Mineral exploration activities since 2010 in a previously poorly explored region of northeastern Guangxi province, South China, confirmed the existence of a potentially large hydrothermal Ni-Co deposit. As a newly discovered deposit, many aspects of this deposit are still unclear, including in which tectonic setting the deposit formed and how the high-grade mineralization developed. To address these questions, in this study, we conduct a comprehensively geochronological and geochemical study on the Dajin pluton (consisting of biotite monzogranite and sericitized monzogranite), which is the only pluton occurring in the district. LA-ICP-MS zircon U-Pb dating shows that two types of granites were emplaced nearly at the same time, with biotite monzogranite and sericitized monzogranite being 449 ± 15 Ma and 448.4 ± 8.8 Ma, respectively. This, combined with previously published studies, confirms that the Dajin pluton was broadly emplaced during the formation of the Longhua deposit. These granites are characterized by high K2O, and noticeable depletion of Eu, Sr, P and Ti, with whole-rock εNd(t) values ranging from −1.99 to −4.26 and zircon εHf(t) values ranging from −0.9 to 4.1. These features support that the studied granites are typical of A2-type granites, which experienced strong fractional crystallization and formed in a post-collisional environment. Although Co and Ni might not have been originally derived from granitic magmas, we suggest that the emplacement of the Dajin pluton facilitated the formation of high-grade Ni-Co mineralization in two ways: (1) melting the Co– and Ni-enriched mafic basement probably existing in the deep crust, and (2) remobilizing and concentrating metals from wall rocks.