Ni-Fe-Cu-PGE ore deposition driven by metasomatic fluids and melt-rock reactions in the deep crust: The ultramafic pipe of Valmaggia, Ivrea-Verbano, Italy

Abstract

The Valmaggia ultramafic pipe belongs to a group of Fe-Ni-Cu-PGE-rich, hydrous ultramafic intrusions emplaced within the Ivrea-Verbano Zone (Italy). According to the most recent geodynamic models these early Triassic ultramafic pipes represent a network of magma conduits derived from partial melting of continental lithospheric mantle metasomatized by fluids and melts related to the Eo-Varisican subduction. New extensive mapping and sampling of the Valmaggia pipe provides evidence of the close relationship between metasomatism and Fe-Ni-Cu-PGE mineralization. New geological data allow a 3D geometrical model of the pipe that intrudes the Main Gabbro domain. The pipe has a sub-vertical, tube-like main body with at least three lateral branches. Along its rims the pipe displays magmatic, soft contacts with an almost continuous meter-sized envelope of a peculiar marginal rock facies (“plagioclasite”) consisting of Na-bearing plagioclase, biotite and apatite and, locally, with pegmatoidal texture. The peridotite pipe consists of a primary anhydrous mineral assemblage comprising coarse-grained olivine, diopside, enstatite and Cr-rich spinel. These minerals are extensively overprinted by a metasomatic assemblage comprising Ti-rich pargasitic amphibole and phlogopite oikocrysts, enstatite, Al-rich spinel and PGE-bearing Fe-Ni-Cu sulfides, with accessory carbonates, Cl-F-rich apatite, zircon and monazite. Sulfide mineralization is preferentially located along pipe margins, and, subordinately, in the nearby plagioclasite contact rock. Sulfides are strictly associated with the metasomatic assemblage and occur as disseminated mm-sized droplets, network-like aggregates and cm-sized amoeboidal nodules rimmed by carbonates. Several features suggest that plagioclasite interacted with the ultramafic pipe, and infiltrated it, as a magma. Plagioclasite in contact with the pipe rock frequently displays a mottled texture, similar to that of gabbro, and where dark nodules, with ultramafic cores and pargasite- spinel-phlogopite-bearing symplectite coronas, are densely disseminated in white plagioclasite-rich groundmass. Metasomatic assemblages and symplectite textures, suggesting disequilibrium between pipe minerals and plagioclasite components, are widely developed also along the pipe margins and interstitial plagioclase pockets within the pipe. The metasomatic minerals provide calculated PT conditions ranging between 680° and 870°C and between 4 and 8kbar. Such conditions are compatible with the latest geodynamic model for the Ivrea Zone pipes proposing their emplacement in the lower crust. These PT constraints also likely apply to the conditions of ore deposition because of the strict relationships between the metasomatic assemblage and the sulfides. Carbonate crystals that are associated with sulfide-rich portions of the pipe are likely the product of CO2 vapor sticking to the sulfide melt surface resulting in their net buoyancy and ascent in the melt. The textural and chemical characteristics of the pipe mineral assemblage and sulfide mineralization suggest a polyphasic evolution dominated by metasomatism during interaction between magmas. At Valmaggia the alkali- and volatile-rich, metasomatizing magma may be analogous to slab-derived melts envisaged in the most recent geodynamic model for IVZ. Percolation of a metasomatic melt favours Valmaggia pipe-like geometries, including a tubular main body with lateral appendages.