HYDROUS MANTLE METASOMATISM AND THE FORMATION OF DEEP LITHOSPHERIC MAGMATIC SULFIDE DEPOSITS

Mantle-derived hydrous fluids are considered to be key ingredients in the formation of magmatic sulfide systems in the lower continental crust. However, the physical and chemical processes through which deep lithospheric metalliferous fluids are generated and mobilized remain poorly constrained.

Here we discuss results of an ongoing study that investigates the role of hydrous fluids in the formation of magmatic sulfide deposits in the lower continental crust in the Ivrea-Verbano Zone (IVZ), Italy. The IVZ is an exposed cross section of the subcontinental lithospheric mantle and lower continental crust that was subvertically uplifted during the Alpine Orogeny. The IVZ can provide unique insight into the role of hydrous fluids in deep lithospheric ore genesis as it allows for a field-based comparison of strongly metasomatized rocks that exhibit different degrees of magmatic Fe-Ni-Cu-(PGE) sulfide mineralization; i.e., a series of 5 strongly sulfide-mineralized lower crustal ultramafic pipes, and the sulfide-poor Finero mantle peridotite.

Our data show that the formation of the pipes is related to dehydration of a subducting slab, which fertilized a depleted mantle. Post orogenic collapse caused partial melting of the fertilized mantle, which created a pipe-like network that (episodically?) connected the upper mantle with the lower crust between (at least?) 278-249 Ma. The harzburgitic pipes were infiltrated by a late-stage fluid that formed secondary phlogopite and amphibole. Geochemical modelling shows that the magma parental to the pipes was metal-poor, thus implying that the late-stage fluid was metalliferous and upgraded the metal contents of the sulfides.

The depleted Finero peridotite underwent significant metasomatism that, similar to the pipes, resulted in the formation of abundant phlogopite and amphibole. Trace element and Cu-isotope data of phlogopite from Finero and the pipes indicate that both systems were metasomatized by the same, or at least a very similar, hydrous fluid. However, whereas the pipes are strongly mineralized, the Finero peridotite contains only minor sulfides. This observation implies that fluid pathways (focused flux vs. more wide-spread/unfocused metasomatism) represent an important control on the formation of deep lithospheric magmatic sulfide ore deposits.