Geology and exploration for Au, Cu and other metals in the Kalkafi-Khuni, northeast of Nakhlak mine, Esfahan
The aim of this study is to do geological and explorational survey in the Kalkafi and Khuni area, located at northeast of Nakhlak mine, Esfahan Province. the region belongs to the Central Iran zone and Anarak-Khur subzone. This folded zone contains exposed horst-like block of basement with various rock units including schists, quartzite, and hornfels that form the Anarak metamorphic complex. The age of complex was determined by TechnoExport (1987) as upper Proterozoic. Igneous bodies expose in the Kalkafi and Khuni areas and cut Cretaceous rocks is some places. Therefore these bodies are late Eocene-Oligocene in age. Their composition varies between acidic and basic. There are three phases of magmatic activity: the first phase was associated with intrusion of granodiorite porphyry-quartz diorite porphyry and monzodiorite porphyry; in the second step quartz monzonite, monzodiorite, monzogranite and quartz monzogranite were intruded, and in the third phase alkaline porphyry granite and porphyry quartz syenite have formed. Kalkafi and Khuni intrusions have sub-volcanic facies. Cu-Mo-Au mineralization is related to second and third phases. Three trends of major faults include:
1- N-S old faults that correlate with the Kalkafi mineralized vein (N005-020) having right lateral movement.
2- NW-SE faults that parallel with geological trend and dip toward NE.
3- NE-SW faults with left lateral movements.
Fluids have concentrated the ore minerals along the fractures. Some parts show stockwork and disseminated textures. Cu, Pb, Zn, Au, W, Mo, Bi, As mineralization has been reported from the region. But this report focuses on Cu, Pb, Ag, Mo and Au. There are old workings in the area. Based on the geochemical analyses data of 50 samples, average copper grade is 0.14% occurred as malachite, azurite, chalcopyrite and bornite. The average grade of lead is 11320 ppm. Ag has low grade that ranges between 0 and 32 ppm. The grade of molybdenum is 14500 ppm. The gold has considerable grade that is exploitable as a by product. The average gold grade is 2.157 ppm. 10% of samples have more than 5 ppm gold.
The correlations of Ag with Pb and Au with Mo are more 0.75. Available alterations in the area include argillic, silicic, sericitic and potassic. Silicic and sericitic alterations are more widespread in the case which the granitoids have numerous silicic veins and veinlets.
1:1000 geological and mineral maps were prepared for the study area showing promising zones. Estimated reserve of Kalkafi is 62400 and Khuni 89700 tons. A vein in the Kalkafi that is 200m long has 4.4 ppm gold. The area is recommended for IP geophysical survey. Average gold grade in the Khuni is 0.14 ppm.