Origin of volcanic-hosted Mn-oxide mineralization from San Pietro Island (SW Sardinia, Italy): An integrated geochemical, mineralogical and isotopic study

The Mn-oxide mineralizations from Sardinia (Italy) represent examples of Mn deposits in a continental environment and, as such, their investigation can shed light on the genesis and characteristics of this deposit type, less studied than their more common oceanic counterpart. In this paper, the genetic mechanisms, the physical-chemical conditions and the origin of the mineralizing fluids are discussed for the volcanic-hosted Mn ores of the San Pietro Island (SW Sardinia), through an integrated geochemical, mineralogical and radiogenic isotope approach. New and published data are here examined on the basis of recent trace element discrimination schemes for Mn ores and a model for the genesis is proposed. The mineralogy of the alteration paragenesis (argillic facies) and the type of observed fluid inclusions (very small monophase liquid inclusions) suggest a circulation of mineralizing fluids characterized by weak acidic conditions (pH indicatively around 5) and relatively low temperature (< 100-120 °C). Geochemical features such as the contents of transition metals (Mn, Fe, Cu, Co, Ni, Zn), and contents and ratios of rare-earth elements and yttrium (REY), suggest that the San Pietro Island Mn metallogenesis is the result of combined hydrothermal and hydrogenetic mechanisms of formation. The Pb isotope compositions of different Mn veins exhibit a significant spread that suggests the contribution of distinct mineralizing fluids, deriving their isotopic signature from both seawater and water-rock interaction processes. The possible model of formation of the San Pietro Mn mineralization can be viewed as a complex process, in which Mn-bearing fluids originated from remobilization of Mn oxides sequestered in shelf and slope sediments and/or from leaching of the older Cenozoic volcanic rocks that form the basement of the island. Fluid rise was due to hydrothermal convection, possibly related to the general thermal anomaly active in the area from 15 to 12 Ma, during the final stage of the intense Cenozoic volcanic activity of the Sulcis area (San Pietro, Sant'Antioco, Isola del Toro volcanic activity). The fluids were probably dominated by seawater, characterized by relatively low temperatures and acidic conditions, due to the input of H2S of hydrothermal origin. The Mn ore deposition occurred in relatively shallow water and oxidizing environment, following the pH neutralization induced by water-rock interaction processes.