The Zn-Pb(-Cu-Fe) mineralization in the northern Sila Massif (Calabria, southern Italy): Genetic constrains from trace element concentrations in sphalerite

The occurrence of Zn-Pb(-Cu-Fe) mineralization in the northern Sila Massif (Calabria), primarily in Longobucco (LGB) and Fonte Argentila (FAR) areas, is not a novelty (e.g., Fregola et al., 2023). However, the genesis of this mineralization and its relation to the geological evolution of the Sila Massif are still unknown. These issues are addressed in the present study. Detailed mineralogical and geochemical analyses were carried out on ore-samples collected from mineralized faulted bodies within the granodiorite of the Sila batholith. The mineral association, common to both areas, consists of sphalerite (Sp), as main ore-mineral, galena (Gn), quartz (Qz) and calcite (Cal). Samples from the FAR site also contain chalcopyrite (Ccp), pyrite (Py), and fluorite (Flr). Mineral composition and microstructures were characterized using SEM, EPMA and m-Raman spectroscopy techniques. In addition, trace element distribution in sphalerite and chalcopyrite were determined with LA-ICP-MS. The sphalerite is Fe-poor (Fe up to 11.3 wt%; 0.21 mol% FeS) and shows the associated color and chemical zonings mainly due to variations in Fe concentrations. The following trace elements have been identified in sphalerite: Mn, Co, Cu, Ga, Ge, Ag, Cd, In, Sn, Sb, Hg, Tl, Pb, Bi. The reconstructed paragenetic sequence comprises five growth stages for both areas. During stage-1, precipitation of massive, light-colored and Fe-poorer Sp1 occurred, whereas in stage-2 a darker, Fe-richer Sp2 formed in association with euhedral Qz1. The latter shows color and chemical zonings related to variations in Al, Na, K, Ca content. Fracturing characterized stage-3, along with precipitation of massive Cal (in LGB) and Flr (in FAR). The calcite in LGB hosts synchysite, with grain sizes between 20 and 60 µm, and ΣREE (Ce, La, Y, Nd, Sm, Pr, Gd, Dy) ranging from 2.40 to 42.6 wt%. Stage-4 was characterized by diffuse recrystallization of colorless Sp3 and Qz2. During the stage-5 an almost pure Gn, Ccp and Py formed at the expense of the previous minerals. Formation temperatures ranging between 150 and 200°C were derived for Sp1 and Sp2 using the GGIMFis sphalerite geothermometer (Frenzel et al., 2016). These predicted values will be compared with those obtained from ongoing fluid inclusion analyses. Anyway, such low formation temperatures are also supported by the Zn/Cd (195–267), Ga/In (190–596), and In/Ge (0.24–1.20) average ratios in the sphalerite, indicative of a low-temperature ore-forming fluid. Our mineralogical and geochemical results are comparable with those of either the vein-type or the MVT deposits, although the examined mineralization in the northern Sila Massif occur in a different geological context and registers slightly higher temperatures than those typical of the MVT deposits (that is, 100–150°C).