Boletín de la Sociedad Geológica Mexicana Volumen conmemorativo del centenario Revisión de algunas tipologías de depósitos Minerales de México Vol. 58, núm. 1, 2006, p. 83-102 http://dx.doi.org/10.18268/BSGM2006v58n1a3

Procesos de mineralización en manantiales hidrotermales submarinos someros. Ejemplos en México

Carles Canet* y Rosa María Prol-Ledesma

Instituto de Geofísica, Universidad Nacional Autónoma de México, Ciudad Universitaria, Delegación Coyoacán, México D.F., 04510, México

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Abstract

Recent mineralogical and geochemical studies on shallow submarine hydrothermal vents (at water depths < 200 mbsl) suggest that their activity could have been responsible for the formation of oxide, sulfide and precious metal-bearing ores. Therefore, shallow submarine vents may be considered as modern analogues of some economic ore deposits.

The boundary between shallow and deep hydrothermal vents can be established at a depth of 200 mbsl, which represents an abrupt change in the environmental parameters and in the structure of the biotic communities. In addition, this depth corresponds to an increase of the slope of the boiling curve of seawater with respect to pressure.

Shallow submarine vents support complex specialized biotic communities, characterized by the coexistence and competition of chemosynthetic and photosynthetic organisms. Some biogeochemical and biomineralization processes related to chemosynthesis are similar to those described in deep ocean hydrothermal vents and in cold seeps.

Hydrothermal shallow vent fluids show intermediate chemical and isotopic characteristics between those of deep vents and of continental geothermal systems. Commonly, vent water has lower salinities than seawater. This fact, along with isotopic compositions, is evidence for large contributions of meteoric water in these vents. Venting of exsolved gas, evidenced by continuous bubbling, is a striking feature of shallow submarine hydrothermal systems. In most cases vent gas is rich in CO2, but it can be rich in N2 and CH4 in vent systems related to thick sedimentary series, and rich in H2S in vents related to volcanic fumaroles.

In Mexico, shallow submarine hydrothermal venting has been reported in Punta Banda and Bahía Concepción in Baja California Peninsula, and in Punta Mita in Nayarit. The tectonic setting of these hydrothermal systems corresponds to continental margins affected by extension, with anomalously high geothermal gradients. These vents do not show obvious links with volcanic activity. Their study has contributed to the understanding of mineralogical and geochemical processes in shallow submarine hydrothermal vents. These systems, in addition, may be a potential source of geothermal energy.

Keywords: Hydrothermal vents, neritic zone, chemosynthesis, ore deposits.