Sulfur Isotope Composition of Kuvaevite (Ir5Ni10S16) and Tolovkite (IrSbS): First Results

Abstract

The results of studying the sulfur isotope system in platinum-group minerals (PGM) are rare and generally limited to S-isotope data for Ru-Os sulfides from dunite-harzburgite massifs. To partially fill this gap, we for the first time characterized features of the S-isotopic composition of kuvaevite (Ir₅Ni₁₀S₁₆) and tolovkite (IrSbS) from the Verkh-Neivinsk dunite-harzburgite massif, a typical representative of the ophiolitic association at the Middle Urals. The study employed a number of analytical techniques, including scanning electron microscopy, electron microprobe analysis and a femtosecond laser ablation with a gas source isotope ratio mass spectrometry. The primary PGM assemblage is formed by osmium and iridium minerals, laurite, kuvaevite and Pt–Fe alloys, which are replaced by As-bearing laurite, irarsite, tolovkite and other PGM of secondary origin. Kuvaevite is characterized by a predominance of Ni over Fe, Cu, and Co (Ni/(Ni + Fe + Cu + Co from 0.56 to 0.58), as well as Ir over other platinum-group elements (PGE) (i.e., Ir/(Ir + Rh + Os + Ru + Pt + Pd) = 1.00); tolovkite is characterized by trace amounts of Pt (0.51–2.86 wt %), Rh (0.58–1.36 wt %), Ru (0.31–1.47 wt %), Ni (0.34–0.74 wt %), Cu (0.06–1.10 wt %), and As (0.06–1.44 wt %). Particularities of the isotopic composition of sulfur in kuvaevite (δ³⁴S from 0.9 to 2.1‰, mean δ³⁴S 1.5 ± 0.5‰, n = 4) are indicative of the mantle source with a chondritic isotope composition. The heavy sulfur isotope composition of tolovkite (δ³⁴S from 5.0 to 7.8‰; mean δ³⁴S 5.9 ± 0.9‰, n = 8) indicates the participation of sulfur of crustal origin (for example, isotopically heavy sulfur derived from host sedimentary rocks), being consistent with the secondary origin of the tolovkite. New data support the conclusion about contrasting sources of sulfur and a multistage evolution of PGE mineralization.