The Kuh Toto volcanic-hosted copper deposit, Semnan Province: geochemical, and C and O isotopic studies

The Kuh Toto deposit is located 25 km to the west of Torud village in the Semnan Province. Malachite is the only ore mineral. It occurs in veinlets, coatings and small patches in Eocene volcanic rocks. Malachite is accompanied by minor calcite, manganese and iron oxides and oxyhydroxides, clay minerals, epidote, quartz, and chrysocolla. The chondrite-normalized REE patterns of malachite and calcite are similar to those of the volcanic host rocks, in terms of being enriched in LREEs, and most of the Ce and Eu anomalies. The volcanic host rocks are enriched in Cu (187 ppm on average). These geochemical evidences strongly support that mineral-forming elements, including Cu, originated from the associated volcanic rocks. The oxygen isotopic geothermometry yielded an average temperature of 41 °C for malachite formation. Available data support the view that Cu was leached as mobile aqueous Cu2+ from the volcanic rocks by oxidizing surface waters. When Cu-enriched fluids entered the underlying groundwater environment, Cu was precipitated as malachite in fractures, via recombination with carbonate ions dissolved as CO2 in meteoric fluids.