“Critical” importance of the Silius vein system (SE Sardinia,Italy)

The late- to post-Hercynian geological evolution of Europe, as well as part of Northern Africa, is characterized by numerous hydrothermal mineralizing events, ranging in age from the end of the orogenic compression (Permian) to the onset of Tethys spreading (Triassic-Jurassic) (Muchez et al., 2005). The Silius vein system (SE Sardinia (Italy) is analogous to other late- to post-Hercynian mineral systems of this type in Europe. Silius consists of two main veins, characterized by several generations of fluorite, barite, carbonates and quartz (Boni et al., 2009). Silius is known for its barite-fluorite, but also galena mineralizations that have been exploited until lately. Distinct fluorspar resources still exist in the mine area. Gangue carbonates, consisting of calcite and ferroan dolomite, contain Rare Earth Elements (REE)- bearing minerals represented by minor xenotime, and especially by synchysite-(Ce) (Mondillo et al., 2016). To check the effective amounts of REE in the Silius orebody, representative samples of the carbonate gangue have been collected in the underground mine. The values of LREE mainly, are in the range of 462-2071 ppm (951 ppm on average). The average volume of the carbonate gangue still in place is considered to be around 532,000 tons, to which may be added more 750,000 tons of carbonates discarded at the flotation plant. The corresponding total REE resource currently occurring at Silius may be about 1,220 tons. The discovery of REE in the Silius mine opens interesting perspectives for the exploration of subeconomic REE concentrations in this deposits type, where REE could be recovered as by-product of the fluorite exploitation. REE geochemistry of carbonates in other post-Hercynian fluorite veins has been widely investigated in the Harz Mts (Germany), in the Valle de Tena (Spain) and in the El Hammam orebodies (Morocco). Even though REE occur in contents comparable to those observed at Silius, the above deposits are not considered economic for this type of commodity till now. Being both REE and fluorite critical raw materials for the European economy, the Silius and the other similar post- Hercynian F-mineralizations should be now reconsidered in their double significance: not only for their fluorspar content, but also for the possibly recoverable REEs contained in the carbonate gangue. Boni M., Balassone G., Fedele L. & Mondillo N. 2009. Post-Variscan hydrothermal activity and ore deposits in southern Sardinia (Italy): selected examples from Gerrei (Silius vein system) and Iglesiente district. Per. Mineral., 78, 19-35. Mondillo N., Boni M., Balassone G., Spoleto S., Stellato F., Marino A., Santoro L. & Spratt J. 2016. Rare earth elements (REE)—Minerals in the Silius fluorite vein system (Sardinia, Italy). Ore Geol. Rev., 74, 211-224. Muchez P., Heijlen W., Banks D., Blundell D., Boni M. & Grandia F. 2005. Extensional tectonics and the timing and formation of basin-hosted deposits in Europe. Ore Geol. Rev., 27, 241-267.