Integrated magnetic studies of the El Romeral iron-ore deposit, Chile: implications for ore genesis and modeling of magnetic anomalies

Abstract

Mineralogical and rock-magnetic studies of iron ores and host rocks in El Romeral Mine are carried out to characterize the magnetic mineralogy and the processes that affect the natural remanent magnetization during emplacement and evolution of the iron-ore deposit. Extremely important is the identification of magnetic mineralogical composition (magnetite and/or titanomagnetite, hematite and/or titanohematite, and titanomaghemite) and grain size. These data permit investigation of magnetic domain state and magnetization acquisition processes and to assess their significance as a source of magnetic anomalies. Chemical remanent magnetization (CRM) seems to be present in most of investigated ore and wall-rock samples, substituting completely or partially the original thermoremanent magnetization (TRM). Magnetite (or Ti-poor titanomagnetite) and titanohematite are commonly found in the ores. Although hematite may carry a stable CRM, no secondary components are detected above 580 °C, which probably attests that oxidation occurred soon after the extrusion and cooling of the ore-bearing magma. The microscopy study under reflected light shows that magnetic carriers are mainly titanomagnetite with significant amounts of ilmenite–hematite minerals. Magmatic titanomagnetite, found in igneous rocks, shows trellis texture, which is compatible with high temperature (deuteric) oxy-exsolution processes. Hydrothermal alteration in ore deposits is indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 μm, and possible magnetic states from single to multidomain, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and isothermal remanent magnetization acquisition suggest a predominance of spinels as magnetic carriers, most probably titanomagnetites with low-Ti content. For quantitative modeling of the magnetic anomaly, we used data on bulk susceptibility and natural remanent intensity for quantifying the relative contributions of induced and remanent magnetization components, and this allows greater control of the geometry of source bodies. The position and geometry of these magnetic sources are shown as ENE-striking tabular bodies, one steeply inclined (75°) to the south and another lying horizontal.