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DETERMINATION OF MODES AND STRUCTURE OF GOLD AND SILVER OCCURRENCE IN HYPERGENE DEPOSITS BY ELECTRON PROBE MICROANALYSIS METHOD (EXEMPLIFIED BY LOOSE SEDIMENTS OF WATER CURRENTS OF THE DUKAT DEPOSIT)
Abstract
This paper reports the results of electron probe x-ray microanalysis (EPMA) aimed at identification of the modes of gold and silver occurrence in loose sediments of lithochemical stream sediments (LSS) of the Dukat gold-silver deposit in northeastern Russia. Modes of occurrence of major ore elements Ag and Au have been investigated and their chemical composition has been identified. The EPMA technique has been elaborated to measure the composition of stream sediments (minerals) containing silver and gold using the microanalyzers JCXA-733 and JXA8200. To minimize the influence of temperature from an electron probe on an analytical signal the stability of samples (minerals and mineral formations) effected by an electron beam was studied and the best compromise conditions for excitation and recording of the analytical signal were selected. The studies indicate that it is possible to determine the element composition of hypergene silver- and gold-containing sediments if the beam power density is in the range from 2.00 to 2.65 $\mu$W*$\mu$m–2. Metrological characteristics of quantitative determinations of hypergene sediment by EPMA comply with the “applied geochemistry” category of performance (category 2). We found, that modes of Ag and Au occurrence in loose alluvial sediments of watercurrents, draining the Dukat Au-Ag deposit are closely related to features in the composition of the studied zones. The major elements both in alluvial sediments of LSS and in Au-Ag ores are native Ag, its intermetallic compounds (electrum, kystelite), sulfides (akantite, sternbergite) and silver sulfosalts (pyrargyrite). Fine-dispersed native Au and the so-called invisible Au are mainly associated with Fe hydroxides and corroded grains of sulfides (pyrite, chalcopyrite and Ag minerals). In terms of the application the modes of Ag and Au occurrence in LSS are an effective indicator of ore-formation belonging of the revealed anomalies and, hence, are an additional criterion of their industrial importance. This indicator can be well applied at all stages of forecast and prospecting both within and outside of the Northeast of Russia.
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