4.1.3. Gold–Silver Adularia in Volcanogenic Strata GIT Fineness of Native Gold

The gold–silver adularia GIT is characterized by native gold of a relatively low fineness with a polymodal distribution on fineness histograms. The spread of the values of this indicator lies in the areas of 200–850‰ (Figure 8).

Deposits in genetic terms belong to the volcanogenic class, but often fall into areas of tectonomagmatic activation with late intrusions, which leads to the superposition of hightemperature processes on ores of early stages, to thermometamorphism of ores, differentiation and redistribution of matter, and staged mineral formation. It is these factors that are reflected in the histograms of gold in the form of polymodal graphs. Heterogeneity in fineness due to thermometamorphism can manifest itself within one grain of native gold (Figure 9).

**Figure 8.** Fineness histograms of native gold for deposits of gold–silver adularia GIT: along the abscissa axis—frequency of occurrence, %; along the *x*-axis, fineness intervals, ‰; in the numerator is the fineness, ‰, in the denominator is the number of determinations.

**Figure 9.** Heterogeneity of native gold within one grain: (**a**) Kupol deposit (from 435‰ to 702‰); (**b**) Dalnee deposit (from 276‰ to 767‰), taken in reflected electrons.

### Trace Elements

It has been established that native gold from deposits of this type constantly contains impurities of Sb, Cu, Hg in noticeable concentrations (Table 4), and the highest concentrations reach Hg up to 1000 g/t in gold from the Dalnee deposit. Quite often, Fe is found, from 1.5 to 26.5 g/t (Kubaka, Burgali, Primorskoye), and also As, from 0.1 to 27.3 g/t (Kupol, Primorskoe, Dalnee).


*Minerals* **2022**, *12*, x FOR PEER REVIEW 16 of 25

**Table 4.** Concentration of microimpurities in native gold from ores of gold–silver adularia GIT deposits.

Notes: —–below detection limit. Dalnee (14) 2.7–12.1 1.5–21.1 0.1–7.3 – – 0.1–1000.0 – –

#### Mineral Intergrowths Notes: –—below detection limit.

The ores of the gold–silver adularia GIT deposits are characterized by a wide variety of mineral intergrowths with Ag sulfides, selenides, and sulfosalts. The sharply gradient conditions for the formation of epithermal Au–Ag deposits predetermine the predominantly fine-grained character of native gold segregations and its intergrowth with a wide range of Cu, Pb, Zn, Fe sulfides and Ag sulfosalts and sulfoselenides. The deposits of the Omolon cratonic terrane show intergrowths of native gold with magnetite and hematite. This is due to the peculiarities of the rocks of the base of volcanic apparatuses, among which Archean-Proterozoic ferruginous quartzites are widely developed. At the same time, volcanic activity manifested itself much later—on the border of the Devonian and Carboniferous periods. Segregation forms are predominantly xenomorphic; in quartz they are interstitial. On the example of only one Olcha deposit, we have shown an exceptional variety of such intergrowths (Figure 10). For other deposits of this GIT, examples of the most common intergrowths of native gold in ores are given—with pyrite, tetrahedrite, sphalerite, galena, acanthite, polybasite (Figure 11). Mineral Intergrowths The ores of the gold–silver adularia GIT deposits are characterized by a wide variety of mineral intergrowths with Ag sulfides, selenides, and sulfosalts. The sharply gradient conditions for the formation of epithermal Au–Ag deposits predetermine the predominantly fine-grained character of native gold segregations and its intergrowth with a wide range of Cu, Pb, Zn, Fe sulfides and Ag sulfosalts and sulfoselenides. The deposits of the Omolon cratonic terrane show intergrowths of native gold with magnetite and hematite. This is due to the peculiarities of the rocks of the base of volcanic apparatuses, among which Archean-Proterozoic ferruginous quartzites are widely developed. At the same time, volcanic activity manifested itself much later—on the border of the Devonian and Carboniferous periods. Segregation forms are predominantly xenomorphic; in quartz they are interstitial. On the example of only one Olcha deposit, we have shown an exceptional variety of such intergrowths (Figure 10). For other deposits of this GIT, examples of the most common intergrowths of native gold in ores are given—with pyrite, tetrahedrite, sphalerite, galena, acanthite, polybasite (Figure 11).

