Bi-minerals occurrence in various ore deposits of Southern Sardinia: a short review

Bismuth is recognized as a Critical Raw Material by the EU Commission and it is found in many ore deposits across the world. In Southwestern (SWS) and Southeastern (SES) Sardinia, Bi-minerals are commonly found in two main groups of ore deposit: 1) late Variscan granite-related orebodies including greisens, W-Mo(-Sn) HT hydrothermal veins, skarns and hornfelses; and 2) late- to post- Variscan five-element (Ni-Co-As-Bi-Ag) LT hydrothermal veins. In the first group, greisens (Flumini Binu prospect, SWS) and HT hydrothermal W-Mo(-Sn) veins (Perd’e Pibera mine and Togoro prospect, SWS; Perda Majori-Bruncu Spangas prospects, SES) tipically host native Bi, bismuthinite and, subordinately, Pb-Ag-Bi-sulfosalts interstitial to molybdenite and/or scattered in the quartz-feldspar(-fluorite-topaz) gangue. Locally, maldonite (Au2Bi), Bi-tellurides (hedleyite Bi7Te3, and Bi2Te) and probable russellite (Bi2WO6) are abundant in wolframite-rich veins (Togoro prospect), associated with native Au. Small grains of native Bi have also been found in some poorly mineralized garnet-vesuvianite-epidote calc-silicate hornfelses (Domus De Maria, SWS). Besides native Bi and bismuthinite, skarn orebodies frequently host wider assemblages consisting of Bi-Pb-Ag-Cu-sulfosalts intergrowths, once again associated with wolframates (scheelite at Monte Tamara prospect and Sa Marchesa mine, SWS) and molybdenite (Monte Tamara, Sa Marchesa and Morettu prospect, SWS). As a reference, the Monte Tamara assemblage includes “phase 88.6” (Cu0.33Pb0.33Bi7.67S12), pekoite (PbCuBi11S16Se2), salzburgite-paarite (Cu1.58-1.67Fe2+ 0.03-0.01Pb1.65-1.72Bi6.38-6.3S12-12.06), gustavite (PbAgBi3S6) xilingolite-lillianite (Pb3Bi2S6), cosalite (Pb2Bi2S6), berryite (Cu3Ag2Pb3Bi7S16), ourayite (Pb4Ag3Bi5S13) and cupropavonite (Cu0.9Ag0.5Pb0.6Bi2.5S5), identified by means of EPMA analyses. Moreover, since high Bi(-Ag-Te) contents have been detected in sulfides (sphalerite, galena, arsenopyrite), micro-inclusions of -sulfosalts and/or -tellurides may also occur. In the same area, wittichenite ((Bi,Cu)2S3) and hammariite (Pb2Cu2Bi4S9) have been previously identified, while schapbachite (AgBiS2) has been reported at the Sa Marchesa mine. The second group of Bi-bearing orebodies includes the five-element veins of the Arburèse district (Pira Inferìda, Acqua Is Prunas and Sa Menga mines, SWS), where native Bi and bismuthinite tipically occur at the core of Ni-Co arsenides-sulfarsenides (e.g. nickeline and gersdorffite-cobaltite) concentric growths. Therefore, the strong affinity of bismuth for granite-related W-Mo(-Sn) deposits of Southern Sardinia indicates that the late-Variscan (Early Permian) granites represent its main metallogenic source. However, the formation of such diverse Bi-minerals assemblages is seemingly controlled by local-scale conditions. In skarn ores, the Bi-Pb-Ag-Cu-sulfosalts intergrowths formed during the sulfide stages, apparently after the interaction between primary Bi-phases and Pb-Ag-Cu-bearing hydrothermal fluids and under oscillatory variations of metals availability and stability. Conversely, in W-Mo(-Sn) hydrothermal veins and greisens, where sulfides are apparently more scarce, the array of Bi-phases is usually more limited. Furthermore, field and analytical data point towards a selective remobilization of bismuth from the primary native and -tellurides assemblage of HT wolframite-quartz veins (Togoro, SWS) by late cross-cutting LT five-element veins, suggesting that multiple, spaced over time hydrothermal-veining events occurred in the same area. In conclusion, bismuth and related mineral phases could serve as important markers, providing useful qualitative indications regarding the source of metals, the ore-forming processes and the relationships between different ore deposits at the district-scale.