiores taxas de radiacao gamaespectrometrica e maiores valores de susceptibilidade magnetica em comparacao com outros tipos de rochas. Os nelsonitos estudados podem ser divididos em N1 e N2. O nelsonito N2 e mais rico em K, U e Th do que o N1. Carbonatitos sao divididos em magneticos (C1) e nao magneticos (C2). Os carbonatitos nao magneticos podem ser subdivididos em C2a e C2b. O carbonatito C2a e mais rico em K, U e Th do que o C2b, o que e consistente com a presenca de apatita e/ou monazita na composicao minerologica. O modelamento 3D revela um corpo principal de nelsonito mineralizado, alongado segundo a direcao E-W. Este e cerca de 100 m de largura e 300 m de comprimento, com uma profundidade maxima de 850 m. Palavras-chave: modelamento 3D de minerio, minerio de niobio, geofisica aplicada, rochas alcalinas, nelsonito.

revista brasileira de geofísica

MAGNETIC SUSCEPTIBILITY AND GAMMA-RAY SPECTROMETRY ON DRILL CORE: LITHOTYPE CHARACTERIZATION AND 3D ORE MODELING OF THE MORRO DO PADRE NIOBIUM DEPOSIT, GOIÁS, BRAZIL