Age and genesis of the Jinshan gold deposit in the Chinese North Tianshan: A link to large-scale strike–slip shearing events

Abstract

Large-scale transcrustal strike–slip shearing, orogenic gold deposits, and magmatic Ni–Cu sulfide deposits are prominent features in the Chinese North Tianshan in the northeastern segment of the Tianshan Orogenic Belt. The relationship between the shearing events and the different styles of mineralization, however, remains ambiguous. Here, structural data, zircon U–Pb–Hf isotopes, bulk-rock geochemistry, muscovite 40Ar/39Ar age dating, and S–Pb isotopes of pyrite from the Jinshan gold deposit are utilized in conjunction with the Pb isotope composition of mineralized mafic–ultramafic rocks of the Huangshandong Ni–Cu sulfide deposit to characterize the link between strike–slip shearing events, and orogenic Au and magmatic Ni–Cu sulfide mineralization. The new zircon U–Pb age results indicate that the host diorite of the gold orebodies formed at ca. 335 Ma. These diorites are enriched in light rare earth elements and large ion lithophile elements (e.g., Ba, U, K, and Pb), but depleted in high field strength elements (e.g., Nb, Ta, and Ti), and have low MgO (4.45–5.17 wt%) contents, Mg# (0.32–0.36), and (La/Yb)CN ratios (4.81–6.90). Based on the lack of ancient crustal basement rocks in the Qoltag Arc, the depleted Nd–Hf isotope composition (εHf(t) = 10.82–15.38; εNd(t) = 6.17–6.67), the juvenile Hf depleted-mantle model ages (350–530 Ma), and the enrichment in Na2O (4.45–5.17 wt%) relative to K2O (1.05–2.20 wt%) of the diorite, it is inferred that the primary magma from which the diorite crystallized was derived from partial melting of metasomatized mantle wedge in an oceanic arc environment. Two muscovite samples separated from an auriferous quartz lode yield 40Ar/39Ar plateau ages of 248.6 ± 0.8 Ma and 249.6 ± 0.9 Ma, representing the age of the gold mineralization event. The similarity in S–Pb isotope composition of pyrite in the Jinshan gold deposit and pyroxenite in the Huangshandong Ni–Cu sulfide deposit suggests that the auriferous fluids required for the formation of the Jinshan gold deposit could represent a mixture of deuteric fluids with limited upper crustal metamorphic fluids from the Kanggur–Huangshan Shear Zone.

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