The solubility of gold and silver in the system AuAgSO 2H 2O at 25°C and 1 atm.

Abstract

During supergene alteration of auriferous carbonate ore, the weathering fluids formed are likely to be alkaline and therefore unsuitable as a medium for gold transport as a chloride complex. Secondary gold remobilization in such deposits can often be attributed instead to gold complexing by sulphur-bearing ligands. Gold and silver solubility in the systems AuSO 2H 2O and AgSO 2H 2O respectively, from the thermodynamic data available, is due to complex formation with thiosulphate and bisulphide ligands. The most stable gold complexes, Au(S 2O 3) 2 3− (at φO 2 > 10 −60) and Au(HS) − 2 ( atφO 2 < 10 −60), exist in neutral or alkaline solutions. Like gold, silver forms a stable thiosulphate complex, Ag(S 2O 3) 3− 2 in moderately oxidizing, and bisulphide complexes, AgHS 0 and Ag(HS) − 2 in reducing, alkaline media. Silver solubility in highly oxidized, neutral or acid solutions is increased by formation of AgS 2O 3 −, Ag + and AgSO 4 − complexes. Colloidal, crystalline and alloyed gold and silver reacted with 0.1 M Na 2S 2O 3 do not, however, demonstrate independent solubility. The rate of gold solubility in 0.1 M Na 2S 2O 3, for example, is increased both by the presence of silver-thiosulphate complexes and alloyed silver. It is possible that such behaviour is due to the formation a mixed metal complex of the type (Au, Ag)(S 2O 3) 2 3−. The nature and mineral association of secondary gold in the oxidized zone of carbonate ore at Wau. in Papua New Guinea, is consistent with prior remobilization as a thiosulphate complex. Here the secondary gold is coarsely crystalline, alloyed with 50–75 at% Ag and enriched at the watertable and with manganese dioxide in the oxidized zone.