Petromagnetic properties in the Naica mining district, Chihuahua, Mexico: Searching for source of mineralization

Abstract

Ore mineral and host lithologies have been sampled at 14 sites (89 oriented samples) in the Naica District, northern Mexico. The following properties were measured to characterise samples: saturation magnetization, high-temperature magnetic susceptibility, remanence intensity, Koenigsberger ratio, Curie temperature and hysteresis parameters. Rock magnetic properties seem to be controlled by variations in pyrrhotite, (titano)magnetite, (titano)hematite and (titano)maghemite content, and hydrothermal alteration. Post-mineralization hydrothermal alteration seems to be the major event that affected the minerals and magnetic properties. Continuous susceptibility measurements with temperature in most cases yield Curie points close to that of almost pure magnetite. Hysteresis curves and associated IRM (isothermal remanent magnetization) acquisition plots, however, in some cases points to the higher coercivity minerals. Hematite or a mixture of hematite-titanomagnetite-titanomaghemite are probably present in the Naica samples although their contribution in remanent magnetization is minor. Judging from the ratios of hysteresis parameters, it seems that all samples fall into the pseudo-single domain (PSD) grain size region. The Koenigsberger ratio (Q) was used as a measure of the relative contribution of remanent and induced magnetization into the magnetic anomalies. Q ranges from 0.05 to 34 and, generally, it is higher than 1, indicating the predominance of remanence over induced magnetism. The location and geometry of the magnetic source is shown as a single long subhorizontal body slightly inclined (∼20°) to the SE. This modelled body suggests an andine-type nature and emplacement of granitic magmas.