Garnet U-Pb geochronology and geochemistry reveal deposit types and fluid evolution: An example from the Dongguashan Cu-Au deposit, eastern China

Abstract

Dongguashan is one of the largest Cu-Au deposits in eastern China, with ubiquitous garnet in the stratiform and skarn orebodies. The stratiform ore-formation remains enigmatic. In this study, we describe the diverse textures and present new U-Pb age and chemical data for the Dongguashan garnet. Garnet in the skarn orebody can be divided into three generations (Grt-1, Grt-2, and Grt-3). Grt-1 and Grt-2 show three zones from core to inner rim to outer rim (a-b-c). Garnet in the stratiform orebody is homogeneous (Grt-s). All garnet types have low MnO, a wide range of Y/Ho ratios, and visible fluid inclusions, indicating that they formed from hydrothermal replacement. Grt-1a, Grt-1b, Grt-2a, Grt-2c, and Grt-3 are oscillatory-zoned, low-Sn, and HREE-rich grandite, which may have formed from reduced HREE-rich fluids. Meanwhile, Grt-1c and Grt-2b are high-Sn, LREE-rich andradite, and may have formed from oxidized LREE-rich fluids. Grt-s is pure, low-Sn, LREE-rich andradite, and that may have formed from oxidized LREE-rich fluids. The Eu behaviors are sensitive to the fluid physicochemical conditions and Cl content. Grt-1, Grt-2, and Grt-s have positive Eu anomalies, and may have formed from acidic and Cl-rich fluids. Grt-3 has negative Eu anomalies and was possibly formed from neutral and Cl-poor fluids. All garnet types at Dongguashan may have formed by infiltration metasomatism in an open system with multiple metasomatic pulses. Grt-1, Grt-2, and Grt-s yielded nearly the same in-situ U-Pb ages of 135.9 ± 2.1 Ma, 135.6 ± 2.2 Ma, and 134.6 ± 1.4 Ma, respectively. We propose that both ore types at Dongguashan deposit are closely related to the Early Cretaceous plutonism.