THE GEOLOGY AND ORES OF THE UDOKAN COPPER DEPOSIT

Abstract

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 cupriferous sandstones of the upper-Sakukan Subformation, which is part of the Sakukan megarhythm in the Naminga brachysyncline. The ore-bearing rocks of the Udokan deposit are metamorphosed polymictic sandstones and siltstones with quartz-micaceous (quartzy sandstones) and carbonate (calcareous sandstones) cement containing interlayers of ferruginous sandstones, mudstones, conglomerate-breccias, and sandy limestones. The rocks occur rhythmically. The washed-out upper part of the rhythm is the base for the above-lying one. Primary ore minerals are bornite, chalcosine, chalcopyrite, pyrite, and magnetite. They occur in the middle parts of the rhythms. Metamorphogenic redistribution 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 recognized. The deposit has an ancient crust of weathering with a related zone of oxidation 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, hydrogoethite, etc. A specific feature of basic sulfates is the presence of water (in crystallohydrates). By the degree of oxidation the ores are divided into sulfide, mixed, and oxidized. Four levels of mineralization at the deposit correspond to the evolution of ore-bearing lithofacies in time. Ore deposits form a series of bands or ribbons of SW strike in concordance with the conditions favorable for ore deposition. The major commercial component is copper, and accessory ones are silver, gold, sulfur, and magnetite. Commercial ores are dominated by chalcosine-bornite, of less importance are brochantite-antlerite-malachite ores. Chalcopyrite-pyrite ores are less abundant. We present various concepts of the deposit genesis. Preference Is given to the hypothesis of sedimentary-diagenetic copper accumulation with participation of various sources of ore substance and its further redistribution during metamorphism. The main factors that governed the formation of unique and large deposits of cupriferous sandstones are synchronous sedimentation and ore deposition, which suggests long-term and multistage supply and deposition of ore material in large long-lived ore-generating structures within geochemically specific provinces in the peripheral parts of planetary taphrogenic structures.