THE ROLE OF CARBONATED HYDROUS FLUIDS IN THE TRANSPORT OF METALS IN THE DEEP EARTH

Despite the importance attributed to fluids in the transport of metals in the upper mantle and lower crust, we still lack a robust understanding of the composition of such fluids and their capacity to transport metals. Here we present results of a multi-disciplinary study that investigates fluid-controlled mass transfer between different levels of the lithosphere as recorded by rocks of the Ivrea Verbano Zone (IVZ), Italy, an exhumed cross section of the lower continental crust and upper lithospheric mantle. The IVZ shows widespread evidence for metal and S transport by fluids, most notably in the form of 5 sulfide mineralized mafic-ultramafic pipes that intruded the IVZ 278-249 Ma ago. The harzburgitic pipes were infiltrated by a late-stage highly volatile-rich melt and/or fluid that reacted with olivine and orthopyroxene to form a secondary hydrous mineral assemblage (amphibole, mica) with locally abundant carbonates. Matrix and nodular sulfide ore (pyrrhotite, chalcopyrite, pentlandite) occurs along the margin of the pipes with metal contents of up to 12 wt% Cu, 11 wt% Ni and S isotopic compositions consistent with a mantle source.

Geochemical and geodynamic modelling show that the hydrous fluid was derived from a subducting slab, which fertilized a dry and depleted mantle. This created volatile- and metal-rich pods that were stored beneath thickened crust for tens of millions of years. Post orogenic collapse allowed for partial melting of the pods, which created an upward flux of heat, volatiles, metals and S within a pipe-like network. Geochemical data show that the parental magma of the pipes was metal-poor, implying that the high metal contents are directly related to the late-stage hydrous fluid. However, experiments that mimic the pipes’ source region (1-2 GPa, 1050-1220°C) show that the capacity of H₂O fluids to transport metals in the lower crust is low relative to the parental melt (Kd <0.01). To explain the high metal contents of the pipes, it is suggested that the carbonate-rich composition of the fluid allowed for a significantly increased metal transport capacity. Although the IVZ does not contain giant deposits, our data show that metasomatism of the lower crust can seed the ground for the genesis of later mineral systems, thus making geodynamic settings similar to the IVZ important future exploration targets.