Gold leaching of an auriferous pyrite ore from the Portovelo deposit in Ecu
ador indicates that noncyanide leaching systems, such as thiocyanate and th
iourea, are not as effective as the traditional cyanide system. For example
, whereas approximately 95% of the gold can be leached from the ore with cy
anide, only about 50% gold dissolution can be achieved with a thiocyanate l
eaching system. Electrochemical measurements indicate that gold is not cath
odically protected by pyrite in these systems. Based on the results of prel
iminary diagnostic experiments, it appears that gold dissolution in thiocya
nate and thiourea systems is limited by passivation due to the formation of
either elemental sulfur or sulfur compounds on the gold surface.