Fluid evolution and genesis of the Sankuanggou Fe-Cu skarn deposit, Duobaoshan ore field, Northeast China: Evidence from fluid inclusions and H-O-S-Pb isotopes

The Sankuanggou Fe-Cu skarn deposit is located in the Duobaoshan ore field in the northern Great Xing'an Range, Notheast China. The ore bodies occurred in the skarn of the contact zone between the Yanshanian granodiorite and the middle Ordovician Duobaoshan Formation marble. According to the field investigations and mineral crosscutting relationships, five paragenetic stages were identified: the anhydrous skarn (Stage I); hydrous skarn (Stage II); chalcopyrite-pyrite-quartz-molybdenite (Stage III); sphalerite-quartz stage (Stage IV) and quartz-calcite (Stage V). In the Stage I and II, the assemblage of fluid inclusions (FIs) in quartz is characterized by the development of daughter mineral-bearing three-phase FIs (S-type), vapor-rich two-phase aqueous FIs (LVtype) and liquid-rich two-phase aqueous FIs (VL-type). VL- and LV-type FIs were found in the Stage III quartz, whereas only VL-type FIs were present in the Stage IV quartz and Stage V calcite. The homogeneous temperatures of the FIs in Stages I to V were 364-495, 295-405, 243-360 and 135-207 degrees C, respectively, with salinities of 4.16-58.42, 4.17-47.99, 3.37-14.2, 1.03-11.11 and 3.53-8.4 wt% NaCl eqv., respectively. In general, the oreforming fluids changed from high-temperature NaCl-H2O system fluids, that contained negligible amounts of CO2 and CH4 and exhibited obvious boiling, to a medium- to low-temperature and low-salinity NaCl-H2O system with fluid mixing. According to the H-O isotopic analysis of Stages I and II (& delta;18OH2O = 0.3-6.3 %o, & delta;18D = -112.6 to -79.9 %o), the ore-forming fluid was identified as magmatic fluid. In contrast, the & delta;18OH2O and & delta;18D values in Stages III and IV gradually decreased (& delta;18OH2O = -9.6 to -2.3 %o, & delta;18D = -133.1 to -118.9 %o), implying a gradual meteoric water input into the fluid. The S isotopic analysis (& delta;34S range from -4 to -0.8 %o) revealed that S mainly originates from the magma during the mineralization process. Further, the Pb isotopic (208Pb/204Pb = 37.64-38.725, 207Pb/204Pb = 15.45-15.606 and 206Pb/204Pb = 18.115 to 19.786) investigations indicated that the ore-forming materials of this deposit are derived from the crust-mantle mixing. Based on the previously reported geochronologic and geodynamic data, we can conclude that the Sankuanggou deposit is a typical Fe-Cu skarn deposit and formed during the Early Jurassic Paleo-Pacific Plate subduction.