Rock-magnetism and ore microscopy of the magnetite-apatite ore deposit from Cerro de Mercado, Mexico

Rock-magnetic and microscopic studies of the iron ores and associated igneo us rocks in the Cerro de Mercado, Mexico, were carried out to determine the magnetic mineralogy and origin of natural remanent magnetization (NRM), re lated to the thermo-chemical processes due to hydrothermalism. Chemical rem anent magnetization (CRM) seems to be present in most of investigated ore a nd wall rock samples, replacing completely or partially an original thermor emanent magnetization (TRM). Magnetite (or Ti-poor titanomagnetite) and hem atite are commonly found in the ores. Although hematite may carry a stable CRM, no secondary components are detected above 580 degrees, which probably attests that oxidation occurred soon enough after the extrusion and coolin g of the ore-bearing magma. NRM polarities for most of the studied units ar e reverse. There is some scatter in the cleaned remanence directions of the ores, which may result from physical movement of the ores during faulting or mining, or from perturbation of the ambient field during remanence acqui sition by inhomogeneous internal fields within these strongly magnetic ore deposits. The microscopy study under reflected light shows that the magneti c carriers are mainly titanomagnetite, with significant amounts of ilmenite -hematite minerals, and goethite-limonite resulting from alteration process es. Magmatic titanomagnetites, which are found in igneous rocks, show trell is, sandwich, and composite textures, which are compatible with high temper ature (deuteric) oxy-exsolution processes. Hydrothermal alteration in ore d eposits is mainly indicated by martitization in oxide minerals. Grain sizes range from a few microns to >100 mum, and possible magnetic state from sin gle to multidomain, in agreement with hysteresis measurements. Thermal spec tra, continuous susceptibility measurements, and IRM (isothermal remanent m agnetization) acquisition suggest a predominance of spinels as magnetic car riers, most probably titanomagnetites with low-Ti content. For quantitative modeling of the aeromagnetic anomalies, we used data on bulk susceptibilit y and natural remanent intensity for quantifying the relative contributions of induced and remanent magnetization components and allow a better contro l of the geometry of source bodies. The position and geometry of this magne tic source are shown as an ENE striking tabular body, steeply inclined (75 degrees) to the south.