Fluid inclusion and isotopic constraints on the origin of the Ag-Pb-Zn polymetallic mineralization at Biliya, Great Xingan Range, NE China

Abstract

The Biliya Ag-Pb-Zn polymetallic (SLZP) deposit (16.5 Mt, @ 52.9 g/t Ag, 2.6% Pb and 2.3% Zn) lies in the Great Xing’an Range, Northeast China. Breccia-, veinlet- and vein-type Ag-Pb-Zn ore bodies are primarily hosted in the quartz porphyry, trachyandesite and rhyolitic tuff. They are spatially and temporally related to andesitic porphyry. Three mineralization processes are identified: stage I: grey quartz - pyrite + tawny sphalerite (high Cd2+), stage II: grayish white quartz - pyrite + grey sphalerite (high Fe2+) + galena + argentite + tetrahedrite, stage III: white quartz-pyrite. The alterations consist of quartz + sericite, illite + quartz, fluorite + calcite + opal and chlorite zone. The silver-lead–zinc mineralization primarily corresponds to stage II. Five sulfide samples from stage II yielded a well-fitted isochron age of 131.3 ± 2.4 Ma (MSWD = 2.4), marking the timing of Ag-Pb-Zn polymetallic mineralization. CO2-bearing NaCl-H2O (C), gaseous CO2 (V); vapour-liquid H2O-NaCl (WL) and fluid-phase H2O-NaCl (L)-type of fluid inclusions were distinguished. Their petrographic and microthermometric features and H-O isotopic data suggest that the hydrothermal fluids were initially originated from magmatic source, with intensive mixing with meteoric water and that their temperatures and salinities are 254–130 ℃ and 7.15–1.22 wt% NaCl eqv, respectively, belonging to a reduced fluid (CO2-H2O-NaCl ± CH4) system. In-situ S and bulk Pb isotope analyses indicate that the metal materials were related to andesitic magma from the lower crust. The deposit geology, fluid inclusions, stable isotopes and chronology results suggest that the Biliya deposit belongs to a low-sulfidation (LS) epithermal SLPZ deposit. Boiling and fluid immiscibility coupled with suddenly decreasing in temperature, pressure, logfS2 of the hydrothermal fluids are likely to play important roles in Biliya SLPZ ore deposition.