Much of global gold production has come from deposits with uneconomic concentrations of base metals, such as copper, lead and zinc1. These 'gold-only' deposits are thought to have formed from hot, aqueous fluids rich in carbon dioxide2, but only minor significance has been attached to the role of the CO2in the process of gold transport. This is because chemical bonding between gold ions and CO2species is not strong3, and so it is unlikely that CO2has a direct role in gold transport. An alternative indirect role for CO2as a weak acid that buffers pH has also appeared unlikely, because previously inferred pH values for such gold-bearing fluids are variable2,4,5,6. Here we show that such calculated pH values are unlikely to record conditions of gold transport, and propose that CO2may 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.