The submarine hydrothermal vents located in the vicinity of Punta Mita contain abundant deposition of calcite, pyrite. Also present are barite, carbonate hydroxyl-apatite, cinnabar and T1-sulfide.Textures indicate direct deposition for calcite and pyrite; however, pyrite also replaces magnetite in the host rock. Thermal water was filtered and microscopic analysis of the particles fiom the filters shows that the shape of the crystals varies euhedral crystals of pyrite to globular aggregates that may be interpreted as a mixture of chemical and biogenic precipitation. Isotopic signature of sulfur in pyrite and carbón in calcite indicates that biogenic deposition is an important phenomenon in this hydrothermal systemPeer ReviewedPostprint (published version

Alfonso Abella, María Pura

Canet Miquel, Carles

Melgarejo Draper, Joan Carles

Prol Ledesma, Rosa Maria

The submarine hydrothermal vents located in the vicinity of Punta Mita contain abundant deposition of calcite, pyrite. Also present are barite, carbonate hydroxyl-apatite, cinnabar and T1-sulfide.Textures indicate direct deposition for calcite and pyrite; however, pyrite also replaces magnetite in the host rock. Thermal water was filtered and microscopic analysis of the particles fiom the filters shows that the shape of the crystals varies euhedral crystals of pyrite to globular aggregates that may be interpreted as a mixture of chemical and biogenic precipitation. Isotopic signature of sulfur in pyrite and carbón in calcite indicates that biogenic deposition is an important phenomenon in this hydrothermal systemPeer Reviewe

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Biogenic vs geochemical precipitation of minerals in the submarine hydrothermal vents of Punta Mita, Mexico

