Sulfur isotope fractionation in metasedimentary rocks during prograde metamorphism

Sulfur is closely associated with various types of ore deposits, particularly orogenic gold (Au) systems, where sulfur-bearing melts and fluids play a critical role in transporting ore-forming elements essential for ore formation. The widely accepted metamorphic devolatilization model suggests that compositionally fertile sedimentary rocks serve as potential gold sources. Therefore, understanding sulfur behavior during prograde metamorphism is essential for elucidating the mechanisms underlying metal activation and mobility. In this study, we conducted in situ sulfur isotope (δ³⁴S) analyses using secondary ion mass spectrometry (SIMS) on samples from the Hongshankou area, a representative Barrovian-type metamorphic sequence characterized by intermediate pressure-temperature (P-T) conditions. This sequence comprises the biotite, garnet, staurolite, and kyanite zones. Our results show a systematic increase in δ³⁴S values (from 3.1‰ to 5.5‰) coupled with a progressive decrease in total sulfur content (from 320 ppm to 165 ppm) as metamorphic grade increases. The most pronounced sulfur mobilization occurs between the garnet and staurolite zones. In all analyzed samples, Au or Au-bearing minerals predominantly occur along the edges or within pyrite grains, highlighting the critical role of pyrite breakdown in controlling gold mobility. Thus, sulfur isotope fractionation provides robust constraints for quantitatively assessing sulfur mobility during metamorphism. These findings reinforce the concept that metasedimentary rocks and their metamorphic fluids represent fertile sources of Au and other metals enriched in orogenic gold deposits.