Zircon Hf–isotopic mapping of Middle–Lower Yangtze River Valley Metallogenic Belt, China: Constraints on crustal properties and ore cluster formation

Abstract

The formation of endogenic mineral deposits is commonly thought to be closely related to the initiation and evolution of magmatic systems, which are controlled by deep structure and dynamic processes. The Middle–Lower Yangtze River Valley Metallogenic Belt (MLYB), a paramount metallogenic belt in eastern China, lies between the Dabie and Jiangnan orogens. Although large-scale mineralization occurred in MLYB during the Mesozoic, it is unclear why numerous metals were enriched in such a narrow region. We integrated geological, geochemical, and geochronological data (specifically zircon UPb dating and LuHf isotope data) for the Mesozoic magmatic rocks in MLYB and adjacent parts of the Dabie and Jiangnan orogens. Then, we first drew regional zircon Hf–isotope compositions and model-age maps from 259 weighted-average data values (64 new and 195 prior published) and marked 98 major deposits of MLYB on the contour maps according to their latitude and longitude.The results showed a wide range of zircon crustal Hf–isotopic model ages (TDMC) of MLYB, indicating that its basement is more likely generated by the mixing of the Jiangnan and Dongling basements. Almost all large deposits in the metallogenic belt were concentrated on these basement interfaces, indicating that the junction zone of two basements is favorable for mineralization.Almost all large Cu–Au–Mo polymetallic deposits of MLYB formed during the compression–extension transition stage from 151 to 138 Ma, and zircon Hf–isotope compositions of related high-K Calc–alkali intrusions evolved from high to low. During 137–126 Ma, numerous magnetite deposits formed, and the amount of Cu–Au–Mo polymetallic deposits rapidly decreased. The zircon Hf–isotope compositions of magmatic rocks in this stage evolved from low to high. The concentration of S and Cu–(Au) in the mother magma might be diluted by a large amount of crustal materials, and the Cu content in the magma was significantly reduced by magnetite formation.There was a wide range of Hf–isotope compositions in Mesozoic high-K calc–alkaline series and shoshonite series magmatic rocks, which are closely related to MLYB mineralization, and almost all Cu–Au–Mo deposits and magnetite–apatite deposits were developed in the εHf(t) isotopic boundary. Therefore, magma mixing played a key role in the formation of deposits in MLYB.The Mesozoic magmatism and mineralization of MLYB migrated from Edongnan to Jiurui ore clusters northeastern with time (151–105 Ma). This phenomenon was related to the rollback of the subducting Paleo–Pacific slab in the Late Jurassic and Early Cretaceous, which led to the earlier heating of the lithospheric mantle base in the southwest than in the northeast when the metasomatic mantle source partially melted.Hf–isotope mapping showed spatiotemporal relationships among crustal structure, crust–mantle interaction, and ore cluster formation process, thereby demonstrating that the structure, nature, and composition of the crust control the location of ore deposits and the migration of ore-forming metals within the terrane.