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

Dolerites from the Sierra de San Pedro (Betic Cordillera, southern Spain) develop three main types of microsystems during hydrothermal alteration: (1) centimetre-sized veins crosscutting the dolerites; (2) in ic ro fractures in feldspar and diopside grains; and (3) alterations involving primary-igneous rnafic phyllosilicate grains. The vein and rnicrofracture 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 inafic 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-ternpcrature stage of the hydrothernial process, In these inicrosites, 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.