Age and genesis of the Lamahanshan Ag-Pb-Zn deposit, southern Great Xing'an Range, northeastern China: Constraints from sphalerite Rb-Sr dating, fluid inclusions and H-O-S-Pb isotopes

Abstract

The Lamahanshan deposit in the eastern Central Asian Orogenic Belt (CAOB) is a newly-discovered Ag-Pb-Zn vein deposit in the southern Great Xing'an Range (GXR). Vein-type Ag-Pb-Zn orebodies are mainly hosted in the fracture zones of gneissic plagiogranite, granite porphyry, and quartz porphyry. Mineralization comprises four stages: (I) quartz-magnetite, (II) quartz-pyrite-chalcopyrite, (III) quartz-polymetallic sulfides, and (IV) quartz ± sphalerite. Sphalerite Rb-Sr dating yielded an isochron age of 137.7 ± 6.1 Ma, coeval with the regional Late Jurassic-Early Cretaceous Pb-Zn polymetallic mineralization. Three types of fluid inclusions (FIs), namely vapor-rich two-phase (LV-type), liquid-rich two-phase (VL-type), and daughter mineral-bearing three-phase (SL-type), are distinguished at Lamahanshan. Stage I and II contain all FI types with homogenization temperatures (Th) of 383–418 °C and 350–392 °C and salinities of 0.9–38.1 wt% and 0.7–35.0 wt%, respectively, whereas stage III contain VL- and LV-type FIs with Th = 318–349 °C and salinity = 0.7–10.5 wt%. Only VL-type FIs occur in stage IV, with Th of 271–320 °C and salinities of 4.3–7.8 wt%. The microthermometric and H-O isotope data (δ18OH2O = −9.4 to 0.9‰ and δD = −137.0 to −124.7‰) indicate that the ore-forming fluids were initially magmatic with later meteoric water incursion. Fluid boiling, cooling, fluid-rock reactions, and meteoric water mixing were the major ore precipitation mechanisms at Lamahanshan. The sulfide S (δ34S = 4.0–6.7‰), Sr ((87Sr/86Sr)i = 0.698957–0.700069), and Pb (206Pb/204Pb = 17.230–18.193, 207Pb/204Pb = 15.440–15.627, and 208Pb/204Pb = 37.664–38.407) isotopes also support that the metals were magmatic-derived. Integrating the available geological, mineralization, fluid inclusion, and H-O-S-Sr-Pb isotope evidence, we propose that the Lamahanshan deposit shares many similar features with distal magmatic-hydrothermal vein-type ores, and was formed in an extensional setting related to the Early Cretaceous Paleo-Pacific subduction rollback.