Generation of Cu-Au skarn mineralization by mafic magma recharge: Insights from the Bozymchak deposit in the Chatkal-Kurama region, Kyrgyzstan, West Tianshan

ABSTRACT Large numbers of igneous intrusions are emplaced in the magmatic arcs above subduction zones, but only a small fraction of them are mineralized with Cu ± Au ± Mo. Despite significant advances in recognizing the importance of magma source, volatile content, and redox condition in controlling mineralization, the magmatic processes required to form a mineral deposit are still poorly understood. Here, we demonstrate that mafic recharge to the magma chamber is critical for Cu-Au mineralization, as exemplified by the Bozymchak Cu-Au skarn deposit in the Chatkal-Kurama arc of Kyrgyzstan, West Tianshan. The Bozymchak intrusion associated with the mineralization comprises monzonite porphyry and granodiorite porphyry. A sensitive high-resolution ion microprobe zircon U-Pb age of 305 ± 2 Ma was obtained for the monzonite porphyry, which is consistent with the U-Pb age of zircon from the granodiorite porphyry (304 Ma) and an Re-Os age for the molybdenite (305 Ma). These ages indicate that both the monzonite porphyry and granodiorite porphyry were coeval with mineralization. The monzonite porphyry exhibits high-K calc-alkaline (K2O = 3.36–3.50 wt%), metaluminous and magnesian-rich characteristics, enrichment in large ion lithophile elements (LILEs; e.g., Cs, Th, Ba, K, and Pb), and negative anomalies in high field strength elements (HFSEs; e.g., Nb, Ta, and Ti). These geochemical signatures, combined with the Sr-Nd-Hf isotopic signatures [ISr(avg) = 0.7059, εNd(t) = −3.5 to −2.8, εHf(t) = −3.8 to +1.4], suggest that the magma was the product of partial melting of a metasomatized mantle wedge and was subjected to subsequent assimilation and fractional crystallization (AFC). The development of disequilibrium textures in both the monzonite porphyry and the granodiorite porphyry, such as acicular apatite, reverse zoning of plagioclase, and coexisting high-Al and low-Al amphiboles, indicates that the magma evolved in an open crustal magma chamber with mafic magma recharge. The magmas were moderately oxidized (~ΔFMQ +1.0; FMQ is the fayalite-magnetite-quartz oxygen buffer) and water-rich, with 5.9 wt% and 6.5 wt% H2O in the granodiorite porphyry and monzonite porphyry, respectively. The repeated influx of hot mafic magma batches prolonged the existence of the magma chamber and promoted the maturation of the arc magma. In addition to introducing more Cu and Au, the replenishment of mafic magma in the chamber also introduced additional H2O and Cl, as shown by the presence of high-Al amphiboles and the elevated oxygen fugacity, which further promoted Cu-Au mineralization.