On the occurrence of selenium in sulfides of the ore deposit of Baccu Locci (Gerrei, SE Sardinia)

In a section of the abandoned Pb-As mine of Baccu Locci (SE Sardinia). where several Se-bearing secondary minerals had been recovered, microscopic and microchemical investigations were performed on the sulfide assemblage, in view to determine their possible Se contents and to single out the source of selenium. The ore in this area consists predominantly of galena, sphalerite, termantite, and minor sulfide and sulfarsenides; there are also some minerals such as wittichenite, cupro-pavonite, pearceite, bismuthinite, native Bi, greenockite, idaite, cubanite, mackinawite, magnetite (included by sphalerite) and gersdorffite, which better characterize the ore assemblage but have not yet been reported on. Microprobe analyses were performed on every ore mineral. Galena continually proved to be the main Se-bearing mineral (usually up to 1.17, and sometimes even up to 13.61 wt% Se). Furthermore, not negligible amounts of Se were also detected in sphalerite, greenockite, cupropavonite, wittichenite, chalcopyrite. Minor amounts of Se are sometimes present in pyrite, whereas termantite, pearceite, arsenopyrite and gersdorffite are always Se-free. The textural patterns and mineral composition in the Baccu Locci deposit strictly resemble those of the ore assemblages linked to volcano-plutonic activity which currently form in the oceans; moreover, the general agreement on the recurring link between selenium and volcanic activity and/or black-shales sequences provides further evidence in favour of a volcano-sedimentary genesis which has already been proposed by some authors.