Sr-Nd-Os-S isotope and PGE geochemistry of the Xiarihamu magmatic sulfide deposit in the Qinghai–Tibet plateau, China

• Published in: Mineralium deposita 2017, Vol.52 (1), p.51-68
• Main Authors: Zhang, Zhaowei, Tang, Qingyan, Li, Chusi, Wang, Yalei, Ripley, Edward M.
• Format: Article
• Language: English
• Subjects: Basalt; Cenozoic; Devonian; Earth and Environmental Science; Earth Sciences; Geochemistry; Geology; Isotope studies; Isotopes; Magma; Marine; Mineral Resources; Mineralization; Mineralogy; Silurian; Sulfides
• ISSN: 0026-4598; 1432-1866
• DOI: 10.1007/s00126-016-0645-0

The newly discovered Xiarihamu Ni-Cu deposit is located in the Eastern Kunlun orogenic belt in the northern part of the Qinghai–Tibet plateau, western China. It is the largest magmatic Ni-Cu sulfide deposit found thus far in an arc setting worldwide and ranks second in China in terms of total Ni resources. Fe-Ni-Cu sulfide mineralization occurs in a small ultramafic body that is part of a larger mafic-ultramafic complex formed by protracted Silurian-Early Devonian basaltic magmatism. The mineralized ultramafic body is composed predominantly of lherzolite and olivine websterite, with minor dunite, websterite and orthopyroxenite. Here we report new PGE (platinum group element) data and the results of a new, integrated Sr-Nd-Os-S isotope study. The initial concentrations of Rh and Pd in the parental magma are estimated to be 0.014 ppb and 0.24 ppb, respectively, which are more than one order of magnitude lower than those in undepleted mantle-derived magmas such as many continental picrites. The observed PGE depletions in the Xiarihamu parental magma are attributed to sulfide retention in the source mantle, because the degree of partial melting required to generate the Xiarihamu primary magma was not high enough for a magma of that composition to dissolve all sulfides in the source. The ( 87 Sr/ 86 Sr) i ratios and ε Nd (t) of the Xiarihamu host rocks range from 0.7062 to 0.7105 and from −1.97 to −5.74, respectively, indicating 5–30 wt% crustal contamination in the Xiarihamu magma. These data also reveal that the source mantle for the Xiarihamu magma is isotopically (Sr-Nd) more enriched than that for the average Cenozoic arc basalt. The γOs(t) and δ 34 S values of sulfide ores from the Xiarihamu deposit range from 78 to 1393 and from 2 to 6‰, respectively. These values clearly indicate addition of crustal Os and S to the Xiarihamu parental magma. Metal tenors such as Ni and Rh are inversely correlated with γOs(t) and δ 34 S values. This indicates that mixing between different pulses of magma with different isotope compositions took place during magma emplacement at Xiarihamu, and that external S played a critical role in the formation of the Xiarihamu magmatic Ni-Cu sulfide deposit.