International Geologiical Congress - Oslo 2008

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MRD-02 Deep sources and signatures of ore forming systems - a tool for new discoveries of mineral deposits

 

Transport of Au, Cu and Sn by vapor phase in hydrothermal systems: Experiments and case study

 

Shumin Hu, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Ronghua Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
Xuetong Zhang, Institute of Mineral Resources, Chinese Academy of Geological Sciences (China)
 

 

Experiments prove that Au, Cu and Sn can be redistributed in vapor phase and liquid phase during separation of the metal bearing supercritical fluids to vapor (V) and liquid (L) phases. Those experimental results can illustrate some ore genesis related to the metal transportations by vapor, where the boiling phenomena of ore fluids were often found.
Experimental system consists of liquid reservoir, pump, main pressure vessel, connection and continuous heating, V-L phase separator, temperature controllers, cooling system, pressure gauges, back pressure regulators, tubing etc. We prepared a 2 ppm gold solution (or 4 ppm Cu solution, or 4 ppm Sn solution), which is a 0.1mol NaHCO3-HCl-H2O solution. Au (Cu, or Sn) bearing NaHCO3-HCl solutions were heated up to more than 350°C in main vessel, then passed through a phase separator in the temperature range from 250° to 300°C, separated into vapor and liquid phase. We collected and analyzed the liquid and vapor sample separately, and found that metals (Au, Cu or Sn) are dissolved and redistributed in vapor phase. In some cases, the concentrations of metals in vapor are higher than those in liquid phase.
The experimental results also show that the vapor phase of the NaHCO3-HCl-H2O system has more HCO3-, CO32- , CO2 and less Na+ and Cl- concentrations than those in liquid phase. Referring to our experiments, Cu ion in vapor phase would behave the cluster form of CuClm(H2O)gn .and Au is the form of AuCl(H2O) gn, or SnClm(H2O)gn.
It is well known that a close association of fluid inclusion of hypersaline brine and lower salinity vapor is an important feature of all porphyry-type copper. Fluid inclusion study suggested that the boiling phenomena were found in some porphyry copper and hydrothermal gold mineral deposits, e.g. gold deposits in the western Shandong province, China. Salinity data of the fluid inclusions in the gold deposit can be divided into two groups: one is high salinity (14. 8 wt%NaCl) and other is low salinity (5 wt% NaCl) with the same homogenization temperature, about 400°C.
The experiments about tin and zinc transport by vapor are also carried out, which indicate that tin could be transported by vapor phase of NaCl-H2O hydrothermal system. But zinc would be transported by a vapor phase of CO2-rich solution in NaCl-H2O-CO2 system. Fluid inclusion observations in Dachang tin deposits, Southern China, provide evidence that vapor phase of a low salinity NaCl-H2O is one of the filling fluids in cassiterite. But fluid inclusions in sphalerite and arsenopyrite belong to a vapor phase of CO2-rich, low salinity NaCl-H2O- CO2. When boiling of ore fluids occurred, the fluids can be separated into vapor phase and liquid phase. Then Au, Cu, Sn are dissolved in vapor and transported by vapor.

 

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