The metallogenic model presented in this paper is based on observations from three Iranian nonsulfide zinc deposits: Mehdi-Abad, Iran-Kuh, and Kuh-e-Surmeh. The emplacement of nonsulfide ore can be generally subdivided into a “sulfide oxidation stage” followed by a “postsulfide oxidation stage.” The formation of carbonate-hosted nonsulfide zinc deposits is primarily controlled by three factors: climate, protore composition, and geology (lithology and structure) of the wall rocks. Typical minerals of the red zinc ore are iron oxyhydroxides, goethite, hematite, hemimorphite, smithsonite, and/or hydrozincite and cerrusite. Common minerals of the white zinc ore are smithsonite or hydrozincite and minor amounts of Fe oxyhydroxides. Metal separation is caused by a gradual change from an acidic oxidation zone to alkaline conditions in the adjacent carbonate wall rock. The formation of an acidic oxidation zone within carbonate host rocks is facilitated by the armoring of calcite by gypsum and hydrous ferric oxides (HFO) and by several pH-buffering reactions. The HFO within the oxide zone additionally adsorb various amounts of Pb and Zn, depending on the pH. The neutralization processes of the sulfuric acid with the carbonates of the host rock during the oxidation stage lead to the formation of CO2(g). Under these conditions most of the zinc precipitates as smithsonite. The partial decreases during the postsulfide oxidation stage and reaches the level of atmospheric , and hydrozincite becomes stable and replaces smithsonite. The postoxidation stage is also associated with the successive formation of local zinc (hydro-) silicates, depending on the availability of SiO2 within the solution.
Figures & Tables
Supergene Environments, Processes, and Products
At least five altered and mineralized porphyry centers related to the cooling of a polyphase Eocene intrusion occur within a 25-km2 "pampa"-type area in the southwestern sector of the Chuquicamata district in northern Chile. These deposits take place 1 to 2 km apart as discrete porphyry "columns" covered by postmineral, poorly consolidated Miocene sedimentary rocks. Such copper oxide and sulfide deposits were discovered and evaluated by drilling done by Codelco from 1996 through 2007 during a brownfield exploration program, driven by the necessity to replace and increase leacheable ore consumed by the Chuquicamata and Radomiro Tomic operations. During this program a resource of more than 20 million metric tons (Mt) Cu was discovered, including 6 Mt Cu of oxide, mixed and secondary sulfide ore, representing one of the largest supergene copper resources discovered worldwide during the last 10 years.
Despite their close location and their genetic relationship to a single, polyphase intrusion mineralization event, the five porphyry centers display contrasting host-rock and structural framework as well as different hypogene alteration and ore mineral assemblages. This picture reaches high levels of complexity because of the different levels of exposure of the mineral systems, resulting from primary emplacement processes and post-mineral faulting. These hypogene features and the effect of landscape and climate evolution controlled supergene alteration, thus generating different profiles in each specific porphyry center. The key controlling factors in the supergene overprint are discussed on the basis of their relationship to ore and gangue mineralogical abundance and occurrence, assemblage distribution, geochemical response, and the broad geologic setting.
As exploration for covered porphyry copper deposits in the southwestern sector of the Chuquicamata district progressed, numerous lessons were learned about the origin of supergene profiles and the analysis and use of supergene effects and their products as a guide for exploration. These lessons, which include geological and geochemical criteria among others, are discussed in the context of the appraisal of the mineral potential of copper oxide-mixed-secondary sulfide blankets and underlying sulfide protore.