| Abstract |
The Fisura de Las Coronas hydrothermal system is located in the Western coast of Mexico. Submarine hot springs were discovered southeast of Punta de Mita at depths from 9 to 11 m. The surveys included mapping 70 m of "Fisura de Las Coronas" which has a 70 NW direction. Photographic, video and spring temperature profiling was carried out in the vent area. A large vent growing from the sandy bottom, a large vent in basaltic rock, and numerous small springs aligned on the sand-gravel bottom were observed. In addition to the mapped sea floor manifestations at the "Fisura de Las Coronas", bubbles have been observed on the ocean surface along a lineament of at least 400 m length. The occurrence of recently reported hydrothermal manifestations on the sea bottom at 11 m depth in western Mexico provides a unique opportunity to study metals deposition and the reactions between the geothermal fluid and the host rock at conditions prevailing at shallow depth in the ocean. The hydrothermal manifestations are located in basaltic rocks on the sea bottom, and result in the replacement of the original rock forming minerals by hydrothermal minerals, as well as direct deposition of carbonates (calcite, aragonite), sulfates (barite), sulfides (Fesulfide, pyrite, and only minor galena) and native elements (copper). The observed hydrothermal alteration sequence is similar to the alteration processes already described in deep active and fossil submarine hydrothermal systems, which occur under more reducing conditions. Sulfides are always restricted to the inner sections of the vents, where conditions may remain more reducing, before interaction with the oxidizing shallow seawater. In the outer parts of the vents carbonates predominate. The absence of anhydrite and the abundance of calcite indicate that the saturation of anhydrite is not achieved in this system, and calcite is deposited as the seawater is heated by the geothermal fluid. |