THE ROLE OF HYDROUS FLUIDS IN THE FORMATION OF MAGMATIC SULFIDE DEPOSITS IN THE LOWER CONTINENTAL CRUST

Most known world-class magmatic sulfide deposits formed relatively close to the Earth’s surface. Therefore, exploration efforts have traditionally been focused on the upper crust. This approach is based on the assumption that assimilation of crustal S is necessary to the genesis of economic sulfide ore deposits. However, recent studies show that sulfide deposits in exhumed lower crustal settings are more frequent than previously thought. Although such deep settings might contain important resources, exploration efforts have been hampered by the lack of a solid understanding of ore forming processes in the deep Earth.

One key area that allows the investigation of ore genetic processes at deep crustal depths is the Ivrea-Verbano Zone (IVZ), Italy, an uplifted cross section of the lower continental crust and upper lithospheric mantle. Here, we present the results of a comprehensive study on magmatic Fe-Ni sulfide ore deposits in the IVZ that are hosted by variably metasomatized Devonian to Triassic pipes and sills. Structural relationships are integrated with geochemical and experimental models and the results are used to reflect on the role of hydrous fluids in the formation of sulfide ore deposits in the deep lithosphere.

The results show that early subduction in the IVZ fertilized a depleted mantle, creating volatile- and metal-rich pods in the deep crust. Post orogenic collapse caused melting of these pods and triggered the migration of metal-rich hydrous fluids. Deposits with the highest ore grades formed in volatile-rich pipes (~300 m across), where focused flux allowed for significant accumulations of sulfides. Sill-hosted deposits have lower ore grades as they lacked an efficient physical concentration mechanism. Although the IVZ does not contain giant deposits, we 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.