Magnetic properties of iron ores from two Ordovician deposits in Northern Portugal: Marão and Moncorvo

The Vila Cova do Marão Mining Field (VCMF) and the Moncorvo Iron Complex (MIC) in northern Portugal are important geo-resources of iron. Here we provide a detailed characterisation of their mineralogy through petrographic observations and magnetic properties. Petrographic observations show the VCMF ore primarily consists of quartz, magnetite, and chlorite, while the MIC ore of hematite and quartz, with rare magnetite occurrences. The magnetic susceptibility (χ) of the VCMF host rock is ~ 2470 × 10^⁻8 m³/kg, while the VCMF ore deposits have χ of ~ 20,500 × 10^⁻8 m³/kg. MIC ores show lower χ values of 520–700 × 10^⁻8 m³/kg. Isothermal Remanent Magnetisation (IRM) component unmixing of the VCMF host rock indicates that the magnetic population is represented by a single component with B_1/2 of 45–55 mT and DP values 0.30–0.35 mT, respectively, typical of magnetite. The VCMF ore is characterised by a single component with B_1/2 of 32–44 mT and DP of 0.30–0.35, typical of magnetite. The MIC ore shows two components: component 1 with B_1/2 of 50 mT and DP values of 0.31–0.37 mT, typical of magnetite, and component 2 with B_1/2 of 500 mT and DP of 0.19–0.26, typical of hematite. IRM at saturation (SIRM) of component 1 is 116–248 A/m for the host rock, 1100–2400 A/m for the VCMF samples, and 209–335 A/m for the MIC samples, providing an approximation of magnetite concentration. The Cisowski test shows the presence of strongly interacting single-domain particles (0.16 < R < 0.24) in all samples. Our data provides useful information for future iron ore resource exploration in the area.