Metallogeny of subcontinental lithospheric mantle driven by sulfide-saturated pyroxenite-forming melts: evidence from the Balmuccia peridotite massif

Abstract

Focused melt flow is a common phenomenon in the subcontinental lithospheric mantle. Although it exerts significant control on the magmatic differentiation of the upper mantle, its role in metal transport remains poorly constrained. To improve our understanding of the subcontinental mantle metallogeny, we investigated the Balmuccia massif of the Ivrea-Verbano Zone (Italian Alps), which consists of fresh mantle peridotites that experienced a prolonged period of multistage melt intrusions. As a result, this massif hosts two suites of pyroxenite dykes, known as Cr-diopside and Al-augite pyroxenites, which enable us to provide undisturbed insights into mantle metallogeny. Here, through scrutiny of the pyroxenite dykes and their contacts with mantle peridotites, we provide insights into the sulfide and associated chalcophile metals (e.g., Cu and Ag) distributions. We demonstrate that the Balmuccia mantle pyroxenites are enriched in magmatic sulfides and sulfide-loving elements compared to the Balmuccia mantle peridotites. In particular, the pyroxenites contain up to 8 times more Cu (on average 227 ± 58 ppm; 1SD; n = 8) than the mantle peridotites (29 ± 20 ppm Cu; n = 20). Additionally, we found that each sulfide phase has distinct S-Fe isotopic signatures among sulfides. Such differentiation indicates that the S-Fe isotopic fractionation is most likely controlled by the mass-dependent fractionation that follows the (re)crystallization under high-T subsolidus magmatic conditions.

The increased amount of sulfides and associated chalcophile metals (Cu and Ag) within the studied pyroxenites evidence the heterogeneous distribution of sulfides and metals in the subcontinental lithospheric mantle, similarly to observations from other pyroxenite dykes within mantle rocks and metasomatized mantle xenoliths. Specifically, we estimate that from 12% to 42% of the Cu and from 11% to 40% of the Ag of the upper mantle inventory could be accumulated within mantle pyroxenites. Our results indicate that mantle pyroxenites constitute a critical metal reservoir for subcontinental lithospheric metallogeny.