Geochronology of the giant Beiya gold-polymetallic deposit in Yunnan Province, Southwest China and its relationship with the petrogenesis of alkaline porphyry

Abstract

The Beiya gold-polymetallic deposit, located at the junction of the “Sanjiang” Tethys–Himalaya orogenic belt and the Yangtze craton in southwestern China, is the largest gold deposit in the region. Mineralization mainly occurs in skarn, which formed along the contacts between porphyry intrusions and limestone host rocks. The ore formation process is suggested to be closely related to the Himalayan age alkaline porphyries. U–Pb dating of magmatic zircons from the Beiya quartz syenite porphyry yields a weighted average 206Pb/238U ages of 36.07±0.43Ma (n=25, MSWD=1.17). Calculated isochron age of five molybdenite samples from Beiya is 34.7±1.6Ma (MSWD=0.99, initial 187Os of 0.21±0.16ppb), which indicates that mineralization is coeval with the intrusion of the Beiya alkaline porphyry. This age is also in agreement with the results of apatite fission track analysis in the porphyry. Magmatic zircon εHf(t) values range from −7.4 to +0.1, which correspond to the two-stage Hf model with ages between 1112Ma and 1588Ma. This finding suggests that the main source of porphyry was probably melting of thickened Mesoproterozoic lower crust triggered by partial melting of the lithospheric mantle. Zircon Ce (IV)/Ce (III) ratios of ore-related porphyry are high (given that all EuN/EuN* ratios are above 0.4), which demonstrates that the magma source of the porphyry was highly oxidized, which was favorable for Cu–Au mineralization. The medium Re concentrations in molybdenite (Re=2.08–20.54ppm) suggest that the ore-forming materials were characterized by mixed crustal–mantle sources. Mineralization in the Beiya deposit was one of the many metallogenic events related to the India–Eurasia collision that was responsible for the fertile deep-sourced magmatism.