Mineralogy and Geochemistry of Secondary Copper Minerals in the Barite-Chalcopyrite Veins of the Ardakan Barite Mine

The Ardakan barite mine, located in 37 km northeast of Ardakan city in the Central Iran, occurs in limestone and dolomitic rocks of the Permian. Secondary copper minerals of the mine include Cu-sulfides, Cu-carbonates and Cu-oxides. In the studied samples, association of pyrite with chalcopyrite and differences in their electric potentials caused a series of galvanic reactions and chalcopyrite alteration. In a chemical battery of pyrite-chalcopyrite, pyrite with higher electric potential remains fresh as a cathode and chalcopyrite with lower electric potential alters to another sulfide phase as an anode. The final product of the chemical reaction is the formation of secondary copper sulfides. Two stages of alteration are observable in the chalcopyrite alteration sequence. The first stage is the alteration of chalcopyrite to covellite, led to a decrease of Fe content with a simultaneous increase of Cu and S. The second stage is the alteration of covellite to anilite with an enrichment of Cu and a corresponding decrease in S. Copper bearing acid solutions were neutralized by carbonate host rock which resulted in the deposition of Cu-carbonates in the ore body.