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Cinnabar deposition was observed in submarine hydrothermal vents at about 10 m depth in the Punta Mita area in western Mexico. The vents occur in basaltic rocks and discharge a mixture of liquid and gas at a temperature of 85°C. Tertiary ignimbrites, basaltic lava flows, and mafic intrusions crop out in Punta Mita; however, at present there is no volcanic activity in the area, and the hydrothermal vents do not possess characteristics compatible with a magmatic heat source. The water discharged by the vents is more dilute than seawater, and precious and base metal contents are below detection limits. The gas is composed mostly of nitrogen and methane, and it contains only trace amounts of He, Ar, H₂, CO₂, H₂S, and O₂.

Deposits of carbonates (calcite and aragonite), sulfides (pyrite, cinnabar, minor thallium sulfide, and galena), sulfates (barite), and phosphates (carbonate-hydroxyl apatite) were …