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Clay Minerals; June 2008; v. 43; no. 2; p. 267-280; DOI: 10.1180/claymin.2008.043.2.09
© 2008 Mineralogical Society of Great Britain and Ireland
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Research Paper

Contrasting alteration processes in hydrothermally altered dolerites from the Betic Cordillera, Spain

J. JIMÉNEZ-MILLÁN1,*, I. ABAD1 and F. NIETO2

1 Departamento de Geología, Universidad de Jaén, 23071 Jaén, Spain, and 2 Departamento de Mineralogía y Petrología and IACT, Universidad de Granada-CSIC, 18002 Granada, Spain

* E-mail: jmillan{at}ujaen.es

(Received 26 September 2007; revised 21 January 2008)

Dolerites from the Sierra de San Pedro (Betic Cordillera, southern Spain) develop three main types of microsystems during hydrothermal alteration: (1) centimetresized veins cross-cutting the dolerites; (2) microfractures in feldspar and diopside grains; and (3) alterations involving primary-igneous mafic phyllosilicate grains. The vein and microfracture sites developed alteration assemblages of randomly oriented smectites (saponite and beidellite) and halloysite. At these sites, the alteration mechanism was governed by complete dissolution of the parent material with subsequent crystallization of the products, with no parent-mineral crystallographic control. Pseudomorphed mafic phyllosilicate sites are characterized by oriented complex mineral assemblages made of chlorite, chlorite/smectite mixed layers, corrensite, saponite and relicts of biotite. These assemblages formed during the initial high-temperature stage of the hydrothermal process. In these microsites, the alteration sequence was controlled by the parent-mineral structure and chemistry, with products determined by structural relations with the parent phyllosilicate. Alteration of one phyllosilicate to the next most stable proceeds via interstratification of the parent and product phyllosilicates.

KEYWORDS: pseudomorphed biotite, interstratification, microfractures, fluid circulation, dissolution






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