MESOZOIC Mo MINERALIZATION IN NORTHEASTERN CHINA DID NOT REQUIRE REGIONAL-SCALE PRE-ENRICHMENT

Northeastern China is one of the richest Mo-mineralized regions in China, with 95 Mesozoic Mo-bearing deposits and a total metal resource of >12.2 Mt Mo. The reasons behind the large-scale Mesozoic Mo mineralization in NE China remain unclear, and whether or not there was any regional-scale pre-enrichment of the source region (e.g., a Mo-rich lower crust) is still a matter of debate. In this study, whole-rock Nd and zircon Hf isotope compositions of the intrusions related to the Mo mineralization have been compiled. The results show that the isotopic compositions are highly heterogeneous among these deposits with different ages and Mo tonnages, indicating that Mo-related magmas could be derived from either ancient lower crust or relatively juvenile lower crust or via mixing of mantle-derived magmas with varying proportions of crustal melts. This suggests that different magmas, independently from their sources, can produce Mo mineralization in NE China, and, therefore, that there was probably not an unusually Mo rich basement underlying NE China. In addition, Monte Carlo simulations have been carried out to explore the magmatic processes potentially associated with the formation of magmatic-hydrothermal (porphyry- or skarn-type) Mo deposits. The results reveal that a variably large magma volume (e.g., >150 km3) was required to provide enough Mo metal to form the large (>0.1 Mt Mo) deposits. In NE China, the Jurassic Mo deposits are much more abundant and larger than the Triassic and Cretaceous deposits, which could be attributed to the specific Jurassic tectonic regime. In the Jurassic, the subduction of the Paleo-Pacific oceanic plate led to regional compression, which favors accumulation of larger amounts of magmas at depth, ultimately resulting in larger Mo deposit formation. In this study, we highlight the importance of magma volume, rather than magma source, in the formation of the Mesozoic Mo deposits in NE China.