Rock-magnetic and oxide microscopy studies of the El Laco iron-ore deposits, Chilean High Andes: implications for magnetometric modeling
Abstract
Microscopy and rock-magnetic studies of the iron oxide-ore and host rocks in the El Laco deposits were carried out to characterize the magnetic mineralogy and the processes that affected the natural remanent magnetization (NRM) during emplacement and evolution of the iron-ore deposits. Particular attention was paid to identify the magnetic mineralogical composition (magnetite and/or titanomagnetite, and hematite and/or titanohematite, and titanomaghemite) and grain size. These data help to investigate the magnetic domain states and the remanence acquisition processes, and to assess their significance as a source of magnetic anomalies. Magnetite or Ti-poor magnetite, maghemite and hematite are commonly found in the ores and host rocks, respectively. The scatter in the natural remanent magnetization and cleaned remanence directions of the ores may result from physical movement of the ores during faulting or mining, or from perturbation of the ambient geomagnetic field during remanence acquisition within these strongly magnetic ores. The microscopy study under reflected light shows that magnetic carriers are mainly magnetite, 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. Supergene reactions in ore deposits at the eruptions conditions (still hot ore magma) is indicated by goethite and hematite oxide minerals. Grain sizes range from a few microns to >100 µm, and dominant magnetic state PSD, in agreement with hysteresis measurements. Thermal spectra, continuous susceptibility measurements, and isothermal remanent magnetization (IRM) acquisition suggest a predominance of some spinels (titanomagnetite or titanomaghemite) with low-Ti content as magnetic carriers. Although the presence of (titano)hematites is evidenced by hysteresis and IRM studies, their contribution on total remanence seem to be minor. For the modeling process of the magnetic anomaly, we used data on bulk susceptibility and intensity and direction of the NRM in order to know the relative contributions of induced and remanent magnetization components and allow a greater control of the source bodies. The position and geometry of the main magnetic source are shown as an ENE-25° striking tabular body, steeply inclined (65°) to the north.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....2043A