Carbonate-hosted lead-zinc deposits: the siliceous crust type (SCT)

Among the carbonate-hosted ore-bearing deposits, the Siliceous Crust Type (SCT), frequently coexisting in different geodynamic and geotectonic settings with the classical Mississippi Valley Type (MVT) and the Alpine Type (APT), assumes an anomalous or a not yet fully recognised position. The SCT deposits are strata-bound, pervasively silicified independent lithological units of reworked allochthonous and authochtonous carbonates and siliciclastic sediments, from dm to tens of m thick, overlying unconformity surfaces evolved on top of carbonate platforms of different ages (from Archean to Tertiary). The transgressive siliciclastic sediments often contain fragments of underlying deposits. Very high SiO2 contents, locally With substantial barite and/or fluorite concentrations, characterize the SCT compositions. The metal content (Pb, Zn, Sb, Hg, Cu, Fe, As, Ni, Cd, Au), as sulphides and/or oxides, is variable from one ore deposit to another and also within the same deposit. The formation of the deposits is restricted within a narrow biostratigraphical gap, i.e. after the deposition of karst-solution landscapes and before the transgression of siliciclastic sediments, as also confirmed by isotopic data: delta O-18 of microcrystalline quartz suggests derivation from diagenetic transformation of clay-rich sequences, and delta S-18 of barite follows the values of the ocean water with geological time. The described Features represent common and persistent geological and compositional parameters of the SCT model over large areas, that point to an important stratigraphical marker for some sedimentary Sequences of different ages, as well as to a worldwide significant geological and metallogenic event.