THE GIANT JIAODONG GOLD DEPOSITS, CHINA: OROGENIC GOLD IN A UNIQUE TECTONIC SETTING OR A UNIQUE GOLD DEPOSIT TYPE?

The ca. 126-120 Ma Au deposits of the Jiaodong Peninsula, eastern China, define the country’s largest Au province with reserves of 2300 t. The vein and disseminated ores are hosted by NE- to NNE-trending brittle normal faults that parallel the margins of 165-150 Ma, deeply emplaced granitoids derived by melting of lower crust. The deposits are sited along the faults for many tens of kilometers and the larger orebodies are associated with dilational jogs. Country rocks to the granites are Neoarchean and Paleoproterozoic high-grade metamorphic rocks located on both sides of the Triassic suture between the North and South China blocks. During early Mesozoic convergent deformation, the ore-hosting structures developed as ductile thrust faults that were subsequently reactivated during Early Cretaceous “Yanshanian” intracontinental extensional deformation and associated Au formation.

Classification of the deposits remains problematic. Many features in Jiaodong resemble those typical of orogenic Au, including their linear structural distribution, mineralization style, ore/alteration assemblages, and ore fluid chemistry. However, if these are defined as orogenic deposits, then important modifications to the commonly accepted ore genesis model are needed. Phanerozoic orogenic Au deposits are formed by prograde metamorphism of accreted oceanic rocks in Cordilleran-style orogens. The Jiaodong deposits, in contrast, formed within Precambrian rocks 2 b.y. after their metamorphic devolatilization, and thus require an alternative fluid and metal source for the ores. A widespread suite of 130-123 Ma granodiorites overlap temporally with the ores, but shows a poor spatial association with the deposits. Furthermore, the deposit distribution and mineralization style are atypical of ores formed from proximal magmatic sources. Fluid focusing during deep-seated magmatism or metamorphism of underplated material is required, which must relate to a combination of coeval lithospheric thinning, asthenospheric upwelling, paleo-Pacific subduction, and/or seismicity along the continental-scale Tan-Lu fault. Regardless of the tectonic trigger, an immense fluid source is required and, at least for the S that mobilized the Au, the isotopically heavy d³⁴S of the ore-related sulfides preclude a dominant mantle source.