Geochemical discrimination between fertile and barren Eocene potassic porphyries in the Jinshajiang Cu–Au–Mo metallogenic belt, SW China: Implications for petrogenesis and metallogeny

Abstract

The Jinshajiang potassic porphyry metallogenic belt represents one of the most important Chinese intrusive belts that hosts Eocene Cu–Au–Mo porphyry systems including the Narigongma Mo, Yulong Cu, and Beiya Au deposits. This metallogenic belt also contains several uneconomic potassic intrusions, such as the Jianchuan, Shigu and Binchuan granite porphyries. Bulk-rock geochemical compositions together with Sr–Nd–Pb isotope ratios, and zircon Hf isotope and trace element compositions are compiled and interpreted to reveal the fundamental differences leading to the different fertility of these potassic porphyries at Jinshajiang. The economic Narigongma, Yulong, Beiya, and barren Jianchuan, Binchuan, Shigu granite porphyries are all formed in the same post-collisional tectonic setting, and all exhibit high SiO2 (61–65 wt%), and total alkali contents (K2O + Na2O > 8 wt%). Based on low Ce contents (<10), FeOT/MgO ratios (<100) ratios and A/CNK values (<1.1), these granite porphyries are classified as potassic I-type granites. High (87Sr/86Sr)i ratios (0.70669 to 0.70786), low bulk-rock εNd(t) values (−8.7 to −0.7), MgO (0.2–1.4 wt%), Cr (0.1–36.4 ppm) and Ni (0.4–26.8 ppm), and radiogenic Pb isotopic compositions suggest that the granite porphyries, both fertile and barren, were most likely generated by partial melting of thickened lower crust with the addition of mantle-derived magmas. However, the fertile Narigongma, Yulong and Beiya granite porphyries have low Y (7.8–16.0 ppm) and Yb (0.6–1.5 ppm) contents, high (La/Yb)N ratios (31.3–98.9) and overall steeper REE profiles, reflecting residual garnet during partial melting of thickened lower crust, whereas the barren Jianchuan, Shigu and Binchuan granite porphyries have high Y (12.3–19.4 ppm) and Yb (1.1–2.5 ppm) contents, low (La/Yb)N ratios (25.2–44.3) and flat HREE patterns, indicating a garnet-poor magma source. Garnet that remains as a residual mineral during partial melting of thickened lower crust can drive magma oxidation, as shown by higher zircon Ce4+/Ce3+ ratios (averaging 75.6) and high fO2 values (averaging ΔFMQ = +3.9) of the fertile Yulong and Beiya granite porphyries than those of the barren Shigu and Jianchuan granite porphyries with average Ce4+/Ce3+ = 18.2 and ΔFMQ = +0.6. Therefore, magma source played an important role in determining the fO2 of the granite porphyry, thus controlling its fertility for Cu–Au–Mo mineralization.