Fluid-rock interaction, skarn genesis, and hydrothermal alteration within an upper crustal fault zone (Island of Elba, Italy)

The Terranera magnetite-hematite-pyrite deposit of the Island of Elba (Italy) is an historical skarn deposit hosted by a fault zone of regional importance (Zuccale Fault) and by its hanging wall rocks. We combine field observations with petrographic data, electron probe microanalyses (EPMA), XRPD data, fluid inclusion microthermometry, and element imaging by Laser Ablation-Inductively Coupled Plasma-Time of Flight Mass Spectrometry (LA-ICP-TOFMS) to define the ore-forming process at Terranera. We show that in this location the fault is made of four levels of mineralized fault rocks having distinct mineral compositions. In these levels, a mineral association made of diopside, clinozoisite, and other Mg-rich minerals is replaced by magnetite, hematite, pyrite, Mg-hornblende, clinochlore, and other Mg-rich phyllosilicates. This paragenesis is overprinted by goethite and clay minerals. Chlorite-quartz geothermometry and fluid inclusion microthermometry show that ore precipitation occurred at 350–180 °C from fluids of distinct bulk salinities, but goethite and clay mineral overprinting progressed at lower T. We propose that Terranera is a magnesian Fe skarn formed due to the interaction between distinct hydrothermal fluids and a dolomitic protolith, which was preserved within the fault zone. These fluids mixed and cooled during protolith metasomatism, causing ore precipitation due to oxidation and desulfidation. A very similar process was described in a large deposit of Elba (Rio Marina). Argillic alteration was widespread within the fault but met permanently intermediate sulfidation conditions. Trace element composition of hematite shows that Terranera has features that overlap those of skarn and epithermal deposits. In particular, elements that are typical of epithermal deposits (Sb, Ga, Ge, As) occur at mass fractions (50–200 μg/g) that are either unreported or not typical of hematite from skarn deposits. These features identify Terranera as formed in an ore environment that was transitional between that of a skarn and of an epithermal deposit. These features are shared by other historical deposits located at Elba and in the massive pyritic ore district of south Tuscany (e.g., Gavorrano, Fenice Capanne). This indicates that a similar environment might have occurred during the Neogene beyond Elba, in a much larger ore district of south Tuscany.