The Kizilcaoren fluorite-barite-Rare Earth Element (REE) deposit occurs as epithermal veins and breccia fillings in altered Triassic metasandstones and Oligocene-Miocene pyroclastics adjacent to alkaline porphyritic trachyte and phonolite. This deposit is the only commercial source of REE and thorium in Turkey. Most of the fluorite-barite-REE mineralisation at Kizilcaoren has been formed by hydrothermal solutions, which are thought to be genetically associated with alkaline volcanism. The occurrence of the ore minerals in vuggy cavities and veins of massive and vuggy silica indicate that the ore stage postdates hydrothermal alteration. The deposit contains evidence of at least three periods of hypogene mineralisation separated by two periods of faulting. The mineral assemblage includes fluorite, barite, quartz, calcite, bastnasite, phlogopite, pyrolusite and hematite as well as minor amounts of plagioclase feldspar, pyrite, psilomelane, braunite, monazite, fluocerite, brockite, goethite, and rutile. Fluid inclusion microthermometry indicates that the barite formed from low salinity (0.4-9.2 equiv. wt% NaCl) fluids at low temperatures, between 105 and 230 degreesC, but fluorite formed from slightly higher salinity (< 12.4 equiv. wt% NaCl) fluids at low and moderate temperatures, between 135-354 degreesC. The depositional temperature of bastnasite is between 143-286 degreesC. The local coexistence of liquid- and vapour-rich inclusions suggests boiling conditions. Many relatively low-salinity (< 10.0 equiv. wt% NaCl), low and moderate temperature (200-300 degreesC) inclusions might be the result of episodic mixing of deep-saline brines with low-salinity meteoric fluids. The narrow range of delta(34)S (pyrite and barite) values (2.89-6.92parts per thousand CDT)suggests that the sulphur source of the hydrothermal fluids are the same and compatible with a volcanogenic sulphate field derived from a magmatic sulphur source. (C) 2002 Elsevier Science Ltd. All rights reserved.