Formation of the Panshi nephrite deposit, NE China: Constraints from mineralogy, in situ zircon U-Pb geochronology, and oxygen isotopes

• Hydrothermal zircons constrain the precise age of nephrite formation. • Monte Carlo simulation quantifies water-rock interactions in nephrite genesis. • A single, prolonged fluid pulse can generate large nephrite deposits. The dolomite-related Panshi nephrite deposit in Jilin Province, northeastern China, represents a significant resource of nephrite. Comprehensive field observations and petrographic investigations have identified six mineral assemblage zones: syenogranite, actinolite-altered syenogranite, diopside–tremolite, nephrite, calcite–tremolite, and dolomitic marble. These zones collectively represent part of a typical magnesium skarn zonation. Within these zones, four types of zircons have been classified to determine the ages and processes of nephrite mineralization. Type-I and Type-Ⅱ zircons, both magmatic from the syenogranite zone and the actinolite-altered syenogranite zone, respectively, exhibit similar compositions and concordia 206 Pb/ 238 U ages of 265 ± 1 Ma and 266 ± 1 Ma, respectively. Type-Ⅲ zircons in the calcite–tremolite zone are inherited from both the syenogranite and other regional rocks. They present 206 Pb/ 238 U ages spanning from 436 to 261 Ma, which are uninformative regarding nephrite mineralization. Type-Ⅳ zircons in the nephrite zone are of hydrothermal origin and yield a concordia 206 Pb/ 238 U age of 254 ± 1 Ma. The U-Pb age of four types of zircons indicates that the syenogranitic magma intruded at ca. 265 Ma while nephrite mineralization occurs at a lagged age of ca. 254 Ma. These ages, coupled with the similar composition and oxygen isotope composition of minerals from nephrites, further suggest a single ∼ 10 Myr prolonged nephrite mineralization event rather than a multi-stage metasomatism. Moreover, differences in non-formula elements, such as rare earth elements (REE), Y, Nb, and Ta, between magmatic and hydrothermal zircons, along with the oxygen isotope variation in six skarn assemblage zones, suggest that the nephrite mineralization is driven by the water–rock interaction which is characterized by the incremental infiltration of external fluids (meteoric water and CO 2 ). Monte Carlo simulations have been conducted to quantify that the water/rock atomic ratio in terms of oxygen ranges from 1.03 to 1.62, with the magmatic water predominating (Xmagmatic water ≥ 0.60). In addition, petrographic observations of the gradual transition of fine- to coarse-grained tremolites from the nephrite orebody to the calcite–tremolite zones, indicate a reduction in the fluid’s supersaturation level with respect to Ca, Mg, and Si. The iron in the fluid was sequestered by Fe-rich actinolite overprinting the syenogranite, thereby reducing the Fe content of the residual fluid and promoting the formation of white nephrite. The study of the Panshi nephrite deposit outlines a new model for nephrite mineralization in which protracted water-rock interactions, initiated by a one-pulse fluid event characterized by a high level of supersaturation with respect to Ca, Mg, and Si in the fluid and low Fe content in the system, can lead to the formation of substantial magnesian skarn nephrite deposits. This model provides an alternative understanding of the genesis of magnesian skarn nephrite deposits, particularly for reported occurrences worldwide that lack clear evidence of multistage metasomatic overprinting.