Metallogenesis of the sediment-hosted copper deposits in the Miocene sandstones from Upper Red Formation, Zanjan, NW Iran

Abstract

The Chehrabad, Qezeljeh, and Sarikand copper deposits, located in the Zanjan district in northwestern Iran, are classified as sediment-hosted copper (SHC) deposit. These deposits mainly consist of fine-grained, disseminated base-metal sulfides cementing grey sandstones from the thick sequences of Miocene red beds (Upper Red Formation: URF). The ore-hosting grey sandstones contain carbonaceous materials (fossil plants) and fine-grained, disseminated framboidal pyrite. Copper, lead, zinc, and iron sulfides replace primary organic material as well as framboidal pyrite. Ore minerals such as chalcocite, galena, sphalerite, argentite, covellite, pyrite and minor chalcopyrite and bornite occur as replacement in organic matter and cement of the host sandstone. Geochemical analyses of the SHC deposits in the Zanjan district show an average of 5 wt% Cu, 6 wt% Pb and 3 wt% Zn, ~0.7 to 128 ppm for Ag, and ~ 0.1 to 821 ppm for Cd in the grey sandstone. This research aims to enhance the comprehension of the genesis of the SHC deposits in the Zanjan district, thereby providing direction for future exploration efforts in the area.

Electron probe micro-analysis (EPMA) was used to determine the mineral chemistry and occurrence of copper, lead, zinc, and silver in sulfide and carbonate minerals. The results reveal that argentite is associated with Cu-bearing minerals (mainly chalcocite and covellite). Furthermore, the presence of montetrisaite mineral, along with minor amounts of cadmium, has been identified. The studied samples from Zanjan SHC deposits show a range of δ³⁴S values from −1.90 ‰ to +12.9 ‰ values, indicating sulfur might have originated from framboidal pyrite, organic matter, evaporite layers, and salt diapirs of URF. The sulfide ore is vertically zoned, with Cu sulfides at lower stratigraphic levels and Pb-Zn-rich ore at the upper part. Ore-stage sulfide minerals were likely formed by the infiltration of ore-forming fluids originating from the underlying red beds into the grey sandstone layers during the Miocene, following the deposition of these rocks. The oxidized, low-temperature, metalliferous brine responsible for mineralization originated from highly porous and permeable sediments expelled during diagenetic events in the basin. Mineralization occurred within a favorable reducing environment, mainly provided by organic matter, framboidal pyrite, and reduction of the oxidized, low-temperature brine in the grey sandstone horizons. The timing of these deposits is typically late diagenetic. Concerning host rock lithology, mineral assemblages, fabric and structure of the ores, and the overall genetic interpretation, they are similar to the sediment-hosted stratiform copper deposits elsewhere such as Lisbon Valley (Utah, USA) and Nacimiento (New Mexico).