In the SE region of Tandilia two types of clay deposits originated by hydrothermal activity have been studied: 1a) Derived from alteration of original fall out pyroclastic material (mostly lapillistone) without material rework, deposited in restricted areas of a platform environment dominated by quartz sandstones (Cerro del Corral area) showing a kaolinite(3T)-rutile-anatase assemblage. The genesis of these deposits intercalated in the sedimentary cover of Tandilia, are clearly related to diabase intrusions. 1b) Deposits involving mainly reworked pyroclastic material are located in the west Barker, Taglioretti and east of Sierra de La Tinta areas displaying an advanced argillic alteration (AAA), which included pyrophyllite, kaolinite, dickite, rutile, alunite, diaspore, hematite and secondary quartz as mineral association. 2) Derived from alteration of deformed migmatites from the igneous-metamorphic basement rocks, showing an AAA represented by pyrophyllite, kaolinite, sericite, diaspore, hematite, alunite, turmaline and rutile. Though in a lesser extent, the sedimentary cover (orthoquartzites, wackes) of these deposits have been also affected (San Manuel and Cerrito de la Cruz areas). Deposits1b) and 2) type show clear evidences of replacement of preexistent rocks and developed mineral zoning due to hydrothermal activity. Trails of secondary fluid inclusions have been found in the orthoquarzite wall rocks. In the secondary quartz associated to 1b) type deposits primary fluid inclusions and solid inclusions (pyrophyllite and/or sericite) can be recognized. Small amounts of aluminum phosphate sulfate (APS) minerals enriched in REE have been detected in these deposits.