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Proceedings 24* Geothermal Workshop 2002BIOGENICVS GEOCHEMICALPRECIPITATIONOFMINERALS IN THESUBMARINEHYDROTHERMALVENTS OFPUNTAMITA, MEXICOR.M. c. P. J.C.Instituto de Geofisica, Universidad Nacional Autonoma de Mexico, Mexico D.F.de Mineralogia i Minerals de Barcelona), Barcelona, Spain.2SUMMARY -The submarine hydrothermal vents located in the vicinity of Punta Mita contain abundant deposition of calcite, pyrite. Also present are barite, carbonate hydroxyl-apatite, cinnabar and T1-sulfide.Textures indicate direct deposition for calcite and pyrite; however, pyrite also replaces magnetite in thehost rock. Thermal water was filtered and microscopic analysis of the particles fiom the filters shows thatthe shape of the crystals varies euhedral crystals of pyrite to globular aggregates that may beinterpreted as a mixture of chemical and biogenic precipitation. Isotopic signature of sulfur in pyrite andcarbon in calcite indicates that biogenic deposition is an important phenomenon in this hydrothermal system.1. INTRODUCTIONThe Las Coronas is located 500 m to thesouth of the Punta Mita peninsula (Fig. 1).Presently, hydrothermal fluid is being dischargedat a temperature of 85°C along this feature. Thisactivity takes place in a shallow ocean bottomfrom 9 to 13m depth. Due to the accessibility ofthe deposit, it was possible to sample thehydrothermal mounds and the host rock, which iscomposed of basalt, and unconsolidatedsediments. Preliminary data on the structures,and seismic activity in the study zone werereported by et al. (2000). Shallowcoastal hydrothermal activity, similar to thatobserved in Punta Mita, has been reported in'Punta Banda in Baja California (Vidal et al.,associated to deep circulation of meteoricwater...... ....... ............. . .................................. ............ .Fig. 1 Location of study area.The occurrence of chemosynthetic microbial activities in the deep hydrothermal vents has beenstudied in detail (Jannasch, 1995). However, thepresence of microbial activity in shallowhydrothermal vents has not been documented.Therefore, this study attempts to document theincidence of microbial activity in the deposition of minerals in the Punta Mita coastal vents.2. PUNTAMITA SURFACEGEOLOGYThe Punta Mita area is located in the western coast of Mexico, in the NW border of the JaliscoBlock. The tectonic history of this region is verycomplex and recent activity is shown as largestructural features. The stratigraphiccontains units fiom the Paleozoic to the Quaternary (Fig. 2). Paleozoic rocks arerepresented by strongly deformed feldspatic and calcareous schist. The Cretaceoussequence contains limestone, and interlayeredsandstone and siltstone with a carbonate matrix. and siltstoneFig. 2 Stratigraphic column in the Punta Mita area.Tertiary rocks include intrusive, volcanic andsedimentary units. The base of the sequenceconsists of basaltic lava, pyroclastic flows and airfall deposits. This basaltic sequence is cut byalmost vertical basaltic dikes that acted as feedersof pillow lava flows. The basalts are veryvesicular and intensely altered to celadonite. Thissequence is overlaid by an unconsolidated well-stratified air fall.The Tertiary conglomerate includes fragments ofgranodiorite, granite, gneiss, ignimbrite andbasalt. Locally, it presents silicification and isaffected by vertical faults, especially between thePunta Mita and the Punta Negra area. TheQuaternary deposits are mostly formed by mediumgrained stratified unconsolidated sandstone thathas been recently uplifted. In the sea bottom,many vents are partially covered byunconsolidated platform sediments, and thedischarge of gas is the only evidence of thehydrothermal activity. In some cases thedeposited minerals form small mounds.3. GEOCHEMISTRYOFTHE FLUIDSwater samples present different degrees ofmixing with seawater; the least mixed are thosewith the largest flow rate. The chemistry of thewater reveals a fluid more dilute than seawater;almost all trace elements are below detectionlimits, except for Mn, Ba and I, which aresignificantly enriched with respect to seawater(Prol-Ledesma et 2002).The sampled gases show a predominance ofnitrogen and methane, with low contents of carbondioxide, argon, helium and hydrogen, and onlytraces of Isotopic composition of carbon inmethane yields a value of this indicates athermogenic origin for the organic gas.4. MINERALOGY OFTHEVENTSHydrothermal precipitates act as cement of thesands and also form mounds nearby the dischargeareas. The fluid discharged by the vents depositslayers of sulfides, calcite and phosphate (Fig. 3).Fig. 3 Vent deposit from the Punta Mitasubmarine hydrothermal area.4.1 CalciteCalcite is by far the most abundant precipitate inthis deposit and presents diverse textures. There isa first calcite generation with fibrous crystals up to250 microns in length. The fibers occur asconcentric aggregates with variable morphology:in the center they are mono-mineralic and lessporous; in the more external zones the aggregatesacquire a lobular morphology and increase theirporosity, in such a way that the most external zonehas arborescent texture. After this stage, a latefine-grained calcite generation appears. Thiscalcite develops euhedral rhombohedral crystals(up to a few tens of microns in size) that cementthe detrital and bioclastic grains. Cathodo-luminiscence and microprobe analyses do notshow any variations in the chemical compositionof all generations of calcite. The calcitecomposition is more than and themagnesium reaches only about 5%; strontium andsodium are present in small quantities, and iron,barium and manganese are not present insignificant amounts. The low ratioobserved in many calcite crystals from the innerpart of the vents indicate that they were notdeposited