Formation of barite in the Ab Torsh deposit, Kerman province, Iran: Insights from rare earth elements, O and S isotopes, and fluid inclusions

Abstract

Barite can form in a variety of geological environments, as it occurs in a wide range of mineral deposits. To determine the origin and physicochemical conditions under which the Ab Torsh barite deposit formed, an extensive study was conducted using petrographic, Rare Earth Element (REE) geochemical, O and S isotopic, and fluid inclusion methods. Barite mineralization occurs at Ab Torsh as a stratabound vein in the Senonian carbonate rock units. Barite-quartz is accompanied by subordinate malachite, chrysocolla, Fe-Mn oxide-hydroxides, galena, azurite, fluorite, pyrite, and bornite. The ∑REE values are very low in barite (5.32–14.56 ppm), with chondrite-normalized patterns showing enrichment of Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE). The low ∑REE content and REE element ratios (Ce/La, (La/La*)_N, and (Gd/Gd*)_N) indicate that seawater with a highly altered geochemical signature (connate water) acted as a Ba ore-forming fluid. The δ¹⁸O and δ³⁴S values in barite (+10.4–+11.1 ‰ and +27.3–+27.8 ‰, respectively) and the δ³⁴S values in galena (+6.3 and + 7.9 ‰) indicate that the sulfate (and thus sulfur) originated from sulfate-bearing connate waters and/or evaporites. Thermochemical Sulfate Reduction (TSR) was the most likely mechanism for the formation of the reduced sulfur in galena. The salinity and homogenization temperatures in the aqueous fluid inclusions of barite and quartz (2.7–19.3 wt% NaCl equivalent and 110–275 °C, respectively) indicate that basinal fluids containing a meteoric water component were the source of the mineralizing solutions. The fluid inclusion data demonstrate that two fluid mixing have occurred: one between the hot basinal brines and cold meteoric waters, and another between heated and cold meteoric waters. It is estimated that the hot fluids derived from a maximum depth of about 9 km. The Ab Torsh deposit is classified here as a structure (unconformity)-related barite deposit. It is concluded that intense faulting and brecciation of the host rocks caused by post-Cretaceous compressional tectonics probably provided the channels necessary for the upward migration of deep mineralizing fluids from a basinal brine source. Barite formed where these ascending hot, Ba-bearing hydrothermal fluids encountered cooler, sulfate-bearing connate waters trapped in the overlying Senonian strata and/or the descending cold meteoric waters that dissolved evaporite-bearing rock units.