Complex sulfur speciation in scapolite – Implications for the role of scapolite as a redox and fluid chemistry buffer in crustal fluids

Scapolite is a common S-, Cl-, and C-bearing mineral in metamorphic terranes, especially those that include meta-evaporites. Fluid interaction with scapolite-bearing rocks can result in albitisation of scapolite, and release of volatile components such as Cl⁻, CO₃^2–, SO₂ and/or SO₄²⁻. Hence, scapolite may play an important role in buffering the oxidation state, salinity and sulfur speciation of the rock and fluids in such terranes, and since Cl, S, and the redox state of S are vital for the transport and deposition of metals, scapolite may exert an important influence on hydrothermal mineralisation processes. We investigated the oxidation state of S and concentrations of S and Cl from 17 scapolite-bearing samples collected from various metamorphic and hydrothermal environments. The highest concentrations of S and Cl in the samples set are respectively 1.11 ± 0.04 wt% S (reported as SO₃, n = 10 points measurements) and 3.84 ± 0.17 wt% (n = 11 points measurements). µ-XANES spectroscopy demonstrates that S chemistry in scapolite is complex, S being present in both oxidised (as sulfate and sulfite) and reduced forms (polysulfides and minor sulfide) in most scapolites. These results confirm that scapolite can buffer fluid oxidation state to sulfate/sulfide coexistence, at acidic to neutral pH. The proportion of reduced S in scapolite increases with increasing metamorphic grade from greenschist to amphibolite facies. Since scapolite contains highly reactive polysulfide and sulfite, its breakdown may promote alteration of sulfide minerals and release of hosted metals. Thus, the presence of scapolitic calc-silicate rocks can strongly influence regional syn-orogenic metallogeny and should allow the mobilisation of metals differently from other bulk compositions.