Behaviour of gold in the weathered mantle at Shewushan, Hubei, China

A supergene gold dispersion halo has been developed at Shewushan, Hubei, ce ntral China, from the weathering of an auriferous limestone under warm, hum id climate conditions. The distributions of gold and associated elements we re determined in soils and in both fresh and weathered rocks. Fresh mineral ized rocks contain 0.1 to 1 g/t An. 55% of the gold occurs as small Ag-bear ing 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 particl es (<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 dow nward 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 th e water-table in response to changes in hydraulic conditions. Weathering ha s promoted gold accumulation in the centre of the halo and dispersion at it s margins. Gold concentrations progressively increase from the surface to t he 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 bot h complexed and particulate gold in groundwater during weathering. (C) 2000 Elsevier Science B.V. All rights reserved.