Geology and ores of the Udokan copper deposit (review)

The Udokan deposit is unique in the reserves of copper and ordinary in its content in ores, averaging 1.56 %. The ores are Lower Proterozoic cuprifero us sandstones of the upper-Sakukan Subformation, which is part of the Sakuk an megarhythm in the Naminga brachysyncline. The ore-bearing rocks of the U dokan deposit are metamorphosed polymictic sandstones and siltstones with q uartz-micaceous (quartzy sandstones) and carbonate (calcareous sandstones) cement containing interlayers of ferruginous sandstones, mudstones, conglom erate-breccias, and sandy limestones. The rocks occur rhythmically. The was hed-out upper part of the rhythm is the base for the above-lying one. Prima ry ore minerals are bornite, chalcosine, chalcopyrite, pyrite, and magnetit e. They occur in the middle parts of the rhythms. Metamorphogenic redistrib ution of copper is accompanied by decalcitization with formation of epidote and plagioclase, albitization, and decrease in the amount of free silica. Primary bornite-chalcosine and pyrite-chalcopyrite ores have been recognize d. The deposit has an ancient crust of weathering with a related zone of ox idation and a modern zone developing under the conditions of cryolithozone. The zone of oxidation abounds mainly in brochantite, antlerite, malachite, azurite, gypsum, covellite, hypergene chalcosine, hematite, goethite, hydr ogoethite, etc. A specific feature of basic sulfates is the presence of wat er (in crystallohydrates). By the degree of oxidation the ores are divided into sulfide, mixed, and oxidized. Four levels of mineralization at the dep osit correspond to the evolution of ore-bearing lithofacies in time. Ore de posits form a series of bands or ribbons of SW strike in concordance with t he conditions favorable for ore deposition. The major commercial component is copper, and accessory ones are silver, gold, sulfur, and magnetite. Comm ercial ores are dominated by chalcosine-bornite, of less importance are bro chantite-antlerite-malachite ores. Chalcopyrite-pyrite ores are less abunda nt. We present various concepts of the deposit genesis. Preference Is given to the hypothesis of sedimentary-diagenetic copper accumulation with parti cipation of various sources of ore substance and its further redistribution during metamorphism. The main factors that governed the formation of uniqu e and large deposits of cupriferous sandstones are synchronous sedimentatio n and ore deposition, which suggests long-term and multistage supply and de position of ore material in large long-lived ore-generating structures with in geochemically specific provinces in the peripheral parts of planetary ta phrogenic structures.