New constraints on the origin of metal enrichment at the crust-mantle boundary from the Ivrea-Verbano Zone, NW Italy

Copper deposits or sulfide enrichment have been found along the crust-mantle transition zones in ophiolites and along the oceanic Moho. However, scarcity of suitable exposures limits our knowledge on the migration of chalcophile metals across the subcontinental crust-mantle boundary. This study aims to provide new constraints on the migration of sulfide-associated chalcophile metals at the transition between the subcontinental mantle peridotites of the Balmuccia massif and lower crustal gabbronorites of the Mafic Complex (Ivrea-Verbano Zone, NW Italy).

An ~80-m-thick zone composed of interlayered pyroxenites and gabbronorites (Contact Series; CS) showing igneous contact with the mantle peridotites was sampled along the Val Sesia river, near the Isola village. We investigated a transect from the mantle peridotites (rich in pentlandite) through the CS to the lower crustal gabbronorites (rich in pyrrhotite or pyrite). The CS zone comprises three sampling sites located 0–5 m (CS1), 65–70 m, and 75–80 m from the mantle peridotites and is characterized by the along-transect Mg# variations (Mg# of 71–57). The mantle peridotites are sulfide poor (average of 0.12 vol.‰), in contrast to the CS rocks (up to 7.8 vol.‰). The enhanced sulfide abundances in mafic rocks of the CS correlate with higher S, Cu, Ag, and Cd contents. This sulfide- and chalcophile-rich metal zone within the CS ends ~75 m away from the margin of mantle peridotites implying a probable thickness of the enrichment zone. Sulfides from mantle peridotites and CS1 are pyrrhotite-(troilite)-chalcopyrite-(cubanite)-pentlandite assemblages of magmatic origin, which is supported by δ³⁴S ranging from –0.6‰ to +1.8‰ (average of 0.0‰; cf., Oeser et al., 2012 – Chemical Geology).

The in-situ Fe isotope signatures of polyphasic sulfide grains from CS1 show a strong fractionation between the various phases. The δ⁵⁶Fe values of pyrrhotites are negative ranging from –0.8‰ to 0.0‰, whereas chalcopyrite exhibit positive values of 1.3–1.7‰. The mass balance calculations of the δ⁵⁶Fe for the bulk composition of the sulfide grains from CS1 show unfractionated (magmatic or mantle) values of 0.0 ± 0.2‰ (cf., Craddock et al., 2013 – EPSL).

The stagnant melts at the crust-mantle boundary extensively react with the mantle yielding enrichment in sulfides and chalcophile elements, which is known to yield enrichment in sulfides (Ciazela et al., 2018 - GCA; Patkó et al., 2021 - Lithos). However, the contact between the Balmuccia mantle peridotites and the lower continental crust of the Mafic Complex is highly heterogeneous with alternating layers of pyroxenites and gabbronorites. These layers may have formed from distinct magma batches as suggested by the along-transect Mg# variations. Therefore, the mechanism of observed enrichment in sulfides and chalcophile elements probably involves several stages of melt-peridotite and melt-pyroxenite reactions. These might explain the exceptionally large ~75-m-thick sulfide-rich horizon observed at the CS. Our results indicate that substantial chalcophile metal inventory is trapped at the CS. Assuming they behave the same at the Moho level, this would explain the relative deficit of these elements in the continental crust when compared its bulk composition to the composition of primitive mantle melts.

This research was funded by the NCN Poland (2018/31/N/ST10/02146)