Mineralizing processes at shallow submarine hydrothermal vents: Examples from México
Published:January 01, 2007
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Carles Canet, Rosa María Prol-Ledesma, 2007. "Mineralizing processes at shallow submarine hydrothermal vents: Examples from México", Geology of Me´xico: Celebrating the Centenary of the Geological Society of Me´xico, Susana A. Alaniz-Álvarez, Ángel F. Nieto-Samaniego
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Recent studies on shallow submarine hydrothermal vents (at water depths <200 m below sea level [mbsl]) suggest that their activity could have been responsible for the formation of oxide, sulfide, and precious metal–bearing ores.
The boundary between shallow and deep hydrothermal vents has been established at a depth of 200 mbsl, which represents an abrupt change in the environmental parameters and in the structure of the biotic communities. Shallow submarine vents support complex biotic communities, characterized by the coexistence and competition of chemosynthetic and photosynthetic organisms. Some biogeochemical and biomineralizing processes related to chemosynthesis are similar to those described in deep hydrothermal vents and in cold seeps.
Frequently, hydrothermal shallow vent water has lower salinity than seawater. This fact, together with the isotopic compositions, is evidence of a meteoric component in vent water. 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 occasionally it can be rich in N2, CH4, and H2S.
In México, shallow submarine hydrothermal venting has been studied in Punta Banda and Bahía Concepción, Baja California Peninsula, and in Punta Mita, Nayarit. The tectonic setting of those hydrothermal systems corresponds to continental margins affected by extension, with 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. Those systems could be a potential source of geothermal energy.
The study of hydrothermal vents, mainly performed during the last three decades, has contributed to the understanding of the nature of ore-forming processes in seafloor environments and in volcanogenic massive sulfide (VMS) deposits (e.g., Graham et al., 1988; Rona, 1988; Herzig and Hannington, 1995; Humphris et al., 1995a; Parson et al., 1995; Scott, 1997). In addition, their study has elucidated key questions related to the cycle of some metals in oceans (e.g., Gamo et al., 2001), and to the metabolism of extremophile and chemosynthetic biotic communities (e.g., Karl et al., 1980; Jannasch, 1984). Thus, the study of hydrothermal vents provided clues to the possible origin of life and the understanding of primitive biosphere (e.g., Corliss et al., 1981; Russell, 1996).
Research on submarine hydrothermal venting dates back to 1965, when hydrothermal brine pools and related metalliferous sediments were discovered in the axial oceanic rift of the Red Sea (Degens and Ross, 1969). These discoveries corresponded to the “Atlantis II Deep” vent site, which is the largest identified submarine metalliferous deposit, with estimated reserves of 94 Mt and grades of 0.45% Cu, 2.07% Zn, 39 ppm Ag, and 0.5 ppm Au (Missack et al., 1989; Herzig, 1999).
In 1976, a low-temperature vent system with associated deposits of nontronite mud and manganese oxides was discovered in the Galapagos Ridge. Later, in 1979, the first black smokers were found at the East Pacific Rise, near latitude 21°N, with vent water rich in dissolved metals at temperatures up to 350 °C (Macdonald et al., 1980). Black smokers are sulfide-rich meter-sized chimney structures that act as discharge conduits for fluids circulating in hydrothermal convection systems within the oceanic crust, and are build up over mounds of sulfates (barite and anhydrite) and base metal sulfides with minor precious metals (Scott, 1997). So far, more than 100 deep-sea hydrothermal vent systems have been discovered in the ocean floor, with fluid temperatures up to 405 °C, mainly in sea-floor spreading settings and, seldom, in seamounts. Most of those sites occur in the Pacific and Atlantic oceans, even though they have been found also in the Indic Ocean and in the Mediterranean Sea (Scott, 1997).
The internal anatomy of deep-sea vents was verified by drilling at the Trans-Atlantic geotraverse (TAG) hydrothermal mound on the Mid-Atlantic Ridge (Humphris et al., 1995b). The structure and morphology, together with the mineralogical and chemical composition observed on deep-sea hydrothermal vent mineralization, suggest their connection to some economic ore deposits that have been important sources of copper, zinc, lead, and gold and also of manganese and iron (e.g., Jorge et al., 1997). Thus, deep-sea hydrothermal vents can be regarded as modern analogues of volcanogenic massive sulfide (VMS) deposits hosted in ophio lites, or Cyprus type VMS deposits (e.g., Sawkins, 1990).
On the other hand, only a few shallow submarine hydrothermal vents (SSHV) have been studied in detail, in spite of being much more accessible and easy to sample. In fact, initially, in comparison with deep-sea vents, research on SSHV did not become known, but now the studies on this subject are growing significantly.
The first publications on SSHV were essentially focused to the specialized biotic communities that colonize these environments (e.g., Tarasov et al., 1985, 1990, 1991, 1993). Results of both these and subsequent studies (e.g., Kamenev et al., 1993, 2004; Hoaki et al., 1995; Tarasov, 2002; Cardigos et al., 2005; Rusch et al., 2005; Tarasov et al., 2005) suggest that specialized prokaryotes are highly diverse and similar to those described in deep-sea vents. In addition, SSHV can be considered natural laboratories to study the interactions between hydrothermal fluids (water and gas), sediments and seabed rocks, and seawater (e.g., Fitzsimons et al., 1997; Botz et al., 1999; Pichler et al., 1999a; Prol-Ledesma et al., 2004; Chen et al., 2005; Forrest et al., 2005; Villanueva-Estrada et al., 2005; Villanueva et al., 2006).
Hydrothermal fluids discharged by shallow submarine vents have lower temperatures than those discharged by black smokers; nevertheless, they can transport dissolved metals in amounts enough high to produce deposits of oxides, sulfides, and precious metals (Martínez-Frías, 1998; Stoffers et al., 1999; Hein et al., 2000; Prol-Ledesma et al., 2002a; Canet et al., 2005b; Jach and Dudek, 2005).
