Evolution and origin of the Bairendaba Ag–Pb–Zn deposit in Inner Mongolia, China: Constraints from infrared micro-thermometry, mineral composition, thermodynamic calculations, and in situ Pb isotope

Abstract

The Bairendaba deposit is a typical large-scale Ag–Pb–Zn vein-type deposit in the Southern Great Hinggan Range, Inner Mongolia, China. Vein-type orebodies are hosted within biotite–plagioclase gneiss and quartz diorite and controlled by EW- and NE-trending faults with extensive hydrothermal alteration. Three primary mineralization/alteration stages have been recognised as the early ore stage, forming ore-barren quartz veins with arsenopyrite and pyrite, the primary ore stage, forming Zn-rich sulphide veins and Pb–Ag-rich sulphide–sulfosalt veins and the late barren stage, characterised by plenty of calcite, muscovite and fluorite veins. Infrared micro-thermometric data and mineral geothermometer show temperatures of ∼300 °C and ∼180 °C for Zn mineralization and Pb–Ag mineralization, respectively. A combination of mineralogical observations and thermodynamic calculations show that Zn mineralization is at pH 5.1 to 7.2 and logfO2 of −39.7 to −35.3, whereas Pb–Ag mineralization is at pH 5.6 to 7.4 and logfO2 of −48.2 to −40.3. The sulfidation state is estimated to be intermediate-low sulfidation. Conductive heat loss and extensive wall rock alteration are inferred to cause the Zn deposition, whereas an influx of meteoric water and wall rock alteration primarily trigger the large-scale Ag deposition. Laser ablation–inductively coupled plasma–mass spectrometry analysis shows that chalcopyrite is anomalously enriched in Sn (>1000 ppm). Sn is hosted in chalcopyrite of Zn mineralization in solid solution, whereas it exists as a stannite group phase in chalcopyrite of Pb–Ag mineralization. The occurrence of stannite group minerals in chalcopyrite of Pb–Ag mineralization might imply the decomposition of the Sn-bearing chalcopyrite at low temperature. In situ Pb isotope ratios of galena (206Pb/204Pb = 18.374–18.399, 207Pb/204Pb = 15.581–15.601 and 208Pb/204Pb = 38.332–38.416) indicate that the ore metals could be derived from the Mesozoic magma. Trace element compositions of sulphides in the Bairendaba deposit share the same characteristics with magma-related systems but differ considerably from non-magmatic and epithermal deposits. These results, combined with geological and isotopic evidence, suggest that the Bairendaba deposit is a magma-related hydrothermal vein-type deposit, indicating further exploration prospects for Sn mineralization near and/or under the Bairendaba mining area.