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Late sulfide saturation in the Valle Fertil deep crustal section, Argentina: implications for chalcophile cycling in arcs

Chetan Nathwani 1, Emanuel Giovanini1, Cyril Chelle-Michou1, Paolo Sossi1, Julien Allaz1, Marine Cotte2,3, Olivier Bachmann1

  • Affiliations: 1Institute for Geochemistry and Petrology, ETH Zurich, Zurich, Switzerland; 2European Synchrotron and Radiation Facility, Grenoble, France; 3Sorbonne Université, CNRS, Laboratoire d'archeologie moleculaire et structurale, Paris, France 

  • Presentation type: Poster

  • Presentation time: Tuesday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 29

  • Programme No: 4.1.20

  • Theme 4 > Session 1

Abstract

The oxidation state of sulfur in arc magmas controls the transport of chalcophile metals through the crust to be vented to the atmosphere or form magmatic-hydrothermal ore deposits. It is posited that early saturation of sulfide in arc magmas generates metal-rich sulfides which depletes ascending magmas in chalcophile elements. Despite this, arc magmas are the primary source of porphyry Cu deposits, suggesting that sulfide saturation in arc magmas is not detrimental to their ore-forming capacity. However, the timing of sulfide saturation relative to deep arc magma differentiation remains poorly described.  We investigated the behaviour of sulfur in the Valle Fertil crustal section, Argentina where the deep to mid-crust of a palaeo-continental arc is exposed. With differentiation, cumulates transition from sulfide- and copper-poor to sulfide- and copper-rich suggesting that sulfides formed relatively late in the crystallization sequence. We tracked the valence state of sulfur across the deep crust using micro X-ray Absorption Near Edge Spectroscopy (μXANES) at the S K-edge in apatite, which can incorporate S2-, S4+ and S6+. We found that the deep crust was dominated by oxidised sulfur (S6+), regardless of melt differentiation or pressure, indicating fO2 exceeding one log unit above the fayalite-magnetite-quartz buffer. We present a sulfide saturation model, which indicates that fO2 ~FMQ+2 can delay sulfide saturation sufficiently to produce intermediate magmas which escape chalcophile depletion through sulfide saturation. Such oxidized and metal-rich magmas may play a key role in delivering metal fluxes to Earth's atmosphere and porphyry-type ore deposits, which remains to be fully tested.