Rock-Magnetic and Oxide Microscopic Studies of the El Laco Iron Ore Deposits, Chilean Andes, and Implications for Magnetic Anomaly Modeling

Microscopic and rock-magnetic studies of the ores and host rocks of the El Laco iron oxide deposits permit us to characterize the magnetic mineralogy and the processes affecting natural remanent magnetization (NRM) during emplacement and evolution of the deposits. Particular attention was devoted to identifying the magnetic mineral composition (magnetite and/or titanomagnetite, and hematite and/or titanohematite, and titanomaghemite) and grain size variations of both ores and host rock. Rock-magnetic data are used to clarify magnetic domain states and remanence acquisition processes, and to assess their significance as a source of magnetic anomalies. Microscopy under reflected light demonstrates that magnetic carriers are mainly magnetite, with significant amounts of ilmenite-hematite minerals. Magmatic titanomagnetites in the andesitic rocks show trellis textures, compatible with high-temperature oxy-exsolution processes. Supergene reactions in ore deposits under eruption conditions are indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 µm. Hysteresis measurements point to pseudo-single-domain states. Thermal spectra, continuous temperature-dependent susceptibility measurements, and isothermal remanent magnetization (IRM) acquisition suggest predominance of spinels (titanomagnetite or titanomaghemite) with low-Ti contents as magnetic carriers. Although the presence of (titano)hematites is indicated by hysteresis and IRM studies, their contribution to the total remanence seems to be minor. The Fe-oxides in the ore are typically poor in Ti, whereas in the rocks they are Ti-bearing. For the modeling of the magnetic anomalies, we used data on bulk susceptibility and NRM intensity and direction in order to constrain the relative contributions of induced and remanent magnetization components, and to obtain improved control regarding depth and geometry of source bodies. The deep magnetic source corresponds to an ENE-striking tabular body, steeply inclined 65° to the north.