The SW end of the southeastern coast polymetallic mineralization belt comprises the Dayaoshan golden metallogenic zone, the Guigang area in Guangxi province. The LA-ICP-MS U-Pb zircon dating for pyrite-mineralized porphyry of the Longtoushan gold deposit from this region yields a weighted mean 206Pb/238U age of 96.13.0Ma, which is consistent with the result obtained from nearby Pingtianshan pluton (96.20.4Ma) within analytical error. Host rock Sr isotope and zircon REE feature of the Longtoushan deposit reveal that the mineralization was coupled with massive fluid activity. The coeval ages of mineralized porphyry and intrusive rock and evidently higher Ce positive anomalies of zircons from the porphyry relative to those from the intrusion imply a rapid regional crust uplifting along with pronounced rise of oxygen fugacity in magmatic system. The two igneous suites show a Nd isotopic signature akin to the low Nd model age belt of South China, indicative of mantle-derived magma involvement. Accordingly, it is suggested that large-scale fluid upward migration, rapid crustal uplift, abrupt cooling of ore-forming fluid along deep-seated fault, and oxygen fugacity increasing are key factors resulted in the metallogenesis of the Longtoushan gold deposit. We thus propose that the low-angle subduction of Pacific plate derived large-scale fluid migration spread widely along the southeastern coast, while the underplating of mantle-derived magma resulted in the ascending of high-solubility mineralized fluid along the tectonic channels, which are the critical setting responsible for the large-scale metallogenesis along the southeastern coast of South China during the Mesozoic.