Ore genesis and hydrothermal evolution of the Huanggang skarn iron-tin polymetallic deposit, southern Great Xing'an Range: Evidence from fluid inclusions and isotope analyses

The southern Great Xing'an Range is one of the most important metallogenic belts in northern China, and contains numerous Pb-Zn-Ag-Cu-Sn-Fe-Mo deposits. The Huanggang iron-tin polymetallic skarn deposit is located in the Sn-polymetallic metallogenic sub-belt. Skarns and iron orebodies occur as lenses along the contact between granite plutons and the Lower Permian Huanggangliang Formation marble or Dashizhai Formation andesite. Field evidence and petrographic observations indicate that the three stages of hydrothermal activity, i.e., skarn, oxide and sulfide stages, all contributed to the formation of the Huanggang deposit. The skarn stage is characterized by the formation of garnet and pyroxene, and high-temperature, hypersaline hydrothermal fluids with isotopic compositions that are similar to those of typical magmatic fluids. These fluids most likely were generated by the separation of brine from a silicate melt instead of being a product of aqueous fluid immiscibility. The iron oxide stage coincides with the replacement of garnet and pyroxene by amphibole, chlorite, quartz and magnetite. The hydrothermal fluids of this stage are represented by L-type fluid inclusions that coexist with V-type inclusions with anomalously low delta D values (approximately -100 to -116 parts per thousand). The decrease in ore fluid delta O-18(H2O) values with time coincides with marked decreases in the fluid salinity and temperature. Based on the fluid inclusion and stable isotopic data, the ore fluid evolved by boiling of the magmatic brine. The sulfide stage is characterized by the development of sphalerite, chalcopyrite, fluorite, and calcite veins, and these veins cut across the skarns and orebodies. The fluids during this stage are represented by inclusions with a variable but continuous sequence of salinities, mainly low-salinity inclusions. These fluids yield the lowest delta O-18(H2O) values and moderate delta D values (-1.6 to -2.8 parts per thousand and -101 to -104 parts per thousand, respectively). The data indicate that the sulfide stage fluids originated from the mixing of residual oxide-stage fluids with various amounts of meteoric water. Boiling occurred during this stage at low temperatures. The sulfur isotope (delta S-34) values of the sulfides are in a narrow range of -6.70 to 450 parts per thousand (mean = -1.01 parts per thousand), and the oxygen isotope (delta O-18) values of the magnetite are in a narrow range of 0.1 to 3.4 parts per thousand. Both of these sets of values suggest that the ore-forming fluid is of magmatic origin. The lead isotope compositions of the ore ((206)pb/(204)pb = 18.252-18.345, (207)pb/(204)pb = 15.511-15.607, and (208)pb/(204)pb = 38.071-38.388) are consistent with those of K-feldspar granites ((206)pb/(204)pb = 18.183-18.495, (207)pb/(204)pb = 15.448-15.602, (208)pb/(204)pb = 37.877-38.325), but significantly differ from those of Permian marble ((206)pb/(204)pb = 18367-18.449, Pb-207/Pb-204 = 15.676-15.695, (208)pb/(204)pb = 38.469-38.465), which also suggests that the ore-forming fluid is of magmatic origin. (C) 2014 Elsevier B.V. All rights reserved.