Constraints on the Genesis of the Jiande Polymetallic Copper Deposit in South China Using Fluid Inclusion and O-H-Pb Isotopes

ABSTRACT The Jiande copper deposit is located in the Qin-Hang metallogenic belt, South China. The deposit is dominated by “stratiform-like” ores, which are hosted in the dolomite of the Upper Carboniferous Huanglong Formation. These ore bodies were previously proposed to be Carboniferous sedimentary exhalative (Sedex) style mineralization, but they also appear to be related to the Late Mesozoic granodiorite porphyry at Jiande. Three stages of mineralization can be observed. The prograde skarn minerals garnet and diopside were formed in the pre-ore stage. The “stratiform-like” ores, with minor quartz-polymetallic veins, were formed in the main mineralization stage. The post-ore stage is characterized by quartz-calcite ± pyrite veins. Fluid inclusions in quartz from the pre-ore skarn, the main stage of mineralization, and post-ore quartz–calcite ± pyrite veins were studied. Fluid inclusion petrography shows that two-phase liquid-rich (Type I), two-phase vapor-rich (Type II), and halite-bearing (Type III) fluid inclusions can be identified in the studied hydrothermal quartz samples. Primary Type II and Type III fluid inclusions only occur in quartz from the the main stage, whereas Type I fluid inclusions are present in all three stages of hydrothermal quartz. Type I fluid inclusions in pre-ore stage have homogenization temperatures of 290-368°C and salinities of 2.6-8.8 wt.% NaCl equiv. Type II and coexisting Type III fluid inclusions in the main stage share similar homogenization temperatures of 293 to 334 °C and 290 to 326 °C,but have two contrasting salinity ranges of 1.2 to 2.2 wt.% and 31.87 to 38.16 wt.% NaCl equiv, respectively. The coexistence of Type II and Type III fluid inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred. Type I fluid inclusions in the post-ore quartz–calcite veins have homogenization temperatures of 202-278 °C and salinities of 0.2-6.5 wt.% NaCl equiv. The hydrogen and oxygen isotopes (δD ranges from -78 ‰ to -61 ‰, δ18OH2O calculated from quartz are from 8.1 ‰ to 10.6 ‰) of fluid inclusions in quartz imply that ore fluids were principally derived from magmatic water. The lead isotopes of sulfide from the ores are close to the orogenic Pb evolution curve and are also similar to those of the Jiande Jurassic igneous rocks, suggesting that the metals were sourced mostly from the Jurassic igneous rocks. The skarn alteration, inferred boiling fluid inclusion assemblages, stable isotopic features of magmatic fluids, and metal source imply that the main stages of mineralization in the Jiande polymetallic copper deposit is of skarn-type, related to Jurassic granodiorites.