Geology, isotope geochemistry, and fluid inclusion investigation of the Robat Zn-Pb-Ba deposit, Malayer-Esfahan metallogenic belt, southwestern Iran

Abstract

The Robat Zn-Pb-Ba deposit in the Arak mining district in the Malayer-Esfahan metallogenic belt (MEMB) has an estimated reserve of 5 million tonnes (Mt) ore grading 2.1 wt% Zn and 1.4 wt% Pb and 4 Mt barite ore. This deposit, hosted by Early Cretaceous carbonate rocks, formed in an extensional back-arc environment between the Sanandaj-Sirjan Zone and Central Iranian Microcontinent. Sulfide and barite ores display a strong lithological control and form stratabound stacked ore lenses, which are hosted in two horizons by silicified and dolomitized limestone. They are generally best developed below marl layers within intercalated shale, marl, and limestone.The textural and mineralogical data indicate two main stages of sulfide and barite mineralization at the Robat deposit. The first stage comprised deposition of volumetrically minor early fine-grained disseminated sulfides and barite near the seafloor in unconsolidated sediments. Subsequently, sub-seafloor replacement of barite, carbonate, and early sulfide laminae/bands occurred by precipitation of late sulfides and other hydrothermal minerals within the Early Cretaceous limestone-dominant sequence. The minerals in this later stage were deposited in a close relationship with quartz veins.Electron microprobe analyses revealed that sphalerite has a very high Fe content averaging about 1.23 wt% and is relatively enriched in As, Ga, Ge, Cd, Ag, and Bi, which is similar to Irish-type carbonate-hosted deposits. In addition, the Ag content of galena is comparatively low, although concentrations of Ga, Ge, Cd, Zn, and Sb are fairly high. Fluid inclusion microthermometric study of hydrothermal quartz revealed two types of primary two-phase (L + V) fluid inclusions in hydrothermal quartz. Type I fluid inclusions with irregular shapes were recorded along crystal growth zones, whereas type II inclusions are observed as isolated inclusions with relatively regular shapes that are less common than type I. Type I and II inclusions exhibit homogenization temperatures of 140–239 °C and 233–290 °C, with salinities of 2.2–11.8 and 5.5–11.5 wt% NaCl equiv., respectively. Fluid inclusions extracted from quartz displayed δDSMOW between −67‰ and −56‰ and measurements of hydrothermal quartz yielded δ18Ofluid (SMOW) of 12.9‰ to 23.5‰. The 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of galena samples fell in the ranges of 18.412–18.427, 15.642–15.647, and 38.556–38.580, respectively. The different ratios for the Pb isotopes suggest a dominant continental crust origin for Pb in the ores. Geological, textural, mineralogical, and isotopic data may be cumulatively interpreted to indicate that the Robat carbonate-hosted deposit formed from sub-seafloor replacement like commonly observed for Irish-type base metal deposits.