Geochemical and mineralogical contrasts between economic and uneconomic granite porphyries in the Beiya giant Au deposit, southwest China: Implications for petrogenesis and Cu–Au mineralization

Abstract

The Beiya giant porphyry Au deposit is the largest Cenozoic Au deposit in the Jinshajiang Cu–Mo–Au metallogenic belt. Many granite porphyries of ~36 Ma were exposed in the Beiya deposit, including the Wandongshan (WDS) and Hongnitang monzogranite porphyries (HNT–MPs) with economically important Au metal resources, the uneconomic WDS biotite MP, and Putaishan granite porphyry (GP), and subeconomic MPs at Nandaping (NDP) and Matouwan (MTW) prospects. To reveal the fundamental factors that lead to the different fertility of these granitic porphyries, this study uses the biotite and zircon geochemical compositions to evaluate their petrogenetic and magmatic conditions (e.g., F content, pressure, fO2). The results show that biotites from economic WDS and HNT–MPs exhibit moderate F contents (0.85%–2.16%), low n(FeT)/n(FeT + Mg) ratios (average of 0.36), and shallow emplacement depths of 0.19–1.76 km. In contrast, the uneconomic granite porphyries are characterized by variable F contents (0.17%–2.75%), high n(FeT)/n(FeT + Mg) ratios (average of 0.43), and deep emplacement depths of 2.76 to 5.02 km. Moreover, the economic WDS and HNT–MPs display higher average zircon fO2 (ΔFMQ [fayalite–magnetite–quartz] of +0.88) than uneconomic granite porphyries (ΔFMQ of −0.19), further confirming a higher fO2 of the economic granitic porphyries. These features indicate that magmatic fO2 is a critical factor controlling Cu–Au mineralization at the Beiya Au deposit. In comparison with the economic WDS and HNT–MPs, the subeconomic MPs in the MTW and NDP prospects exhibit higher average biotite n(FeT)/n(FeT + Mg) ratios of 0.47 and 0.45, and lower average zircon ΔFMQ of −0.05 and −0.23, suggesting lower magmatic fO2 and thus have relatively weak potentials to form Cu–Au mineralization. Notably, the economic WDS and HNT–MPs have higher biotite XMg ratios than those of the subeconomic and uneconomic granitic porphyries, indicating that the economic MPs were formed by the partial melting of mixed lower crustal materials with more mantle‐derived materials. Therefore, the addition of mantle‐derived materials played an important role in determining the fO2 of the granitic porphyry, thus primarily controlling its fertility of Cu–Au mineralization.