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Boletín de la Sociedad Geológica Mexicana

versión impresa ISSN 1405-3322

Bol. Soc. Geol. Mex vol.59 no.1 México jun. 2007

http://dx.doi.org/10.18268/bsgm2007v59n1a7 

Artículos

Disolución y precipitación de carbonatos en sistemas hidrotermales. Implicaciones en la génesis de depósitos tipo MVT

Dissolution and precipitation of carbonates in hydrothermal systems. Implications in the genesis of MVT deposits

Mercè Corbella 1  

Esteve Cardellach 1  

Carlos Ayora 2  

1Departament de Geologia, Facultat de Ciéncies, Universitat Autónoma de Barcelona, 08193-Bellaterra, Cataluña, España. merce.corbella@uab.es.

2Institut de Ciències de la Terra Jaume Almera, Consejo Superior de Investigaciones Científicas, c/ Martí i Franquès s/n, 08028-Barcelona, Cataluña, España

Resumen

El origen de la porosidad secundaria en carbonatos profundos, observada tanto en reservorios de hidrocarburos como en depósitos de Zn-Pb de tipo Mississippi Valley (MVT), es difícil de justificar, ya que los datos cinéticos y termodinámicos sugieren que las soluciones calientes que circulan por carbonatos están equilibradas con ellos y, por lo tanto, no los disuelven. Esta aparente paradoja puede ser explicada a partir del estudio e interpretación de las texturas minerales en los depósitos MVT. Algunas de ellas sugieren simultaneidad entre los fenómenos de disolución de la roca encajante carbonatada y de precipitación de sulfuros, sulfatos y carbonatos, en tanto que otras texturas indican un crecimiento de cristales muy rápido y en espacios abiertos. La mezcla de dos soluciones hidrotermales concilia las observaciones texturales con los datos experimentales y teóricos.

Se han realizado los cálculos correspondientes de transporte reactivo en el contexto de formación de depósitos MVT. Los resultados obtenidos muestran que la mezcla de una salmuera ácida, saturada en carbonato, con un agua subterránea más diluida y alcalina, también saturada en carbonato, es otra solución de química intermedia pero mayoritariamente subsaturada en carbonato, y, por lo tanto, capaz de disolver la roca y formar cavidades en algunas decenas de miles de años. Cuando los fluidos que se mezclan transportan metales y sulfhídrico, precipitan sulfuros alrededor de las cavidades; la porosidad generada a partir de la reacción acoplada entre la precipitación de sulfuros y disolución de carbonatos no es suficiente para generar las cavidades observadas, pero sí para evitar el blindaje de ésta por sulfuros. Si la solución rica en azufre contiene más sulfato que sulfhídrico (es ligeramente oxidante) pueden depositarse también sulfatos en el espacio abierto generado. Diferentes proporciones de los fluidos extremos de la mezcla dan lugar a cavidades de formas diferentes, las cuales tienden a alargarse en las direcciones de menor flujo. El mismo proceso de mezcla de soluciones hidrotermales en una roca carbonatada puede resultar en la formación de cavidades simultáneamente a la precipitación de sulfuros y relleno de sulfatos.

Palabras clave: texturas en MVT; disolución de carbonatos; karst hidrotermal; simulaciones numéricas; transporte reactivo

Abstract

The formation of secondary porosity in deep carbonates as observed in hydrocarbon reservoirs or Zn-Pb Mississippi Valley type (MVT) deposits is difficult to explain as kinetic and thermodynamic data suggest that low temperatures hydrothermal solutions flowing through carbonate rocks are in equilibrium with them and dissolution cannot occur. Textural studies in MVT deposits provide the clue to the paradox. Some textures indicate that dissolution of the carbonate host rock was concomitant with sulfide, sulfate and carbonate porosity filling; however, other textures point to a rapid growth of crystals in open spaces. The mixing of two hydrothermal solutions conciliates the observational features with experimental and theoretical data. Numerical methods used to perform the calculations of reactive transport in the context of MVT ore formation show that mixing between an acidic brine with dilute and alkaline groundwater, both independently saturated with respect to carbonate forms with an intermediate chemistry but mostly undersaturated with respect to carbonate. Therefore, the mixture is carbonate-corrosive and is able to build cavities within a time span of some tens of thousands of years. Sulfides precipitate surrounding cavity walls when the mixing fluids carry metals and sulfur; this reaction is concomitant with an increase in porosity. Such porosity is not large enough to explain the developed cavity but is sufficient to prevent its armoring by sulfides. Sulfate may precipitate in the open spaces formed whenever the sulfur-rich fluid carries more sulfate than sulfide (a slightly oxidizing fluid). Mixing of different proportions of end-member fluids results in cavities of uneven shapes, as cavities tend to enlarge towards the smaller flux direction. From the textural and reactive transport study in MVTs we conclude that cavity formation, sulfide precipitation and sulfate filling may be generated by the same major process of hydrothermal fluid mixing.

Keywords: MVT textures; carbonate dissolution; hydrothermal karst; numerical simulations; reactive transport

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Recibido: 25 de Mayo de 2007; Revisado: 09 de Agosto de 2007; Aprobado: 15 de Agosto de 2007

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