Sediment hosted stratiform barite – (Cu-Zn-Pb) deposits in the southwest Mahabad, Iran; implications for geology, ore textural, compositional, and S-O isotopes geochemistry

Abstract

The southwestern part of Mahabad town in the northwestern part of the Sanandaj-Sirjan metamorphic zone (SSZ) of Iran hosts seven barite-base metal deposits. They have occurred within the Precambrian-aged metasedimentary succession including black color fine-grained shale, slate, and phyllite with minor calcareous intercalation. This investigation is focused on three main ore deposits including Meraneh, Sorkhab, and Shekarbeig. The mineralized units and host rocks have been metamorphosed to lower greenschist facies in the Post-Cretaceous.Five different stages of mineralization were distinguished based on the field observations, mineralogical and textural relationships, morphology of the orebodies, and mineral chemistry studies. The followings are the stages of evolution: (1) precipitation of framboidal, colloform, and monomineralic pyrite bands and fine-grained barite (2) stratiform accumulation of barite with sulfide and un-mineralized sediment strata (3) stockwork mineralization and associated alteration (4) formation of discrete vent complex associated with massive sulfides and barite in replacement zone, and (5) post-mineralization deformation and shearing process, contemporaneous with regional metamorphism.Pyrite, chalcopyrite, sphalerite, and galena are the major sulfide minerals, in order of abundance, found in the studied deposits, accompanied by the minor amounts of tetrahedrite and covellite. The stratiform, massive and stockwork ores are indicative of the main ore-forming event and are volumetrically the most important facies. The electron probe micro-analyzer (EPMA) data of sulfide minerals indicates a distinct variation in trace element concentrations during the ore formation. Besides, this study focuses on distributions of δ 34S value in sulfide minerals, as well as δ 34S and δ 18O value of barite mineralization in stratiform, stratabound, and stockwork ore zones. The paragenetic relationships, mineral chemistry and isotopic study of minerals reveal that the stages 1 and 2 can be considered as the pre-date hydrothermal input of the stage 3 in all the three ore deposits. In addition, the mineralization event during stage 1 of the ore formation was found out to have occurred in a low temperature environment within the unconsolidated sediments, instantly under the sea floor. During stage 2, the stratiform ore has been deposited due to the diagenetic processes. The up-flow exhalative hydrothermal fluids resulted in the thermal alteration of the organic-rich host rock during stage 3, and formation of the massive mineralized bodies in stage 4. Finally, the lower greenschist facies metamorphism affected both the ore and host rocks during stage 5.On the basis of mineral chemistry inspections, all types of pyrites have high Ni and low Co contents and great majority of them show the Co:Ni ratios of lower than 1. Stratiform pyrites were fine-grained and enriched in As, Au, and Co, in contrast to hydrothermally pyrites. Ni, Pb, Zn, Bi, Hg, and Cu were mostly concentrated in coarse-grained hydrothermally pyrits. Also, the temperature and sulfur fugacity increased from stratiform stage to hydrothermal stage. The Se content of chalcopyrite as well as Bi and Ag contents of Galena increased from feeder to massive and stratiform ore. Sphalerite was generally iron-poor and its Fe content decreases from feeder to massive and stratiform ore. The Co/Ni ratio of pyrite and Zn/Cd ratio of sphalerite can be used to discriminate the VMS, magmatic or SEDEX origins for ore deposits. The Zn/Cd ratio of the sphalerites and Co/Ni ratio of pyrites provided the evidence for SEDEX style mineralization in the studied deposits.Barites with various textures and morphologies show overlapped distributions of δ34S and δ18O values from the stages 2, 3, and 4, range from + 34.1 to + 41.3‰ and + 10.2 to + 12.6‰, respectively. The d34S versus d18O patterns for the stratiform barite resemble the patterns that indicate sulfate is consumed by AOM. Sulfides have different isotopic compositions: stratiform with markedly more positive δ 34S values of + 28‰ to + 33‰; stockwork with δ 34S values of + 8‰ to + 26‰; and massive pyrite with δ 34S between + 8‰ and + 26‰. We suggest that the isotopic relationships (δ34S pyrite ≈ δ34S barite) in the stratiform ore zones may be related to anaerobic oxidation of methane coupled to sulfate reduction (AOM-SR), which has been developed during open system diagenesis. Textural and isotopic evidence indicates that the stockwork and massive ores has grown by thermochemical sulfate reduction (TSR) and reductively dissolving barite. On the other hand, the trend between d18O and δ34S suggest that the growth of the stockwork and massive barite is related to fluid seep on modern continental margins. Textural evidence documented during the different mineralization stages together with concentration and the distribution pattern of trace elements and isotopic investigation in the stratiform, massive, and stockwork ore zones signify that the SEDEX-type mineralization has been occurred in this district.