Porphyry and Epithermal Au-Cu Systems of the Southern Caucasus and Northern Iran

This article presents tangible geological evidence for coexistence of porphyry copper and epithermal gold systems within single polygenic deposits and provides a paleothermophysical model for their origins. Brief metallogenic analysis of the Southern Caucasus and Northern Iran has shown that such deposits are confined to long-living calc-alkaline island arcs and were formed during their orogenesis. Examples of complex Sonajil (Iran), Gharta, and Merisi (Georgia) deposits are considered. Investigation has shown that for combined porphyry and epithermal ore formation some preconditions are suggested to exist: (i) Source of anomalous energy, which exceeds thermodynamics of the enclosing environment; (ii) Existence of temperature gradient, which determines conventional flows of fluids composed of endogenous and meteoric constituents (proven by rhythmical zoning of ore lodes); (iii) Stability of such conditions for a period of sulfide ore formation. However, such a process of sulfide ore formation cannot explain formation of high sulfidation gold deposits. Mass precipitation of free gold requires phreatic collapse in the ore conduit channel already after formation of hydrothermally altered rocks, and this event results in creation of either hydrothermal breccias, often with jigsaw-fit texture or brecciated vuggy silica where host rocks and hydrothermally altered rocks are cemented by a gold-bearing quartz matrix.