Lithosphere | 2019
Microstructural constraints on magma emplacement and sulfide transport mechanisms
Abstract
The poorly constrained nature of the physical transfer of sulfides along magmatic conduits \nhas implications for the genesis and localization of mineral deposits as well as for understanding the \nlarge-scale mobility of volatiles and metals across different geochemical reservoirs. Our natural \nlaboratory to address this topic is a sulfide-bearing ultramafic pipe emplaced in the deep crust of the \nIvrea-Verbano Zone, northwest Italy. The pipe comprises a volatile-rich peridotite, that contains \ndisseminated, blebby and semi-massive sulfides enriched in nickel, copper and platinum-group \nelements. The integration of electron backscatter diffraction orientation data and 3D X-ray computed \ntomography analyses from this study indicate that 1) most of olivine crystallized upon emplacement \nof the magma; and 2) the shape and texture of the intra-granular sulfide blebs principally hosted \nwithin the central portions of the pipe reflect early sulfide saturation and 3) that marginal areas \n1recorded higher strain compare to the cores of the pipes. The differences in the size distribution of \nthe sulfide grains between the central and marginal areas of the pipe are due to magma emplacement \ndynamics. The larger sulfide aggregates forming the bulk of the Ni-Cu-PGE sulfide mineralization \nalong the margins of the pipe are interpreted to form by coalescence of a large number of smaller \nsulfide droplets. The observed sulfide size distribution between the central and marginal areas of the \nValmaggia pipe is principally due to the dynamics of the magma upon emplacement, and locally \nrecord the role of water- and carbon dioxide-bearing volatiles in the physical entrainment of dense \nsulfide liquids. These processes provide a viable mechanism to transport sulfides enriched in \nchalcophile and siderophile metals from the upper mantle into the lower continental crust, where \nthey may be available for later remobilization into ore systems that may form subsequently in the \nmiddle and upper crust.