Geology and geochronology of the Banchang distal Cu-Mo skarn deposit, Central China

Abstract

Banchang is the largest Cu-Mo deposit (348 Mt @ 0.32% Cu and 428 Mt @ 0.07% Mo) in the Qinling orogenic belt, Central China. Orebodies are hosted in the contact between several granitoid dikes and graphite-bearing marble of the Neoproterozoic Yanlinggou Formation. Wallrock alteration comprises garnet skarn, pyroxene skarn, chlorite skarn, and stockwork chlorite-altered marble. Three hydrothermal stages are indicated: (I) prograde skarn stage, (II) retrograde skarn stage, and (III) main sulfides stage. New zircon U-Pb data show two magmatic events, including early Paleozoic granite porphyry (442 − 427 Ma), and late Mesozoic biotite monzogranite porphyry and late Mesozoic granite porphyry (147 − 146 Ma). The zircon trace element compositions show that the late Mesozoic granitoids with ∆FMQ = -0.7 to 2.3 (avg. 0.5; Eu_N/Eu_N* > 0.6) resemble the Cu ore-related granitoid in the Qinling orogenic belt and indicate high oxygen fugacity and water contents in the magma. The early Paleozoic granitoids have ∆FMQ= -2.4 to 0.6 (avg. -0.7; Eu_N/Eu_N* < 0.2). Electron Probe Microanalysis (EPMA) on the prograde garnet and pyroxene reveal predominantly andradite (And_{40 − 94}Gro₀₋₅₁) and diopside (Di_{53 − 98}Hd₁₀₋₅₅) compositions, respectively. The garnet Fe³⁺ contents decreases whereas the pyroxene Fe²⁺ contents increases from the late Mesozoic granitoid dikes to the marble. This suggests a gradual evolution of the skarn from an oxidized to a reduced state. Stage III sulfide minerals have δ³⁴S = -2.1 to 4.8‰, indicating a magmatic origin. The temporal-spatial relationships, magmatic oxygen fugacity and water contents, zoning of prograde skarn minerals, and the source of sulfur indicate a genetic link between the skarn mineralization and late Mesozoic granitoid dikes. This study proposes a distal Cu-Mo skarn ore deposit model associated with granitoid dikes and some implications for mineral exploration in the Qinling orogenic belt and elsewhere.