Rock magnetic properties and ore microscopy of the iron ore deposit of Las Truchas, Michoacan, Mexico

Abstract

Iron ore and host rocks have been sampled (90 oriented samples from 19 sites) from the Las Truchas mine, western Mexico. A broad range of magnetic parameters have been studied to characterize the samples: saturation magnetization, Curie temperature, density, susceptibility, remanence intensity, Koenigsberger ratio, and hysteresis parameters. Magnetic properties are controlled by variations in titanomagnetite content, deuteric oxidation, and hydrothermal alteration. Las Truchas deposit formed by contact metasomatism in a Mesozoic volcano-sedimentary sequence intruded by a batholith, and titanomagnetites underwent intermediate degrees of deuteric oxidation. Post-mineralization hydrothermal alteration, evidenced by pyrite, epidote, sericite, and kaolin, seems to be the major event that affected the minerals and magnetic properties. Magnetite grain sizes in iron ores range from 5 to >200 μm, which suggest dominance of multidomain (MD) states. Curie temperatures are 580±5°C, characteristic of magnetite. Hysteresis parameters indicate that most samples have MD magnetite, some samples pseudo-single domain (PSD), and just a few single domain (SD) particles. AF demagnetization and IRM acquisition indicate that NRM and laboratory remanences are carried by MD magnetite in iron ores and PSD–SD magnetite in host rocks. The Koenigsberger ratio falls in a narrow range between 0.1 and 10, indicating the significance of MD and PSD magnetites.