Weathered Mantle Hosted Gold Deposit at Shewushan, Hubei, Central China

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 exceeding 1.0×10^-6(average 0.6×10^-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 δ(¹⁸O) values and measured δ(D) data of the hypogene fluids indicate a meteoric origin. These fluids underwent extreme ¹⁸O 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.