Characterization of the CRM occurrences in ore deposits of Sardinia

This thesis is entitled “Characterization of the CRM occurrences in ore deposits of Sardinia” and its aim is to further study the geological occurrence of economic raw materials in various mineral deposits of southern and central Sardinia, with a focus on a specific class of deposit named “skarns”. In the last decades concerns have increased about the supply for specific elements and minerals, essential to high-end industrial applications and for the green-technologies transition. Starting from 2017, and updated to 2020, the European Commission defined a list of elements, namely the Critical Raw Materials (CRM) by means of two factors: economic importance and supply risk. This list includes, among others, elements such as Rare Earth Elements (REE) and lithium (Li) but also indium (In), germanium (Ge), tungsten (W), molybdenum (Mo), bismuth (Bi), niobium (Nb), tantalum (Ta), nickel (Ni) and cobalt (Co) whose supply in Europe is almost totally dependent on the import from non-EU nations, most notably China (accounting for the 69% and 93% of global W and Bi production respectively). Other elements, most notably Sn, are likely to be inducted to the CRM list in the next years. These elements usually occur in small concentrations in the crust and in most orebodies are generally subordinated to base metals like iron (Fe), lead (Pb), zinc (Zn) and copper (Cu). As a reflection of the contextualized economic demand and technologies of the past, most mining activities were mostly oriented towards the exploitation of base metals ores, usually overlooking the minor and trace concentrations of metals that are now considered critical. A re-evaluation of the CRM occurrences in old mining districts and mine wastes of Europe currently represents one of the most promising frontiers in mineral exploration and may assume a key role in reducing the supply risk. In the framework of the re-evaluation of CRM occurrences in mine districts of the past, Sardinia, which was one of the most important mining district of Italy, is a very interesting study area due to its abundant resources, extensively mined during the 18th and 19th century, especially for Zn-Pb-Cu-Fe. The most significant mining districts were in Southern Sardinia and included the Iglesiente-Sulcis and Montevecchio (Arburèse region) mines, hosting several giant Mississippi Valley Type (MVT) deposits and their associated non-sulfide ores and a 15 km long hydrothermal Zn-Pb vein system respectively. Aside from Zn-Pb MVT deposits, southern Sardinia is also characterized by several Au-As(-Sb-W), Ag-Sb, F-Ba-Pb and epithermal Cu-Au-Ag vein deposits. A further class of ore deposits is represented by Late Variscan granite-related deposits such as greisen and hydrothermal veins, locally exploited for their molybdenite, cassiterite, wolframite and arsenopyrite mineralization and skarn deposits. Indeed, skarns are probably the most recurrent type of ore deposits in SW Sardinia (Sulcis district), and to a lesser extent in SE and central Sardinia (Sarrabus-Gerrei; Barbagia). They typically hosted high-grade Zn-Pb-Cu-Fe mineralization, for which were historically explored and exploited. In the last years, the revitalised interest for CRM in Sardinia lead to a clearer definition of the metallogenic potential of some variscan granitic intrusion, especially for elements like Sn, Mo and W (Naitza et al., 2017). Therefore, this project was conceived to gain further knowledge on the geological relationships, mineralogy, chemical composition and mechanisms of formation of skarn deposits of southern and central Sardinia, and to investigate their W-, Sn-, Mo-, Bi-, In- and Ge-bearing potential. Sampling and analyses have been done at the University of Cagliari in collaboration with the University of Milano, and during the six months spent abroad in Switzerland (University of Geneva, ETH Zürich) and Germany (Karlsruhe Institute of Technology, University of Tübingen).