A supergene gold dispersion halo has been developed at Shewushan, Hubei, central China, from the weathering of an auriferous limestone under warm, humid climate conditions. The distributions of gold and associated elements were determined in soils and in both fresh and weathered rocks. Fresh mineralized rocks contain 0.1 to 1 g/t An. 55% of the gold occurs as small Ag-bearing particles (<0.02 mu m) associated with illite and kaolinite, and 45% is incorporated within the lattice of pyrite. The Au content of the supergene deposits ranges from 1.05 to 19.49 g/t, again mostly as very small particles (<0.02 mu m) in kaolinite and illite and partly as particles of <0.6 mu m within limonite. In the pH range of 5 to 8, which is similar to surficial conditions at Shewushan, bisulphide and thiosulphate ligands are possibly present in significant concentrations to complex Au. Particulate Au and Au within pyrite would both dissolve as ionic complexes and be transported downward in groundwater percolating through the clay-rich rock. Most of the Au complexes are adsorbed by clays and Mn and Fe oxyhydroxides, resulting in secondary concentration. Some of the small native gold particles associated with clay minerals could also migrate downwards and accumulate close to the water-table in response to changes in hydraulic conditions. Weathering has promoted gold accumulation in the centre of the halo and dispersion at its margins. Gold concentrations progressively increase from the surface to the base of the weathering mantle and decrease from the centre to the margin of the halo. The dispersion halo results from the lateral migration of both complexed and particulate gold in groundwater during weathering. (C) 2000 Elsevier Science B.V. All rights reserved.
