Contribution of juvenile lower crustal melting to the Chaganhua Mo deposit on the northern margin of the North China Craton

Numerous Late Permian to Triassic porphyry molybdenum deposits have been identified along the northern margin of the North China Craton, distributed across both the North China Craton and the Central Asian Orogenic Belt units. However, significant variations exist in the sources of the ore-forming magmas for these deposits, and the cause of these variations remains poorly constrained. This study focuses on the Chaganhua Mo deposit, the largest in the northern segment of the Nuru-Langshan tectonic zone, and presents a comprehensive analysis of the ore-forming porphyry, including petrography, zircon U-Pb geochronology and Hf isotopes, whole-rock geochemistry, and Sr-Nd isotopes. The results indicate that the zircon U-Pb age of the Chaganhua porphyritic granite is 242±1Ma, which is consistent with the previously reported molybdenite Re-Os isochron age of 241.6±2.5Ma, suggesting that the granite and the Mo mineralization are contemporaneously formed. Geochemically, the analyzed samples of the rocks are characterized by high SiO2 (71.43%~73.16%) and low MgO (0.41%~0.52%), belonging to the high-K calc-alkaline and weakly peraluminous series. They are enriched in light rare earth elements and large-ion lithophile elements (e.g., Rb, Th, U, K, Sr), depleted in heavy rare earth elements and high-field-strength elements (e.g., Nb, Ta, P, Ti), and exhibit a weak negative Eu anomaly (Eu/Eu*=0.59~0.67). Additionally, they show high Sr contents (282×10-6~320×10-6), high Sr/Y (32.91~42.43) and (La/Yb)N (22.57~28.01) ratios, indicating adakitic affinities. Although the samples exhibit relatively high initial 87Sr/86Sr ratios (0.705948~0.709485) and negative εNd(t) values (-7.0~-6.4), their Sr-Nd isotopic compositions distinctly differ from those of the ancient lower crust of the North China Craton. In addition, they have high K2O/Na2O ratios, zircon εHf(t) values (+0.0~+4.5), low CaO/Al2O3 ratios and Mg# values, as well as relatively young zircon Hf two-stage model ages (891~1131Ma). Integrated with reconstructions of regional crustal thickness evolution, we propose that the Chaganhua porphyritic granite is likely originated from the partial melting of thickened juvenile lower crust during the post-subduction tectonic stage, and underwent moderate crustal contamination during its evolution. Thus, the juvenile lower crust within the Central Asian Orogenic Belt can serve as a significant reservoir for porphyry Mo deposits. Comparative results indicate that the Mo-mineralizing magmas derived from this reservoir are markedly distinct, in terms of Nd-Hf isotopic and geochemical characteristics, from those generated by the melting of ancient continental crust within the North China Craton.