from seawater. The carbon isotopiccomposition of the calcite varies from toThe lowest value indicates that some calciteis produced by oxidation of methane and thevalues closer to 0 are evidence that some crystalswere formed with seawater carbon.4.2 BariteBarite is developed as tabular, euhedral crystalshaving up to 50 microns in length that are usuallyarranged in radial or bow-tie groups. These groupsoccur during all the calcite deposition stages andwithin the vugs or in veins.4.3 Carbonate-hydroxylapatiteCarbonate-hydroxylapatite was distinguished inthe optical microscope and identified byanalysis. It is deposited in the inner part of thefluid channels, forming a series of bands inalternation with calcite. It also appears as a latefilling in the porosity of the calcite aggregates.4.4 PyritePyrite is the main sulfide mineral and occurs asthin colloform coatings (Fig. 4) in the inner part ofthe discharge channels and surrounding the ventopenings.Pyrite covers shells and pebbles that are cementedby calcite in the vents, and it was also observed asframboidal aggregates (up to 100 micrometers)disseminated in the calcite cement. Detritalmagnetite grains are also locally replaced bypyrite. Backscattering electron images in theelectronic microscope (SEM-BSE) show finelayering of the pyrite coatings (one micrometerapproximate width) that alternate with carbonatein the thicker crusts.236Fig. 4 Micrograph that shows layers of pyritewithin calcite.The range of values in pyrite from the Punta Mita samples is from 10.7 to and thesulphate sulphide) is from +32 toThese values indicate that sulphur formpyrite in Punta Mita comes fi-om the bacteriogenicreduction of seawater sulphate (Alfonso et al., 2002).4.5 Other sulphidesCinnabar occurs in close association with pyrite,as aggregates up to 10 microns in size (brightminerals within the pyrite layers in figure 4),disseminated between pyrite and calcite layers(Prol-Ledesma et al., 2002). Small grains of anunidentified thallium mineral (up to 5 microns)'-were also observed in association with cinnabar.The only other sulphide present in the vents isgalena, which is rather scarce and only few smallgrains have been identified within the pyritelayers.4.6 Particulate matterParticulate matter was collected withpolycarbonate membrane filters and preserved indesiccators at a fridge temperature until analysisonshore. Filters containing hydrothermalparticulate matter were surveyed under thescanning electron microscope in order to identifythe chemistry of the particles. SEM examinationrevealed the presence of particles of different kindthat suggests high degree of mixing andsuspension: organisms and organicmatter appeared to be abundant; 25 micronparticles of different carbonate faces andinorganic particles of silicates were alsorecognized. Barite crystals were also observed inthe analysed filters. Distinctive euhedral grains of pyrite were identified as the result of precipitationfkom the solution (Fig. 5 ) .237Fig. 5 Micrograph of the particulate matter in afilter from the Punta Mita water samples thatshows a euhedral pyrite crystal.Pyrite was also observed as globules that werepossibly deposited by bacteriogenic reduction of seawater sulphate (Fig. 6). Cinnabar particles 8micron in size are consistent with themineralization conditions, observed in thedischarge precipitate as well in the inner walls of the vent.Fig. 6Micrographof theparticulatematter from a filterthe Punta Mita watersamples.The pyritein this case deposited thermal fluid. 5. DISCUSION AND CONCLUSIONSThe textures of calcite and pyrite indicate that theyare consistent with the, occurrence of microbialactivity in the vents. Also, the carbonate (fluoro)apatite (CFA) precipitation may be favoured bythe presence of bacterial products that wouldprovide surfaces for its precipitation (Reimers etal., 1996). The isotopic values obtained forcarbon in calcite down to , and for pyritefrom 10.7 to cannot be explained on thebasis of sole isotopic fractionation based onchemical deposition of these minerals.Some of the methane is oxidized and calcite withcarbon isotopic values similar to the methane isdeposited in the vents. In addition to methaneoxidation, calcite is deposited also from seawatercarbonateSulphide is produced by bacterial reduction of the seawater sulphate. This causes deposition ofpyrite, cinnabar and T1-sulphide.6. REFERENCESAlfonso, P., Prol-Ledesma, R.M., Canet, C.,Melgarejo, J.C., Fallick, A.E. (2002). Sulphurisotope geochemistry of the submarinecoastal vents of Punta Mita, Mexico.(in press)Jannasch, H.W. (1995). Microbial Interactionswith hydrothermal fluids. In: SeafloorHydrothermal Systems: Physical, Chemical,Biological, and Geological Interactions, S .W .R.A. Zierenber, L.S. andR.E. Thomson (Eds)., Geophys. Monograph 91 .American Geophysical Union, Washington, pp.273-296.F.J., Prol-Ledesma, R.M.,A. and Suarez-Placencia, C., (2000).Near-shore submarine hydrothermal activity in .Bahia de Banderas, western Mexico.39, 171-178.Prol-Ledesma, R.M., Canet, C., Melgarejo, J.C.,Tolson, G., Rubio-Ramos, M.A.,J.C., Ortega-Osorio, A., Torres-Vera, M.A. andReyes, A. (2002). Cinnabar deposition insubmarine coastal hydrotermal vents, PacificMargin of central Mexico. Econ. Geol.,(in press). Reimers, Ruttenberg, K.C., Canfield, D.E.,M.B. and Martin, J.B. (1996).Porewater and authigenicphases formed in theuppermost sediments of the Barbara Basin..Geochim.Cosmochim. Acta, Vol.Vidal, V.M.V., Vidal, F.V., and Isaacs, J.D.,1978,Coastal submarine hydrothermal activity offnorthern Baja California: Journal of GeophysicalResearch, 83-B, p. 1757-1774.238

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Biogenic vs geochemical precipitation of minerals in the submarine hydrothermal vents of Punta Mita, Mexico

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