Cumulate compaction promoting sulfide migration: A case study from the Xiangshandong intrusion in the Eastern Tianshan

Magmatic Ni-Cu sulfide deposits refer to deposits rich in Ni, Cu and PGEs (platinum group elements) and associated with mafic-ultramafic rocks. The key process of the formation of such deposits can be summarized as three basic stages of sulfide: segregation, enrichment and migration. However, there are still many unknowns regarding the migration mechanism of sulfides, particularly the percolation and accumulation behavior of sulfides when in crystal mush system. Aimed to investigate the migration behavior of sulfides in cumulate layers and its potential controlling factors, this study focuses on the eastern part of the Xiangshan intrusion in the Huangshan-Jing'erquan Ni-Cu metallogenic belt in Eastern Tianshan, combining petrology, mineralogy, chalcophile element geochemical analysis and simulation calculations. Field and microscopic observations show that sulfide abundances in the peridotites of the Xiangshandong intrusion are generally low. However, olivine of the Xiangshandong intrusion commonly exhibits significant Ni depletion (as low as 370×10-6). Additionally, its whole-rock Cu/Pd ratios are much higher than those of the primitive mantle and typical basaltic magmas. These features indicate that extensive sulfide segregation must have occurred in the parental magma during evolution, which is contrary to the actual observation results. Moreover, the stratigraphic variations of olivine Fo-Ni contents show negative correlation, suggesting extensive Fe-Ni exchange may have occurred between olivine and sulfide melt. Calculation based on whole-rock composition and olivine-sulfide cotectic ratio shows that the theoretical Ni content of the olivine-sulfide system is much higher than the measured whole-rock Ni content. This result confirms that substantial sulfide melt must have migrated out of the magma system, resulting in a sharp decrease in whole-rock Ni content. Petrographic observations reveal that Ni-depleted olivine commonly exhibits undulatory extinction and subgrain boundaries, and mechanical twins can be frequently observed in associated clinopyroxene. These textures indicate that the crystal mush experienced considerable plastic deformation and compaction during cumulating stage. Based on the results above, we propose that compaction is a potential mechanism promoting sulfide migration within crystal mush. By promoting sulfide percolation and compacting the cumulate texture to create migration pathways, compaction helps sulfide droplets continuously migrate through pore channels and eventually accumulate in low-stress zones or structural weak areas to form ore bodies. Therefore, structural fractures near the deep margins of the Xiangshandong intrusion and laterally expanded sections of the intrusion have high metallogenic potential. This mechanism has broad implications for understanding the formation of magmatic Ni-Cu sulfide deposits.