In situ LA-ICP-MS trace element analysis of magnetite as a vector towards mineral exploration: A comparative case study of Fe-skarn deposits from SW Iberia (Ossa-Morena Zone)

Abstract

The Azenhas and Alvito Fe-deposits are located at SW of the Iberian Variscan belt, in a wide Fe-Zn ore district (Montemor-Ficalho Belt), part of the Ossa-Morena Zone. Both deposits are dominantly composed of magnetite ores that display distinct ore formation processes and, at the Azenhas deposit, massive magnetite ores are mainly hosted in amphibolites (Middle Cambrian – Ordovician), with a genesis arguably associated with metamorphic- metasomatic reactions promoted by fluid circulation through several thrust faults that led to and expressive tectonic pilling. Contrastingly, the Alvito massive magnetite ores are formed by the emplacement of a gabbro- dioritic suite in contact with calcite-dolomite marbles, constituting a typical calcic exoskarn Fe-skarn deposit. Primary and secondary magnetite were identified and characterized, and further selected for in situ laser ablation coupled inductively mass spectroscopy (LA-ICP-MS) trace element analysis. Results show that primary magnetite (Mag I) from the Azenhas deposit is discriminated by higher concentrations of Mg, Cr, Mn, Zn, Co, and Sn, whereas secondary magnetite (Mag II) is depleted in most trace elements, although displaying relatively higher concentrations of V, Ga, Mo, and Pb. The application of TMg-Mag geothermometer reinforced the discriminatory indexes, with Mag I displaying higher-temperature estimations (ca. 770 ◦C) when compared to Mag II (ca. 420 ◦C), thus suggesting that primary ores are associated to higher temperature processes than previously assumed. Characterization of the ores from Alvito deposit revealed primary magnetite characterized by abundant ilmenite and Al-spinel oxy-exsolutions which led to high measured Al concentrations in magnetite. The trace element content indicates high-temperature hydrothermal magnetite, underlining the contribution of hot hy- drothermal fluids from the igneous body emplacement, also supported by the TMg-Mag geothermometer (ca. 685 ◦ C). The anomalous presence of Co (max. 156 ppm) and Ni (max. 100 ppm) concentrations in Mag I from Alvito, along with pentlandite-magnetite assemblages, suggest that prone conditions were sustained for the development of Ni-bearing ores and that magnetite composition could be an interesting geochemical proxy for the exploration of such mineralization. The Mn + Al versus Ti + V discriminant diagram shows that primary ores from both deposits plot in the skarn field. The combination of magnetite analyses with whole-rock geochemistry and field geology, allowed us to define criteria that can be applied in the discrimination of Fe-deposits and contribute to improve the geological models of the studied ore deposits, thus beneficiating future exploration in SW Iberia