Geology and geochemistry of the sediment-hosted stratabound red bed-type Cu-Pb (Zn-Ag) mineralization in the Dozkand-Moshampa Area, NW Zanjan, Iran

摘要

The Dozkand-Moshampa area is located in the northwest of Central Iran. The several-hundred-m-thick Miocene Upper Red Formation (URF) in northwestern Iran hosts stratiform copper mineralization. The main lithological units in the Dozkand-Moshampa area are Cenozoic sedimentary rocks. The stratigraphic sequence includes evaporites, continental red beds and limestone, similar to other sediment-hosted Cu belts around the world. Copper mineralization in Dozkand-Moshampa area appears as disseminated, laminated and replaced copper sulfides along a redox boundary between (a) gray sandstone, siltstone and microconglomerate, and (b) hematitic sandstones, siltstones and shaly marl. Mineralization is closely related to plant fossils (wood fragments) and the main ore minerals include chalcocite, galena, sphalerite, chalcopyrite, pyrite, covellite and malachite. Copper sulfides occur mainly as replacement of diagenetic pyrite. which, in turn, replaced organic matter (wood fragments). Replacement, dissemination, banding and veinlet, are the most common mode of occurrence of ores, which suggest an origin related to diagenesis. Investigations by electron microprobe analyses of chalcocite, sphalerite and galena revealed high Ag contents (up to 0.16, 0.11 and 0.01 wt%, respectively), whereas chalcopyrite and pyrite have only trace amounts of silver. The abundances of Cu and Ag in the chalcocite are up to 76.93 and 0.22 wt% respectively. Negative values of delta S-34 (0.2 to-32.7 %o) show that sulfur was introduced by bacterial reduction of sulfate. Copper mineralization is mainly controlled by organic matter content and paleopermeability (from intragranular pore space and fractures), enhanced by calcite dissolution. Geology, ore paragenesis, mineral chemistry and sulfur isotopes suggest that the Dozkand-Moshampa Cu-Pb (Zn-Ag) deposits may be ascribed to the Red bed.