Geology, geochemistry and genesis of the giant Maoping carbonate-hosted Pb-Zn-(Ag-Ge) deposit in northeastern Yunnan Province, SW China

Abstract

The Maoping Pb-Zn deposit is spatially associated with the late Permian Emeishan basalts and structurally controlled by the NE-trending reverse fault-anticline tectonic system. Nevertheless, there remains limited comprehension of its genetic association with the Emeishan magmatism, as well as the coupling relationship between the overall metallogenic processes and geodynamic evolution. In order to clarify these scientific issues, the syn-ore hydrothermal calcites, sulfides, and sulfates were systematically collected from the representative No. I ore body at different elevations in the Maoping deposit for bulk C-O-S and in situ S-Pb isotopic analyses. Bulk C-O isotopic compositions of syn-ore calcite (δ13CV-PDB = − 4.94 to −0.81‰, δ18OSMOW = +17.66 to +21.16‰) and associated fluids (δ13CV-PDB = −4.56 to −0.48‰, δ18OSMOW = +8.63 to +12.13‰) indicate that C in fluids was mainly derived from ore-bearing carbonate rocks, whereas O was generated by fluid/rock (W/R) interaction process between initial fluids and carbonate rocks. Such process also triggered carbonate dissolution and syn-ore calcite precipitation. In situ Pb isotopic ratios of galena (208Pb/204Pb = 39.116–39.425, 207Pb/204Pb = 15.742–15.767 and 206Pb/204Pb = 18.603–18.753) are more comparable with those of ore-bearing sedimentary rocks, while in situ and bulk S isotopic compositions (δ34SPy+Sp = +20.06 to +23.86‰; δ34SGn = +17.26 to +18.44‰) of sulfides exhibit high consistency with those of sulfates (δ34SBrt = +17.01 to +19.07‰) within the ore-bearing strata. This suggests that the majority of metallogenic materials and reduced sulfur originated from this source. Meanwhile, the S-Pb isotopic data and relatively high ore-forming temperature (206 ℃) further reveal that the ore-bearing fluids underwent gradual migrated and evolved from the basement to the ore-hosting strata. The Emeishan magmatism not only provided the majority of the thermal power for the fluid circulation, but also contributed a portion of the ore-forming fluids and materials. During the late Triassic, ore-forming fluids migrated along compresso-shear reverse faults to coeval secondary extensional ore-hosting structures due to regional compressive tectonic stresses caused by the Indosinian orogeny and Xuefengshan intraplate orogeny. The mixing of metal-rich hydrothermal fluids with the marine evaporites (mainly gypsum) within sedimentary rocks first triggered thermos-chemical sulfate reduction (TSR) process. The subsequent continuous W/R interaction resulted in a decrease in fluid acidity and temperature, ultimately leading to the precipitation of metal sulfides.