CRMs prospecting through comparative ore deposit modelling and the case of SW Sardinia

Keywords: ore geology, structural geology, prospection, exploration The requirement for Critical Raw Materials (CRMs) of the last years brought the need for systematic and large-scale prospecting in fast and efficient ways. An example is the ISPRA – UniCa project developing the Metallogenic Map of Sardinia, Italy, drawing upon classifying CRMs’ occurrences. In Sardinia, recent studies highlighted the discrete structural controls displayed by two types of CRMs-bearing ore deposits: a) mixed sulphides (Cu, Pb, Zn, Fe) skarn deposits with variable contents of W-Sn-Bi, Ni-Co, and F minerals, set along shear zones associated with the several thrusts marking the emplacement of the Variscan Nappes with a low-angled, top-to-the SW transport direction; b) F-Ba (±Pb±Zn) low-temperature hydrothermal veins hosting REEs and subordinate Ni-Co-Bi sulfides-sulfarsenides filling normal, high-angled, N-S and E-W striking faults with strike-slip components, crosscutting any Variscan structure. We present two case studies in southwestern Sardinia prospected by comparative modelling: the Rosas Shear Zone (RSZ) and the Santa Lucia-Su Zurfuru (SLZ) mining areas. In the RSZ, the skarn’s protolith is the Cambrian limestones embedded within the Ordovician succession by a system of anastomosing NW-SE Variscan thrusts, enhancing the circulation of the mineralizing fluids and the structurally passive skarn’s emplacement. N-S striking F-Ba veins cut the thrusts. In the SLZ, the protolith and host rocks of skarn are analogous to the RSZ: km-sized Cambrian limestone bodies are metasomatised at their peripheries and show a distinct lithological zonation: limestone, marble, and chlorite-skarn. Preliminary field surveys indicate that the Cambrian limestones outcrop at the cores of NW-SE striking anticlines, whose limbs have been reactivated at the stratigraphic contact by later high-angled faults, which hosts the F-Ba (Pb-Zn) hydrothermal veins. Decimetre-thick veinlets striking N-S also crosscut the limestone bodies. The vein system displays a generally good continuity, and is distinctly zoned: a) the northern zone is barite-rich and characterized by breccia textures, with several generations of fluorite cementing brecciated marble/skarn, rare galena and sphalerite; b) the central zone is dominated by banded fluorite and galena; here, the main system is intercepted by a two kilometres E-W quartz vein that ends up in a barite deposit at the contact with the limestones; c) the southern zone is fluorite-rich: it cuts across the skarn ore at depth. The comparison between RSZ and SLZ helps us to investigate the metallogenic processes that led to skarn and F-Ba hydrothermal systems emplacements, providing new guidelines for regional-scale mineral explorations and potential evaluation. The RSZ skarn model gave us a key to understanding the structural setting of SLZ. The SLZ veins model suggests some mineralogical potential left in the area and guides us in prospecting the same type of deposit by verifying and comparing zonation, continuity, and geometries. Globally, skarn and hydrothermal veins are common and great bearers of CRMs, and the structural-mineralogical comparative modelling we leveraged is an extremely efficient, quick, and low-cost way for prospecting and exploring, meeting all the requisites for modern economic needs.