Using apatite to discriminate synchronous ore-associated and barren granitoid rocks: A case study from the Edong metallogenic district, South China

Abstract

In order to find criteria to discriminate the synchronous ore-associated and barren granitoid rocks, we have determined apatite chemistry associated with ore-associated (Cu-Au) and barren granitoid rocks in the Edong district of the Middle and Lower Yangtze River metallogenic belt, South China. Both rock types give zircon U-Pb ages between 135.0 and 138.7 Ma. Apatite has a higher volatile and Li content (Cl: 0.19–0.57 wt%, average 0.35 wt%, SO3: 0.08–0.71 wt%, average 0.32 wt%, Li: 0.49–7.99 ppm, average 3.23 ppm) in ore-associated rocks than those in barren rocks (Cl: 0.09–0.31 wt%, average 0.16 wt%, SO3: 0.06–0.28 wt%, average 0.16 wt%, Li: 0.15–0.89 ppm, average 0.36 ppm). Apatite (La/Yb)N ratios and Eu/Eu⁎ values are relatively high and show wider variation in ore-associated rocks than those in barren rocks. Apatite (La/Sm)N and (Yb/Sm)N show positive correlation in ore-associated rocks but negative in barren rocks. The higher volatile content occurs in ore-associated magma, favoring Cu-Au transportation and deposition. Furthermore, amphibole fractional crystallization in ore-associated magma further enriched the ore elements in the residual melt. Barren rocks may have undergone fluid exsolution before emplacement, which makes it barren in Cl, S and ore elements (Cu, S). These signatures emphases the significance of volatile and magma evolution in mineralization and indicate that analyses of magmatic apatite can serve to distinguish ore-associated from barren intrusions.