Geology, Fluid Inclusions and Stable Isotopes of the Xialiugou Polymetallic Deposit in North Qilian, Northwest China: Constraints on its Metallogenesis

The Xialiugou polymetallic deposit is located in the North Qilian Orogenic Belt, Northwest China, of which the main ore-bearing strata are the Middle Cambrian Heicigou Group. The mineralization is zoned with “black” orebodies (galena–sphalerite), which are stratigraphically above the “yellow” orebodies (pyrite–chalcopyrite–tennantite) at the lower zone, corresponding to the alteration assemblages of quartz–sericite in the ore-proximal zone and chlorite in the ore-distal zone. The Xialiugou mineralization can be divided into three stages: (1) Stage I (pyrite); (2) Stage II (chalcopyrite–tennantite–sphalerite); and (3) Stage III (galena–sphalerite). Fluid inclusions data indicate that the physicochemical conditions that lead to ore formation were the medium–low temperature (157–350 °C) and low salinity (0.17–6.87 wt % NaCleqv), and that the ore-forming temperature tended to decrease with the successive mineralization processes. Taking the H–O isotopic compositions (δDV-SMOW = −51.0‰ to −40.5‰, δ18OH2O = −0.4‰ to 8.6‰) into consideration, the ore-forming fluids were most likely derived from seawater with a small amount of magmatic- and meteoric-fluids input. In addition, the combined S (−3.70‰ to 0.10‰) and Pb isotopic (206Pb/204Pb = 18.357 to 18.422, 207Pb/204Pb = 15.615 to 15.687, 208Pb/204Pb = 38.056 to 38.248) data of pyrite indicate that the ore-bearing volcanic rocks may be an important source of ore-forming materials. Finally, we inferred that the Xialiugou deposit shares similarities with the most important volcanogenic massive sulfide (VMS) deposits (Baiyinchang ore field) in China and typical “black ore” type VMS deposits worldwide.