Discovery of the 35 Ma granite porphyry dikes in the giant Dachang Sn ore field: Tectonic and metallogenic implications

The world's second-largest Dachang Sn ore field in the southwest of China exhibits genetic connections to the Late Cretaceous highly evolved S-type granitoids (93-91 Ma). In this study, we report the newly identified Eocene granite porphyry dikes with zircon U-Pb ages of 35-33 Ma at the Gaofeng Sn deposit from the Dachang ore field. The petrogenesis and mineralization potential are also constrained by employing whole-rock and zircon geochemical data. These Eocene dikes coincide with the emplacements of granitoids (35-30 Ma) along the Ailaoshan-Red River fault and its neighboring regions, indicating that they could be related to a similar lithospheric extension setting after the Indo-Asian collision. However, unlike those of the latter granitoids (35-30 Ma), which are grouped into I-type granites, the high-silica granite porphyry dikes show elevated ASI values (1.32-1.38) with high P2O5 (0.41-0.44 wt%) and normative-CIPW corundum (4.15-4.71 wt%) contents and could be classified as S-type granites. The strongly negative whole-rock Sr, Ba, and Eu anomalies signify separation of plagioclase and K-feldspar during magma crystallization, which is also confirmed by the zircon trace elements compositions. The Gaofeng Eocene granite porphyry dikes have a high initial zircon Tti-zr of 855 degree celsius, high Ba/Pb ratios (11.5-16.8), and low Pb contents (7.56-9.98 ppm) with low zircon epsilon(Hf)(t) values (-7.4 to -0.6), hinting at the dehydration melting of biotite from the metasedimentary basement. Additionally, the dikes are characterized by low oxygen fugacity (zircon Ce/Ce* <70; Delta FMQ from -6.0 to +0.6 with an average of -2.0) and show evidence of magmatic-hydrothermal interaction (whole-rock Nb/Ta < 5), in line with those of the Dachang Late Cretaceous fertile granites, suggestive of Sn mineralization potential. Therefore, it is recommended that attention should be paid to the Eocene granite porphyry dikes during future prospecting.