Polymetallic droplets within trapped globules in a quartz diorite porphyry from Gangcha–Kemo gold deposit, West Qinling orogen, China: Implications for petrogenesis and prospecting

As immiscibility in magmas appears most significant in magmatic–hydrothermal processes, studies of magmatic fluid immiscibility should focus on the precipitation of metals. A metallogenic belt along the northern margin of the Western Qinling Belt (China) hosts several porphyry-type Cu—Au deposits. Detailed field investigations reveal that abundant ellipsoidal and sub-spheroidal polymetallic globules exist in a quartz diorite porphyry associated with the Gangcha–Kemo gold deposit, which occurs at the eastern contact of the Dewulu intrusion. To reveal the characteristics and composition of these globules, we present mineralogical and whole-rock major- and trace-element data. The globule formation temperature, which was determined using by chlorite and two-feldspar geothermometers, is estimated at 297–429 °C. This temperature is much lower than the host quartz diorite porphyry, which is estimated to be 860–979 °C using an amphibole geothermometer. Based on the geochemistry of the globules, their polymetallic droplets, and the host quartz diorite porphyry, we infer immiscible separation from the volatile-rich quartz diorite porphyry and precipitation of the polymetallic droplets within the globules. Rapid migration and condensation of the fluids with limited escape of a fluid phase are inferred, with the globules representing trapped ore-forming elements. We propose that the fluid globules were exsolved from a rising magma carrying abundant ore-forming metals and that the globules with sulfide droplets can be used as an exploration tool for interpreting the genesis of mineralized rocks and related ore deposits.