Factors Controlling Deposition of Metallic Minerals in the Meng’entaolegai Ag-Pb-Zn Deposit, Inner Mongolia, China: Evidence from Fluid Inclusions, Isotope Systematics, and Thermodynamic Model

Abstract

The Meng’entaolegai Ag-Pb-Zn vein-type deposit in Inner Mongolia, NE China is hosted in biotite/muscovite granite. This deposit includes the western (Zn-rich, deepest), middle (Zn-Pb rich) and eastern (Pb-Ag-rich, shallowest) ore-blocks. To better understand the metallogenic processes in ore district, we have undertaken a series of studies including fluid inclusion microthermometry, H-O-S-Pb isotope compositions and thermodynamic modeling. Based on fluid inclusion petrography, microthermometry results and HO isotope compositions, the ore-forming H2O-NaCl fluid inclusions are characterized by medium temperature and medium salinity. And two kinds of fluid processes (boiling in western and middle ore-block and fluid mixing in the eastern ore-block) were identified to explain the ore fluid evolution. More importantly, log ƒO2- pH diagrams of δ34S contours with the stability fields of Fe- and Cu-, Zn-, Pb-, and Ag-bearing minerals were constructed to restore the physicochemical conditions of ore-forming fluid in the western (270 °C and 80 bars), middle (250 °C and 70 bars), and eastern (230 °C and 50 bars) ore-blocks. As a result, the ore-forming conditions in the western and middle ore-block were similar. In the eastern ore-block, the fluids may have changed from acidic, S-poor and δ34S(ΣS)≈2.8 to neutral, S-richer and δ34S(ΣS)≈0.5, which imply that neutral S-rich meteoric water was mixed with the magmatic fluid. Meanwhile, the activity of Ag+ was estimated to be about 10 ppm–9 ppm in the middle ore-block, but in the eastern ore-block it was about ~10 ppm–12 ppm. We proposed that the key for Ag ore deposition was likely to be neutralization led by fluid mixing.