A porphyritic granite intrusion was recently discovered in the Zhuanzhilian section of the Dongping gold deposit. There is as many as one tonnage of Au in the fractured shear zone within the porphyritic granite intrusion, but no relevant reports concerning the origin and age of the intrusion has been published as yet. In this paper, zircon U-Pb dating is used to study the geochronology of porphyritic granite, in order to find out the evidence of age and the relationship with gold mineralization. There are two groups of zircon 207Pb/235U-206Pb/238U concordant ages of porphyritic granites: The concordant age of 373.0 ± 3.5 Ma, with the weighted mean age of 373.0 ± 6.4 Ma; and the concordant age of 142.02 ± 1.2 Ma with the weighted mean age of 142.06 ± 0.84 Ma. We believe that the first group might represent the age of residual zircon of alkaline complex, while the second group might be related with main gold mineralization. The obtained results of the petrography and electron probe analysis indicate that the porphyritic quartz and porphyritic granite, as well as gold mineralization, might be products of a late replacement of tectonic-hydrothermal fluid, which was rich in Si, Na and K originally and later yielded gold-forming fluids.

The Bailinchuan alkaline syenite (BAS) is located in the easternmost part of the Triassic alkaline magmatic belt along the northern North China Craton (NCC). Based on a detailed study of the zircon U–Pb age, petrological, and geochemical data of the complex, the characteristics of the magmas system, petrogenesis and the nature of mantle source provide new constraints on the origin and tectonic setting of the Triassic alkaline belt. The BAS is composed of alkaline syenite and/or aegirine-nepheline syenite, with zircon U–Pb age of 226–229 Ma. Aegirine, Na-rich augite, biotite, orthoclase, and nepheline are the major minerals. Most of the zircons selected for the analysis show fine-scale to weak oscillatory growth zoning in CL images, suggesting a magmatic origin. Mineralogy, petrology and geochemical studies show that the parental magma of the BAS is SiO₂-undersaturated, potassic, and is characterized by high contents of CaO, Fe₂O₃, K₂O, Na₂O. The BAS originated from a phlogopite-rich, enriched lithospheric mantle source in a garnet-stable area. The occurrence of the BAS, together with many other alkaline complexes of similar ages (235–209 Ma) in the northern NCC during the Late Triassic implies that the lithospheric mantle beneath the northern NCC was previously metasomatized by melts/fluids. Bailinchuan Late Triassic syenites were formed in a post-collisional extensional setting, which provides time constraints on the major geodynamic setting at the northern NCC.