Computer Science
Scientific paper
Sep 1986
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986gecoa..50.1837w&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 50, Issue 9, pp.1837-1845
Computer Science
8
Scientific paper
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 2 O and Ag---S---O 2 ---H 2 O respectively, from the thermodynamic data available, is due to complex formation with thiosulphate and bisulphide ligands. The most stable gold complexes, Au(S 2 O 3 ) 2 3- (at O 2 > 10 -60 ) and Au(HS) - 2 ( at O 2 < 10 -60 ), exist in neutral or alkaline solutions. Like gold, silver forms a stable thiosulphate complex, Ag(S 2 O 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 2 O 3 - , Ag + and AgSO 4 - complexes. Colloidal, crystalline and alloyed gold and silver reacted with 0.1 M Na 2 S 2 O 3 do not, however, demonstrate independent solubility. The rate of gold solubility in 0.1 M Na 2 S 2 O 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 2 O 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.
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