Ore Genesis of the Bagenheigeqier Pb-Zn Deposit, Inner Mongolia: Constraints from Fluid Inclusions, H-O Isotopes and In Situ Trace Element Compositions of Sphalerite

The Bagenheigqier medium-sized Pb-Zn deposit is located in central-southern segment of Great Xing’an Range, northeastern China, where its vein-type orebodies are hosted within the structural contact zone between the Lower Permian Dashi Formation and granite porphyry intrusions. Five mineralization stages are divided into skarn (I), oxide (II), quartz-pyrite-arsenopyrite (III), quartz-polymetallic sulfide (IV), and quartz-calcite-pyrite (V). Three types of fluid inclusions (FIs) are identified in Bagenheigeqier Pb-Zn deposit, including daughter mineral-bearing three-phase (SL-type), vapor–liquid two-phase (VL-type), and vapor-rich two-phase (LV-type) FIs. All FI types occur in Stages I–III, with homogenization temperatures (Th) of 423–486, 389–441, 362–408 °C, and salinities of 1.1–49.2, 0.9–43.9 and 0.9–38.8 wt.% NaCl equiv, respectively. Stage IV hosts only VL- and LV-type FIs (Th: 277–319 °C; salinity: 2.1–8.7 wt.% NaCl equiv), whereas Stage V contains exclusively VL-type FIs with Th of 173–214 °C and salinity of 1.2–5.7 wt.% NaCl equiv. The H-O isotopic results of quartz in stage II–IV (δD = −103.5‰–−99.1‰, −115.7‰–−107.8‰ and −121.5‰–−117.2‰; δ18OH2O = 4.4‰–7.1‰, 1.1‰–3.5‰ and −4.6‰–−3.5‰) indicate the ore-forming fluids are predominantly of magmatic origin with subordinate meteoric water mixing. Fluid boiling and the mixing of meteoric water may lead to the precipitation of metal. The in situ trace elements analyses indicate that sphalerites in main mineralization stage are enriched in Fe, Mn, Co and In and depleted in Ga and Ge. The calculation results suggest that the sphalerites crystallized under moderate temperature conditions (286–330 °C) and intermediate fS2 (−10.5 to −9.2) conditions. The geological, fluid inclusion, isotopic and trace element evidences indicate that the Bagenheigeqier deposit is classified as a skarn-type deposit.