COARSE GOLD AND NUGGETS IN THE AMUR REGION

This paper investigates the aggregation forms, sizes and composition of native gold present in the ore deposits and placers of the Amur province. The province corresponds to the zone of the Late Mesozoic collision of geoblocks of the South-Eastern framing of the Siberian craton, Amur composite massif and Mongol-Okhotsk folded system. The collision was accompanied by an intrusive and volcanic activity, as well as by the formation of gold mineralisation superimposed on block matrices. Further erosion processes led to the formation of numerous placers. Eight metallogenic zones with 80 ore-placer nodes have been identified in the province, which contain about 1,500 placers and 45 gold deposits. It has been established that the placers in the Cis-Amur region contain gold of mostly small- and medium-size classes. Coarse gold appears less frequently, with the discovered nuggets weighing from first grams to several kilograms. The form of the gold deposits in the placers is diverse — from predominantly flat, pancake-like, lumpy and wire-shaped to crystalline, including dendritic. The crystals and intergrowths of gold with quartz are occasionally observed. Gold is frequently covered with a film of iron hydroxides. In terms of composition, the placer gold in the Cis-Amur region belongs to that of moderately high-grade (800—899‰) and high-grade (900—1000‰). Low-grade gold (799—700‰) and electrum are less common. The coarsest gold and nuggets are found in placers in the central part of the Amur province. It is shown that the gold-quartz formation is the main placer formation. The frontal, middle and basal parts of the ore column are distinguished in the deposits of this formation. An increase in the gold grade and size of its aggregates in the frontal part of the ore column has been established. Most of the nuggets found in placers are assumed to be located in the upper part of ore deposits. This is evidenced by a large number of nuggets in terrace placers as compared to the channel ones. In the Unya-Bomsky ore-placer cluster rich in coarse gold and nuggets, native gold in placers and ores has a high grade and a significant concentration of trace mercury, reaching the level of first percent.

native gold, nugget, placer, deposit, gold grade

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