A supergene gold deposit is located at Shewushan, south Hubei, Central China. The Au mass fraction of the supergene deposits ranges from 1.0.10-6 to 19.5 10-6, with an average of about 2.2 10-6. Primary gold mineralization is closely associated with the well developed faults and fractures mainly on the crest of the reverse anticline with gold mass fraction exceeding1.0 X 10-6(average 0.6 X 10-6 ), the gold occurs as small Ag-bearing particles in association with illite and kaolinite, and partly incorporated within the lattice of pyrite. Fluid inclusion studies on the gold mineralization yield trapping temperatures and pressures from 110 to 290 and 410 105 to 460 105 pa, respectively. The salinities of mineralization fluids range from 2. 6 % to 8. 4 % w(NaCl, equivalent). Calculated (18O) values and measured (D) data of the hypogene fluids indicate a meteoric origin. These fluids underwent extreme 18 O enrichment by reacting with country carbonate rock. Supergene gold minerali- zation is confined largely to the lower portion at the weathered mantle; the gold occurs as small native gold particles accompanied by clay minerals, kaolinite and illite and Mn and Fe oxides. Geochemical investigation shows that Au, together with As and Sb, decreases progressively from the top of ore body to some 2 kilometers away and define a lateral dispersion halo. It is proposed that the gold is mobilized by surficial leaching and concentrated at the lower portion of the weathering profile, and thio-sulfate may play an active role in gold dissolution and re-precipitation.
Hong Hanlie Wang Qinyan The Center for Material Research and Testing, Wuhan University of Technology, Wuhan 430070 Bian Qiujuan Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074
The density function and discrete variation method ( DFT - DVM) is used to study the interaction between kaolinite and gold. The correlation among the structure, chemical bond and stability is discussed. Several models are selected without gold and with gold in different directions and sites . The results show that the models with gold on the edge of kaolinite basal layer are more stable than those with gold above or under the layer, the models ivith gold near to [ A1O2 ( OH)4 ] octahedra are more stable than those with gold near to the vacancy without aluminium. The interaction between gold and the surface ions of kaolinite is strong enough to form the surface complexes.
The correlation among structure,chemical bond and stability of system of kaolnite gold is studied by using density function and discrete variation method (DET DVM).Ten models are selected without Au and with Au in different directions and sites.The results show that the models with Au in the side of kaolnite layers are more stable than those with Au above or bellow the layer where the difference of Au O covalent bond strength is the main effect.In the models with Au in the side,the models with Au near to Al are more stable than those with Au near to the vacancy without Al,to which the differences of the charge distribution for all atoms in the models,Au O ionic bond and Si O covalent bond strength are all the important effecte.
