Overview of CRM-bearing minerals in the Co-Ni-As mineralisation of the Punta Corna mining complex, Northern Italy

The Punta Corna mining district, located in the Lanzo Valleys area of Western Alps (Northern Italy), is a system of hydrothermal veins which were exploited in the past for Fe, Ag and later Co. These veins are hosted within metaophiolitic rocks (metabasalts with minor metagabbros, ultramafic rocks and minor metasediments) representing disrupted sections of oceanic lithosphere of Jurassic age. The mineralization is characterized by Co-Ni arsenides in the Eastern sector of Punta Corna, and by siderite in the Western sector, where Co phases are less abundant. Four deposition stages have been recognized (Moroni et al., 2019): i) siderite, ii) baryte, iii) Co-Ni-Fe arsenide, and vi) sulfides and sulfosalts. This work focuses on the mineralogy and mineral chemistry of CRM-bearing minerals, deposited mainly during the third and fourth stages, in selected mining sites of Punta Corna. The selected mining sites are: San Francesco and Sant’Andrea in the Western part of Punta Corna; Vittorio Amedeo, Nido dell’Aquila, San Giovanni, Carlo Emanuele, Santa Maria, Speranza and Colle Fortuna in the Eastern part; Santa Barbara in the Southern part. The main CRM phases are: ferroskutterudite (Fe,Co)As3, safflorite (Co,Fe)As2, clinosafflorite (Co,Fe,Ni) As2, tetrahedrite (Cu,Fe)12Sb4S13, bismuthian tetrahedrite (Cu,Fe)12(Sb,Bi)4S13, bournonite PbCuSbS3, pyrargyrite Ag3SbS3, a Bi-rich variety of chalcostibite CuSbS2 and an Sb-Bi alloy. Moreover, tiny xenotime (YPO4) crystals have been detected in some samples. Co-bearing minerals occur mainly in Eastern Punta Corna, and were not detected in the western part. Ferroskutterudite is abundant in several mining sites, and shows variable Co contents, up to 13.51 wt.%. Safflorite and clinosafflorite (Co up to 16.26 wt.%) are less abundant and were detected only in the San Giovanni mine. Sb and Bi minerals are widespread in all Punta Corna. Tetrahedrite shows variable Sb contents, up to 30.24 wt.%. Bi partially replaces Sb in the tetrahedrite structure. Bi-tetrahedrite has Sb contents up to 17.33 wt.% and Bi contents up to 15.54 wt.%. Bournonite is not as widespread as tetrahedrite, and was found only in the Santa Barbara mine. Pyrargyrite is rare and limited to the Speranza mining site. Bi-rich varieties of chalcostibite (Sb up to 47.27 wt.%, Bi up to 16.23 wt.%), and a Sb-Bi alloy were detected only in the Carlo Emanuele mining site. Sb and Bi-bearing phases are lollingite, arsenopyrite and gersdorffite. A mineral approaching lollingite stoichiometry shows variable As substitution by Sb (Sb up to 8.72 wt.%). Arsenopyrite and gersdorffite also display Sb fluctuations, with contents from below detection limit to 13.19 wt.%. Finally, Co and Sb-rich secondary phases are also present in the complex. They are mostly undetermined oxides and hydroxides with Co contents up to 10.34 wt.% and Sb contents up to 20.06 wt.%. Tiny xenotime crystals (~10 μm) were detected at Santa Barbara and Santa Maria mining sites. Moroni M., Rossetti P., Naitza S., Magnani L., Ruggieri G., Aquino A., Tartarotti P., Franklin A., Ferrari E., Castelli D., Oggiano G. & Secchi F. (2019) - Factors controlling hydrothermal nickel and cobalt mineralization - Some suggestions from historical ore deposits in Italy. Minerals, 9(7), 429.