-Results are presented from experimental investigations of oscillatory phenomena in an electron beam with a virtual cathode in a diode gap with a decelerating field. Experiments have revealed a stochastic broadband generation of the microwave oscillations of a virtual cathode in a decelerating field. Numerical simulations based on a simple one-dimensional model have shown that the onset of the stochastic generation and the broadening of the oscillation spectrum with increasing beam deceleration rate are governed by the processes of regrouping of the electrons in a beam with a virtual cathode.

We have revealed early productive disseminated pyrite-arsenopyrite mineralization with invisible gold in tectonic zones and late nest-veinlet gold-polysulfide mineralization with free gold in zones of silicified rocks at the Suzdal’ (Suzdal’skoe), Zherek, and Bol’shevik deposits in eastern Kazakhstan, localized within black shales. Two varieties of arsenopyrite differing in morphology, chemical composition, and gold contents have been established in them: acicular-prismatic and tabular. Gold was determined by a specially elaborated technique ensuring a detection limit of 30 ppm Au. Acicular-prismatic arsenopyrite is the main ore mineral of the early productive stage of mineralization; it has high gold contents (1400–5360 ppm) and a nonstoichiometric composition (S/As = 1.2) and is slightly depleted in Fe. The absence of correlation between the contents of the main arsenopyrite components and gold, and the strongly uneven distribution of gold among the mineral grains, and within a grain point to the presence of invisible gold, as elemental particles deposited together with arsenopyrite. Tabular arsenopyrite is abundant at the Suzdal’ deposit, where gold-polysulfide mineralization and argillization are widespread. It has low gold and high antimony contents and a stoichiometric composition. Visible gold usually grows over tabular arsenopyrite. The isotopic composition of sulfur of acicular-prismatic arsenopyrite and globular-crystalline pyrite, formed at the early mineralization stages, is characterized by δ34S = 0.0 to –3.3‰ and evidences a mantle source of sulfur with a partial borrowing of crustal sulfur. The tabular arsenopyrite and other sulfides of the second productive mineralization show a lighter isotopic composition of sulfur (δ34S = –7.7 to –10.2‰), which is due to sulfur fractionation under high oxygen fugacity at the late ore deposition stage. The coexistence of two sets of arsenopyrite of different morphologic varieties and compositions at the deposit points to a long ore deposition, the coexistence of mineralization formed at different stages, and the evolution of physicochemical parameters.

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––A long-term study of the typomorphism of native gold under exogenous conditions gave an insight into its evolution in time and space. The morphology, internal structure, and chemical composition of native gold change depending on the duration of its occurrence under near-surface conditions and on the thermodynamic parameters of the environment. Along with the known facts of gold transformation in the exogenous environment, we consider some of the first identified aspects of the evolution of native gold in weathering crusts and the hydrodynamic, eolian, and anthropogenic environments. Additional attention is given to the transformation of gold in ancient gold-bearing conglomerates under lithostatic pressure and in metamorphic strata depending on the P–T environmental conditions. The paper is based on the results of field work, experiments, and analytical studies of the mineralogy of native gold. The objects of study were gold placers of the eastern Siberian Platform, Tuva, and Mongolia, gold-bearing conglomerates of the Timan Ridge, anthropogenic gold placers of the Yenisei Ridge, kaolinite and laterite weathering crusts of Salair, Kazakhstan, and the Republic of Guinea, and the collections of placer gold from the A.E. Fersman Museum (Moscow), the Central Research Institute of Geological Prospecting for Base and Precious Metals (Moscow), the Moscow Mining Academy, and ZAO VNESHMET (Moscow). In the course of experimental studies, we investigated for the fist time the mechanical transformation of gold particles under the impact of sand–air flow in the eolian conditions and under the lithostatic pressure of the overlying strata on ancient gold-bearing conglomerates. We also used a number of mineralogical and geochemical methods to study the typomorphic features of native gold. The evolution of gold under exogenous conditions depends on the ambient environment. Examination of weathering crust has revealed gold nano- and microparticles resulted from the decomposition of sulfides, tellurides, and other unstable gold-containing compounds. Newly formed gold nano- and microparticles in the form of finest crystals, dendrites, and globules are deposited on the surface of primary endogenous gold. The gold formed in weathering crust is spongy and nodular; the fineness of primary endogenous gold increases, the content of impurity elements in it decreases, and a high-fineness porous shell forms at the edges of the gold particles. In the hydrodynamic environment, placer gold, independently of its form (hemihedral, euhedral, interstitial, etc.), flattens and undergoes a simple deformation, but its chemical composition and internal structure change little; they depend on the stage of ore formation and on the mineragenic type of the gold ore source. We have established that the chemical composition and internal structure of gold change during its long occurrence in the environment and under its repeated redeposition from ancient (Precambrian) to younger (Quaternary) deposits. Based on the obtained results of experimental and mineralogical studies, we have proved that eolian processes change not only the shape of native gold but also its chemical composition and microhardness. In the eolian environment, placer gold of different shapes tends to become a globule with a film-fibrous surface. The change in the shape of gold is accompanied by an increase in its fineness, a decrease in the content of impurity elements, and, as a result, decrease in the gold microhardness. In ancient conglomerates (ancient fossil placers), placer gold subjected to the lithostatic pressure of overlying deposits transforms into pseudo-ore gold. In metamorphic strata with constant temperatures and pressures, gold becomes refined. The identified indicators of placer gold of different exogenous environments make it possible to reconstruct the geologic and geomorphologic conditions of gold placer formation, namely, to determine the genetic type of placers (related to weathering crusts, alluvial, eolian, etc.) and to define the source areas (intermediate or primary sources). This helps to find a more correct technique for the search for gold placer and ore deposits.

-The effect of the electron transverse and longitudinal velocity spread at the entrance to the interaction space on wide-band chaotic oscillations in intense multiple-velocity beams is studied theoretically and numerically under the conditions of formation of a virtual cathode. It is found that an increase in the electron velocity spread causes chaotization of virtual cathode oscillations. An insight into physical processes taking place in a virtual-cathode multiple-velocity beam is gained by numerical simulation. The chaotization of the oscillations is shown to be associated with additional electron structures, which were separated out by constructing charged particle distribution functions.PACS numbers: 05.45.-a, 41.75.Fr

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