Role of CO2 in the formation of gold deposits

Much of global gold production has come from deposits with uneconomic concentrations of base metals, such as copper, lead and zinc 1 . These ‘gold-only’ deposits are thought to have formed from hot, aqueous fluids rich in carbon dioxide 2 , but only minor significance has been attached to the role of the CO 2 in the process of gold transport. This is because chemical bonding between gold ions and CO 2 species is not strong 3 , and so it is unlikely that CO 2 has a direct role in gold transport. An alternative indirect role for CO 2 as a weak acid that buffers pH has also appeared unlikely, because previously inferred pH values for such gold-bearing fluids are variable 2 , 4 , 5 , 6 . Here we show that such calculated pH values are unlikely to record conditions of gold transport, and propose that CO 2 may play a critical role during gold transport by buffering the fluid in a pH range where elevated gold concentration can be maintained by complexation with reduced sulphur. Our conclusions, which are supported by geochemical modelling, may provide a platform for new gold exploration methods.