Nakhlak carbonate-hosted Pb(Ag) deposit, Isfahan province, Iran: a geological, mineralogical, geochemical, fluid inclusion, and sulfur isotope study

- Stratabound, epigenetic Nakhlak Pb(Ag) deposit is Southeast Missouri MVT.
- Galena (main sulfide) contains trace elements in mineral inclusions.
- Pre- to syn-mineralization dolomite is widespread close to veins.
- Ore-forming fluids were low-T basinal type.
- Thermochemical reduction changed sulfate to sulfide.

The Upper Cretaceous Nakhlak epigenetic vein-type Pb(Ag) deposit is located 55 km northeast of the town of Anarak in Isfahan Province, Iran. The deposit contains 7 Mt of galena-barite ore with an average grade of 8.33% Pb, 0.38% Zn, and 72 ppm Ag. The ore mineralization occurs as stratabound, epigenetic, steeply dipping, east-west-trending veins in faulted- or fracture-controlled Upper Cretaceous Sadar carbonates. Galena and barite are the primary minerals. Minor sphalerite, tennantite-tetrahedrite, pyrite, and chalcopyrite occur as inclusions in galena. Cerussite with minor amounts of anglesite and plattnerite formed in the oxidized supergene zone. The ore and ore-related minerals were deposited in the hydrothermally dolomitized carbonate host rock containing saddle-shaped dolomite. Geochemically, the dolomitized carbonate host rocks are enriched in MgO, Fe2O3, MnO, Pb, Zn, and Ba, but depleted in CaO. The galena concentrate contains high values of Ag (932 ppm), Sb (342 ppm), Cu (422 ppm), As (91 ppm), and Zn (296 ppm); the presence of these trace elements indicates a low-temperature type of galena mineralization. This interpretation is corroborated by fluid inclusions containing 12.98 wt.% NaCl equivalent salinity; the inclusions homogenize at the low temperature of about 152.1 °C. The similarity between δ34S(V-CDT) values in Nakhlak barite and Permian-Triassic δ34S marine sulfate values indicates that the Nakhlak sulfur was probably provided from evaporates of Permian-Triassic age. The δ34S(V-CDT) values of galena and barite samples occupy the ranges of − 1.04‰ to + 8.62‰ and + 10.95‰ to + 13.71‰, respectively, and are similar to Mississippi Valley-type (MVT) deposits. The low-temperature basinal fluids, evaporate-originated sulfur, and fault- or fracture-controlled galena-rich veins in the Nakhlak deposit resemble the type of geological features documented in Pb-rich MVT deposits.