Estudio de minerales de fierro y titanio y magnetismo de rocas en el distrito ferrífero El Encino, Jalisco, México

Abstract

Results of mineralogical and paleomagnetic studies of volcano-sedimentary units and iron mineralization in El Encino, District Jalisco arc reported. Studies concentrated on: (a) relationships between paleomagnetic parameters and opaque mineral microscopy observations and (b) inferences concerning genesis, degree and type of oxidation processes, grain sizes, emplacement conditions, and metamorphic and hydrothermal processes. Reflected light microscopy observations of the Fc-Ti-O system document the abundance of iron oxides with large grain sizes and magnetic multidomain states, characteristic of slow cooling and low-degree dcuteric oxidation. Members of the titano-magnctitc series with low-Ti contents and ilmcnitcs and hematites of the titano-hcmatitc series arc dominant as shown also by major oxide chemical analyses (minor quantities of TiO₂ respect to FcO and Fc₂O₃). Remanent magnetizations arc mostly of normal polarity as expected for the Normal Aptian-Albian Chron. Angular dispersion is higher for the iron ore than for other lithologies in the District. This may be due to oxidation, metamorphic and hydrothermal alteration processes after mineralization. Magnetic properties in the iron ore arc dominated by large grain-size multidomain titano-magnctite. The intrusive bodies show susceptibility values below 1.3 x 10^-3 SI, in the range of ilmenite-series granitoids. Blocking temperature spectra, NRM vectorial composition diagrams and acquisition spectra of isothermal remanent magnetization (IRM) all correlate with the ore petrography observations and agree with the predominance of titanomagnctitc minerals with variable contents ofulvospincl and magnetite. Secondary remanence components may reflect effects of hydrothermal and metamorphic processes. There may be some contribution of viscous and isothermal remanent magnetizations.