Microstructural constraints on magma emplacement and sulfide transport mechanisms

Abstract

The poorly constrained nature of the physical transfer of sulfides along magmatic conduits has implications for the genesis and localization of mineral deposits as well as for understanding the large-scale mobility of volatiles and metals across different geochemical reservoirs. Our natural laboratory to address this topic is a sulfide-bearing ultramafic pipe emplaced in the deep crust of the Ivrea-Verbano Zone, northwest Italy. The pipe comprises a volatile-rich peridotite, which contains disseminated, blebby and semi-massive sulfides enriched in nickel, copper and platinum-group elements. The integration of electron backscatter diffraction orientation data and 3D X-ray computed tomography analyses from this study indicate that 1) olivine crystallized upon emplacement of the magma; and 2) the shape and texture of the intra-granular sulfide blebs principally hosted within the central portions of the pipe reflect early sulfide saturation. The differences in the size distribution of the sulfide grains between the central and marginal areas of the pipe are due to the dynamics of the magma upon emplacement. The larger sulfide aggregates forming the bulk of the Ni-Cu-PGE sulfide mineralization along the margins of the pipe are interpreted to form by coalescence of a large number of smaller sulfide droplets. These aggregates would be variably deformed upon emplacement due to drag friction during ascent of the silicate magma transporting its sulfide cargo. These processes provide a viable mechanism to transport sulfides enriched in chalcophile and siderophile metals from the upper mantle to the base of the continental crust, where they may be available for later remobilization into ore systems that may form subsequently in the middle and upper crust.