The genesis and mineralization process of Xiaobaliang VHMS Cu-Au deposit, Central Asian Orogenic Belt: constraints from the pyrite geochemistry and zircon geochronology

The Xiaobaliang Cu-Au deposit is located in the Hegenshan ophiolite complex belt of the western Xing’an–Mongolia Orogenic Belt (XMOB). Strata-bound sulfide ores are discovered within the Carboniferous volcanic tuff, with the oxidized Au-Cu ores at the top and Cu (-Au) sulfide ores at the bottom. Zircon U-Pb dating on volcanic tuff has yielded a concordia age of 326 ± 3 Ma (n = 21, MSWD = 5.6). Four types of pyrite have been identified, including lamellar pyrite (Py1) in the wall rocks, framboidal pyrite (Py2) in the Zn-rich massive ores, colloform pyrite (Py3) in the massive Cu ores, and euhedral pyrite (Py4) in the fractured Cu ores. Chalcopyrite comprises two types: early massive chalcopyrite aggregates (Cp1) and late chalcopyrite veinlets (Cp2). Here we present the in situ trace elements and sulfur isotopes analyses of different sulfide generations. Elements like Se (44.1 ppm), Co (227 ppm) and Cu (3211 ppm) are enriched in pyrites from Cu ores (Py3&Py4), whereas the pyrites from altered rocks (Py1) take high Mn (5027 ppm), Ti (134 ppm), V (13.0 ppm), Ni (41.9 ppm) and Au (5.07 ppm) concentrations. Similar elemental patterns are observed in chalcopyrite analyses where Co (312 ppm), Se (566 ppm) and Te (31.5 ppm) are enriched in massive Cu ores (Cp1), but Mn (147 ppm), V (174 ppm) and Ti (42.2 ppm) contents are mainly concentrated in chalcopyrite veinlets (Cp2). In situ S isotope analyses exhibit a large variation in Py1 (−30.70 to −1.50‰) and Cp2 (−7.81 ∼ −4.50‰) but are constant in Py2, Py3, Py4 and Cp1 (0.44 to 3.15 ‰). The high Co/Ni ratios and relatively lower δ34S values in Py1 suggest a diagenetic environment where the lower-temperature and extremely reduced conditions facilitate the enrichment of Mn, V and Au. The progressive enrichment of Sn, Te, Bi, Se and Cu in hydrothermal Py3 and Py4 then points out the rising of temperature in the weakly reduced environment. The negative sulfur isotopes in altered rocks (Py1) and fractured ores (Cp2) point out the contribution of the volcanic-sedimentary sequence. These compositional features, combined with sulfur isotopes, have demonstrated that Cu was mainly derived from magmatic-hydrothermal fluids whereas Au was released from Au-rich sediments on the seafloor. The Xiaobaliang Cu-Au deposit belongs to a Cyprus-type VHMS deposit formed in the Carboniferous extensional setting.