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

摘要

The Silius vein system, located in SE Sardinia (Italy) is analogous to other late- to post-Hercynianmineral systems\udof this type in Europe. The Silius system consists of two main veins, characterized by several generations of fluorite,\udcalcite and quartz,with initial ribbon-like geometries, followed by breccias and cockade-like textures. In this\udstudy, aimed at investigating the REE concentrations in the Silius vein system, a REE average of ~800 ppm(locally\udΣREE N1500 ppm) has been observed in the carbonate gangue of the fluorite orebody. These amounts are related\udto the presence of the REE-bearing minerals synchysite-(Ce) and xenotime-(Y). The chemical composition of\udsynchysite-(Ce) has been obtained by wavelength dispersive spectrometry (WDS). The average synchysite-\ud(Ce) formula, built on the basis of (CO3)2F and 5 negative charges, is Ca1.07(La0.19,Ce0.36,Pr0.04,Nd0.15,\udSm0.03,Gd0.03,Y0.13)(CO3)2F. From their geochemical characteristics, and their textural relationships with other\udgangue phases, it is likely that synchysite-(Ce) and xenotime-(Y) formed at the same P-T-X conditions as the\udother minerals of the Silius fluorite mineralization. Synchysite-(Ce) and xenotime-(Y) at Silius could be related\udto a local circulation phenomenon, where the REE are derived from a REE-bearing source rock in the basement\udof southeastern Sardinia,which has been leached by the same fluids precipitating the fluorite/calcite mineralization.\udREE concentrations contained in the carbonate gangue of still unexploited parts of the Silius vein deposit, as\udwell as in dumps and tailings accumulated during past fluorite processing, could possibly represent a sub-economic\udby-product of the fluorite exploitation.