Potential field and structural control of a Sn-W rich area, W Iberia (Central Iberian Zone)

The Sn-W belt of western Iberia coincides with large-scale magnetic anomalies, namely the Porto-Veira-Guarda and the Central System Magnetic Anomalies (PVGMA and CSMA) challenging the well-known relationship between Sn ores and ilmenite (non-magnetic) granites. In fact, these magnetic anomalies overlap a series of gneiss domes developed in the latest stages of Variscan evolution and often hosting local, but abundant critical mineral ores.  Paradoxically, these domes are cored by non-magnetic granites and their by-products, rising the question about the source of the magnetism.

With the goal of unravelling the relationship between mineralization, tectonics and magnetic anomalies, we have carried out a 50 x 50 km² ground potential field survey in the Martinamor Gneiss Dome and its surroundings, to the southwest of Salamanca. Results from analytical processing and modelling show that shallow and local magnetic anomalies respond to the existence of highly magnetic, albeit uncommon, Upper-Proterozoic to Ordovician metasediments that are not related to the younger (338-300 Ma) Sn-W mineralization. Contrarily, the ores appear at the non-magnetic core of the dome and are frequently related to high gradient zones within potential field data. The latter coincide with the location of extensional detachments that must have acted as pathways for mineralizing fluids. To the southwest of the study area and at higher depths, conspicuous magnetic maxima coincide with Bouguer gravity anomaly maxima and with high shear-wave velocity anomalies, pointing out to the existence of non-outcropping mafic rocks. These lithologies might be progressively more common at depth and be the source of the long wavelength PVGMA and CSMA.  

The present dataset indicates that, as it has been generally acknowledged, magnetic rocks do not host Sn (and W) mineralization but regardless of this evidence, in western Iberia, there might be a common mechanism that triggers mineralization and magnetization. Constraining the age of the latter is key to further interpret this area.

This research has been funded by project SA066P24 from the JCYL