Ni-Co-As-Bi minerals in the Silius vein system, (SE Sardinia, Italy): The polymetallic evolution of a world-class fluorspar deposit

The world-class Silius deposit (NE Sardinia, Italy), exploited for decades, represents a huge ENE WSW to NE-SW directed vein system, characterized by abundant fluorite, minor barite, and Pb-(Zn-Cu-Fe) sulphides in quartz-carbonate gangue. This deposit shows many similarities to other European unconformity-related F-Ba districts linked to the regional-scale event of the breakup of Pangea, such as ore textures, low-temperature high-salinity fluids, and observed mineral assemblages. Besides fluorite, the mineralogy includes other critical raw materials (CRM) such as disseminated tiny LREE minerals and intergrowths of Ni-Co-Fe arsenides-sulpharsenides such as nickeline (NiAs), rammelsbergite (NiAs2), gersdorffite (NiAsS), cobaltite (CoAsS) and arsenopyrite (FeAsS). Ni-Co assemblages, which were recently discovered in this deposit, were investigated by optical microscopy and SEM-EDS.