K-Ar, d 18 O and REE constraints on the genesis of ammonium illite from the Harghita Ba ˜i hydrothermal system, Romania N. CLAUER 1, *, N. LIEWIG 2 AND I. BOBOS 3 1 Laboratoire d’Hydrologie et de Ge ´ochimie de Strasbourg (CNRS-UdS), 1 rue Blessig, 67084 Strasbourg, France, 2 Institut Pluridisciplinaire Hubert Curien, UMR 7178, 23 rue Becquerel, 67087 Strasbourg, France, and 3 GIMEF- Departamento de Geologia, Universidade do Porto, 4099-002 Porto, Portugal (Received 30 August 2009; revised 9 March 2010; Editor: John Adams) ABSTRACT: Ammonium illite and ammonium illite-smectite mixed layers, together with potassium illite, smectite and minute amounts of kaolinite were identified in hydrothermally altered andesite rocks from the Harghita Ba ˜i area of the Eastern Carpathians, Romania. K-Ar dating and oxygen isotope tracing, as well as rare-earth elemental analyses were made to provide new information on the timing and crystal-chemical processes characterizing the crystallization and further evolution of these illite-type mineral phases. The combined results suggest the occurrence of hydrothermal activity in two distinct episodes with nucleation of two generations of illite-type particles of different chemistry and morphology. About 9.5 Ma ago, potassium illite crystallized in alteration halos of the porphyry Cu system, probably at a temperature of ~270ºC from fluids having a d 18 O of ~2.9% (V-SMOW). Associated smectite seems to have precipitated slightly later in external alteration halos at a similar temperature, but from fluids depleted in alkalis and with a different d 18 O. Alternately, ammonium-rich illite-smectite mixed layers formed very recently, less than ~1 million years ago at a temperature of ~90ºC from fluids of probable meteoric origin that altered the previously crystallized potassium illite, resulting in the crystallization of a new generation of ammonium illite-smectite mixed layers. Evidence of this dissolution-precipitation process is provided by a significant increase in the d 18 O of the mixed-layer structures and by a significant change in their REE contents and distribution patterns. Occurrence of potassium in the ammonium-rich mixed layers probably relates to the progressive alteration of the first-generation potassium illite and a discrete concomitant take up of released K by the new NH 4 - rich interlayers of the ammonium mixed layered sequence. KEYWORDS: ammonium illite and illite-smectite mixed layers, K-Ar, d 18 O, rare-earth elements, X-ray diffraction, scanning electron microscopy, Eastern Carpathians, Harghita Ba ˜i, Romania, hydrothermal activity. K-Ar dating of phyllosilicates that crystallized during hydrothermal mineralization processes has been long used to constrain the evolution of porphyry copper systems (Lipman et al., 1976; Eberl et al., 1987), Kupferschiefer mineralizations (Bechtel et al., 1996, 1999), Fe-Mn sulphide deposits (Marumo & Sawai, 1986; Marumo & Hattori, 1993), and Au and U concentrations of metasedimentary sequences (Clauer et al., 1985; * E-mail: nclauer@illite.u-strasbg.fr DOI: 10.1180/claymin.2010.045.3.393 Clay Minerals, (2010) 45, 393–411 # 2010 The Mineralogical Society