Geodynamics and gold potential of the Kolyma region (Russia)

The study in the Upper Kolyma gold-bearing province has not yielded a prope r explanation of the "Kolyma Gold" phenomenon. Our approach is based on the study of conditions of the initial gold accumulation in the asthenosphere before its migration throughout the lithosphere to the Earth's crust with f luid flows. The formation of the Upper Kolyma gold-bearing province include s two stages, The first preparatory stage comprises the accumulation of gol d-bearing fluids in the innermost, moderately heated parts of the Koni-Murg al island are (P-2-J(3)) The great efficiency of this stage was caused by p rolonged (about 100 Ma), relatively slow quasi-steady subduction of oceanic lithosphere, which provided slow but efficient separation of low-sulfide a queous fluids, where the gold was almost completely isolated from the assoc iated ore components. These processes caused the relatively high fineness o f the Kolyma gold, the rarity of the associated silver mineralization, and the zoned distribution of the gold fineness over the ore province (the fine ness increases to the northeast, i.e., to the inmost part of the Koni-Murga l are). The structural setting (coincidence in space and time of the back p art of the Koni-Murgal are with the opening In'yali-Debin trough) favored t he concentration of fluid flows. This phenomenon was caused by the screenin g effect of thick lithosphere blocks bounding the trough on the routes of f luid migration in the asthenosphere. The second, final stage of ore formation is related to the initiation of th e continental collision, when the local extension zones originated during t he general compression caused by the diagonal interaction of microplates. T he lithosphere beneath the rear part of the Koni-Murgal are was transformed into a giant tectonic breccia, The rock brecciation was accompanied by the formation of a system of numerous, generally diagonal, deep-seated faults, the means for discharge of tectonic stress and lithostatic overpressure. T hese faults also initiated magma generation, which was induced by decompres sion (the distinctly bimodal intrusive complex of the Nera-Bokhapchinsk) an d enabled influx of mantle gold-bearing fluids to the surface. These fluids impregnated the brecciated rocks and saturated the melts. The subsequent m elting of the upper crust and formation of granite batholiths significantly occluded the routes for ascent of the mantle fluids. The gold potential of the asthenosphere could be depleted at that time. According to our scheme, the unique gold potential of the Kolyma region is a consequence of a rare combination of the following three geodynamic factors: prolonged separation of gold under favorable conditions in the asthenosphere, lateral rheologic al heterogeneity of the mantle, providing accumulation of the fluid phase w ithin the local domain; and efficient fluid migration along the asthenosphe ric channelways, simultaneously with brecciation of the lithosphere caused by collision. The general triangular shape of the gold-bearing province is defined by fau lts and a collage of thick lithospheric blocks bounding the area of the gol d-bearing fluid accumulation. The cuneiform Gerbinsk block moved northwest and was a son of forcer that accelerated migration of the fluid-saturated m agma along the axial zone of the In'yali-Debin trough, The gold-bearing pro vince outlined by geological surveying is virtually a projection of an asth enospheric trap onto the Earth's surface. The asymmetry of the gold-bearing province (the ore mineralization is absen t near the Darpirsk fault) is related to the gradient of the collisional st ress, when sufficient permeability of the lithosphere for fluids was mainta ined within the central and southwestern parts of the trough and zones of t he paleolistric faults, such as the Chai-Yuryuinsk, Khakchano-Villcinsk, an d Ten'kinsk faults. The well-known uneven distribution of gold accumulations within the ore pro vince resulted from variable permeability of the crust which underwent brec ciation during the collision. Ore-bearing hydrothermal solutions circulated along the boundary areas between the individual crustal blocks and along t he fractures within these blocks, while the largest consolidated blocks rem ained barren.