Sulfur and metal fluxes in the oceanic crust: the Samail ophiolite as proxy for fast spreading ridges.

On-going work on the Samail Ophiolite Volcanogenic Massive Sulfide (VMS) deposits and Oman Drilling Project (OmanDP) drill cores provide insights on sulfur and metal cycling during hydrothermal alteration and critical differences between ophiolitic and modern oceanic crust.

The volcanic section is pervasively overprinted by low-T oceanic metasomatism leading to variably depleted sulfur in sulfide (TS_sulf) concentrations reflecting leaching and oxidation of magmatic sulfides. Secondary sulfides incorporated mostly basaltic sulfur with minor sulfur addition via open-system bacterial sulfate reduction (BSR).

The sheeted dyke-gabbro transition (OmanDP GT3 drillhole) records a change from BSR open-system processes (d³⁴S>-12.8‰) towards addition of heavy hydrothermal sulfur via thermochemical sulfate reduction ~4km above the Moho Transition Zone-MTZ. Downward progression from d³⁴S=+13.6‰ to ~MORB values suggest decreasing water/rock ratios during hydrothermal alteration. Here, near complete recrystallization under greenschist/amphibolitic facies conditions (no magmatic sulfides), coupled with strong sulfur (TS_sulf>2 ppm) and copper leaching (>1 ppm), document the high-T reaction zone of the hydrothermal system overlying the axial melt lens, where S and metals are sourced to form VMS deposits. Although multiple sulfur isotope systematics for Oman VMS ores indicates a deep S-source within the range of GT3 reaction zone (d³⁴S ~4‰), REE patterns and trace metal endowments in the ores suggest that the footwall lavas are also a source of metals, in addition to those leached from the deep reaction zone. Crucially, metal leaching and S-isotopic shifts are far more extensive than those reported on in-situ oceanic crust, implying a net addition of seawater-S ~30% to the upper crustal section.

Differences between in-situ and ophiolitic lower crustal sections are seemingly less pronounced: the foliated and layered gabbros (GT2-GT1 drillholes) preserve small S-isotopic shifts relative to MORB, implying that formation of secondary sulfides involved minor S-seawater input (~7%) and mostly redistribution of magmatic-S. Wide fault zones of convincing oceanic origin preserve sulfates with composition similar to Cretaceous seawater (d³⁴S~+18‰) supporting the role of focused fluid flow corridors during deep crustal cooling. TS_sulf and Cu+Ni concentrations increase throughout the lower crust while strong Cu+S leaching characterize tectonized and low-T hydrothermally overprinted domains. Above the MTZ, the primitive layered gabbros and intercalated ultramafics (CM1 drillhole- Sequence SI) record metal and TS_sulf enrichments related with magmatic sulfide saturation/segregation from mantle melts upon entering the crust. Incompatible element rich pegmatoidal dikelets crosscutting SI include late, high-fS₂ sulfides formed during low-T BSR (δ³⁴S>-25.8‰).

The MTZ comprises 90m of fully serpentinised dunite (SII) underlain by dunite with rodingitized gabbro (SIII). The SII-dunites show vanishing TS_sulf and Cu concentrations, consistent with desulfurization producing alloy-bearing mineral assemblages formed during extremely low fS₂-fO₂ conditions, typical of early serpentinization stages. The dunites mark the onset of increasing S-isotopic shifts towards the SIII-rodingites The occurrence of both sulfides (δ³⁴S₌+1.4, +56.9‰) and sulfates (δ³⁴S_SO4=+19.4, +36.5‰) with δ³⁴S>>Cretaceous seawater sulfate can be explained by input of fluids at the top of SII-dunites which composition progressed towards extreme heavy values during closed-system, multi-staged evolution.

AJ acknowledges WWU International Visiting Scholars and EU-H2020 Marie Sklodowska-Curie #894599 Fellowships, and FCT I.P./MCTES PIDDAC–UIDB/50019/2020- IDL.