Geochemistry and zircon U-Pb age of volcanic rocks from the Shaquanzi Fe-Cu deposit in East Tianshan Mountains and their geological significance

Abstract: Located along the Aqishan-Yamansu belt of East Tianshan Mountains, the Shaquanzi Fe-Cu deposit is one of the deposits hosted in Paleozoic volcanic rocks. The deposit occurs in the second and third lithologic memberss of the Dikaner Formation consisting of volcanic rocks intercalated with sedimentary rocks. The iron mineralization also occurs at the contact zone between diorite-porphyrite and wall rocks. The tectonic setting and age of the volcanic rocks are key factors in the reconstruction of the ore-forming process. In this paper, representative samples of Dikaner Formation volcanic rocks and diorite-porphyrite in the Shaquanzi Fe-Cu ore district were analyzed for major and trace elements and zircon U-Pb dating was conducted to investigate the tectonic setting and formation ages of these rocks. LA-ICP-MS analyses of zircons with well-defined oscillatory zoning yielded mean 206Pb/238U ages of (321.7±1.7) Ma and (322.7±1.7) Ma, respectively, indicating that they were formed during late Early Carboniferous. The geochemical data suggest that most of the volcanic rocks belong to calc-alkaline series with the enrichment of LILE, LREE and depletion of Nb, Ta, and Ti, similar to the characteristics of arc volcanic rocks. The zircons from rhyolite and diorite-porphyrite have εHf(t) values of +6.28~+15.51 and ?0.22~+13.9, respectively. The rhyolites were derived from juvenile crustal materials and basalts, which came from mantle wedge modified by fluids from subducted materials that formed andesite and diorite-porphyrite through crystal fractionation. Regional geology and geochemical evidence indicates that the Dikaner Formation volcanic rocks and diorite-porphyrite were formed simultaneously through island arc magmatism during the late stage of subduction. The Shaquanzi Fe-Cu deposit hosted in the Dikaner Formation were formed after 322 Ma.