Several published studies have been focused on processes of mineral precipitation in SSHV (Pichler et al., 1999b; Stoffers et al., 1999; Prol-Ledesma et al., 2002a; Canet et al., 2003, 2005a, 2005b; and Alfonso et al., 2005). Those of Canet et al. (2003, 2005a, 2005b) and Alfonso et al. (2005) note the key role of biogeochemical processes in the formation hydrothermal precipitates around SSHV.
Conversely, the consequences of shallow submarine hydrothermal venting on the mass balance in coastal environments, and its supply of hazardous elements to environment, such as arsenic, are not sufficiently known. Likewise, the potential of SSHV as sources of geothermal power has not been evaluated in detail.
DISTRIBUTION OF SHALLOW SUBMARINE HYDROTHERMAL VENTS
SSHV are found in various tectonic settings, mostly in relation to plate margins. Thus, they are located along island arcs and in shallow segments of mid-ocean ridges under the influence of hot spots (Fig. 1) (Fricke et al., 1989; Dando and Leahy, 1993; Hoaki et al., 1995; Fitzsimons et al., 1997; Scott, 1997; Savelli et al., 1999; Stoffers et al., 1999; Geptner et al., 2002). In addition, it is likely that they occur in relation to oceanic intraplate volcanism, where deep-sea vents have been reported, for example, in Loihi seamount, Hawaii (Moyer et al., 1998).
In island arcs, SSHV have been described in Kraternaya Bight, in the Kurile Arc, Russia (Tarasov et al., 1985); in the Bay of Plenty, New Zealand (Stoffers et al., 1999); in the New Britain Islands (Tarasov et al., 1999) and in Ambitle and Lihir (Pichler et al., 1999a, 1998b) in the Bismarck Archipelago, Papua New Guinea; in the Aegean Sea (Sedwick and Stuben, 1996; Dando et al., 1999; Dando et al., 2000) and in the Aeolian Islands, in eastern and central Mediterranean, respectively (Rusch et al., 2005); in Dominica, in the Antilles (Bright, 2004); and along the western margins of the Pacific Ocean (Ferguson and Lambert, 1972; Tarasov et al., 1985, 1990, 1999, 2005; Sarano et al., 1989; Hashimoto et al., 1993; Kamenev et al., 1993; Chen et al., 2005).
In areas with mid-oceanic ridge–hot-spot interaction, SSHV have been reported in Kolbeinsey Ridge, Iceland (Benjamínsson, 1988; Botz et al., 1999), and in João de Castro Seamount, in the Azores Islands (Cardigos et al., 2005).
The geological settings of the above-mentioned sites agree with the close link between most SSHV and recent volcanism. For instance, there are several SSHV along the Kermadec Arc, north of New Zealand, in the active submarine volcanoes of Rumble III, Rumble V, Macauley Cone, Giggenbach, Ngatoroirangi, Monowai, and Vulkanolog, at depths greater than 130 mbsl (de Ronde et al., 2001; C.E.J. de Ronde, 2004, personal commun.). However, there are also SSHV in continental margins actively affected by extension tectonic processes; for example, in Bahía Concepción (Prol-Ledesma et al., 2004) and Punta Banda (Vidal et al., 1978), both in the Baja California Peninsula; in Punta Mita in Nayarit State, México (Figs. 2 and 3) (Prol-Ledesma et al., 2002a, 2002b); and in White Point in California, USA (Stein, 1984). Likewise, some lakes, placed in intra-continental rift basins, contain systems of underwater hot springs; for example, lakes Baringo in Kenya (Renaut et al., 2002) and Tanganyika in Tanzania (Barrat et al., 2000), and Lake Baikal in Russia (Crane et al., 1991). Similar hydrothermal features have been reported in crater lakes in Taupo, New Zealand (de Ronde et al., 2002), and in Crater Lake in Oregon, USA (Dymond et al., 1989). Even though SSVH can show intermediate geochemical and mineralogical characteristics between deep-sea vents and sublacustrine hot springs (e.g., Schwarz-Schampera et al., 2001; Canet et al., 2003), the latter are clearly closer to continental geothermal systems (Prol-Ledesma et al., 2005).
It has been considered that the maximum depth for SSHV would be ∼200 m (Prol-Ledesma et al., 2005). At this depth, which also corresponds to the lower limit of the neritic zone, sharp changes in the environmental parameters and in the structure of the biotic communities occur. Thus, biotic communities inhabiting SSHV clearly differ from those of deep-sea vents by the presence of diatoms, bacterial and algal mats, and phytoplankton.
On the other hand, in comparison to SSHV, deep-sea vents host a vast diversity of highly specialized and symbiotrophic species, and their communities show a more complex structure, with vertical and lateral zoning and a greater biomass (Tarasov et al., 2005). Moreover, formation of large sulfide-rich structures, such as mounds and chimneys, is a distinguishing feature of deep-sea vents.
In addition, the 200 mbsl depth coincides with a sharp change in the slope of the boiling temperature curve for seawater with respect to pressure, which is ∼20 bars at this depth (Bischoff and Rosenbauer, 1984; Butterfield et al., 1990).
The genesis of several ore deposits has been related to SSHV activity. The main examples reported in bibliography are listed here.
