Texture and trace element geochemistry of quartz in skarn system: perspective from Jiguanzui Cu-Au skarn deposit, Eastern China

The Jiguanzui Cu–Au deposit is an important skarn deposit in the Edong ore district of the Middle–Lower Yangtze River Valley Metallogenic Belt (MLYRB), Eastern China. The skarn mineralization occurs dominantly in the intrusive contact between quartz diorite and marble of the Lower-Middle Triassic Jialingjiang Formation. The main mineralization stage (quartz-sericite-pyrite-chalcopyrite) is characterized by abundant quartz, which can be grouped as quartz in orebodies (Qtz1), quartz in quartz-pyrite hydrothermal veins in hornfels of the Middle Triassic Puqi Formation and quartz diorite (Qtz2 and Qtz3, respectively). Cathodoluminescent (CL) imaging reveals core-rim texture in Qtz1 and oscillatory zoning in Qtz2 and Qtz3, suggesting that Qtz1 was formed under low crystallization rate, in a relatively stable closed system, whereas Qtz2 and Qtz3 were formed under high crystallization rate, in an open system with oscillatory physicochemical fluctuations. As the main trace element in the Jiguanzui quartz, aluminum is incorporated into quartz as trivalent Al³⁺ to substitute Si⁴⁺. Monovalent alkali metal (esp. Li⁺) and divalent cations (Ge²⁺, Sr²⁺) act as charge compensators for the Al3+ substitution. Titanium concentrations (mostly <10 ppm) and the clear positive Al vs. Li correlation indicate the main mineralization at Jiguanzui formed under low temperature (<350 °C). Additionally, CL reflectivity of Qtz1 increases from core to rim, suggesting neutralization of the acidic ore fluids, which is interpreted to be an important cause of metal sulfide precipitation at Jiguanzui.