Dynamic classification of alluvial gold placers in the northeast of Russia |
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| Authors: | Yu I Goldfarb |
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| Institution: | (1) Northeast Interdisciplinary Scientific Research Institute, Far East Division, Russian Academy of Sciences, Portovaya ul. 16, Magadan, 685010, Russia |
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| Abstract: | The comprehensive study of alluvial gold placers, fluvial sediments, and river valleys allowed division of these placers into subtypes, categories, and varieties on the basis of genetic principles. The comparison of observation results with reconstructions of placer-forming fluvial processes demonstrates the distinct dependence of all basic properties of placers on the dynamics of their formation, i.e., on the hydraulic size of gold grains, the dimensions of streams, and the phase of the elementary erosion cycle (PEC). Four PECs are distinguished: erosion, abrasion, equilibrium, and aggradation. A lithodynamic alluvial complex (LDAC) of the same name corresponds to each PEC. Any fractions of free gold can be transported by streams, and the mode of transportation depends on the PEC. Placers of eight dynamic categories, brush, erosion, perluvial, apron, bar, equilibrium, aggradational, and gravitational, are syngenetic to certain lithofacies belonging to different LDACs. The first four categories are combined into the channel-line subtype, which embraces all traditional placers. The nontraditional bar and equilibrium (beach subtype) and aggradational and gravitational categories (representing two subtypes of the same name (?)) are so far poorly studied but may be regarded as highly promising. The size, morphology, thickness, structure, and localization of placers; average and modal values of size and roundness of gold grains; degree of their sorting and concentration; three-dimensional distribution of gold; composition of alluvium; relationships of placers with alluvium and the gutter, river valleys of various sizes, and types of morphostructures; and other features are distinct for placers pertaining to specific categories and make up natural groups individualized for each category. The correlation of placer properties in each category makes these properties predictable at the early stages of geological exploration and enhances the efficiency of forecasting and prospecting. In high-grade gold clusters, closely spaced placers group into composite deposits. The subdivision of these placers into categories makes it possible to apply rational methods and tools of exploration and mining of placers; to estimate the character, volume, and location of previous losses of gold; and to select targets for their remining. The dynamic analysis of recently formed and ancient placers is helpful for implementation of panning and interpretation of the results obtained in the context of prospecting for gold. The proposed classification may be used not only in the study of stream gold but also in the exploration of some other placer deposits. |
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