The Eocene Poshteh Ba–Pb–Zn–Cu deposit, Iran: geological, mineral–chemical, fluid inclusion, and S–O isotopic analysis of a Kuroko-type VMS deposit with paleo-hydrothermal vent chimneys

Abstract

The Poshteh deposit is the first documented paleo-hydrothermal chimney-bearing volcanogenic massive sulfide (VMS) deposit in Iran. It occurs in the form of well-developed lensoid orebodies within the Eocene volcano-sedimentary sequence, and comprises of laminated, brecciated, banded, massive, and disseminated textures. The stratiform part of this deposit is approximately 500\xa0m long and up to 20\xa0m thick, and consists of massive to semi-massive sulfides and barites, some of which are banded, which are overlain by barite and siliceous exhalites. The stringer zone stratigraphically underlies the stratiform mineralization. Veins within the stringer zone are composed of quartz, barite, pyrite, galena, sphalerite, and chalcopyrite. Three thin layers of exhalative Mn–Fe-bearing chert overlies the sulfide–sulfate orebodies. Four types of fluid inclusions were recognized in barite and quartz encompassing monophase vapor, monophase liquid, two-phase liquid-rich inclusions, and two-phase vapor-rich fluid inclusions. The homogenization temperatures of fluid inclusions mainly vary between 86.2 and 241.9\xa0°C, whereas their salinities focus on 0.99–21.23\xa0wt.% NaCl equiv. The ore-forming fluids of the barite and quartz minerals are characterized by low-to-medium temperature and low-to-medium salinity. The δ 34 S values of barite associated with the stratiform and stringer barite orebody range from 8.37 to 21.83\xa0‰, indicating that the sulfur was derived from the seawater. The sulfur isotope values of sulfide (−\xa05.18 to 9.33\xa0‰) are similar to those of VMS deposits from other parts of the world. The δ 18 O data suggest that the ore-forming fluids of the Poshteh deposit were mainly derived from deep subseafloor circulation of seawater. The well-preserved chimneys typically contain distinct concentric outer and inner sulfide- and axial sulfate-rich zones. The outer zones of the chimneys contain mainly pyrite and chalcopyrite. The sulfides within the inner zone consist predominantly of sphalerite, argentite, and galena. The axial conduits are commonly dominated by barite and calcite. The sulfide-rich chimney zones are also characterized by significantly higher metal contents. The presence of paleo-hydrothermal vent chimneys in the Poshteh\xa0Kuroko-type deposit and their trace-element contents also provides significant data for understanding the physicochemical conditions of VMS mineralization.