13.15 – Geochemistry of Hydrothermal Gold Deposits

Hydrothermal gold deposits (epithermal, Carlin-type, and orogenic) have accounted for > 75% of lode-gold production. Although these deposits form in distinctly different geologic settings and by different ore-forming processes, they have a number of geochemical similarities. These include their temperature of formation over ~ 200 °C range, bulk composition of ore-forming solutions, and elemental associations. The ores predominantly form by either igneous or metamorphic hydrothermal processes, but Carlin-type ores may also require a sedimentary hydrothermal contribution. Gold is transported by the ore-forming solutions as aqueous H 2 S(aq) complexes or its dissociated equivalent, bisulfide, and dissolved gold concentrations may reach the ppm level. Gold in gold-rich ore-forming solutions locally may form nanoparticles that are physically transported to the place of ore deposition. Other elements that also have an affinity for dissolved H 2 S in hydrothermal ore-forming solutions include silver, arsenic, tellurium, selenium, mercury, antimony, and thallium, and they may be enriched in gold ores and be useful elements for exploration geochemistry. In particular, arsenic is almost universally enriched in all three of the hydrothermal deposits discussed here.