Trace elements and REYs signature of ore and gangue minerals in the Zn-Pb(-Cu-Fe) mineralization of Longobucco and Fonte Argentila (Sila Massif, Calabria, southern Italy)

Here we illustrate key mineralogical and geochemical features of some Zn-Pb(-Cu-Fe) mineral deposits of the Sila Massif (Calabria, southern Italy) that were historically exploited (Fregola et al., 2023), unveiling their hidden genesis. We have collected and analysed samples from apparently vein-type mineralization associated with fault zones affecting the granodiorite of the Sila batholith at Longobucco (LGB) and Fonte Argentila (FAR). The mineral association comprises sphalerite (Sp) as primary ore mineral, along with galena (Gn), quartz (Qz) and calcite (Cal) in both areas. Samples from FAR site also contain chalcopyrite (Ccp), pyrite (Py) and fluorite (Flr). Mineral composition and microstructures were determined by petrographic observation, SEM, EPMA, and μ-Raman spectroscopy. Trace element distribution in sphalerite and chalcopyrite, as well as REY contents in calcite and fluorite, were also analysed through LA-ICP-MS. Sphalerite exhibits colour and chemical zonings mainly due to variations in Fe contents (up to 11.3 wt.%; 0.21 mol FeS). Trace elements identified in sphalerite include Mn, Co, Cu, Ga, Ge, Ag, Cd, In, Sn, Sb, Hg, Tl, Pb, and Bi. The reconstructed paragenetic sequence comprises five growth stages for both areas. Stages 1 and 2 register the main mineralization event, with the formation of Sp1, Sp2, and Qz1. In stage 3, following brittle deformation, massive precipitation of calcite (in LGB) and fluorite (in FAR) occurred. The REYs concentrations in Cal and Flr suggest mixing between two different fluids, one of likely igneous origin and the other diagenetic, also indicating a change from reducing to oxidizing conditions of the system. In LGB, calcite hosts synchysite with grain sizes in the range 20-60 μm, and LREE-Y contents from 2.40 to 42.6 wt.%. Furthermore, colourless Sp3 formed as a result of the recrystallization of Sp1 and Sp2 due to oxidizing conditions. A diffuse recrystallization of Qz2 occurred in stage 4 while, in stage 5, almost pure Gn formed along with Ccp and Py at the expense of previous minerals. Formation temperatures derived from Sp1 and Sp2 using the GGIMFis geothermometer (Frenzel et al., 2016) range between 170 and 220°C. This temperature range is in good agreement with that obtained from preliminary fluid inclusion analyses. In addition, the Zn/Cd (195-267), Ga/In (190-596), and In/Ge (0.24- 1.20) average ratios in sphalerite, are indicative of a low-temperature ore-forming fluid. Our geochemical data indicate an MVT-SHMS ore formation, with no evidence of hydrothermal processes involved in the system, contrasting with the apparently vein-type character of the mineralization observed in the field. A genetic model connecting the paragenetic stages and geochemical data obtained from ore and gangue minerals for LGB and FAR Zn-Pb(-Cu-Fe) mineralization is presented here. Fregola R.A. et al. (2023) - Review of polymetallic mineralization in the Sila and Serre Massifs (Calabria, southern Italy). Minerals, 13, 439, https://doi.org/org/10.3390/min13030439. Frenzel M. et al. (2016) - Gallium, germanium, indium, and other trace and minor elements in sphalerite as a function of deposit type - A meta-analysis. Ore Geol. Rev., 76, 52-78, https://doi.org/org/10.1016/j.oregeorev.2015.12.017.