Earth Planets Space, Vol. 61 (No. 1), pp. 161-171, 2009
Luis M. Alva-Valdivia1,2, M. Perrin2, María L. Rivas-Sánchez1, Avto Goguitchaichvili1, Héctor López-Loera3, Omar Ferreira Lopes4, and Thiago Bastos Bonás5
1Laboratorio de Paleomagnetismo y Geofisica Nuclear, Instituto de Geofisica, UNAM, Ciudad Universitaria, 04510 México DF, Mexico
2Géosciences Montpellier, Université Montpellier II, 34095 Montpellier Cedex 05, France
3Instituto Potosino de Investigación Científica y Tecnológica A. C., Depto. Geología Económica, San Luis Potosí, S. L. P., Mexico
4Universidade Federal do Paraná, Departamento de Geologia, Jardim das Américas, Curitiba, Paraná, Brazil
5Planejamento de Lavra/Geologia, BUNGE Fertilizantes S/A Unidade Cajati, Sao Paulo, Brazil
(Received November 10, 2007; Revised July 20, 2008; Accepted July 21, 2008; Online published January 23, 2009)
This study of the Cajati deposit provides evidence that the ore was neither purely hydrothermal, nor volcanic in origin, as previous workers have proposed. The ores were formed from magnetite-rich magmas, hydrothermally altered and intruded at an indicated crustal depth in excess of 500 m. The mineralogical and textural association between magnetite and magnesioferrite in the carbonatite, and between the titanomagnetite and magnesioferrite-Ti mineralization in the pyroxenite of hedenbergite, seems to be analog mineralizations strongly related to the ionic substitution of Fe2+ by Mg. Relatively high Q ratios (≥5) for Jacupirangite-pyroxenite may indicate a thermo remanent magnetization (TRM) by the ore during post-metamorphic cooling, however it can also be developed from chemical remanent magnetization (CRM). Vector plots for the pyroxenite samples show reasonably linear and stable magnetic components. The intensity decay curves show that only two components of magnetizations are likely present. Continuous susceptibility measurements with increasing temperature show that the main magnetic phase seems to be magnetite. Maghemite is probably produced during the cooling process. Susceptibility recorded from low temperature (liquid nitrogen (-196°C)) to room temperature produces typical curves, indicating Verwey transition of magnetite. Hysteresis parameters point out that nearly all values fall in a novel region of the Day plot, parallel to but below magnetite SD + MD mixing curves.
Key words: Rock-magnetism, microscopy, Jacupiranga complex, Brazil.