Sulfur cycling in the gabbroic section of the Oman ophiolite

Abstract

We present sulfur mineralogy and isotope geochemistry from the gabbro transect of the Oman Drilling Project to unravel the sulfur cycle during hydrothermal alteration of the plutonic oceanic crust. The sheeted dike–gabbro transition (Hole GT3A) shows low sulfide‑sulfur concentrations (GT3A_median = 178 ppm, σ = 4873 ppm) but with great sulfur isotope variability (δ³⁴S = −12.8 to 14.4 ‰ V-CDT, weighted average + 5.8 ‰) and unusually heavy compositions relative to in-situ or ophiolitic crust. These features are consistent with abiogenic thermochemical sulfate reduction during intense hydrothermal alteration under greenschist facies conditions which formed a low-variance and relatively high-fS₂ assemblage of pyrite ± chalcopyrite ± bornite. The heaviest isotope compositions (+10 to +14 ‰) occur within 10 m of the uppermost gabbro screen suggesting focused fluid-rock exchange with isotope enrichment relative to seawater due to closed-system reservoir effects. The change in isotope compositions from +5 to 0 ‰ in the overlying sheeted dike reflect fluids gradually buffered by magmatic sulfur to signatures similar to the Oman Volcanogenic Massive Sulfide deposits. Hole GT3A represents a deep hydrothermal reaction zone with extensive S and base metal losses and incorporation of up to ∼80 % seawater-derived sulfate. The amount of Cu and Zn released in a 1 km³ crustal section similar to Hole GT3A is ∼3 times greater than the average contents of Omani VMS deposits.

The mid to lower crustal section (Holes GT2A and GT1A) mostly preserves MORB sulfur isotope compositions but highly variable sulfide‑sulfur contents (GT2A_median = 454, σ = 693 ppm, GT1A_median = 114, σ = 277 ppm). Away from fault zones, silicate microvein networks enabled variable sulfide and metal remobilization. Magmatic sulfides persist as remobilized remnants along with sulfidation reactions and mild isotopic enrichments (<+2.7 ‰) in secondary sulfides (millerite + siegenite-polydimite_ss + pyrite). The mid-lower crustal section experienced redistribution of magmatic sulfur mixed with minor inputs of seawater-derived sulfur (<10 %), under very low fluid/rock ratios and moderate sulfur fugacities, that chiefly preserved base metal abundances in secondary sulfides. The many faulted intervals present in Holes GT1A and GT2A record near complete sulfur and metal leaching of magmatic sulfides without the deposition of secondary sulfides, but preserve sulfate with a Cretaceous seawater sulfate‑sulfur isotope signature (+16.1 to +17.3 ‰). These structures are the expression of crustal scale channeled hydrothermal recharge fluid flow and record a previously unaccounted sulfur budget introduced in the deep crust.