HYDROTHERMAL ASSEMBLAGE OF Cl-, F- AND OH-BEARING APATITE-GROUP MINERALS FROM MAGLOVEC, NEAR PRESOV, SLOVAKIA

Abstract

We document a mineral assemblage consisting of columnar and fibrous apatite associated with asbestiform amphibole and opal in a tectonic zone in a hydrothermally altered andesite near Vysna Sebestova, in the Slanske vrchy Mountains, eastern Slovakia. The assemblage was examined by chemical and crystal-structure analysis. Apatite consists of three varieties: clear, yellow columnar fluorapatite (apatite I), surrounded and embayed by fibrous fluorapatite and chlorapatite, verging to borderline hydroxylapatite (apatite II), which is in turn intergrown with extremely finely fibrous carbonate-hydroxylapatite (apatite III). Apatite I does not reveal any departures from space group P 6 3 / m. X-ray powder-diffraction patterns of the finely fibrous apatite II, volumetrically dominant over apatite III, which could not be physically separated from each other, confirm the presence of at least two apatite phases. The Cl-rich variety of apatite II and apatite III seems to be monoclinic. Fibrous amphibole found as felt-like aggregates corresponds to tremolite. Opal occurs in the central channel of apatite I and also hosts the asbestiform tremolite. It is H 2 O-rich (close to 40 wt.%) and contains significant adsorbed impurities. The mineral assemblage examined is of hydrothermal origin, mostly postdating subsolidus alteration of the host volcanic rocks. Hydrothermal fluids could have promoted the mobilization of phosphorus from the andesite host. Crystallization of apatite I followed that of tremolite. Subsequently, apatite I was subject to partial dissolution, and its relics were overgrown and in part replaced by finely fibrous apatite II and III. Deposition of opal in the central channels of apatite I may have been coeval with these late generations of apatite.