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. © 2011 Elsevier B.V.