Tellurides, selenides and Bi-mineral assemblages from the Río Narcea Gold Belt, Asturias, Spain: genetic implications in Cu–Au and Au skarns

Gold ores in skarns from the Rio Narcea Gold Belt are associated with Bi-Te(-Se)-bearing minerals. These mineral assemblages have been used to compare two different skarns from this belt, a Cu-Au skarn (calcic and magnesian) from the El Valle deposit, and a Au-reduced calcic skarn from the Ortosa deposit. In the former, gold mineralization occurs associated with Cu-(Fe)-sulfides (chalcopyrite, bornite, chalcocite-digenite), commonly in the presence of magnetite. Gold occurs mainly as native gold and electrum. Au-tellurides (petzite, sylvanite, calaverite) are locally present; other tellurides are hessite, clausthalite and coloradoite. The Bi-bearing minerals related to gold are Bi-sulfosalts (wittichenite, emplectite, aikinite, bismuthinite), native bismuth, and Bi-tellurides and selenides (tetradymite, kawazulite, tsumoite). The speciation of Bi-tellurides with Bi/Te(Se + S) < 1, the presence of magnetite and the abundance of precious metal tellurides and clausthalite indicate f O 2 conditions within the magnetite stability field that locally overlap the magnetite-hematite buffer. In Ortosa deposit, gold essentially occurs as native gold and maldonite and is commonly related to pyrrhotite and to the replacement of lollingite by arsenopyrite, indicating lower f O 2 conditions for gold mineralization than those for El Valle deposit. This fact is confirmed by the speciation of Bi-tellurides and selenides (hedleyite, joseite-B, joseite-A, ikunolite-laitakarite) with Bi/Te(+ Se + S) > 1. [PUBLICATION ABSTRACT]