Bi-minerals occurrence in various ore deposits of&#160;Southern Sardinia: a short review.
Authors: Deidda Matteo Luca , Fancello Dario , Stefano Naitza , Moroni Marilena , Scano Ignazio 
Published: March 2022

&lt;p&gt;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 &amp;#160;hornfelses; and 2) late- to post-Variscan five-element (Ni-Co-As-Bi-Ag) LT hydrothermal veins.&lt;/p&gt;&lt;p&gt;In the first group, greisens (Flumini Binu prospect, SWS) and HT hydrothermal W-Mo(-Sn) veins (Perd&amp;#8217;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 (Au&lt;sub&gt;2&lt;/sub&gt;Bi), Bi-tellurides (hedleyite Bi&lt;sub&gt;7&lt;/sub&gt;Te&lt;sub&gt;3&lt;/sub&gt;, and Bi&lt;sub&gt;2&lt;/sub&gt;Te) and probable russellite (Bi&lt;sub&gt;2&lt;/sub&gt;WO&lt;sub&gt;6&lt;/sub&gt;) 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 &amp;#8220;phase 88.6&amp;#8221; (Cu&lt;sub&gt;0.33&lt;/sub&gt;Pb&lt;sub&gt;0.33&lt;/sub&gt;Bi&lt;sub&gt;7.67&lt;/sub&gt;S&lt;sub&gt;12&lt;/sub&gt;), pekoite (PbCuBi&lt;sub&gt;11&lt;/sub&gt;S&lt;sub&gt;16&lt;/sub&gt;Se&lt;sub&gt;2&lt;/sub&gt;), salzburgite-paarite (Cu&lt;sub&gt;1.58-1.67&lt;/sub&gt;Fe&lt;sup&gt;2+&lt;/sup&gt;&lt;sub&gt;0.03-0.01&lt;/sub&gt;Pb&lt;sub&gt;1.65-1.72&lt;/sub&gt;Bi&lt;sub&gt;6.38-6.3&lt;/sub&gt;S&lt;sub&gt;12-12.06&lt;/sub&gt;), gustavite (PbAgBi&lt;sub&gt;3&lt;/sub&gt;S&lt;sub&gt;6&lt;/sub&gt;) xilingolite-lillianite (Pb&lt;sub&gt;3&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;6&lt;/sub&gt;), cosalite (Pb&lt;sub&gt;2&lt;/sub&gt;Bi&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;6&lt;/sub&gt;), berryite (Cu&lt;sub&gt;3&lt;/sub&gt;Ag&lt;sub&gt;2&lt;/sub&gt;Pb&lt;sub&gt;3&lt;/sub&gt;Bi&lt;sub&gt;7&lt;/sub&gt;S&lt;sub&gt;16&lt;/sub&gt;), ourayite (Pb&lt;sub&gt;4&lt;/sub&gt;Ag&lt;sub&gt;3&lt;/sub&gt;Bi&lt;sub&gt;5&lt;/sub&gt;S&lt;sub&gt;13&lt;/sub&gt;) and cupropavonite (Cu&lt;sub&gt;0.9&lt;/sub&gt;Ag&lt;sub&gt;0.5&lt;/sub&gt;Pb&lt;sub&gt;0.6&lt;/sub&gt;Bi&lt;sub&gt;2.5&lt;/sub&gt;S&lt;sub&gt;5&lt;/sub&gt;), 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)&lt;sub&gt;2&lt;/sub&gt;S&lt;sub&gt;3&lt;/sub&gt;) and hammariite (Pb&lt;sub&gt;2&lt;/sub&gt;Cu&lt;sub&gt;2&lt;/sub&gt;Bi&lt;sub&gt;4&lt;/sub&gt;S&lt;sub&gt;9&lt;/sub&gt;) have been previously identified, while schapbachite (AgBiS&lt;sub&gt;2&lt;/sub&gt;) has been reported at the Sa Marchesa mine.&lt;br&gt;The second group of Bi-bearing orebodies includes the five-element veins of the Arbur&amp;#232;se district (Pira Infer&amp;#236;da, 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.&lt;/p&gt;&lt;p&gt;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. &lt;br&gt;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.&lt;/p&gt;