Rare earth element and Sr-Nd isotopic evidence for the origin of fluorite from the Silius vein deposit (southeastern Sardinia, Italy)

Abstract

Rare earth element (REE) and Y contents along with Sr-Nd isotopic ratios have been determined in fluorite and calcite samples from the two main veins of the Silius deposit, hosted in Lower Paleozoic metavolcanic rocks, in southern Sardinia. Fluorite from the late San Giuseppe vein contains higher (107–336 mg/kg) (REE + Y) than fluorite from the early San Giorgio vein (29–94 mg/kg), whereas calcite exhibits a range (175–440 mg/kg) overlapping with that of San Giuseppe fluorite. Irrespective of either vein, the 87Sr/86Sr ratios of fluorite range from 0.71106 to 0.71566, overlapping with those of calcite (0.71257–0.71406), whereas the 143Nd/144Nd ratios are slightly higher for fluorite (0.51221–0.51236) than calcite (0.51211–0.5122). Moreover, the 147Sm/144Nd versus 143Nd/144Nd ratio relationship suggests that San Giuseppe fluorites were deposited by 294 ± 40 Ma, whereas it yields no information for San Giorgio fluorites. Assuming 270 Ma as the most probable age of emplacement, the initial Sr-Nd isotopic ratios and (REE + Y) contents in fluorite indicate the crustal origin of Silius mineralizing fluids, likely from mixing in different proportions of evaporated seawater brines with fluids from the Paleozoic siliciclastic basement and Late Variscan/post-Variscan magmatic rocks. Mineralizing fluids evolved their chemical and isotopic composition through interaction with wallrocks and/or dilution with meteoric waters circulating through the Paleozoic basement. As a whole, the chemical and isotopic composition of fluorite from the Silius deposit resembles that of other fluorite deposits from Variscan Europe.