A FLUID INCLUSION STUDY OF A LARGE MVT BARITE-FLUORITE DEPOSIT: KOMSHECHEH, CENTRAL IRAN

The Komshecheh barite-fluorite deposit is the product of epigenetic hydrothermal mineralization
in dolomitized limestone of Middle Triassic age. The deposit is structurally and lithologically controlled and
occurs predominantly as bedded stratabound replacement sheets, although the veins and solution-collapse
breccias constitute important modes of occurrence. Brecciation, dolomitization, silicification and
mineralization are interrelated. Post-Eocene intrusions and NW-SE and NE-SW trending faults are important
ore controls. The ore mineralogy is simple and consists of barite and fluorite with very minor galena, pyrite,
chalcopyrite, malachite and azurite. Sulfides are extremely rare in the deposit. At least two episodes of
deposition have been identified: an early stage of fluorite precipitation and a later barite-fluorite
mineralization event with spatial and temporal variations in salinity and temperature. Thermometric
investigations indicate that homogenization temperatures (TH) for primary and pseudosecondary fluid
inclusions in the fluorite range from 89 to 244°C with a mode at 150°C, whereas those for primary fluid
inclusions in barite range from 119 to 323°C with two modes at 150 and 250°C. Evidence for boiling is seen
during barite deposition. The mean salinities measured are 12 and 10 equivalent weight percent NaCl for
fluids in fluorite and barite, respectively. Hydrocarbons in the inclusions hosted by fluorite samples were also
detected. Chemical analyses of fluids extracted from inclusions in fluorite and barite show compositions
dominated by Na, Ca and Mg ionic species. Barite shows δ34S values between 22.6 and 26.7 per mil. The
comparatively narrow spread in δ34S values suggests very uniform environmental conditions throughout the
mineralization field. Consideration of these data in their geologic context favors a basinal source for the oreforming fluids which have been significantly affected by heating during intrusive emplacement. The
deposition of barite and fluorite resulted from physicochemical changes in Ba-F-rich hydrothermal brines
during mixing with sulfate-rich formational water and wall rock interaction.