Open Access

Rock magnetism and microscopy of the Jacupiranga alkaline-carbonatitic complex, southern Brazil

  • Luis M. Alva-Valdivia118, 218Email author,
  • M. Perrin218,
  • María L. Rivas-Sánchez118,
  • Avto Goguitchaichvili118,
  • Héctor López-Loera318,
  • Omar Ferreira Lopes418 and
  • Thiago Bastos Bonás518
Earth, Planets and Space200961:BF03352896

https://doi.org/10.1186/BF03352896

Received: 10 November 2007

Accepted: 21 July 2008

Published: 23 January 2009

Abstract

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