R. G. Kravtsova

Modes of Au, Pt, and Pd occurrence in arsenopyrite from the Natalkinskoe deposit, NE Russia

Pioneering data obtained by studying arsenopyrite from the Natalkinskoe deposit indicate that the mineral contains equidistributed Au, Pt, and Pd in two major non-mineral modes: structural and surface-bound. High Au, Pt, and Pd concentrations in the arsenopyrite are proved to be related to the surface-bound mode. No individual Pt and Pd minerals were detected, while Au occurs mostly in the form of native metal. Variably sized native Au grains (mostly small ones) form aggregates with arsenopyrite and quartz and occur in them as small inclusions. The situation with Pt and Pd is different: arsenopyrite is likely the main concentrator of the elements. Gold-bearing arsenopyrite seems to accommodate all admixtures according to a single mechanism, in which an active role is played by the surface of crystals of the mineral and its surface defects. In view of this, it is possible to recover refractory gold without disintegrating the structure of the mineral, a fact that should be taken into account at, for example, processing ores containing sulfides, for instance, arsenopyrite.

Rare earth elements in the ores of epithermal gold-silver and silver deposits, Russia’s northeast

Relations between mineralization and magmatic rocks and sources of ore-forming fluids are presently among the most important problems in endogenous ore formation. Geochemical methods provide a valuable insight into these problems. The estimation of ore potential and search for reliable criteria for the determination of sources of ore elements are based mainly on the study and comparison of the ore-element compositions of rocks and ores. In this study, we attempted to solve these problems by a nontraditional way, studying REE distribution in ores and comparing it with the behavior of these elements in igneous rocks. The distribution of REEs in ores was studied by the example of epithermal deposits and occurrences of gold‐silver (Au‐Ag), silver‐base metal (Ag‐Pb), and tin‐silver (Sn‐Ag) associations. All the objects are located within the largest ore-bearing structures in the central part of the Okhotsk‐Chukchi volcanic belt (OCVB): the Turomchin (Dal’nee and Kvartsevaya Sopka Au‐Ag deposits and Al’dygich occurrence) and Arman (Katamken Au‐Ag deposit) ring volcanic structures and the Dukat volcanoplutonic uplift (Dukat Au − Ag deposit, Mechta and Tidit Ag‐Pb deposits, Malyi Ken Sn‐Ag deposit, and Final’noe occurrence). The geology, mineralogy, and geochemistry of the deposits and occurrences were reported in [1‐10 etc.]. However, there is no information on REE distribution in epithermal Au‐Ag, Ag‐Pb, and Sn‐Ag ores, both in the Russian and international literature. This is the first such investigation.

Surface Typochemistry of Native Gold

Using the methods of electron spectroscopy of the surface and SEM–EDS, it is shown that native gold of the deposit related to the epithermal Au–Ag ore formation contains oxidized gold with an oxidation degree of Au (I) or higher on the surface. A thin layer (~15 nm) with high concentrations of Ag and S and an underlying SiO2-bearing layer with a thickness of ~30–60 nm play a protective role providing preservation of Ag and Au sulfides in the surface parts of the Au–Ag grains under the oxidizing conditions. S-rich marginal parts of native gold particles may be represented by solid solutions Ag2–xAuxS or (with a lack of S) by agglomerates of AgnAumS clusters. The formation of surface zoning in the nanoscale on the surface of native Au is abundant in nature and may be applied in prospecting.

