From a fluorite vein system to a five-element-type polymetallic vein system? The first evidence of Ni-Co minerals from the world-class, CRMs-bearing Silius deposit (SE Sardinia, Italy)

In recent years mineral sources of Critical Raw Materials (CRMs), whose supply is strategic and irreplaceable in many technological and industrial applications, have become the subject of growing interest throughout Europe. An excellent example is the Silius vein system (SE Sardinia), in which the mining activities, after several years of standby, are going to be resumed to exploit fluorite, galena and LREE minerals. This world-class deposit (2.2 Mt of proven reserves) consists of two main parallel, steeply dipping, ENE-WSW to NE-SW directed veins which coalesce at depth, hosted in Middle Ordovician metavolcanites, Upper Ordovician metasediments and Silurian black shales belonging to the Variscan  Nappe Zone of SE Sardinia. The vein filling displays dominantly banded textures with abundant fluorite, quartz, calcite, galena, barite, and accessory base-metal sulfides (sphalerite, chalcopyrite and pyrite-marcasite), LREE carbonate (synchisite-Ce) and xenotime-Y. Sulfide contents in the ore increase downward. Knowledge about the trace elements content of sulfides in the deposits is still scarce, thus new underground surveys and samplings have been performed in three different levels of the mine. In samples collected from the deepest level (level 100 m a.s.l., Muscadroxiu sector), tiny inclusions of Ni-Co minerals have been found for the first time ever by optical microscopy (reflected light) and SEM-EDS analyses. They consist of very complex intergrowths dispersed in chalcopyrite and the quartz gangue, containing various associations of Ni-Co-Fe arsenides and sulfarsenides such as nickeline, rammelsbergite, gersdorffite, cobaltite and arsenopyrite. Remarkably, this assemblage is very similar to that found in the shallower parts of the nearby “Sarrabus Silver Lode”, a similar, regional-scale vein system exploited in the past for silver. Thus, it may be speculated that it not only could be a general indication of possible Ni-Co enrichments below the 100 level of the mine, but also an exploration tool and a proxy of possible Ag enrichments of the ore at depth. Moreover, the Ni-Co assemblages and LREE minerals found in Silius are similar to those found in the polymetallic (Ni-Co-As-Ag-Bi) veins of Southern Arburèse district (SW Sardinia), the five-element branch of the Montevecchio vein system, in the past a major Pb-Zn source in Italy. In addition to partly similar mineral assemblages, the Silius and Arburèse vein systems share a common derivation from low-temperature and high-saline fluids, supporting their attribution to the unconformity-related group of five-element and fluorite-barite deposits well-known throughout central and western Europe (Erzgebirge, Odenwald, Schwarzwald, French Central Massif, Catalonian Coastal Ranges, Pennidic Alps, etc.). These deposits have been collectively connected to metallogenic events associated with the Late Paleozoic-Mesozoic breakup of the supercontinent Pangea, so Silius, the Arburèse and other similar Sardinian deposits could be related to this regional-scale event. In conclusion, the Silius vein system is a key area to understand the regional events that occurred after the Variscan orogeny in Sardinia; the deposit may represent a relevant source of CRMs and metals in Italy, where besides fluorite, base-metal sulfides and LREE, some concentrations of Ni-Co and, possibly, Ag minerals might be present at depth.