The producing Late Cretaceous Madneuli polymetallic deposit is a major deposit in the Bolnisi district, Lesser Caucasus, Georgia. It is interpreted as a felsic maar – diatreme system with rhyodacite dome, located out of the crater rim. The lower part of the tuff ring consist of a non- bedded pyroclastic sequence, where ore bodies are located. Several volcanic events were followed by the phreatomagmatic eruption, formation of volcanic domes, rhyodacitic extrusion and ignimbrites cropping out in the open pit, accompanied by hydrothermal fluid circulation and different types of mineralization. Detailed facies-oriented analyses allow us to suggest a shallow marine depositional environment for the host rocks of the Madneuli deposit. Matrix- to clast-supported phreatomagmatic breccia is described for the first time in the Madneuli open pit. According to SEM mineralogical studies of the breccia, mineralization is distributed differently in the matrix than in rock clasts, which enabled us to provide different possible models of formation of phretomagmatic breccias in the open pit and different stage of mineralization.