Occurrence Forms of Carbon, Sulfur, and Noble Metals in Deposits of the Black-Shale Formation by the Example of the Degdekan Gold-Ore Deposit (Northeastern Russia)

Pyrite crystals and ore-bearing shales of the Degdekan deposit were studied by means of XPS, SEM–EDX, EPMA, and AAS. Five peaks of carbon organic forms were identified, conforming to polymer compounds containing either double bonds of carbon or alkyne groups and compounds containing C–OH and C=O bonds, as well as, probably, small amounts of S-containing compounds and those with functional groups of carboxylic acids. Sulfate prevails over sulfite in pyrites; among the surface sulfide forms, disulfide prevails over monosulfide; the presence of polysulfide is registered. The occurrence of various chemical forms of sulfur on the surface might provide for concentrating of microelements including the noble metals (NMs) in their surface-bound forms. The regular behavior of NMs (Au, Pt, Pd, and Ru) depending on the grain sizes (specific surfaces) of pyrite crystals along with the narrow range of the ratios of structural and surface components of the concentrations of different NMs points to NM coprecipitation with pyrite during the same productive stage. No capture of NM-containing carbonaceous phases took place, which should violate the regularity of Au distribution in pyrites of the Sukhoi Log deposit.

Specific composition of native silver from the Rogovik Au–Ag deposit, Northeastern Russia

The first data on native silver from the Rogovik Au–Ag deposit in northeastern Russia are presented. The deposit is situated in central part of the Okhotsk–Chukchi Volcanic Belt (OCVB) in the territory of the Omsukchan Trough, unique in its silver resources. Native silver in the studied ore makes up finely dispersed inclusions no larger than 50 μm in size, which are hosted in quartz; fills microfractures and interstices in association with küstelite, electrum, acanthite, silver sulfosalts and selenides, argyrodite, and pyrite. It has been shown that the chemical composition of native silver, along with its typomorphic features, is a stable indication of the various stages of deposit formation and types of mineralization: gold–silver (Au–Ag), silver–base metal (Ag–Pb), and gold–silver–base metal (Au–Ag–Pb). The specificity of native silver is expressed in the amount of trace elements and their concentrations. In Au–Ag ore, the following trace elements have been established in native silver (wt %): up to 2.72 S, up to 1.86 Au, up to 1.70 Hg, up to 1.75 Sb, and up to 1.01 Se. Native silver in Ag–Pb ore is characterized by the absence of Au, high Hg concentrations (up to 12.62 wt %), and an increase in Sb, Se, and S contents; the appearance of Te, Cu, Zn, and Fe is notable. All previously established trace elements—Hg, Au, Sb, Se, Te, Cu, Zn, Fe, and S—are contained in native silver of Au–Ag–Pb ore. In addition, Pb appears, and silver and gold amalgams are widespread, as well as up to 24.61 wt % Hg and 11.02 wt % Au. Comparison of trace element concentrations in native silver at the Rogovik deposit with the literature data, based on their solubility in solid silver, shows that the content of chalcogenides (S, Se, Te) exceeds saturated concentrations. Possible mechanisms by which elevated concentrations of these elements are achieved in native silver are discussed. It is suggested that the appearance of silver amalgams, which is unusual for Au–Ag mineralization not only in the Omsukchan Trough, but also in OCVB as a whole, is caused by superposition of the younger Dogda–Erikit Hg-bearing belt on the older Ag-bearing Omsukchan Trough. In practice, the results can be used to determine the general line of prospecting and geological exploration at objects of this type.

First discovery of high-mercury silver in ores of the Rogovik gold–silver deposit (Northeastern Russia)

New data on mercurial mineralization are presented, and a detailed characteristic is given for the first discovery of mercurous silver in ores of the Rogovik gold–silver deposit (the Omsukchan trough, Northeastern Russia). It was found that native silver in the examined ores occurs as finely-dispersed inclusions in quartz filling microcracks and interstitions. It also occurs in associations with kustelite, Ag sulfosalts and selenides, selenitic acanthite, and argyrodite. The mercury admixture varies from “not detected” in the central parts of grains to 0.22–1.70 wt % along the edges, or, in independent grains, to the appearance of Ag amalgams containing 10.20–24.61 wt % of Hg. The xenomorph form of grains of 50 μm or less in size prevails. It is assumed that the appearance of mercurial mineralization is caused by the superposition of products of the young Hg-bearing Dogda–Erikit belt upon the more ancient Ag-bearing Omsukchan trough.