In Milos, Greece, there are manganese deposits whose origin has been linked to SSHV, even if they show some features typical of epithermal veins (Hein et al., 2000; Liakopoulos et al., 2001; Naden et al., 2005). They consist in veins of manganese oxides with barite, hosted in volcaniclastic rocks of Pliocene to Pleistocene age, and are located in the coastal area around Cape Vani. In addition, ore mineralization occurs as crusts, cementing sandstone, and replacing limestone. The manganese minerals of the deposits are pyrolusite MnO2, ramsdellite MnO2, cryptomelane K(Mn4+, Mn2+)8O16, hollandite (Ba, K)Mn8O16, coronadite PbMn8O16, and hydrohetaerolite Zn2Mn4O8·(H2O) (Liakopoulos et al., 2001). The ores, with manganese grades up to 60 wt% MnO, have high contents in lead (up to 3.4 wt% Pb), barium (up to 3.1 wt% BaO), zinc (up to 0.8 wt% Zn), arsenic (up to 0.3 wt% As), antimony (up to 0.2 wt% Sb), and silver (up to 10 ppm). These manganese deposits are in the vicinity of Profitis Ilias, a low sulfidation epithermal deposit rich in precious and base metals. The resemblance in style of mineralization, host rock, and geologic setting between both deposits suggests that they would be genetically related (Hein et al., 2000).
Another example of an ore deposit believed to be a fossil SSHV is located in Wafangzi, China, and is the largest manganese deposit known in this country. According to Fan et al. (1999), microbial activity mediated the mineral precipitation processes that led to ore formation.
Jach and Dudek (2005) described a stratabound manganese deposit hosted in encrinites deposited as tempestites during the Early Jurassic, in the Tatra Mountains, Poland. Its mineralogical and geochemical features, together with the presence of specific microbial structures and the inferred sedimentary environment, suggest that this ore deposit formed as a result of venting of hydrothermal fluids in the neritic zone, through synsedimentary faults. The essential minerals of the ores are braunite Mn7SiO12, caryopilite (Mn, Mg, Zn, Fe2+)3(Si, As)2O5(OH, Cl)4, manganiferous calcite, and rhodochrosite MnCO3. The ore body is rich in manganese (up to 62.8 wt% MnO) and barium (up to 4500 ppm), and has low contents of transition metals (Co+Ni+Cu < 0.01 wt%).
The stratabound Ba-Sb-Ag-Fe-Hg ores of Las Herrerías and Valle del Azogue occur in the Neogene-Quaternary volcanic province of SE Spain (Martínez-Frías, 1998). Orebodies attain up to 10 m in thickness and contain some fossil structures that have been interpreted as vent chimneys (Martínez-Frías, 1998). They are made up of barite and jasper with iron and manganese oxides and hydroxides, with disseminated pyrite, sphalerite, galena, cinnabar, and native silver. The structure of the deposits, the occurrence of chimney-like structures, and their chemical and mineralogical compositions suggest that they formed due to exhalative sub marine activity at depths near 200 mbsl (Martínez-Frías, 1998). Those deposits are spatially and temporally related to the epithermal Au, Ag, Fe-Mn, Hg-Sb, and base metal veins of Sierra Almagrera.
In México, there are several manganese deposits close to Bahía Concepción (Baja California Sur) that occur as veins, breccias, and stockworks. They are composed of manganese oxides (pyro-lusite, coronadite, and romanechite (Ba, H2O, O)2Mn5O10), with dolomite, quartz, and barite, and are hosted principally in Tertiary andesites and, locally, in detrital and carbonate rocks of Pliocene age (Rodríguez-Díaz, 2004). The mineralization arrangement is controlled by Miocene normal faults of NW-SE trend. In the vicinity of these deposits there are numerous manifestations of a SSHV system, at depths from the sea surface to 15 mbsl, with vent temperatures up to 87 °C that are hosted in the same andesitic rocks and related to the same NW-SE faults (Prol-Ledesma et al., 2004).
As a result of the modern hydrothermal venting, there are patches of mineral precipitates, made up of poorly crystallized phases chemically analogous to todorokite (Mn, Ca, Mg)Mn3O7·H2O and romanechite, with barite, calcite, and opal-A (Canet et al., 2005b). Taking into account the similarities between both fossil and modern mineralization, and the importance of their structural NW-SE fault control, it can be concluded that the metallogenesis of these Late Miocene to Pliocene manganese deposits is analogous to the modern SSHV systems. Likewise, in the same area there is a chert bed of Late Pliocene age, whose origin has been attributed to hydrothermal venting of silica-rich fluids in a mangrove marsh (Ledesma-Vázquez et al., 1997).
ORIGIN AND CIRCULATION OF HYDROTHERMAL FLUIDS
Fluids vented from SSHV show transitional chemical and isotopic compositions between deep-sea vents and continental geothermal systems, both subaerial and sublacustrine (Dymond et al., 1989; Barrat et al., 2000; Schwarz-Schampera et al., 2001, de Ronde et al., 2002; Prol-Ledesma et al., 2002a, 2002b, 2004, 2005; Renaut et al., 2002), and are strongly constrained by the geological and tectonic setting (Prol-Ledesma et al., 2005).
Sampling of vent water in SSHV unavoidably implies mixing with seawater; thus, it is suitable to assume a simple mixing model for estimating the composition of vent water. In this way, a thermal end-member is calculated, which represents vent water previous to the seawater addition through sampling (Table 1) (Prol-Ledesma et al., 2002a, 2002b). The principal hypothesis in the mixing model is that the concentration of magnesium in the thermal end-member is near zero, in agreement with the results of heating seawater experiments carried out by Bischoff and Seyfried (1978). Their results point to an almost complete loss of magnesium in solution due to the precipitation of minerals that scavenge and immobilize it. Depending on the sampling method, it is possible to obtain samples of vent water with <10% of sea-water (Villanueva et al., 2006).
Generally, vent water salinities and densities are lower in SSHV than in deep-sea vents (Table 1). This fact, together with the isotopic compositions, point to a meteoric component in SSHV fluids (Prol-Ledesma et al., 2003, 2004, 2005; Villanueva-Estrada et al., 2005). Numerical and conceptual models obtained from several SSHV corroborate that vent water contains meteoric water that interacted in variable degree with the underlying rocks (Fig. 4) (Vidal et al., 1981; Prol-Ledesma et al., 2002b, 2004; Villanueva-Estrada et al., 2005). Nevertheless, SSHV water shows chemical similarities with that from deep-sea vents, both for its concentration in some major ions and for its contents in rare earth elements (Prol-Ledesma, 2003). So, in both cases, vent water is enriched in Si, Ba, Mn, B, and Fe with respect to seawater, it shows enrichment in Eu, and does not exhibit a negative Ce anomaly, since they are reducing fluids (Michard, 1989). The rare earth element (REE) patterns in SSHV can be inherited by the mineral precipitates formed around the vents (Canet et al., 2005b), and they clearly differ from those of hydrothermal fluids in orogenic and continental settings (Michard and Albarède, 1986).
