Geochemistry and evolution of ore-forming fluids of the Yueshan Cu-Au skarn- and vein-type deposits, Anhui Province, South China

摘要

The Yueshan mineral belt is geotectonically located at the centre of the Changjiang deep fracture zone or depression of the lower Yangtze platform. Two main types of ore deposits occur in the Yueshan orefield: Cu-Au-(Fe) skarn deposits and Cu-Mo-Au-(Pb-Zn) hydrothermal vein-type deposits. Almost all deposits of economic interest are concentrated within and around the eastern and northern branches of the Yueshan dioritic intrusion. In the vicinity of the Zongpu and Wuhen intrusions, there are many Cu-Pb-Zn-Au-(S) vein-type and a few Cu-Fe-(Au) skarn-type occurrences. Fluid inclusion studies show that the ore-forming fluids are characterised by a Cl~-(S)-Na~+-K~+ chemical association. Hydrothermal activity associated with the above two deposit types was related to the Yueshan intrusion. The fluid salinity was high during the mineralisation processes and the fluid also underwent boiling and mixed with meteoric water. In comparison, the hydrothermal activity related to the Zongpu and Wuhen intrusions was characterised by low salinity fluids. Chlorine and sulphur species played an important role in the transport of ore-forming components. Hydrogen- and oxygen-isotope data also suggest that the ore-forming fluids in the Yueshan mineral belt consisted of magmatic water, mixed in various proportions with meteoric water. The enrichment of ore-forming components in the magmatic waters resulted from fluid-melt partitioning. The ore fluids of magmatic origin formed large Cu-Au deposits, whereas ore fluids of mixed magmatic-meteoric origin formed small- to medium-sized deposits. The sulphur isotopic composition of the skarn- and vein-type deposits varies from -11.3 per thousand to +19.2 per thousand and from +4.2 per thousand to + 10.0 per thousand, respectively. These variations do not appear to have been resulted from changes of physicochemical conditions, rather due to compositional variation of sulphur at the source(s) and by water-rock interaction. Complex water-rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulphur mixing. Lead and silicon isotopic compositions of the two deposit types and host rocks provide similar indications for the sources and evolution of the ore-forming fluids. Hydrodynamic calculations show that magmatic ore-forming fluids were channelled upwards into faults, fractures and porous media with velocities of 1.4 m/s, 9.8 X 10~(-1) to 9.8 X 10~(-7) m/s and 3.6 X 10~(-7) to 4.6 X 10~(-7) m/s, respectively. A decrease of fluid migration velocity in porous media or tiny fractures in the contact zones between the intrusive rocks and the Triassic sedimentary rocks led to the deposition of the ore-forming components. The major species responsible for Cu transport are deduced to have been CuCl, CuCl_2~-, CuCl_3~(2-) and CuClOH, whereas Au was transported as Au_2(HS)_2S~(2-), Au(HS)_2~-, AuHS and AuH_3SiO_4 complexes. Cooling and a decrease in chloride ion concentration caused by fluid boiling and mixing were the principal causes of Cu deposition. Gold deposition was related to decrease of pH, total sulphur concentration and fO_2 which resulted from fluid boiling and mixing. Geological and geochemical characteristics of the two deposit types in the Yueshan mineral belt suggest that there is a close genetic relationship with the dioritic magmatism. Geochronological data show that the magmatic activity and the mineralisation took place between 130 and 136 Ma and represent a continuous process during the Yanshanian time. The cooling of the intrusions and the mineralisation event might have lasted about 6 Ma. The cooling rate of the magmatic intrusions was 80 to 120 deg C my~(-1), which permitted sufficient heat supply by magma to the ore-forming system.