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Rare earth elements (REE)—Minerals in the Silius fluorite vein system (Sardinia, Italy)

N. Mondillo a,b, M. Boni a, G. Balassone a, S. Spoleto a, F. Stellato a, A. Marino c,  L. Santoro b, J. Spratt b

aDipartimento di Scienze della Terra, dell'Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, via Mezzocannone 8, 80134 Napoli, Italy
b Department of Earth Sciences, Natural History Museum, Cromwell Road, SW7 5BD London, UK
cFluorite di Silius S.p.A. Viale Merello 14, 09023 Cagliari, Italy
Ore Geology Reviews, v. 74, 211–224.
The Silius vein system, located in SE Sardinia (Italy) is analogous to other late- to post-Hercynianmineral systems of this type in Europe. The Silius system consists of two main veins, characterized by several generations of fluorite, calcite and quartz,with initial ribbon-like geometries, followed by breccias and cockade-like textures. In this study, aimed at investigating the REE concentrations in the Silius vein system, a REE average of ~800 ppm (locally ΣREE N1500 ppm) has been observed in the carbonate gangue of the fluorite orebody. These amounts are related to the presence of the REE-bearing minerals synchysite-(Ce) and xenotime-(Y). The chemical composition of synchysite-(Ce) has been obtained by wavelength dispersive spectrometry (WDS). The average synchysite-(Ce) formula, built on the basis of (CO3)2F and 5 negative charges, is:
Ca1.07(La0.19,Ce0.36,Pr0.04,Nd0.15, Sm0.03,Gd0.03,Y0.13)(CO3)2F.
From their geochemical characteristics, and their textural relationships with other gangue phases, it is likely that synchysite-(Ce) and xenotime-(Y) formed at the same P-T-X conditions as the other minerals of the Silius fluorite mineralization. Synchysite-(Ce) and xenotime-(Y) at Silius could be related to a local circulation phenomenon, where the REE are derived from a REE-bearing source rock in the basement of southeastern Sardinia,which has been leached by the same fluids precipitating the fluorite/calcite mineralization. REE concentrations contained in the carbonate gangue of still unexploited parts of the Silius vein deposit, as well as in dumps and tailings accumulated during past fluorite processing, could possibly represent a sub-economic by-product of the fluorite exploitation.