Hydrothermal fluids from deep-sea vents placed on areas of oceanic crust overlaid by thick sedimentary covers; for example, in the Guaymas Basin, Gulf of California, fluids are enriched in Ca and I with respect to seawater (Von Damm et al., 1985). Similarly, in some cases, SSHV fluids can be enriched in these elements (Prol-Ledesma, 2003).
A striking feature of SSHV is the presence of exsolved gas, producing a conspicuous bubbling (Fig. 3; Table 2). Accordingly, some authors (e.g., Tarasov et al., 1990) name the SSHV gaso-hydrothermal vents. The composition of the gas phase in SSHV is variable, although it is roughly similar to that of the dissolved gases in deep-sea hydrothermal fluids. Thus, the gas is mainly CO2 with minor H2S and CH4 (Dando et al., 2000; Botz et al., 2002; Amend et al., 2003). Conversely, for vent gas in SSHV systems whose fluids interacted with sedimentary sequences, as in Punta Mita (México), the main exsolved gases are N2 and CH4 (Prol-Ledesma et al., 2005), and in SSHV systems directly related to volcanic activity the predominant gas is H2S (de Ronde et al., 2001).
MINERALOGY AND GEOCHEMISTRY
Potentially, SSHV systems can be expected to generate ore deposits with oxides, sulfides, and precious metals (Figs. 5 and 6) (Martínez-Frías, 1998; Hein et al., 1999; Stoffers et al., 1999; Prol-Ledesma et al., 2002a, 2002b). In SSHV, pressure from the water column (up to 20 bars) is much lower than in deep-sea vents. This fact could trigger the deposition of some metals below the seafloor as a result of deep boiling of hydrothermal fluids.
Mineral assemblages produced by venting of hydrothermal fluids in shallow submarine environments are comparable to those of low to medium temperature deep-sea vents (Canet et al., 2005b), given that they used to be rich in barite, manganese oxides, and iron oxyhydroxides. In addition, they show some resemblances to some low-sulfidation epithermal deposits. Thus, many SSHV deposits are enriched in S, Hg, As, and Sb, a suite of elements that can be found in epithermal deposits (e.g., Bornhorst et al., 1995), although it is also characteristic of mineralization that formed from low-temperature hydrothermal fluids unrelated to volcanism and rich in organic matter (Tritlla and Cardellach, 1997).
In the shallowest manifestations of the SSHV system of Bahía Concepción (Baja California Sur, México) there are sinter-like silicic deposits and banded veins of chalcedony, calcite, and barite (Canet et al., 2005a, 2005b), mineralogically and texturally similar to the sinters and veins that are usually found in association with low sulfidation epithermal deposits (e.g., Hedenquist et al., 1996; Camprubí and Albinson, 2006). Consequently, many SSHV can be considered as a transition between deep-sea vents and epithermal deposits (Schwarz-Schampera et al., 2001).
Poorly crystalline and amorphous phases are prevalent in SSHV deposits. Iron oxyhydroxides, both amorphous and with low crystallinity (protoferrihydrite and ferrihydrite, respectively), have been described in Bahía Concepción, Baja California Sur, México (Canet et al., 2005b), and in Ambitle, Papua New Guinea (Pichler and Veizer, 1999). Iron oxyhydroxides are common in other hydrothermal environments; for example, in deep-sea hydrothermal vents, forming replacive surface crusts over sulfide mounds (e.g., Haymon and Kastner, 1981; Marchig et al., 1999; Hannington et al., 2001; Lüders et al., 2001), in metalliferous sediments, accompanied by manganese oxides (e.g., Daesslé et al., 2000), and in low-temperature hydrothermal vents (Bogdanov et al., 1997).
Precipitation of iron oxyhydroxides depends on pH, Eh, temperature, iron concentration, and microbial activity (Puteanus et al., 1991; Savelli et al., 1999; Mills et al., 2001), whereas the low crystallinity is due to rapid precipitation kinetics (Chao and Theobald, 1976).
Manganese oxides also have been described in SSHV (Canet et al., 2005b), and more often, in metalliferous sediments (Daesslé et al., 2000) and in deep-sea vents (Glasby et al., 1997), where they are usually associated with iron oxyhydroxides and precipitate from low-temperature (up to 25 °C) hydrothermal fluids (Burgath and von Stackelberg, 1995). On the other hand, manganese oxides form hydrogenous deep-sea deposits of nodules and crusts (Nicholson, 1992). In SSHV deposits, poorly crystalline phases chemically equivalent to romanechite and todorokite have been described (Canet et al., 2005b), whereas there are crystalline species as birnessite Na4Mn14O27·9H2O and todorokite in relation to deep-sea hydrothermal vents (Marchig et al., 1999).
In some near shore SSHV, finely laminated sinter-like deposits occur with variable composition: silicic and silica-carbonated (Canet et al., 2005a) and calcareous (Canet et al., 2003). The chemical character of these deposits depends on fluid composition, pH, and temperature, and their precipitation is usually mediated to some degree by biological activity. Opal is the main component of the silicic deposits formed by exhalation of hydrothermal fluids. It has been reported in relation to deep-sea venting of silica-rich hydrothermal fluids; for example, in the Aeolian Arc and in the Tyrrhenian Sea (Savelli et al., 1999) and near the Rodrigues triple point in the Indic Ocean (Halbach et al., 2002). However, silicic deposits are much more abundant in lacustrine hydrothermal vents (e.g., Eugster, 1969; Renaut et al., 2002) and, especially, in subaerial hot springs, where they build up the archetypical sinters; for instance, in the Taupo volcanic zone, New Zealand (Mountain et al., 2003; Rodgers et al., 2004), and in Krisuvik, Iceland (Konhauser et al., 2001). These deposits develop from sodium-chloride thermal waters of neutral, alkaline, or, rarely, acidic pH (e.g., Fournier and Rowe, 1966; Rodgers et al., 2004), with reservoir temperatures over 175 °C (Fournier and Rowe, 1966).
