Sulfur isotope evolution in sulfide ores from Western Alps: Assessing the influence of subduction-related metamorphism

Abstract

Sulfides entering subduction zones can play an important role in the release of sulfur and metals to the mantle wedge and contribute to the formation of volcanic arc-associated ores. Fractionation of stable sulfur isotopes recorded by sulfides during metamorphism can provide evidence of fluid-rock interactions during metamorphism and give insights on sulfur mobilization. A detailed microtextural and geochemical study was performed on mineralized samples from two ocean floor-related sulfide deposits (Servette and Beth-Ghinivert) in high-pressure units of the Italian Western Alps, which underwent different metamorphic evolutions. The combination of microtextural investigations with d 34 S values from in situ ion probe analyses within individual pyrite and chalcopyrite grains allowed evaluation of the effectiveness of metamorphism in modifying the isotopic record and mobilizing sulfur and metals and have insights on fluid circulation within the slab. Textures and isotopic compositions inherited from the protolith are recorded at Beth-Ghinivert, where limited metamorphic recrystallization is attributed to limited interaction with metamorphic fluids. Isotopic modification by metamorphic processes occurred only at the submillimeter scale at Servette, where local interactions with infiltrating hydrothermal fluid are recorded by metamorphic grains. Notwithstanding the differences recorded by the two deposits, neither underwent intensive isotopic reequilibration or records evidence of intense fluid-rock interaction and S mobilization during metamorphism. Therefore, subducted sulfide deposits dominated by pyrite and chalcopyrite are unlikely to release significant quantities of sulfur to the mantle wedge and to arc magmatism sources at metamorphic grades below the lower eclogite facies.

Ocean floor and subduction record in the Zermatt‐Saas rodingites, Valtournanche, Western Alps

“…A similar hypothesis that includes different degrees of oceanic metasomatism in rodingites has been inferred for metaophiolite from the Betic Cordillera that underwent Alpine subduction (Puga et al ., ). In the Alps, oceanic rodingitization has also been inferred in the Mount Avic massif (Panseri et al ., ), and additional relicts of oceanic metasomatism that survived the Alpine subduction are represented by Mn‐bearing ore deposits at the Praborna mine (Tumiati et al ., ) and sulphide deposits at the Beth and Servette mines (Giacometti et al ., ).…”

Isotopic Compositions of Sulfides in Exhumed High‐Pressure Terranes: Implications for Sulfur Cycling in Subduction Zones

“…Sulfur isotope compositions >+10 ‰ for pyrite in garnet from Syros (sample SY403) are surprisingly high, but are consistent with the total range of δ 34 S values reported for AOC. Giacometti et al (2014) reported a~21 ‰ range (-6.7 ‰ to +13.9 ‰) for metabasite-hosted sulfides in two metamorphosed ocean floor-related sulfide deposits in the Italian Western Alps. The data of Giacometti et al (2014) largely cluster between 0 ‰ and +10 ‰, similar to sulfides in many modern mid-ocean ridge, back arc, and arc hydrothermal systems (e.g., Herzig et al, 1998;McDermott et al, 2015;Peters et al, 2010).…”

“…Giacometti et al (2014) reported a~21 ‰ range (-6.7 ‰ to +13.9 ‰) for metabasite-hosted sulfides in two metamorphosed ocean floor-related sulfide deposits in the Italian Western Alps. The data of Giacometti et al (2014) largely cluster between 0 ‰ and +10 ‰, similar to sulfides in many modern mid-ocean ridge, back arc, and arc hydrothermal systems (e.g., Herzig et al, 1998;McDermott et al, 2015;Peters et al, 2010). These comparisons suggest that the seafloor character of δ 34 S values of metamorphic sulfides in metabasic rocks are preserved throughout the metamorphic cycle.…”

“…Letters refer to each sample as described inTable 2. Data in open boxes fromBebout (1995),Crossley et al (2018),Evans et al (2014),Giacometti et al (2014), andShimizu et al (2013). Lower panel: Summary of sulfur isotope compositions of seafloor rocks.…”

Separate zones of sulfate and sulfide release from subducted mafic oceanic crust

“…Pyrite has been recorded in equilibrium with eclogite facies silicate assemblages in samples from New Caledonia , the Zermatt-Saas (Fee) Ophiolite Belt in Italy and Switzerland Giacometti et al, 2014). These sample localities record a range of metamorphic peak conditions from 600 • C, 1.9 GPa in New Caledonia to 600-630 • C, 2.7-2.9 GPa at Lago de Cignana (see references in Evans et al, 2014).…”