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” (Cu_0.33Pb_0.33Bi_7.67S₁₂), pekoite (PbCuBi₁₁S₁₆Se₂), salzburgite-paarite (Cu_1.58-1.67Fe²⁺_0.03-0.01Pb_1.65-1.72Bi_6.38-6.3S_12-12.06), gustavite (PbAgBi₃S₆) xilingolite-lillianite (Pb₃Bi₂S₆), cosalite (Pb₂Bi₂S₆), berryite (Cu₃Ag₂Pb₃Bi₇S₁₆), ourayite (Pb₄Ag₃Bi₅S₁₃) and cupropavonite (Cu_0.9Ag_0.5Pb_0.6Bi_2.5S₅), 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.