In contrast, silica-carbonated deposits are much more scarce, as they have been found only in the continental geothermal fields of Waikite (Jones et al., 2000) and Ngatamariki (Campbell et al., 2002) in New Zealand, and in the intertidal hot springs of the SSHV system of Bahía Concepción, in Baja California Sur, México (Canet et al., 2005a). In this site, supersaturation of the fluid with respect to silica is attained by cooling and causes the precipitation of opal-A, whereas CO2 loss triggers supersaturation in calcium carbonate (Canet et al., 2005a).
Calcareous sinters, also called calcareous tufa or hydrothermal travertines, have been reported mainly in subaerial and lacustrine hot springs; for example, in Pyramid Lake, Nevada, USA (Arp et al., 1999), and Yellowstone, Wyoming, USA (Fouke et al., 2000). In these settings, calcite precipitates from alkaline to neutral waters supersaturated in calcium carbonate (Jones et al., 2000). An analogous example from SSHV is found in Punta Mita, in Nayarit, México, that consists in fine-layered calcite aggregates forming at ∼10 mbsl (Canet et al., 2003).
Sulfides occur in accessory amounts in SSHV deposits, whereas in deep-sea vents they build up chimney structures and accumulate in massive mounds. The reported sulfides in modern SSHV are pyrite, marcasite, cinnabar, carlinite, realgar, and orpiment (Stoffers et al., 1999; Canet et al., 2005a, 2005b). The occurrence of cinnabar has been confirmed in several SSHV deposits (Halbach et al., 1993; Stoffers et al., 1999; Prol-Ledesma et al., 2002a; Canet et al., 2005b).
In addition, SSHV deposits can contain sulfates (anhydrite, gypsum, and barite), phosphates (hydroxylapatite), and native sulfur and mercury (Stoffers et al., 1999; Canet et al., 2005a, 2005b). Barite precipitation is caused by mixing barium-rich reducing thermal water and SO4-rich cold seawater (Canet et al., 2005a).
BIOTIC COMMUNITIES AND BIOMINERALIZATION PROCESSES
Generally, in both subaerial and submarine hydrothermal exhalations, mineral deposits form from solutions at temperatures below the upper limit for life development (Jones and Renaut, 1996; Konhauser et al., 2001). The close relationship between hydrothermal venting mineralization and microbial activity has been confirmed in numerous cases (Konhauser et al., 2001). Thermophilic life forms, similar to those that develop in hot spring environments, have been proposed as the common ancestor of life on Earth (Stetter et al., 1990). Therefore, the study of biomineralization processes and their resulting deposits in hydrothermal vents can yield information about life in extreme conditions (Walter and Des Marais, 1993) and about early life forms. In this way, the study of deep-sea black smoker fields and, more recently, of cold seeps have shown complex ecosystems based in chemosynthesis that do not directly depend on the sun's energy supply.
Biogeochemical processes in SSHV sites are significantly different from those of deep-sea hydrothermal vents, since SSHV environments fulfill conditions suitable for both photosynthetic and chemosynthetic metabolisms (Tarasov et al., 1993; Tarasov, 2002). For that reason, SSHV yield an opportunity for studying the relationships and competition interactions between chemosynthetic and photosynthetic organisms.
In SSHV environments, biotic communities are specifically adapted to survive in chemically modified habitats, tolerating high sulfur and heavy metal concentrations and high temperatures (Tarasov, 1991). These communities contain photosynthetic species (Dando et al., 1999) and, in some cases, they are hotspots of eukaryote diversity (Morri et al., 1999).
The effects of SSHV activity on planktonic and benthonic communities are not known yet, and biotic communities of SSHV are not as well characterized as those of deep-sea hydrothermal vents.
Some textural, mineralogical, and geochemical characteristics of mineral assemblages formed in SSHV suggest that many mineralizing processes are more or less influenced by microbial activity (Canet et al., 2003, 2005a, 2005b; Alfonso et al., 2005). Thus, microbial activity can promote the precipitation of iron oxyhydroxides (Juniper and Tebo, 1995). In addition, Mita et al. (1994) demonstrated that microbes decisively mediate the precipitation of manganese oxides in subaerial hot springs in Japan. In some cases, microbial activity can cause silica precipitation in sinters (Hinman and Lindstrom, 1996), and it often influences the fabric and textures of these deposits (Canet et al., 2005a). SSHV deposits formed under these influences, for example, the stromatolitic intertidal sinters of Bahía Concepción, in Baja California Sur (México), are similar to those described in lacustrine environments (Walter et al., 1976; Renaut et al., 2002).
In the case of the carbonated deposits of Punta Mita, Nayarit (México), which are related to methane-rich fluids venting, calcite precipitation takes place as a consequence of microbial oxidation of methane (Canet et al., 2003). Besides calcite, these deposits contain pyrite, whose precipitation is caused by a microbial reaction of seawater sulfate reduction coupled to methane oxidation (Alfonso et al., 2005).
SHALLOW SUBMARINE HYDROTHERMAL VENTS IN MÉXICO
Off the Pacific coast of México, the SSHV systems have been reported off Punta Banda (Vidal et al., 1978) and Bahía Concepción (Prol-Ledesma et al., 2004) in Baja California Peninsula, and Punta Mita, in Nayarit (Prol-Ledesma et al., 2002a, 2002b) (Fig. 2).
