International Geologiical Congress - Oslo 2008


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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts


Local prediction of large-volume gold deposits in black shales strata using analysis of gold placers and crusts of weathering in their development field


R.S. Rodin, Siberian Research Institute of Geology, Geophysics and Mineral Resources (SNIIGGiMS) (Russian Federation)
L.V. Gess, Siberian Research Institute of Geology, Geophysics and Mineral Resources (SNIIGGiMS) (Russian Federation)


For revealing deposits of the stated type on the National mineral resources map with a scale of 1:200000 localities, composed of black shale strata (BSS) are marked, within them gold placers are mapped and red crusts of weathering are developed. As is well known, gold particles size in BSS is mainly less than 0.05 mm and they are not gravitated, i.e. they are not able to form placers, but, having come under influence of hypergenesis process, gold particles size increases, they turn into placer class, forming placers. Published works exist in this respect, in which the conclusion about gold enlargement in weathering crusts is made (Crater and others, 1958; Petrovskaya, 1973 and others).
As a result of research the source (in this instance, BSS), from which metal enters in drainage system is determined, and the main thing -is that the possibility to determine the scale of auric object in first approximation is arisen, as the area, drainable by drainage system with developed gold placer, in fact is auric BSS area.
This is proved by the data on ore Yegoryev region in Salair Ridge (Central Siberia). Commercial gold placers extending for 140 km are mapped here, in the Suenga R. basin, within Lower Cambrian carbonaceous- terrigenous-volcanogenic BSS. All attempts to discover endogenous original gold sources failed. At the same time ubiquitously mapped lateral crusts of weathering are possibly of Cretaceous -Palaeogene age. Their influence on gold enclosing (BSS) rocks and directly on the gold proper is manifested in the following: mechanical rock disintegration occurs during weathering; relative metal concentration increases due to solution and migration of rock-forming components from host rocks (oxides of alkaline and alkaline-earth metals, phosphorus, manganese, silica to a lesser degree; gold quality increases during it solution and regeneration; gold particles consolidation occurs and it becomes gravitated.
Green-violet formation (Middle Cambrian-Lower Ordovician) rocks lie in entourage of Lower Cambrian BSS in Yegorev region. They are not black shale, products of weathering crust are mapped by them, however, commercial placers are practically lacking at their area. The most significant conclusion from given data consists in discovering of large-volume gold ore objects, confined to BSS, as according to approximate estimations, taking into account a diversity of loss coefficients, only in one described Yegorev region, prediction gold resources amount to 100 tons.


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