Dehydration of hot oceanic slab at depth 30–50 km: KEY to formation of Irankuh-Emarat Pb[sbnd]Zn MVT belt, Central Iran

Abstract

Irankuh-Emarat is an important district of PbZn MVT-type deposits in Iran. The deposits are hosted by Lower and Upper Cretaceous dolostone in which and several stages of mineralization were identified. The deposition of sulfide minerals occurred spatially within or close to dolostone and shale-siltstone units with replacement and open space filling textures. Field observations, mineralogical and petrographic studies show different degrees of alteration related to mineralization. Dolomite, particularly Fe-rich dolomite, ankerite, bitumen and silicified rocks are associated with the principal sulfide mineralization. Hydrothermal fluids at Irankuh had low sulfidation to form Fe-rich dolomite and ankerite within dolostone and pyrite in shale formations. Chemical analysis of representative samples within the belt show that in sphalerite, Fe content varies between 0.3 and 5.2%, Cd 150–2000 ppm. In galena, Ag varies between 150–1200 ppm and Sb 140–1200 ppm. Fluid inclusion study shows homogenization temperatures (Th) between 150–260 °C and fluid salinities 9–21% (NaCl, wt% equiv). Field observations showed that high PbZn ore was deposited as replacement within the dolostone member favourable for chemical reaction. In this paper, we present a conceptual model for the formation of MVT deposits in the Irankuh-Emarat district. We suggest that the hydrothermal fluid originated from the dehydration of an oceanic subduction slab, which liberated Pb, Zn, and other metals, and may have removed metals from rocks and organic material of the continental crust. The down dip thermal gradient of the oceanic slab was about 18 °C/km (hot slab). More than 90% of all the water within the oceanic slab was released in the depth zone of the forearc region (depth of 30 to 50 km). Pb isotopic compositions of Irankuh-Emarat PbZn deposits are 206Pb/204Pb from 18.42 to 18.45; 208Pb/204Pb from 38.53 to 38.62 and 207Pb/204Pb from 15.63 to 15.65. Very low radiogenic Pb-isotopes indicate that Pb-isotopes had an oceanic slab source and were slightly contaminated by the basement rocks. Deep-seated thrust faults formed during the early stages of subduction, (Late Cretaceous?) and played an important role in the upward migration of hydrothermal fluids from the basement to shallow depths.