Gol'tsovoe Ag-Pb-Zn Deposit (Northeastern Russia): Geological Setting, Mineralogy, Geochemistry, and Ore Formation Conditions

The Gol'tsovoe Ag-Pb-Zn-deposit (1600 t silver in ores with an average grade of 1025 g/t Ag) is located on the southeastern flank of the Dukat mining district, at the junction of the Balygychan-Sugoi rift trough and the Okhotsk-Chukotka Volcanic Belt (OCVB). The Dukat district is the largest silver producer in Russia. The Gol'tsovoe deposit is located 2 km north of the Pestraya granitoid intrusion, in the structurally complex zone of a fault contact of the Upper Triassic terrigenous sediments and Late Cretaceous volcanic rocks. The contact consists of a series of fault planes within a 0.5-1 km wide belt, along which the western block, composed of felsic volcanics, has been downthrown more than 1 km. The host Upper Cretaceous rhyolitic and rhyodacitic ignimbrites are intruded by subvolcanic nevadite bodies. A characteristic feature of the deposit is the occurrence of en-echelon galena veins and veinlets with high Ag grades at the upper levels of orebodies. Massive, coarsely banded, and spotted ore structures are predominant. Ore geochemistry agrees well with the mineral composition. The ores are enriched in a wide spectrum of elements: Ag, Sb, Pb, Cd, As, Zn, Bi, In, W, Sn, Mn, Cu, Li, Tl, and Cs. They are characterized by a low total REE abundance (33.34-50.69) and a negative lanthanide distribution type. Commercially significant potential by-products are Cd, In, and Bi. Galena is the main Ag mineral concentrator: the high Ag concentrations in this mineral are due to the occurrence of small inclusions of a large group of independent Ag-bearing minerals, argentotetrahedrite and pyrargyrite especially, and a small amount of myargirite, polybasite, etc. Argentite and native Ag occur in limited amounts. Tin minerals are represented by stannite, canfieldite, and less frequent cassiterite. Oxidation zone has been tracked over the entire mineralization depth and displays a pronounced linear trend. Oxidized ores are dominated by Zn, Pb, Fe, Ca, and Ba sulfates; less abundant Fe, Mn, and Zn oxides; and subordinate carbonates and clays; total sulfides and native metals (Cu, Ag) do not exceed 2%. Ore oxidation degree is 40.3%. The sulfide-quartz-chlorite veins of the Gol'tsovoe deposit formed at moderate temperatures (276-138 degrees C) from low-concentration chloride hydrothermal fluids (4.2-0.2 wt % NaCl eq.), saturated with Na and K cations and Mg in subordinate amount. The obtained data allow the Gol'tsovoe deposit to be classified as an epithermal intermediate sulfidation deposit. A two-stage ore emplacement model is proposed. During the first stage Ag-Pb-Zn ore was emplaced as a result of the functioning of a near-surface hydrothermal system of volcanic origin. During the second stage, imprinted Sn mineralization was emplaced in the Ag-Pb-Zn ore veins as a result of tin-bearing magmatogenic fluid inflow at the lower levels of the deposit.