Deposited precipitates have built chimneys up to 33 m tall, composed of iron, copper, and zinc sulfides, and include some precious metals. Many of the chimneys on the Alarcón Rise were found to be high-temperature black smokers, so-called because of the particles of metal-sulfide that precipitate from the hot fluid as it cools. Autonomous underwater vehicles were used to map the region, followed by dives with remotely operated vehicles to observe and sample features on the seafloor. Hydrothermal chimneys on two previously unexplored spreading ridges, the Alarcón Rise and southern Pescadero Basin, were discovered on Monterey Bay Aquarium Research Institute expeditions to the Gulf of California. The paucity of sulfide minerals suggests subsurface deposition of metals. The fluids are compositionally similar to those in Guaymas Basin, produced by high-temperature basalt-seawater interaction followed by reaction with sediment. They have deposited massive mounds of calcite with minor sulfide. Discharging fluids are clear, contain hydrocarbons, and have neutral pH, elevated salinity, and temperatures up to 291 ☌. In southern Pescadero Basin, the Auka field is on the margin of a sediment-filled graben at 3,670-m depth. The inactive field is 10 km north-northeast along the rift axis, and most sulfide chimneys are enriched in Zn and associated elements that are transported at lower temperature compared to the more Cu-rich active fields. The Tzab-ek field is 850 m west of the volcanic axis, and active chimneys rise up to 33 m above a broad sulfide mound. Vent fluids up to 360 ☌ from Meyibó have compositions similar to northern East Pacific Rise vents. The Ja Sít, Pericú, and Meyibó active fields are near the eruptive fissure of an extensive young lava flow. Vent fields were interpreted as active from temperature anomalies in water column data and observed and sampled during remotely operated vehicle dives. Individual chimneys were identified using the bathymetric data. The Alarcón Rise spreading segment was mapped at 1-m resolution by an autonomous underwater vehicle. Hydrothermal vent fields located in the gap between known sites in Guaymas Basin and 21°N on the East Pacific Rise were discovered on the Alarcón Rise and in southern Pescadero Basin.

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