Garnet geochemistry of the giant Beiya gold–polymetallic deposit in SW China: Insights into fluid evolution during skarn formation

The Beiya deposit in the central Jinshajiang–Ailaoshan metallogenic belt of SW China is the country’s largest gold skarn and third largest gold deposit. In this study of the Beiya deposit, we use garnet mineralogy and geochemistry to delineate three skarn alteration zones (i.e., contact, proximal and distal skarn). We investigate the possible controls on the rare earth element (REE) incorporation into skarn-type garnet, the ore-fluid evolution, and metasomatic dynamics. Garnet in the Beiya deposit belongs to the grossular–andradite solid solution series, and comprises three types: (1) Grt1 andradite (Adr67–88Grs9–32) in the contact skarn, exhibiting light REE (LREE) enrichments, heavy REE (HREE) depletions, and positive Eu anomalies; (2) Oscillatory-zoned Grt2 andradite (Adr55–98Grs1–44) in proximal skarn, which has moderate to low LREE/HREE enrichments and varying Eu anomalies; (3) Grt3 andradite (Adr29–76Grs22–69) in distal skarn with low LREE/HREE enrichments and minor negative Eu anomalies. Grt1, Grt2 and Grt3 yielded lower intercept U–Pb ages of 36.4 ± 1.8 Ma (MSWD = 0.87, n = 67), 34.8 ± 0.7 Ma (MSWD = 1.8, n = 31) and 33.4 ± 1.0 Ma (MSWD = 1.2, n = 25). Garnet shows compositional changes from Grt1 to Grt3 due to the fluid-wallrock reactions. With decreasing andradite and increasing grossular contents, the (La/Yb)N ratio decreases. Ore-forming fluids changed from diffusive metasomatism to infiltration metasomatism, and then again to diffusive metasomatism. Meanwhile, the ore-fluid physicochemical conditions changed from acidic to near-neutral (both oxidised), whilst the water/rock ratio first increased and then decreased. The proximal skarn recorded high water/rock ratio with intermittent boiling events, leading to fluctuating oxygen fugacity, which is conducive to ore deposition. Therefore, the skarn-type garnet composition from the various alteration zones can reveal the fluid activity and physicochemical fluid evolution path, preserving rich information about the mineralisation process.