Geological and Geophysical Studies of Sulfide Copper Mineralization in the Dochileh Area: An Example of Manto-Type Deposit in the Sabzevar Zone, Iran

摘要

The Dochileh stratiform copper deposit in the Sabzevar Zone of northeastern Iran is hosted in the basaltic sequence of the Upper Eocene age. The host rock displays two hydrothermal events: zeolite-carbonate alteration that is a stratigraphic-lithologic feature and chlorite and chlorite/ferruginous alterations in the local mineralized structures. Ore formation is related to both hydrothermal events and occurs in both stratiform and vein mineralization types. Mineralization consists of main chalcocite with variable amounts of bornite, chalcopyrite, native copper, malachite, and cuprite minerals, which occur as hydrothermal breccias, and disseminated, vein, and veinlet forms. Geophysical field studies using resistivity and induction polarization (IP) methods were conducted along nine survey lines in the area. As a result of modeling and interpretation of the acquired geophysical data, high values of IP and resistivity corresponding to mineralization were observed at two depth levels: 0-20 m and more than 40 m. Based on these geological and geophysical investigations, six locations for drilling exploration boreholes were proposed. Drilling data confirmed the mineralization containing high copper values in the two depth levels: the vein-type mineralization in the surface and shallow depth level, and the stratiform mineralization at the deeper level. Fluid inclusion studies in calcite and quartz from stratiform- and vein-type mineralization show the evidence of mixing, and a linear dilution trend during the ore formation occurred at a wide range of temperatures: 121-308 degrees C and 80-284 degrees C, respectively, and varying salinities of between 3.2-16.8 and 0.8-22 wt% NaCl equivalents. The stable isotope composition of delta S-34 that falls in a range of -2.4 to +25.0 parts per thousand could be considered biogenetic sulfur from bacterial sulfate reduction and leaching of sulfur from hosting basalt. The delta C-13 values of calcite vary between -0.6 and -7.6 parts per thousand, suggesting a major contribution of marine carbonates associated with igneous carbonates, and the delta O-18(SMOW) values of calcite are between +15.2 and +19.9 parts per thousand, suggesting a contribution of delta O-18-rich sedimentary rocks and delta O-18-poor meteoric water. Copper and sulfide-rich hydrothermal fluid have flowed upward through the local faults and permeable interbeds within the Eocene volcanic sequence and have formed the mineralized veins and horizons. The geophysical results have detected the local faults as the channel ways for mineralization.