Origin and tectonic implications of the Zhaxikang Pb–Zn–Sb–Ag deposit in northern Himalaya: evidence from structures, Re–Os–Pb–S isotopes, and fluid inclusions

The Zhaxikang Pb–Zn–Sb–Ag–(Au) deposits, located in the eastern part of northern Himalaya, totally contain more than 1.146 million tonnes (Mt) of Pb, 1.407 Mt of Zn, 0.345 Mt of Sb, and 3 kilotonnes (kt) of Ag. Our field observations suggest that these deposits are controlled by N–S trending and west- and steep-dipping normal faults, suggesting a hydrothermal rather than a syngenetic sedimentary origin. The Pb–Zn–Sb–Ag–(Cu–Au) mineralization formed in the Eocene as indicated by a Re–Os isochron age of 43.1 ± 2.5 Ma. Sulfide minerals have varying initial Pb isotopic compositions, with (206Pb/204Pb)i of 19.04–19.68, (207Pb/204Pb)i of 15.75–15.88, and (208Pb/204Pb)i of 39.66–40.31. Sulfur isotopic values display a narrow δ34S interval of +7.8–+12.2‰. These Pb–S isotopic data suggest that the Zhaxikang sources of Pb and S should be mainly from the coeval felsic magmas and partly from the surrounding Mesozoic strata including metasedimentary rocks and layered felsic volcanic rocks. Fluid inclusion studies indicate that the hydrothermal fluids have medium temperatures (200–336 °C) but varying salinities (1.40–18.25 wt.% NaCl equiv.) with densities of 0.75–0.95 g/cm3, possibly suggesting an evolution mixing between a high salinity fluid, perhaps of magmatic origin, with meteoric water.