Abstract
The Precambrian sedimentary-volcanic sequence of Kahar Formation (KF), located in the Mishu mountain, NW Iran, hosts numerous strata-bound and stratiform Barite-(Pb-Cu-Zn) deposits. These deposits include Daryan, Shanjan, Almas, and Aralan-Pyrbala that are hosted by black shale, siltstone, and fine-grained sandstone of the KF. Available evidence such as clastic rocks, bimodal volcanism (rhyolitic and mafic flows) and geochemistry investigations shows that sedimentary host rocks of KF were formed in an extensional environment in the active continental margin. Based on petrographic studies, three different ore facies have been distinguished: 1) a stockwork (feeder) zone including five types of veins and veinlet’s; 2) a massive ore consisting of barite and sulfide (galena, chalcopyrite, pyrite, sphalerite and minor tetrahedrite and tennantite) minerals, and (3) a bedded ore that includes laminated sulfides with sedimentary structures and sulfides with barite minerals. High contents of redox-sensitive elements, such as Fe, Mn, V, and U in the host rocks (KF) indicate that mineralization occured in oxic-anoxic ambient environment. The Electron Prob Micro Analyzer (EPMA) data indicate that the Se content of chalcopyrite in the bedded ore facies is higher than that in stockwork and massive ore zones, which indicate a decrease of temperature from feeder zone towards massive and bedded ores. The trace-elements distribution among the minerals from the three ore facies, indicate evidence of a submarine hydrothermal exhalative origin or SEDEX-type mineralization at the Mishu deposits (especially Co and Ni contents and Co/Ni ratios in pyrite and Zn/Cd ratios in sphalerite). The δ34S values of the sulfides range from +17.7 to +35.8‰ within wich the highest δ34S values correspond to the bedded ore (+28.7 to +35.8‰), and the lowest δ34S values belong to the feeder zones (+22.4 to +25.4‰) and the massive ore (+17.7 to +21.6‰). The overall range of δ34S suggests that sulfides were formed by the reduction of Precambrian seawater sulfate due to bacteriogenic sulfate reduction in a closed or semiclosed basin. Briefly, the sedimentary host rocks, types of mineralization (bedded, feeder, and massive ore), mineralogical pieces of evidence, along with geochemical and sulfur isotope signatures, indicate that Mishu Barite-(Pb-Cu-Zn) deposits are very similar to SEDEX-type deposits.
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