The genetic link between rhyolitic magmas and low-temperature hydrothermal molybdenum mineralization: Insights from the Zhangmajing Mo-U deposit, Northeast China

Abstract Hydrothermal molybdenum mineralization can occur as molybdenite in porphyry deposits at high-temperature conditions (mostly 300–500 °C) or as jordisite in low-temperature hydrothermal deposits (≤250 °C). Previous studies mainly focused on the origin of porphyry Mo deposits that are generally considered to be produced from felsic magmas. However, little is known about the mechanism responsible for molybdenum enrichment in low-temperature systems. To address this knowledge gap, we investigated the Zhangmajing deposit in Northeast China. It is one of the world’s largest known jordisite deposits containing uranium mineralization, with resources totaling 110,000 tonnes at 0.339% Mo and 5000 tonnes at 0.1%–0.3% U. This deposit is mainly hosted by Early Cretaceous rhyolite porphyry, with minor mineralization in volcanic rocks of the Zhangjiakou Formation. The Re-Os isotope results indicate that the mineralization occurred during the Early Cretaceous (ca. 138 Ma), which overlaps with the emplacement ages of the host rocks. The melt inclusion analysis suggests that the rhyolite porphyry corresponds to highly evolved rhyolitic magmas generated in a within-plate tectonic environment, analogous to ore-forming magmas of Climax-type porphyry Mo deposits. Compared with Mo-fertile intrusions, however, zircon chemistry reveals that the Zhangmajing magmas have relatively low oxygen fugacity (average ΔFMQ = 0.7, where FMQ is the reference buffer fayalite-magnetite-quartz) and water contents, showing an affinity with barren igneous suites. Molybdenum partitions strongly into magmatic minerals and residual melts in its tetravalent state. This sequestration hinders the efficient extraction of Mo into exsolving hydrothermal fluids, thereby preventing the formation of a porphyry Mo deposit. We propose that ore formation in Zhangmajing was related to the Early Cretaceous magmatism, which provided the thermal energy for the circulation of fluids—mainly meteoric water. These fluids scavenged ore metals from volcanic–subvolcanic rocks and possibly the underlying basement. The proposed model for low-temperature Mo mineralization provides a new perspective on the genesis of hydrothermal Mo deposits.