Unlike SSHV systems off the Western Pacific Margin, which are distributed along island arcs, Mexican examples do not show clear links with modern volcanism. The three described SSHV systems are located in continental margins presently affected by intense tectonic extension, with anomalously high geothermal gradients.
About 500 m off Punta Mita, in the northern part of Bahía de Banderas (Nayarit), there is a SSHV system. The area lies near the NW limit of the Jalisco Block and in the western end of the Mexican Volcanic Belt. It is affected by the tectonic extension that results from the Rivera Plate displacement (Allan et al., 1991; Ferrari et al., 1994; Kostoglodov and Bandy, 1995). In this area, anomalously high heat flow values have been calculated (Prol-Ledesma and Juárez, 1986).
Hydrothermal activity consists of fluid discharge (gas and water) at 85 °C through a NE-SW–trending fissure that is hosted in basaltic rocks and is partially covered by unconsolidated platform sediments. This activity affects an area of ∼1 km2 at a depth of 10 mbsl (Prol-Ledesma et al., 2002a, 2002b; Canet et al., 2003).
Thermal exsolved gas is composed mainly of N2 (88%) and CH4 (12%); it also contains H2S, CO2, H2, Ar, and He in trace amounts (Table 2) (Prol-Ledesma et al., 2002a). Stable isotopic analysis of carbon in methane yields a δ13C value of −42.8‰ (Vienna Peedee belemnite [VPDB]) (Prol-Ledesma et al., 2002a), suggesting that this gas is not biogenic in origin, but thermogenic (Rooney et al., 1995).
Vent water is less saline than seawater (the calculated salinity of the thermal end-member is 1.51 wt%). It is significantly enriched in Si, Ca, Li, B, Ba, Rb, Fe, Mn, and As with respect to seawater and has lower concentrations of Na, K, Cl, HCO3−, SO42−, and Br (Table 3). These depleted elements are mainly contributed to the vent discharge by seawater (the calculated SO42− concentration in the thermal end-member is 2.81 mmol/kg; Prol-Ledesma et al., 2002a). δD ranges between −11.2 and −16.2‰ (Vienna standard mean ocean water [VSMOW]), and δ18O between −1.9 and −3.5‰ (VSMOW) (Prol-Ledesma et al., 2002b). These isotopic values of the thermal water agree with a model of seawater mixing with deeply circulating meteoric water whose recharge zone is the adjacent mountain range in the peninsula of Punta Mita.
Water circulation takes place through fault systems related to modern extensive tectonics that affect the entire region. During this process, water interacts with Si- and Ba-rich granitic rocks and with sedimentary layers with organic matter, which act as the source of some elements as Ca, I, and Hg (Prol-Ledesma et al., 2002a) and where methane is generated, similar to deep-sea hydrothermal vents in sediment-covered ocean crust areas (Scott, 1997). Thus, SSHV sites in Punta Mita show temperatures and water compositions similar to those of deep-sea vents and, at the same time, they vent a methane-rich gas, as happens in cold seeps.
Discharging of hydrothermal fluids takes place through discrete seafloor springs separated by some meters and roughly located along a NE-SW–trending fissure. As a consequence, several mounds of calcareous tufa with Ba, Hg, and Tl mineralization occur (Fig. 5). These mounds are made up of fine-layered calcite aggregates, with arborescent textures developed in the outer area (Fig. 5).
Farther from main active discharge orifices, mineralized structures consist of accumulations of detrital and bioclastic grains cemented by fine-grained calcite. The degree of development of mineralized mounds depends on the venting flow and its age. The largest recognized mound is ∼2.5 m in diameter and 0.75 m in height. Currents and waves that at these depths disturb the seafloor constrain the morphology and vertical growth of the mineralized mounds (Canet et al., 2003).
Calcite crystals that compose the mounds are acicular, up to 250 μm in length, and they are low-magnesium calcite (MgCO3 ∼4% molar). In addition, there is a later, fine-grained calcite generation, which consists of small rhombohedral crystals of a few microns in length. It cements both detrital grains (basically plagioclase, quartz, and magnetite) and bioclasts (fragments of bivalves, gastropods, foraminifers, echinoderms, and red algae) and, locally, replaces the acicular calcite crystals.
Stable isotopic analyses in calcite aggregates yield δ13C values between −39.2 and +0.9‰ (VPDB) and δ18O values between −12.8 and −1.5‰ (VPDB) (Canet et al., 2003). This exceptional depletion of 13C suggests that calcite precipitation is a consequence of microbial oxidation of methane (Canet et al., 2003). On the other hand, δ18O values are in agreement with a precipitation from thermal water (with δ18O of −1.9 to −3.5‰ VSMOW; Prol-Ledesma et al., 2002b) at temperatures ranging between 70 °C and 81 °C.
After calcite, the most abundant mineral in the mounds is pyrite, which forms thin layered coatings, up to 500 μm in thickness, covering calcite aggregates, and seldom bivalve fragments, in the inner part of the discharge conduits. In addition, pyrite forms framboidal and spherulitic aggregates up to 100 μm in diameter. It is also present in the suspended particulate material around the discharge areas (Ortega-Osorio et al., 2001).
Sulfur isotopic composition (δ34S) of pyrite grains ranges between −13.3 and −4.9‰ (Alfonso et al., 2005). Both pyrite textures and isotopic composition are consistent with a biogenic precipitation. The isotopic fractionation sulfate-sulfide that results from the analyzed δ34S values is consistent with a biogenic reduction of seawater sulfate (Ohmoto and Rye, 1979). Therefore, the combination of two coupled microbially mediated reactions, summarized as (1) oxidation of thermal methane, and (2) reduction of seawater sulfate, is the cause of the precipitation of calcite and pyrite, respectively, in the same way as in cold seeps (Kohn et al., 1998; Canet et al., 2006) but at much higher temperatures.
