Distribution and modes of occurrence of Au, Ag, and associated elements in the sediment streams of Au–Ag zones at the Dukat deposit (northeastern Russia)

We studied the distribution and modes of occurrence of the main (Au and Ag) and major associated elements indicating mineralization (Hg, Sb, As, Pb, Zn, and Cu) in samples of loose sediments from lithochemical sediment streams of Au–Ag zones at the Dukat deposit. The major mineral and nonmineral modes of occurrence of elements indicating Au–Ag mineralization have been detected with POLAM-312 and POLAR-3 ore microscopes and by X-ray electron probe microanalysis with JCXA-733 and JXA-8200 (JEOL, Japan) microprobes. The distribution and modes of occurrence of ore elements in the loose (alluvial) sediments of streams draining the Au–Ag zones are closely related to the ore composition. As in the ores, the main elements indicating mineralization are Au and Ag, while the major associated elements are Hg, Sb, As, Cu, Pb, and Zn. Gold in alluvial sediments occurs as electrum, fine (dispersed, ultradispersed, and colloid–dispersed) particles, and the so-called “bound” (predominantly with sulfides) invisible Au (sorption component and its derivates). Silver occurs not only as hypergene forms, microinclusions, and ultrasized particles but also in native form and as partly preserved sulfides and sulfosalts. The other elements (Hg, Sb, As, Pb, Zn, and Cu) are observed as secondary minerals of complex composition and differently preserved corroded fragments of pyrite, chalcopyrite, galena, and sphalerite grains, dominated by oxides and hydroxides (mainly those of iron). Salts and water-soluble sulfates of Fe, Cu, Pb, and Zn are less widespread. In study of the conditions of sediment stream formation in cryolithogenesis zones, where physical weathering is regarded as the major process, chemical factors (first of all, chemisorption) should also be taken into account. This is proven by the enrichment of “fine” loose material in Au, Ag, and ore elements in general. The high percentage of mobile forms of elements, which are easily leached out by ordinary acids, in ores and aureoles, together with the widespread occurrence of clay minerals and hydroxides, has a significant influence on chemisorption. For example, the portion of fine and “bound” Au, the element considered to be the least prone to chemical weathering, reaches 50–60% in the riverbed sediments of the stream head. Native Au (electrum) makes up 40–50%. The material is diluted during its transport. As Au content decreases, the portion of fine and “bound” Au in the sediment stream increases to 95%. Evidently, these two Au forms are predominant in streams as compared with ores. The modes of occurrence of elements indicating Au–Ag mineralization in secondary environments, as in primary ones, serve as an effective criterion for assessment of the potential of geochemical anomalies detected in sediment streams. They are a reliable indicator of ore associations, which is of practical significance. We think that modes of occurrence can sometimes be used to estimate the erosion level of drained zones and sites (i.e., to predict mineralization to depth). This criterion can be successfully applied at all stages of geochemical study of ore-bearing areas, from the prediction of the potential of anomalies detected in sediment streams in poorly studied areas to the detailed exploration of known ore objects.