Geochemical behavior of Mo and precious metals during supergene enrichment in the SarCheshmeh porphyry Cu deposit, Iran

Abstract

Supergene oxidation-leaching and secondary sulfide enrichment in the SarCheshmeh porphyry Cu-Mo deposit generated a supergene profile to depths of 20 to 80 meters with oxide ore reserve about 22 Mt. @1.08% Cu and sulfides ore reserve~106Mt.@1.27% Cu. Geochemical distribution and behavior of trace metals associated with Cu namely Mo, Au, and Ag in the supergene profile of the SarCheshmeh were poorly understood and documented. To this aim, using a relatively large geochemical dataset along with theoretical data allowed distribution of the metals in the supergene profile to be clarified, and to be discussed behavior of Mo, Au, and Ag during supergene enrichment process of the SarCheshmeh deposit. Constraints on distribution and mobility of trace metals in the supergene environment of the SarCheshmeh deposit indicated that Au strongly enriched in the chalcocite zone. During supergene oxidation and leaching, Mo significantly remained in the leached cap, whereas Ag tends to be partially fixed in oxidized ore zone and locally enriched in chalcocite zone. Observed difference in the geochemical mobility of the metals is function of change in pH, Eh and metal-transporting capacity of supergene solutions. Under high acidic condition of supergene solutions which corresponded to oxidation of pyrite-rich primary ores (e.g., sericitized samples), mobility of Mo is greatly restricted with formation of ferrimolybdite in leached zone which has prevented supergene sulfide enrichment of Mo. This caused local enrichment of Mo (average~0.020 %) associated with Fe-oxides in leached ores which are supported by moderately positive correlation between Mo and Fe2O3 (r=+0.75) in such ores. Under such conditions, Au is strongly mobilized possibly as auric-chloride complexes rather than thiosulfates, and appreciably enriched in chalcocite zone. This is consistent with the low neutralizing capacity of the pyrite-rich primary ores along supergene solutions pathway and also semi-arid climate of the SarCheshmeh region. Unlike Au, soluble Ag in high acidic supergene solutions tends to be locally fixed in oxidized zone and to some extend enriched in chalcocite zone. This could relate to rapid decomposition of the halide complexes, consequently leaving silver as Ag-halide minerals, arjento-jarosite and/or native silver in oxidized ores without gold. This is supported by the negative correlation between Ag and Au values in oxidized ores (r= -0.28). Partial enrichment of Mo in chalcocite zone indicate that pH of supergene solutions derived from oxidation of pyrite-poor ores (biotitized ores) could remain above the stability field of ferrimolybdite (pH=5); this led to mobilization of Mo and locally enrichment of Mo in chalcocite zone. Where these conditions apply there may be local retention of Au in the form of native gold within leached ores.