There are disseminated grains of cinnabar, up to 10 μm in size, and carlinite Tl2S, up to 5 μm in size, associated to layered pyrite aggregates. According to Prol-Ledesma et al. (2002a) thallium forming carlinite is probably scavenged from seawater. As well, there are barite, hydroxylapatite, and baritocalcite BaCa(CO3)2 in accessory amounts.
Near the sites of hydrothermal discharge, basaltic rocks of the seabed show hydrothermal alteration. Their plagioclase phenocrysts are replaced by zeolites (heulandite and analcime), whereas the augite grains and the groundmass are replaced by a cryptocrystalline assemblage of celadonite and pyrite.
The Bahía Concepción, nearly 40 km in length, is a fault-controlled bay placed in the eastern coast of the Baja California Peninsula (Fig. 2) that hosts a SSHV system that is revealed by several submarine vents (at depths down to 15 mbsl) and intertidal and subaerial hot springs. It is a semi-closed bay, whose NW-SE semigraben configuration results from the extensive tectonics that affected the Gulf of California Province during the Late Miocene (Ledesma-Vázquez and Johnson, 2001).
The area with submarine and intertidal hydrothermal activity extends 700 m along a stretch of rocky shoreline in the western coast of the bay. The cliffs are configured by a system of normal faults that allows hydrothermal fluids to rise to the surface (Forrest et al., 2003).
Two types of the SSHV system surface manifestations are found in this area: (1) a zone of diffuse hydrothermal fluids submarine venting (gas and water) through the sediments of the seabed, at depths between 5 and 15 mbsl; and (2) a cluster of hydrothermal springs and bubbling vents in the intertidal zone (Canet et al., 2005a, 2005b). These manifestations are all NW-SE aligned.
The temperature of vent discharge is up to 87 °C in the submarine diffuse area and 62 °C in the intertidal hot springs, and pH is 5.9 and 6.7, respectively.
Water and gas chemical compositions are shown in Tables 4 and 2, respectively. The exsolved gas is composed largely by CO2 (44%) and N2 (54%), although it contains CH4, Ar, He, H2, and O2 in minor amounts (Table 2) (Forrest et al., 2003). δ13 C in CO2 is −6‰ (Forrest and Melwani, 2003) and−34.3‰ in CH4. This value is in agreement with a thermogenic origin (Forrest et al., 2005). On the other hand, isotopic composition of N2, with an average δ15N value of 1.7‰, suggests that this gas is thermally produced from organic matter in immature sediments, whereas helium isotopes (R/Ra = 1.32) are compatible with a mantle-derived component (Forrest et al., 2005).
Thermal water is a sodium-chloride type and shows high concentrations of Ca, Mn, Si, Ba, B, As, Hg, I, Fe, Li, HCO3−, and Sr with respect to seawater (Table 4) (Prol-Ledesma et al., 2004). Chemical geothermometers (Na/Li, Na-K-Ca and Si) yield reservoir temperatures of ∼200 °C (Prol-Ledesma et al., 2004). Calculations of the saturation state indicate that the fluid vented by the intertidal hot springs is supersaturated in barite and silica, and that the mixture of this fluid with seawater is subsaturated in calcite and silica and supersaturated in barite (Canet et al., 2005a). These calculations validate the observed mineralogical composition of the vent precipitates in the area. Thus, amorphous silica is the main precipitate in the intertidal hot springs, whereas it lacks in the submarine vents, where mixing between hydrothermal fluid and seawater is more extensive. On the other hand, supersaturation of the fluid with respect to calcite, which is associated to amorphous silica in the intertidal hot springs, is caused by a process of CO2 loss (Canet et al., 2005a).
δ18O and δD (VSMOW) values of thermal water range between −0.3‰ and −3.1‰ and between −0.3‰ and −25.5‰, respectively (Prol-Ledesma et al., 2004). The range of iso topic compositions of the fluid suggests that it has a large meteoric water component, even though mixing with seawater has a decisive role in fluid evolution. From its content in dissolved magnesium, it can be deduced that thermal water discharged in the submarine diffuse vents undergoes mixing with seawater near the ocean bottom in the unconsolidated sediments (Prol-Ledesma et al., 2004).
Neither seafloor mounds nor chimney-like mineralized structures form as a consequence of the submarine discharge of hydrothermal fluids. Around the more active vent areas, millimetric crusts of iron oxyhydroxides form over the detrital blocks of basaltic andesites accumulated in the seabed. Additionally, the sandy sediments are locally covered by yellowish microbial mats. Crusts of iron oxyhydroxide are composed of poorly crystallized ferrihydrite with minor amounts of pyrite and cinnabar. Pyrite forms fine-layered coatings, up to 20 μm thick, and cinnabar occurs as small grains, up to 10 μm, in close association to pyrite (Canet et al., 2005b).
In contrast, around the intertidal hot springs there are conspicuous accumulations of hydrothermal precipitates, forming irregular pavements up to 10 m2, that remain partially exposed during low tides (Figs. 3A and 3B). These deposits consist of (1) massive aggregates of detrital and bioclastic fragments cemented by opal-A with minor amounts of calcite and barite, (2) manganese oxides, and (3) stromatolitic crusts of silica-carbonate sinter (Fig. 6).
Manganese oxides form moss-like porous aggregates and are composed of poorly crystalline phases chemically equivalent to barium-rich todorokite (>2 wt% BaO) and to romanechite (Ba, K, Na, Ca, H2O)2 (Mn4+, Mn3+, Fe3+, Mg, Al, Si)5O10 with ∼10 wt% BaO (Canet et al., 2005b).
Silica-carbonate sinter crusts overlie the above mentioned precipitates and deposits directly over volcanic cobbles and blocks. They consist of fine-layered bulbous to undulant aggregates, composed essentially of opal-A, calcite, and barite. At the microscale, opal-A forms porous aggregates built up by attached microspheres up to 300 nm in diameter (Canet et al., 2005a).
