Early cretaceous molybdenum-bearing felsic magmatic rocks in the tongmushan district, northern great Xing’an range: implications for mesozoic porphyry metallogenesis

The Nenjiang-Heihe metallogenic belt of the northern Great Xing’an Range is renowned for Mesozoic epithermal Au deposits, however, apart from the Duobaoshan and Tongshan super-large porphyry deposits in the Paleozoic, no superlarge porphyry copper deposits have been discovered in the Mesozoic. The metallogenic potential of Mesozoic porphyry Cu-Mo deposits in this belt remains controversial. Recent deep drilling in the Tongmushan district intersected a molybdenite-bearing quartz diorite porphyry, representing a breakthrough in exploring for porphyry ore beneath epithermal systems. Here, we present an integrated study of this newly discovered intrusion, integrating petrography, whole-rock geochemistry, zircon U-Pb and molybdenite Re-Os geochronologies. The results show that molybdenite Re-Os dating yields 115.4 ± 2.7 Ma, which is coeval with zircon U-Pb ages of 120–116 Ma, establishing an Early Cretaceous magmatic-hydrothermal event. The ore-bearing rocks exhibit high-K calc-alkaline affinities with elevated Sr/Y (>30) and Al 2 O 3 /TiO 2 (>20) ratios, and plot in the hydrous magma field. High Fe 2 O 3 /FeO ratios (1.58–2.44) and magnetite-series characteristics indicate strongly oxidized conditions favorable for porphyry mineralization, comparable to Cretaceous ore-bearing porphyries in NE China. Geochemical discrimination indicates their formation in an active continental margin setting, locally overprinted by Early Cretaceous extension. The temporal and geochemical affinities between Tongmushan porphyry Cu-Mo and regional epithermal Au systems suggests a continuum from deep porphyry to shallow epithermal environments. This study provides the first direct evidence for Early Cretaceous porphyry potential in this belt.