Geological, geochemical, and fluid inclusion evidences for the origin of the Ravanj Pb?Ba?Ag deposit, north of Delijan city, Markazi Province, Iran

Authors: MOSTAFA NEJADHADAD, BATOUL TAGHIPOUR, ALIREZA ZARASVANDI, ALIREZA KARIMZADEH SOMARIN

Abstract: The Lower Cretaceous sequences of the Ravanj anticline in Iran host the Ravanj Pb-Ba-Ag mineralization. Economic orebodies are restricted to the thrust zone within the brecciated massive limestone and immediately above the Jurassic shale and/or shale-limestone intercalations of the Lower Cretaceous. Paragenetic sequence and distinct zoning of mineral assemblages indicate that ore-forming fluid migrated through thrust zones along the NE-trending faults. The REE pattern of mineralized host rock is characterized by HREE-enrichment ((La/Lu)_PAAS = 0.24). The Ce/Ce* ratio of mineralized host samples shows negative Ce anomalies, which is most likely inherited from seawater. The positive Eu/Eu* anomaly suggests high ?O₂ during ore deposition. Negative Δ³⁴S values of the Ravanj sulfide minerals (-27? to -11?) suggest bacteriogenic sulfate reduction, whereas positive Δ³⁴S values of barite (+20?) fall in the range of Tertiary marine sulfates. Multiple isotopic sulfur sources of sulfides and sulfate minerals support mixing of a reduced negative isotopic sulfur-bearing fluid and a positive isotopic sulfate-bearing fluid. The average of homogenization temperatures of fluid inclusions from the early and late-stage mineralization calcites are 165 and 160 °C, respectively. The salinity of fluid inclusions varies between 0.66 and 18 wt% NaCl equivalent with an outlier at 22.2. Wide variation in the salinity of fluid inclusions can be explained by fluid mixing between a higher salinity group with 14-18 wt% NaCl equivalent and a lower salinity group with 0.66-8 wt% NaCl equivalent. In the Ravanj, fine grained sulfide minerals are consistent with a sulfur supersaturated fluid. High concentrations of Pb can be present in oxidized, chlorine-bearing fluids if the concentration of total H₂S is very low. Therefore, mixing of two geochemically different fluids could precipitate both galena and barite. These data show that the Ravanj Pb-Ba-Ag deposit is comparable with Pb-rich Mississippi Valley-type deposits such as the Viburnum Trend district in the USA.

Keywords: Ravanj Pb?Ba?Ag deposit, rare earth elements, multiple isotopic sulfur sources, microthermometry, fluid mixing

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