Rare earth elements geochemistry, O and S isotopic compositions, and microthermometric data of barite from the Kuh–Ghalagheh deposit, Markazi Province, Iran

Abstract

The Kuh–Ghalagheh barite deposit is located 43 km southwest of the city of Mahallat in Markazi Province, Iran. The barite mineralization occurs as a stratabound, lenticular massive orebody in siliciclastic rocks of Eocene age. Barite is accompanied by calcite, Fe- and Mn-oxides, quartz, and minor malachite. The concentration of total REEs (∑REE) is very low in barite, ranging from 0.38 to 6.26 ppm. Chondrite-normalized REE patterns show enrichment of LREE relative to HREE, and dominantly negative La, Gd, and Ce anomalies. The δ18O and δ34S isotopic values of the barite samples fall within narrow ranges of +4.0 to +5.6‰ and +26.2 to +27.8‰, respectively, which do not match those of contemporaneous seawater (Eocene). This suggests that isotopically modified seawater sulfate is the source of oxygen and sulfur in barite. Three types of aqueous, aqueous–carbonic, and carbonic fluid inclusions were found in barite, suggesting that two immiscible fluids (aqueous and carbonic) were involved in the deposition of barite. Salinity values in the aqueous fluid inclusions ranged from 1.57 to 13.40 wt% NaCl equivalent. The aqueous–carbonic inclusions have salinity values ranging from 9.43 to 16.84 wt% NaCl equivalent. The salinity ranges of the aqueous and aqueous–carbonic inclusions show that two fluids have mixed, one with moderate salinity (>15 wt% NaCl equivalent) from basinal origin and a second with low salinity (<5 wt% NaCl equivalent) from seawater. The broad homogenization temperature ranges of 130–258 °C for aqueous and 118–290 °C for aqueous–carbonic inclusions do not represent the actual temperatures of the trapped fluids, but were generated by thermal re-equilibration of the inclusions due to deep burial of barite. The available data are consistent with a cold seeps model for barite formation in the Kuh–Ghalagheh deposit. The entry of Ba-rich basinal brines into the sedimentary basin resulted in the reaction of Ba with seawater sulfate, and the deposition of barite in siliciclastic sediments.