K-Ar, δ18O and REE constraints on the genesis of ammonium illite from the Harghita Bãi hydrothermal system, Romania
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 ∼270°C from fluids having a δ18O 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 δ18O. Alternately, ammonium-rich illite-smectite mixed layers formed very recently, less than ∼1 million years ago at a temperature of ∼90°C 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 δ18O 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 NH4-rich interlayers of the ammonium mixed layered sequence.
Keywords: AMMONIUM ILLITE AND ILLITE-SMECTITE MIXED LAYERS; DELTA18O; EASTERN CARPATHIANS; HARGHITA BAI; HYDROTHERMAL ACTIVITY; K-AR; RARE-EARTH ELEMENTS; ROMANIA; SCANNING ELECTRON MICROSCOPY; X-RAY DIFFRACTION
Document Type: Research Article
Publication date: 2010-09-01