Magnetite from the Cogne serpentinites (Piemonte ophiolite nappe, Italy). Insights into seafloor fluid–rock interaction

Abstract: In the Cogne area (Aosta Valley, Western Italian Alps), magnetite mineralization occurs associated with different types of serpentinite (pseudomorphic, rodingitic and magnetite-rich serpentinites), which are also heterogeneous in terms of bulk chemical composition, mineral assemblage and mineral chemistry. The mineralization is hosted by a lizardite-, chrysotile-bearing serpentinite showing a pseudomorphic texture after oceanic peridotite. It contains fine-grained magnetite showing oscillatory zoning with Cr-poor cores ( 0.4 wt.% Cr2O3), which are surrounded by Cr-rich inner euhedral rims (up to 17.0 wt.% Cr2O3), mantled in turn by partially resorbed Cr-bearing outer rims (2.0 wt.% Cr2O3). This dramatic variation of Cr content in magnetite is described by the exchange vector Fe3þ Fe2þ Cr3þ -1 (Mn, Zn, Ni) 2þ -1. The absence of Cr-rich spinel cores suggests that the source of chromium was the Cr-Tschermak component of the orthopyroxene, made available during the serpentinization-related breakdown. In these rocks, lizardite, the primary serpentine phase, is partially overgrown by prograde antigorite, indicating that serpentinization occurred under relatively low-T conditions in a preorogenic (i.e., oceanic) setting, and was followed by an Alpine metamorphic re-equilibration. Rodingitic serpentinite and serpentinite rich in magnetite are composed of, in highly variable amounts, diopside (locally with inclusions of partially disordered graphite), calcite, magnetite, andradite/hydroandradite, prograde olivine, brucite and serpentine (mostly antigorite but also relict lizardite), with minor chlorite and accessory apatite. Magnetite content can be very high. In these rocks, magnetite is exceptionally Cr-poor (almost Cr-free). Such almost Cr-free magnetite is characterized by significant Fe2þ$Mg substitution (2–15 mol.% MgFe2O4). Crystal-chemical investigations confirm that Mg, as in magnesioferrite, shows a strong preference for the octahedral site. Due to Fe2þ $ Mg substitution, the Fe2þ content available for Fe2þ – Fe3þ electron hopping in the octahedral site decreases leaving Fe3þ in excess and giving rise to a charge increase. Additionally, a small Fe3þ - Fe2þ disorder is present at the tetrahedral site (Fe2þT ¼ 0.07 apfu). The bulk-rock compositions and the mineral associations of rodingitic serpentinite and serpentinite rich in magnetite point to a strong metasomatism and iron mobilization sustained by CO2-bearing fluids, developed at the expense of the host serpentinite. These processes likely occurred on the Tethyian seafloor, mediated by fluids sourcing from hydrothermal vents. Thermodynamic modelling suggests that mineral assemblages observed in the Cogne area are consistent with a process of serpentinization coupled with Ca (Al) metasomatism at 300–360C. This might have been driven by carbon-saturated C-O-H fluids characterized by CO2 contents comparable to present-day seawater, capable of fixing the redox potential of rocks close to the fayalite–magnetite–quartz buffer. Nevertheless, magnetite and associated Fe-Ni sulphides suggest slight variations in fO2 (and fS2) conditions recorded, ranging from relatively reducing (magnetite–heazlewoodite–pentlandite pyrrhotite) towards rather oxidizing (magnetite–millerite–pyrite) assemblages.