Nature 342, 793 - 795 (14 December 1989); doi:10.1038/342793a0

Role of methane and carbon dioxide in gold deposition

Jonathan Naden & Thomas J. Shepherd

British Geological Survey, 64 Grays Inn Road, London WC1X 8NG, UK

VEIN-HOSTED gold deposits in low- to medium-grade metamorphic terrains are commonly associated with low-salinity hydrothermal fluids rich in CO₂ and/or CH₄ ^1–12. Fluid inclusion studies of gold mineralization indicate that the ore fluid comprised co-existing CO₂/CH₄-rich and H₂O-rich phases, and that phase separation played an integral part in gold deposition^3,5,6,8–12. In hydrothermal solution gold is present as the Au(HS)₂ ^- complex¹³. Precipitation of gold caused by decreasing ligand activity involving the formation of iron sulphides from wall-rock iron oxides and silicates¹³ is clearly relevant to gold deposits associated with iron formations and iron-rich igneous rock^14,15. It cannot, however, be used to explain the common association of gold deposits with black shales or schists^3,5,7,8,16, where wall-rock iron is in the form of sulphides, and therefore generally in equilibrium with the hydrothermal fluid, or with granitoids or felsic volcanics, where the amount of iron is low. This latter association may be explained by the partitioning of H₂S into the non-aqueous phase during fluid immiscibility^10,12, but the general applicability of this mechanism is not known. Here we present a new synthesis of experimental data from a variety of sources which puts this mechanism on a semi-quantitative basis, and suggest that it may be applicable to a wide variety of hydrothermal gold environments.

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