Modelling and Exploration of CRMs-bearing greisen systems of Northern Sardinia

Northern Sardinia, Italy, shows substantial potential for W-Sn-Mo-Bi and F-REE granite-related ore deposits. In the Oschiri area, mineralized greisen and hydrothermal veins occur at intrusive contacts between late-Variscan granites and high-grade metamorphic rocks [1].

The greisen consists of K-feldspar, plagioclase, rutile, biotite, and zoned muscovite rimmed by quartz, sericite, chlorite, and ore minerals such as molybdenite, rare interstitial Bi-Te alloy, scheelite, columbite, partially hematitized pyrite, LREE phosphates and fluorocarbonates (monazite, synchysite-Ce), and pyrochlore-(Y). Hydrothermal veins are primarily composed of fluorite and quartz, with rare barite, galena and apatite. Together, these ore bodies form an "F-rich open system" [2] where greisenization occurred at ~400–300°C, introducing an early phase of ore minerals. Subsequent HF-rich hydrothermal fluids circulation below 300°C produced brecciated and argillitic bands in granite and greisen. The Nb-Y-F signature suggests a metallogenic affinity with NYF (Niobium-Yttrium-Fluorine) pegmatites.

Electron microprobe analyses of muscovite zonation were performed on five greisen samples, with 146 core analyses and 163 rim analyses for F, Na, Mg, Si, Al, Ba, K, Cl, Ca, Mn, Ti, Ni, Fe, and Cr. These analyses revealed an average increase in Si (3.08–3.13 apfu), Fe (0.06–0.15 apfu), Mg (0.01–0.02 apfu), Mn (0.003–0.007 apfu), and Ti (0.002–0.005 apfu), and a decrease in Al (2.75–2.56 apfu) toward the rims, consistent with a Tschermak-type substitution. Additionally, an increase in OH (1.82–1.84 apfu) and a decrease in F (0.18–0.15 apfu) suggest more hydrous conditions at the rims. This chemical zonation, combined with the textural relationships between muscovite and ore minerals, indicates alteration driven by the mineralizing fluid circulating during greisenization [3]. Dating these altered rims will enhance the genetic model by constraining the timing of the mineralization [4], providing a valuable interpretative framework for similar geological settings and offering a novel approach to dating granite-related ores.

The identification of economically significant elements such as W, Mo, LREE, and F within the greisen and hydrothermal system, coupled with the possible potential for NYF pegmatite occurrences, underlines the area's untapped resource potential. This study highlights Northern Sardinia as a promising region for future exploration and resource development, offering critical guidance for targeted exploration strategies.

 

 

References 

[1] Cossu, D., Attardi, A., Deidda, M. L., Naitza, S. (2024). Studio giacimentologico sulle mineralizzazioni legate ai granitoidi dell’area di Oschiri-Alà dei Sardi (Sardegna Nord-orientale). [Master’s Thesis], University of Cagliari, 121.

[2] Pirajno, F. (2009). Hydrothermal processes and mineral systems. Springer, 1250.

[3] Attardi, A., Cossu, D., Naitza, S., Majka, J., 2024. The role of micas in modelling ore-bearing greisen in Sardinia, Italy. Mineralogia – Special Papers, 52, 33.

[4] Rösel, D., Zack, T. (2021). LA-ICP-MS/MS single-spot Rb-Sr dating. Geostandards and Geoanalytical Research, 46(2), 143–168. https:// doi. org/ 10. 1111/ ggr.12414