Selective crustal contamination and decoupling of lithophile and chalcophile element isotopes in sulfide-bearing mafic intrusions: An example from the Jingbulake Intrusion, Xinjiang, NW China

Abstract

The Jingbulake mafic–ultramafic intrusion in the South Tianshan orogenic belt, Xinjiang, NW China, is a zoned intrusive body composed of gabbro-diorite, olivine gabbro and wehrlite, locally intruded by pyroxenite, within a major W–E structure. Both olivine gabbro and wehrlite contain disseminated Ni–Cu sulfides, whereas pyroxenite hosts an ore body containing massive, net-textured and disseminated sulfide ores. Both silicate rocks and sulfide ores from the Jingbulake intrusion have low Pd/Ir ratios (10–63). The silicate rocks contain Ni-poor olivine (generally <0.1wt.% NiO) and have low Pt (0.48–2.3ppb) and Pd (0.52–4.8ppb) contents and high Cu/Pd ratios (128,000 to 200,000), indicative of significant chalcophile element depletion. The disseminated sulfide ores from No. 1 and 2 mineralization zones have 0.28–1.5ppb Pt and 2.8–8.1ppb Pd, and Cu/Pd ratios (200,000–300,000). The disseminated and net-textured sulfide ores from No. 8 mineralization zone have Pt (39–315ppb) and Pd (76–1100ppb), and intermediate to high Cu/Pd ratios (6850–138,000). The variably high Cu/Pd ratios indicate that the parental magma may have reached earlier sulfide saturation and removal. The low Os contents (0.0146 to 0.276ppb) and high Re/Os (29 to 292) and 187Os/188Os(i) ratios (0.277 to 0.916) of the silicate rocks are consistent with a major contribution of Os from crustal materials. On the other hand, the net-textured sulfide ores from No. 8 mineralization zone have high Os contents (15.1 to 32.6ppb), low Re/Os (2.0 to 2.3) and low 187Os/188Os(i) ratios (0.125 to 0.141), indicating a mantle-derived origin. Modeling indicates that >30% crustal contamination is required to explain the radiogenic Os isotopic composition if average upper crust is adopted as the crustal contaminant, or 5%–25% if sulfide-rich crustal rock is used as the crustal contaminant. However only 5–10% crustal contamination is required to explain the Nd and Sr isotopes. We propose that the decoupling of Os from Sr–Nd isotopes in the silicate rocks was due to selective crustal contamination. Addition of external sulfur from a crustal source may be the key factor triggering sulfide saturation. The silicate magma that underwent sulfide extraction before emplacement to shallow depth preserved a crustal Os isotope signature. Continued reaction of sulfide melts with new pulses of mantle-derived magma increased the Os content and decreased the 187Os/188Os(i) ratios, effectively masking the crustal Os contribution in the sulfides.