TYPOMORPHISM OF PLACER GOLD IN THE KASMINSKO-URSKY ORE DISTRICT (northeastern Salair)
V.V. Kolpakov, P.A. Nevolko, V.S. Vesnin, P.A. Fominykh
V.S. Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia
Keywords: Gold, placer, typomorphism, morphology, fineness, trace element, hypergene alteration, mineral inclusion, primary source, isoferroplatinum, Salair
Abstract
The morphology, chemical composition, and ore and heavy-mineral associations of gold from placers in the Ursky ore cluster and the less-studied Kasminsko-Chesnokovsky prospective area in northeastern Salair indicate numerous and diverse primary sources, as well as the presence of gold in intermediate reservoirs. Gold with a fineness of 970-1000‰ is predominantly hypergenically altered. The placers have formed primarily from quartz, quartz-carbonate, and quartz-(carbonate)-sulfide veins and stockworks of the main gold mineralization stage, including mineralization superimposed on barite-polymetallic ores with fine and finely dispersed gold in the Ursky ore cluster, and on basic dikes. Another source of gold is metasomatite (carbonate-mica, quartzite, and quartz-albite-epidote rocks with rutile), which either accompanies gold ore bodies or is indirectly associated with them. Differences in the typomorphic properties of the placer gold are due to varying contributions from these primary sources. In the placers of the upper reaches of the Ur and Zvonchikha rivers, the source is likely gold mineralization predominantly superimposed on basic dikes. This gold exhibits high fineness (>910‰) and contains copper impurities (up to 1-6 wt.%), as well as an inclusion of Pd telluride in one instance. Quartzite is another potential source of Au (as observed at the Kopna deposit). In the lower part of the Ur River placer, the fineness of Au and the proportion of copper-bearing gold decrease, while gold with mercury impurities is more frequently encountered. The gold mineralization here is probably superimposed on polymetallic mineralization or localized within rocks of the Pecherkinsky complex. The presence of gold with fineness below 820‰ (as at the Iyunsky deposit) indicates polymetallic gold-bearing mineralization. Sources for the gold in the Chesnokovka and Kurnichikha river placers, besides mineralization associated with basic dikes, include mineralization characterized by gold with a fineness of 820-910‰ (including rather small and fine gold), which typically contains mercury impurities. These could be quartz veins and metasomatites developed in terrigenous rocks of the Suenginsky Formation, known within the area. Inclusions in gold from placers of the Kasminsko-Ursky ore district are represented by minerals from sulfide ores, metasomatites, and weathering crusts. Unlike gold from the Ursky ore cluster, gold from the Kasminsko-Chesnokovsky area contains no inclusions of copper minerals (chalcopyrite, bornite, covellite, and tennantite), which are common in pyrite-polymetallic ores. The typomorphic properties of gold from the Aprelsky deposit reflect multiple sources, including quartz veins, metasomatites in rocks of the Pecherkinsky Formation, and possibly mineralization superimposed on polymetallic ores. The sources of gold for the Khristinovskaya Yama placer are likely diverse and similar to those for the Ur and Zvonchikha river placers. An isoferroplatinum grain from the Khristinovskaya Yama placer can be classified as the Vilyui type.
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