Isotope analyses of calcite crystals from silica-carbonate sinter show enrichment in 13C with respect to marine carbonates, with δ13C values up to +9.3‰ (VPDB) and δ18O values between −2.6‰ and −10.0‰ (VPDB) (Canet et al., 2005a). The 13C enrichment is caused by CO2 loss by thermal water, the same process that triggers calcite precipitation. δ18O values are consistent with precipitation of calcite from a mixture of thermal and marine water.
At Punta Banda, near Ensenada, in the NW coast of Baja California Peninsula (Fig. 2), there is a SSHV coastal system that presents manifestations in two zones of shallow submarine hydrothermal venting at 40 mbsl and in subaerial hot springs.
The discharge of hydrothermal fluids in the submarine vents takes place at a temperature of 102 °C (Vidal et al., 1978). Chemical compositions of vent water and gas are shown in Tables 1 and 2, respectively (Vidal et al., 1978, 1981).
Thermal gas is chemically similar to that of the Punta Mita SSHV, with N2 and N4 as principal components. In Punta Banda, however, methane is more abundant, reaching up to 51.4% of the exsolved gas (Vidal et al., 1981).
Thermal water is enriched in SiO2, HCO3−, Ca, K, Li, B, Ba, Rb, Fe, Mn, As, and Zn with respect to seawater (Vidal et al., 1981). Stable isotopes (δ18O and δD) indicate that thermal water is a mixture of meteoric and marine water. Estimated reservoir temperatures range between 190 °C and 213 °C (Vidal et al., 1981).
According to the geological setting and geochemical characteristics of the hydrothermal fluids, Vidal et al. (1981) reject the influence of magmatic sources and suggest that thermal water is a mixture of ∼1:1 local meteoric water and “fossil” seawater.
The most abundant minerals in the submarine hydrothermal deposits are pyrite and gypsum, with high contents of As, Hg, Sb, and Tl in whole rock analysis (Vidal et al., 1978).
CONCLUSIONS AND RECOMMENDATIONS
Most SSHV systems are distributed along volcanic island arcs and show a close relationship to volcanic activity. However, there are also some SSHV systems in continental margins actively affected by tectonic extension and not linked to volcanism. This is the setting of the three main examples of SSHV in México: Punta Banda and Bahía Concepción, in Baja California Peninsula, and Punta Mita, in Nayarit.
The maximum depth in which a submarine hydrothermal system can be considered as a SSHV is 200 mbsl. This depth limit implies sharp changes in the ecological and environmental parameters and corresponds to an increase in the slope of the boiling curve for seawater.
SSHV systems can originate ore deposits rich in Mn, Ba, Pb, Zn, As, Sb, Ag, Hg, and Tl, with oxides, sulfates, sulfides, and native elements.
Thermal water in SSHV can show chemical and isotopic characteristics between deep-sea vents and continental geothermal fields. Generally, it involves a significant component of meteoric water, although the process of mixing with seawater constrains its chemical and isotopic composition.
Venting of exsolved gas, evidenced by continuous bubbling, is a distinctive feature of SSHV systems. Usually, exsolved gas is CO2-rich, although in particular cases it can be H2S- or CH4-rich.
The most common SSHV systems, those directly connected to active volcanism, can be considered transitional systems between epithermal deposits and deep-sea hydrothermal vents.
Mineral assemblages forming in SSHV sites are similar to those from low-temperature deep-sea hydrothermal systems and to those from low-sulfidation epithermal deposits. Amorphous and poorly crystalline phases, including iron oxyhydroxides, manganese oxides, and opal, are widespread in SSHV deposits. In addition, they may contain sulfides (pyrite, cinnabar, carlinite, realgar, and orpiment), sulfates (anhydrite, gypsum, and barite), carbonates (calcite and baritocalcite), phosphates (hydroxylapatite), and native elements (sulfur and mercury).
SSHV environments fulfill appropriate conditions for both photosynthetic and chemosynthetic metabolism. Biogeochemical processes in SSHV unquestionably influence the formation of mineral deposits.
A detailed study of SSHV systems will contribute to understanding processes pertaining to ore formation, to microbially mediated mineral precipitation, to the geochemical cycle of some toxic metals in the oceans (e.g., mercury and arsenic), and to water-rock interaction. Furthermore, the potential of SSHV as sources of geothermal energy should be evaluated.
Funding for the studies that resulted in this article came from the Mexican projects Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica [PAPIIT] IN-122604 and IN-107003, Consejo Nacional de Ciencia y Tecnología [CONACyT] J-51127-I, 32510, and SEP-2004-C01-46172. Antoni Camprubí Cano encouraged us to write this article, and his comments and exhaustive revision significantly contributed to the improvement of the manuscript. Also, comments from Jordi Tritlla Cambra were very helpful. We thank A. Camprubí, D. Blanco Florido, M. Dando, P. Dando, M.J. Forrest, J. Ledesma Vázquez, A. López Sánchez, A. Melwani, A.A. Rodríguez Díaz, M.A. Torres Vera, and R.E. Villanueva Estrada for helping in different field campaigns. S.I. Franco Sánchez is thanked for her observations on the manuscript. Scanning electron microscope images were obtained from the Serveis Científico-Tècnics of the Universitat de Barcelona (Spain) and from the Instituto de Geofísica and Instituto de Geología of the Universidad Nacional Autónoma de México, with the assistance of R. Fontarnau, C. Linares López, and M. Reyes Salas, respectively.
Figures & Tables
Geology of Me´xico: Celebrating the Centenary of the Geological Society of Me´xico
- aliphatic hydrocarbons
- Baja California
- carbon dioxide
- chemical composition
- continental margin
- geothermal energy
- geothermal gradient
- heat flow
- hydrogen sulfide
- mineral composition
- modern analogs
- organic compounds
- shallow-water environment
- Bahia Concepcion