Mantle Metasomatism and Crustal Assimilation in the Genesis of the Giant Xiarihamu Magmatic Ni-Co Sulfide Deposit in the East Kunlun Orogen, China

Abstract The Xiarihamu magmatic Ni-Co sulfide deposit in the Paleozoic East Kunlun orogenic belt, western China, is the largest magmatic sulfide deposit in an orogenic setting (157 Mt at 0.65% Ni, 0.013% Co, and 0.14% Cu). In this study, mercury isotope analyses of ores and sulfide separates from the Xiarihamu deposit reveal negative Δ199Hg values (–0.11 ±0.08‰ standard deviation [SD]). These values are distinct from the near-zero Δ199Hg values for the primitive mantle (0.00 ±0.05‰ SD) and positive Δ199Hg values for the subcontinental lithospheric mantle modified by oceanic subduction (0.16 ±0.17‰ SD) but within the range of terrestrial materials (–0.23 ±0.19‰ SD). Primitive harzburgite cumulates display negative Δ199Hg values of –0.09 ± 0.06‰ (SD), suggesting that the mantle source was modified by subducted continental materials via continental subduction after the closure of the Proto-Tethys Ocean. Olivine orthopyroxenites, orthopyroxenites, websterites, and gabbronorites also display negative Δ199Hg values (–0.12 ± 0.09‰, SD), suggesting assimilation of terrestrial material into the parental magma at the crustal level. The results demonstrate that mantle metasomatism and crustal assimilation are both critical factors for the metallogenesis of magmatic sulfide deposits in orogenic belts.