Sulfide mineral chemistry investigation of sediment-hosted stratiform copper deposits, Nahand-Ivand area, NW Iran

•Copper deposits in the Nahand-Ivand area are hosted by gray sandstone.•Detailed EPMA indicates that the syn-sedimentary to early-diagenetic framboidal pyrite (PyI) has low values of Cu, Pb and Ag. Successive generations of pyrite (Py II and Py III) formed as a consequence of overgrowths and diagenetic processes.•Geochemical studies indicate that the Nahand-Ivand deposits are rich in copper and silver, reaching up to 35 wt.% and 730 ppm, respectively. The EPMA studies show that two-phase covellite-yarrowite associations are most widespread and, where mixtures of Cu-sulfides coexist, Ag is preferentially partitioned into covellite over co-existing digenite.•Several features, including framboidal pyrite and cuprian pyrite, the nonequilibrium assemblage of pyrite and Cu-rich sulfide, basket-weave intergrowth texture of chalcopyrite and bornite, the presence of roxbyite, replacement and open space filling texture, and concentration of copper mineralization associated with organic matter, suggest that the Nahand-Ivand deposits are low-temperature sediment-hosted stratiform copper deposits.

Copper mineralization in Nahand-Ivand area, NW Iran, appears as disseminated copper sulfides along a redox boundary between gray sandstone and microconglomerate and hematitic sandstones, siltstones and shaly marl. Geochemical analyses of the Nahand-Ivand deposits show as much 35 wt.% Cu and 730 ppm Ag in the gray sandstone. Electron probe micro-analysis (EPMA) was used to determine the mineralogical composition and distribution patterns of copper and silver-bearing phases mainly in pyrite and Cu-bearing sulfides. The EPMA data were essential for evaluating the distribution and partitioning of potential economically-valuable components between co-existing minerals. Furthermore, they contribute to a better understanding of the genesis of the Nahand­Ivand copper deposits and will guide further exploration in the region.The EPMA results from different types of pyrite reveal Cu contents as high as 0.32 wt.%, 1.10 wt.%, and 2.88 wt.% for framboidal pyrite (PyI), overgrowths on framboidal pyrite (PyII), and diagenetic pyrite (PyIII), respectively. This successive increase of copper from PyI to PyIII is attributed to a continuous supply of copper that replaces framboidal pyrite in turn by the more copper-rich digenetic pyrite. Because of hydrothermal overprinting, pyrite has been replaced by djurleite, roxbyite, and other nonstoichiometric Cu-S minerals that include covellite. The EPMA study indicates that covellite contains significant concentrations of Ag (locally > 1 wt.%). In contrast. only trace amounts of silver have been detected in pyrite and other copper sulfides, indicating that covellite is the major Ag-carrier in the ore. According to textural relationships, such open space filling, impregnation, and replacement textures, and the EPMA results, later stage copper-bearing fluids were responsible for the silver enrichment in the Nahand-Ivand deposits.

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