Fractionation of trace elements and sulfur isotopes during fluid evolution in the Darasun goldfield

Abstract The Darasun goldfield in the Eastern Transbaikalia comprises three gold deposits, Talatui, Teremkyn and Darasun, considered to represent progressively periphery parts of an intrusion-related hydrothermal system with porphyry-style features. The trace element and sulfur isotope (δ 34 S) compositions of pyrite, chalcopyrite, arsenopyrite, galena, sulfosalts, and sphalerite were investigated using electron probe microanalysis (EPMA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and both in situ and bulk sulfur isotope analyses. Cooling and fractionation away from the granitoid intrusion are expressed by systematic decrease in Co, W, and Co/Ni and increase in Sb, Sn, and Au in sulfides. The concentrations of mineral-controlled elements in pyrite show systematic but non-monotonic variations from close-to-source Talatui to outlying Darasun deposit. Copper, Zn, In, and Ag contents in pyrite increase towards the periphery, whereas Ga, Se, Cd, and Te decrease, with pronounced depletion at the Teremkyn, reflecting change in sulfide assemblages and fluid evolution. At the Darasun deposit, three pyrite generations are identified. Pyrite 1 preserves an early, high-temperature signatures comparable to that of close-to-source Talatui. Pyrite 2 represents the main sulfide-forming stage and contains abundant sphalerite and sulfosalts inclusions, together with the highest As/Co ratio, and contents of Au and As. Fine-grained Py3 is enriched in Zn, In, Pb, Bi, Ag, Sn, and Tl, with Zn and In exceeding those of coexisting chalcopyrite, consistent with late reworking of earlier Py2. Based on our new and published data, δ 34 S values of sulfides from all three deposits are near-magmatic and become lighter from Talatui (median 0‰) to Teremkyn (–1.1‰), and Darasun (–1.0‰), which is consistent with cooling and increasing isotope fractionation from homogeneous magmatic fluid. The Darasun deposit sulfides show a bimodal distribution with modes at − 1.5‰ and + 0.8‰. The dominant negative population formed from a reduced magmatic-hydrothermal fluid with δ 34 S ∑S ≈ 0‰, whereas the subordinate positive population is confined to veins close to anhydrite-bearing domains, requiring localized interaction between the reduced fluid and an isotopically heavy sulfate reservoir. These results show that preexisting sulfates can supply heavy sulfur to later sulfides, producing local 34 S-enrched signatures.