Underplated melts control sulfide segregation at the continental crust-mantle transition

Sulfides play an important role in lithosphere metallogeny, hosting critical metals for sustainable development. Investigation of the crust-mantle transition in the Ivrea-Verbano Zone revealed sulfide enrichment attributed to crustal cumulates, addressing the paradox of the Cu missing reservoir.
Published in Earth & Environment
Underplated melts control sulfide segregation at the continental crust-mantle transition
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The majority of economically significant ore deposits are formed by sulfides. Hence, an enhanced understanding of sulfide-controlled metal segregation allows accurate metal prospecting in the future. Whereas most Cu deposits are attributed to porphyry deposits located in the upper crust, the primary source of the metal has to be situated deeper in the lithosphere. Copper from the deep repositories may be remobilized and transferred upwards through the lithosphere, ultimately forming ores. As the mantle-derived melts beneath the oceanic and continental crust typically have high Cu contents (100–120 ppm), the reduced Cu concentrations in the rocks of lower continental crust (on average 26 ppm) suggest a crucial role of magmatic differentiation at the crust-mantle boundary in sulfide segregation. However, previous studies of sulfide distribution at the Earth’s crust-mantle transition zones were limited to small-scale or altered samples, thereby implying that the role of the crust-mantle transition for metal storage is fundamentally unknown.

One of the most complete examples of the continental lower crustal section exhumed at Earth's surface is represented by the Ivrea-Verbano Zone (IVZ) from the Italian Alps. The southwestern domain of the IVZ includes the mantle-derived Balmuccia peridotitic massif, whose external margin is highlighted by a magmatic contact (called Contact Series) with the Permian lower crustal gabbronorites of the Mafic Complex. The Contact Series is up to 150-m-thick and mostly consists of an exceptionally fresh pyroxenite-gabbronorite sequence. Although the Balmuccia mantle massif was extensively studied, its external contact was almost untouched by scientists, with the few studies dealing with the Contact Series conducted more than 40 years ago. Nevertheless, the origin of this contact was debated through decades, with the latest studies of the IVZ suggesting that it represents a fossil, continental crust-mantle transition zone. With this interpretation, the Contact Series is an exceptional large-scale field laboratory for studying processes, including metallogeny, occurring at the continental crust-mantle boundary. Therefore, we sampled a >100 m-long transect across the Contact Series exposures along the Sesia River, to collect rocks representing the entire crust-mantle transect, starting from the Balmuccia mantle massif through the Contact Series and the Mafic Complex gabbronorites. The collected set of samples allowed us to address the role of cumulates underplated at the crust-mantle boundary in the metallogeny of sulfides and to provide new insights into the evolution of the crust-mantle transition.

(a) Simplified geological map of the Ivrea-Verbano Zone. The insert displays the close-up of the geological map of the Balmuccia mantle massif and the adjacent Mafic Complex. (b) Geological map displaying the external margin of the Balmuccia massif with sampling sites corresponding to the lithological profile. (c) Field image of the Contact Series site proximal to Balmuccia massif, displaying the primary contact between mantle peridotites and Contact Series rocks.

Geological setting of the Ivrea-Verbano Zone (IVZ) and the studied eastern margin of the Balmuccia mantle massif. (a) Simplified geological map of the Ivrea-Verbano Zone. The insert displays the close-up of the geological map of the Balmuccia mantle massif and the adjacent Mafic Complex. (b) Geological map displaying the external margin of the Balmuccia massif with sampling sites corresponding to the lithological profile. (c) Field image of the Contact Series site proximal to Balmuccia massif, displaying the primary contact between mantle peridotites and Contact Series rocks.

The Contact Series rocks sampled up to 80 meters away from the magmatic contact with the Balmuccia mantle massif are anomalously enriched in Cu (up to 380 ppm) compared to both mantle peridotites (~19 ppm) and crustal gabbronorites (~1 ppm). The Cu enrichment was stated for two Contact Series sampling sites differing from each other by distance from the mantle peridotites, sulfide assemblage, and S isotope composition. The Contact Series section proximal to the mantle (CS1; 0–5 m from the mantle) comprises pyrrhotite-chalcopyrite-pentlandite (Fe, Cu, and Ni+Co sulfides, respectively) assemblages and S isotopes of typical magmatic origin. In the section located ~75 m away from the contact with mantle rocks (CS2), the primary sulfides are partially or completely replaced by pyrite (Fe sulfide), documenting the occurrence of secondary processes that re-homogenized the primary S isotope signature. Nevertheless, the in-situ trace element fingerprint of sulfides provided evidence that pyrite inherited the metal contents from its magmatic precursors. The third sampled section (CS3; ~80 m away from the contact), the most distant from mantle peridotites, is depleted in sulfides and associated metals due to alteration, as evidenced by the presence of hydroxides on the rims on sulfides. Upon such a complete alteration, the base and precious metals are easily remobilized, making the interpretation of primary processes difficult. Therefore, these observations demonstrate that at least 80 meter-thick zone of Contact Series was originally enriched in sulfides due to the sulfide segregation during magmatic processes. Additionally, the whole-rock data (S/Se ratio) and trace element compositions of pentlandite document that the assimilation of continental crust material is a critical mechanism driving sulfide segregation and sulfide-controlled metal storage.

(a-b) Examples of sulfide assemblages described in the Contact Series rocks and their corresponding modes (c) across the studied transect. (d) Copper, Ag, and S contents, (e) sulfide abundances together with bulk rock S/Se ratios, and (f) S isotope signatures across the crust-mantle transition zone. Note the correlation between the higher S/Se ratios and higher sulfides and associated metal contents.

Summary of observations and geochemical results provided within the scope of this study. (a-b) Examples of sulfide assemblages described in the Contact Series rocks and their corresponding modes (c) across the studied transect. (d) Copper, Ag, and S contents, (e) sulfide abundances together with bulk rock S/Se ratios, and (f) S isotope signatures across the crust-mantle transition zone. Note the correlation between the higher S/Se ratios and higher sulfides and associated metal contents.

Our study documents sulfide enrichment attributed to the crustal cumulates emplaced at the base of the IVZ lower crust, indicating a critical role of mantle-derived melts and their interaction with crustal rocks in the upper lithospheric metallogeny. Although sulfides in the outer domain of the Contact Series sequence were pyritized, they constitute an important Cu repository for the lower continental crust. Nevertheless, such Cu enrichment does not fully explain the discrepancy between the estimated Cu content in the bulk continental crust and in the mantle-derived melts. Hence, we suggest that a fraction of the missing Cu content may be also distributed within sulfide-rich ultramafic magmatic rocks dispersed through the Mafic Complex.

Simplified model to track sulfide saturation during magma differentiation and later sulfide remobilization. The extent of crustal contamination likely controlled the sulfide precipitation resulting in the enrichment of sulfur-loving metals. The inset presents a simplified reconstruction of the processes responsible for the sulfide differentiation along the subcontinental crust-mantle transition zone. See the published paper for details.

We conclude that IVZ constitutes an ideal exposure for defining the substantial contribution of mafic-ultramafic cumulates in the Cu metal budget of the lower continental crust, which was previously overlooked and not accounted for global calculations and still needs to be addressed to resolve the missing Cu paradox.

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Petrology
Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Petrology
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Physical Sciences > Earth and Environmental Sciences > Earth Sciences > Geochemistry
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