Ore-hosting igneous rocks in the Xiahe–Hezuo district, West Qinling orogen, China, and their relationships with gold mineralization

Abstract

• The Gulangdi Formation in West Qinling orogen (WQO) was formed prior to Triassic. • Gold-bearing igneous rocks in WQO were crystallized prior to gold mineralization . • Gold-bearing igneous rocks were generated by subduction of the Mian–Lue Ocean. • The Mian–Lue Ocean started the subduction process since ca. 251 Ma. • The gold metallogeny in WQO postdates regional magmatism by 40 to 20 m.y. The Xiahe–Hezuo district in the West Qinling orogen hosts a large number of gold deposits, most of which are spatially associated with igneous rocks. However, the nature and origin of these igneous rocks and their possible relation with gold mineralization remain poorly understood. In this study, the ore-hosting igneous rocks from the Zaozigou, Yidi’nan, and Gangcha gold deposits are selected to perform geochronological and geochemical analyses, aiming to clarify their petrogenesis and their role in gold mineralization. The quartz diorite porphyry from the Zaozigou gold deposit, the quartz diorite from the Yidi’nan gold deposit, and the andesite from the Gangcha gold deposit yield LA-ICP-MS zircon U-Pb ages of 250 ± 2, 251 ± 1, and 247 ± 1 Ma, respectively, representing the earliest Triassic magmatic rocks in the West Qinling orogen. The quartz diorite porphyry and quartz diorite display relatively high ( 87 Sr/ 86 Sr) i ratios (0.707188–0.710693), negative ε Nd (t) (−11.7 to −6.2) and zircon ε Hf (t) values (−13.0 to −1.6), and ancient two-stage Nd and Hf model ages (2073–1357 Ma), indicating they were generated by partial melting of ancient crustal materials. The andesite exhibits slightly positive ε Nd (t) values (0.8–1.8) and negative ε Hf (t) values (−20.6 to −2.2), indicating it was derived by partial melting of a metasomatized lithospheric mantle with significant crustal contamination. All of these igneous rocks show enrichments in large ion lithophile and light rare earth elements, depletions in high field strength elements, and high Mg # values (>50), indicating their generation in a subduction-related magmatic arc. These imply that the northward subduction of the Mian–Lue Ocean beneath the West Qinling orogen may had already started since the earliest Triassic. The geological and geochronological evidences suggest these igneous rocks were not genetically associated with the ore-forming process, and they merely provided the space for gold deposition.