Geochemistry, Geochronology, and Hf-S-Pb Isotopes of the Akechukesai IV Mafic-Ultramafic Complex, Western China

Abstract

The Akechukesai IV mafic–ultramafic complex, located in the western segment of the eastern Kunlun orogenic belt (EKOB), represents a newly-discovered complex, containing Ni ores at grades of up to 0.98% Ni. It is dominated by olivine pyroxenite, pyroxenite, and gabbro units. The gabbros are enriched in lithophile elements (e.g., Rb, U, and K) and light rare-earth elements (LREE), with negative anomalies in high field-strength elements, except Zr, Ta. Nb/Ta(∼5) and Zr/Hf (∼10) ratios lower than the primitive mantle and chondrites, respectively, indicate the influence of the mantle metasomatic process or fractionation of accessory mineral phases. Zircon U–Pb dating of the gabbro yielded an age of 423.9 ± 2.6 Ma, indicating that the complex formed contemporaneously with the Xiarihamu Ni deposit (423 ± 1 Ma). The gabbro has negative εHf(t) values (−11.3 to −1.2) with corresponding TDM1 ages of 1535–1092 Ma. The vein-like and disseminated mineralization (i.e., pyrite and pyrrhotite) have δ34S values of 13.1‰–13.4‰ and 5.0‰–8.5‰, respectively, suggesting that the magmas that formed the complex assimilated crustal sulfur. They yield 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb values of 17.323–18.472, 15.422–15.626, and 37.610–38.327, respectively, indicating Pb derived from multiple sources (i.e., mantle crustal sources). Geochemical and Hf–S–Pb isotopic characteristics suggest that the complex formed from a primitive magma derived by partial melting of a spinel- and garnet-bearing lherzolite mantle at variable degree of 5%–10%. This source region was geochemically enriched by previous interaction with slab-related fluids. Tectonic reconstruction suggests that the Akechukesai IV complex was generated in a post-collisional extensional environment.

The parental magma composition, crustal contamination process, and metallogenesis of the Shitoukengde Ni‐Cu sulfide deposit in the Eastern Kunlun Orogenic Belt, NW China

“…It exhibits resources of Ni (1,183,000 t), Co (42,900 t), and Cu (238,300 t) (Li, 2018), which indicates the great exploration potential of Ni–Cu–Co in the EKOB and gives geologists great confidence in exploration. Zircon U–Pb dating of the mafic–ultramafic rocks suggests that the Xiarihamu Cu–Ni deposit formed in the middle Silurian‐middle Devonian period (394–432 Ma; Table S7), and researchers are paying more attention to the mafic–ultramafic magmatism and associated Ni–Cu mineralization in this period (Li et al ., 2015; Song et al ., 2016; Li, 2018; Liu et al, 2018; Zhang et al ., 2018; Yan et al ., 2019a). A series of Ni–Cu deposits with a large number of mafic–ultramafic intrusions in the Silurian‐Devonian period have been discovered in the EKOB in recent years (Figure 1; Table S7), such as Shitoukengde (418–426 Ma; Table S7), Akechukesai (416–424 Ma; Table S7), Langmuri (438–440 Ma; Unpublished data), Binggounan (427 Ma; He et al ., 2017), and Gayahe (425 Ma; Unpublished data).…”

“…There are great amounts of mafic–ultramafic complexes in the Eastern Kunlun Orogenic Belt (EKOB) with varied Ni–Cu mineralization. Since the discovery of the Xiarihamu Cu–Ni deposit in 2011 (Li et al ., 2012; Wang et al ., 2014, the EKOB has become an important Cu–Ni resource base in China, and many Ni–Cu sulfide deposits with great prospecting potential have been found in recent years, such as Akechukesai, Gayahe, Shitoukengde, and Langmuri (Figure 1; Zhou et al ., 2016; Li et al ., 2018; Shi et al ., 2018; Yan et al ., 2019a; Mo et al ., 2020). They are distributed in the EKOB in a narrow strip, especially in the basement‐uplifting and granite belt of Middle Eastern Kunlun (BGMK; Figure 1).…”