CRMs from the LREE-F-rich belt of the Roman Comagmatic Province (Central Italy)

Recent advancements in green and sustainable technologies, particularly for decarbonisation and the energy transition, have led to a growing demand for Critical Raw Materials (CRMs). In response, the European Commission is encouraging exploration for economically viable CRMs deposits. The LREE-F-rich belt of the Roman Comagmatic Province (RCP; Washington, 1906) has emerged as a promising target (Mastrangelo, 1976; Stoppa et al., 2016, 2019), hosting key sites: the Pianciano deposit, which features a fluoritite–F-Ca carbonatite orebody, currently the largest underdeveloped fluorite resource in Western Europe, and the Santa Maria di Sala deposit, characterised by F-Ca carbonatite. Both deposits formed from carbothermal fluids derived from carbonatite magmas and exhibit interesting concentrations of LREEs (La, Ce, Pr, Nd) and barite, suggesting their potential for CRMs economically viable recovery.

Here we present the preliminary results of a comprehensive geological and geochemical survey carried out over these study areas, including the first detailed mapping of the Santa Maria di Sala deposit.

The Santa Maria di Sala deposit, which covers an area of approximately 2.5 km², formed in a shallow water basin, with paleosoils intercalations, lignite fragments and travertine beds or layers, suggesting a lacustrine environment. In its northern sector, the deposit is almost continuously exposed, most likely set on a single stratigraphic level, whereas in the southern sector it has a more heteropic character, intercalated with volcanic and fluvial deposits, and developed in at least two distinct series. The deposit is characterised by a very fine grain size, from 200 µm to <1 µm, and displays variable compositions and relative abundances of primary constituent minerals.

XRPD and SEM analyses of samples from Pianciano and Santa Maria di Sala deposits reveal that the dominant mineral phases in both deposits are fluorite (up to 90% in fluoritite and 50% in carbonatite), calcite, and barite, with subordinate apatite, clay minerals, (Pb, Mn)-carbonates, diopside, zeolites, spinel, rare microcline, zircon, garnet, vesuvianite, Fe-horneblende,  and occasional Ce-wakefieldite, vanadinite and titanite.

XRF and ICP-MS dataindicate LREEs concentrations of ~1000–2700 ppm in fluoritites and ~200–800 ppm in carbonatites, hosted principally in apatite and Pb+Mn-carbonates which may contain ~0.5-4% of them.

Preliminary LREEs extraction tests using HCl or citric acid, under variable conditions of time, temperature, concentration and quantity of reagent, showed excellent extraction rates for carbonatites (up to 100%), and lower rates for fluoritites (up to 30%). Although the extraction rate from fluoritites is much lower, the amount of LREEs recovered with citric acid for a given mass of sample is broadly similar due to the higher initial concentration. Optimization of extraction protocols for more efficient LREEs recovery, especially for fluoritites, is ongoing.