Magmatic Controls on Contrasting Molybdenum Fertility of Caledonian Granitoids in Eastern Guangxi, South China

Caledonian hydrothermal vein-type Mo deposits are largely restricted to eastern Guangxi, South China, yet the magmatic controls on molybdenum (Mo) mineralization remain poorly constrained. The adjacent Guiling (Mo–mineralized) and Daning (Mo–barren) plutons in this region represent a natural pair for deciphering these issues. Zircon U–Pb dating, whole-rock geochemistry, and mineralogical characteristics indicate that the Daning pluton (~440 Ma) is an I-type granite, whereas the younger Guiling pluton (~425 Ma) is a more evolved I-type granite, with higher SiO2 contents (avg. 71.17 wt.% vs. 65.88 wt.%), stronger negative Eu anomalies (avg. 0.53 vs. 0.63), lower Nb/Ta ratios (avg. 6.5 vs. 9.9), and Zr/Hf ratios (avg. 30.6 vs. 33.4), as well as higher Rb/Sr ratios (avg. 3.6 vs. 1.1). Sr–Nd–Hf isotopes are consistent with a Paleoproterozoic crustal source for both plutons, whereas coeval mafic microgranular enclaves in the Guiling pluton exhibit positive εNd(t) values (+3.43 to +3.58), providing evidence for mantle input. Biotite geochemistry shows that the Guiling pluton is enriched in F and Cl, as reflected by lower log(ƒH2O/ƒHF)fluid values and higher log(ƒH2O/ƒHCl)fluid values. Our integrated study suggests that the Daning and Guiling plutons were emplaced in an intracontinental orogenic setting during the transition from compression to extension, but the more pronounced extension facilitated a greater input of mantle-derived heat and volatiles into the Guiling pluton, favoring Mo enrichment. However, the similar biotite crystallization temperatures and moderate oxygen fugacities of both plutons suggest that these factors were not the decisive factors. Collectively, we propose that the contrasting Mo fertility between the two plutons reflects the interplay of magmatic and structural factors. Magmatically, crust–mantle source mixing, high degrees of fractionation, and volatile enrichment promoted Mo concentration, whereas ductile shear zones likely served as fluid conduits that facilitated mineralization. Accordingly, the southwestern segment of the Yingyangguan (YYG) ductile shear zone between the Daning and Guiling plutons could be a prospective target for Mo prospecting in eastern Guangxi.