Hydromagmatic PGE telluride-rich Ni-Fe-Cu sulfide mineralization related to melt-rock reaction processes in presence of carbonated hydrous fluids: examples from sulfide-rich ultramafic intrusions of the Ivrea-Verbano Zone

The Ivrea-Verbano Complex hosts several sulfide-bearing ultramafic intrusions which are substantially enriched in volatiles including carbon. In particular, the Valmaggia ultramafic pipe displays widespread intergrowths between coarse nodular Ni-Cu-PGE sulfide mineralization, carbonates and the magmatic silicate-oxide assemblage. The latter bears evidence of a melt-rock reaction process between a peridotitic crystal mush and a volatile-rich percolating melt similar to adakite (Sessa et al., 2017). The intrusion displays progressive reaction fronts where augite, olivine, enstatite and spinel I were replaced by pargasite amphibole oikocrysts, enstatite II, phlogopite and spinel II by reacting with plagioclase-rich hydrous metasomatizing melt at PT conditions of 5÷8 kbar and 700÷900°C. Both PGE-bearing sulfide mineralization and carbonates are strictly related to the metasomatic assemblage. Dolomite blebs are interstitial to silicates but most carbonates occur as substantial intergrowths with the coarse sulfide nodules and their peculiar aureoles of sulfide-silicate symplectites. Carbonates make up bubble-like meniscus aggregates between concave rims of sulfide nodules and nearby silicates. Carbonates (Fe-calcite, dolomite, siderite) vary in composition according to the nearby phase (silicate, sulfide). Ni-Fe-Cu sulfide nodules are often concentrically zoned, with FeS cores giving way to Fe-Ni-Cu sulfide- and magnetite-rich rims. Pt-Pd tellurides only occurs along the nodule rims as well as in the Fe-Ni sulfide blebs of the nearby sulfide-silicate symplectite haloes, especially where sulfides are intimately intergrown with carbonates and hydrous silicates. Textural relationships suggest metal-rich sulfide (± telluride) melt segregation ensuing from sulfur saturation during melt-rock reaction and coexisting with immiscible carbon-bearing vapour. In this context Pt- and Pd telluride deposition (or Pt-Pd remobilization) appears to be favoured as a late crystallization product of the fluid-drenched sulfide melt blobs in hydromagmatic conditions. The Valmaggia carbonates do not show carbonatite-like trace element composition, yet they display C and O isotope mantle-like signature, as is the case for the carbonates from the pyrossenite intrusion of Campello Monti (CM), which is considered for comparison. The CM intrusion is highly enriched in Fe-Ni sulfides but is low in hydrous silicates. Like at Valmaggia, the CM carbonates are intergrown with sulfides and silicates (including minor amphibole) and also enclosed in olivine, although the abundant Fe-Ni sulfides at CM are PGE-poor. This further suggests that a key factor for PGE mineralization might be the coupling of carbon and hydrous magmatic fluids.