**Figure 10.** Types of intergrowths of native gold with ore minerals in quartz-sericite vein material at the Olcha deposit: (**a**) gold with hematite; (**b**) inclusion of kustelite in chalcopyrite; (**c**) close intergrowths of gold with argentotetrahedrite; (**d**) vein-like intergrowths of gold with argentotetrahedrite; (**e**) gold with bornite; (**f**) gold with acanthite; (**g**) inclusion of chalcopyrite in gold; (**h**) inclusion of gold in chalcopyrite; (**i**) intergrowth of gold with acanthite and stromeyerite; **Figure 10.** Types of intergrowths of native gold with ore minerals in quartz-sericite vein material at the Olcha deposit: (**a**) gold with hematite; (**b**) inclusion of kustelite in chalcopyrite; (**c**) close intergrowths of gold with argentotetrahedrite; (**d**) vein-like intergrowths of gold with argentotetrahedrite; (**e**) gold with bornite; (**f**) gold with acanthite; (**g**) inclusion of chalcopyrite in gold; (**h**) inclusion of gold in chalcopyrite; (**i**) intergrowth of gold with acanthite and stromeyerite; (**j**) gold with freibergite; (**k**) complex intergrowth of gold with naumannite and stromeyerite; (**l**) gold with acanthite; (**m**) gold-freibergite + jalpaite; (**n**) gold with chalcopyrite. (All taken at 100× magnification).

**Figure 11.** Mineral intergrowths of native gold from ores of gold–silver adularia GIT deposits in quartz: (**a**) with pyrite and tetrahedrite (Dalnee deposit); (**b**) with acanthite and sphalerite (Dalnee); (**c**) with galena (Primorskoe); (**d**) with polybasite and acanthite (Burgali); (**e**) with polybasite (Burgali); (**f**) heterogeneous in fineness of native gold with the inclusion of galena (Kupol deposit).

#### Internal Structures

The internal structures of native gold for gold–silver adularia GIT are predominantly zonal and are well identified by structural etching (Figure 12). In the case of thermometamorphism of ores, patchy heterogeneity often appears (Figure 13a) and there are granulation structures with the expansion of boundaries (Figures 12a and 13b,d) with expansion of grain boundaries. At the Olcha deposit, native gold has an unusual gulf-like shape with a zonal structure. This is due to the fact that it crystallized in quartz vacuoles filled with highly concentrated hydrothermal solutions (Figure 13).

**Figure 12.** Internal structures of native gold from ores of gold–silver adularia GIT deposits: (**a**) spotted (Kubaka); (**b**) clear zonal (Dalnee); (**c**) zonal (Olcha); (**d**) zonal with a break in the boundaries (Kupol); (**e**) zonal (Burgali); (**f**) zonal (Primorskoe).

**Figure 13.** Filling of vacuoles with native gold in quartz at the Olcha deposit ((**a**) quartz in transmitted light without analyzer, black—vacuoles; (**b**–**d**) in reflected light, native gold in quartz, structural etching).

#### **5. Discussion**

All considered GIT deposits are hydrothermal and most of the features of native gold are associated with the depths of formation of these deposits. It is the depth that determines the duration, stability, and temperature zoning of mineral formation. To no lesser extent, the metallogeny of ore regions affects the formation of certain GIT deposits and their specific mineral types.

In genetic terms, the three most common GITs are generally accepted:


At medium-deep and deep deposits in GITs 1 and 2, the hydrothermal system is in a relatively stable state with a long cooling process. This creates conditions for the growth of large individuals during the crystallization of both native gold and any other mineral. A different picture in the shallow type of deposits is GIT type 3, where, during the outburst of volcanoes, sharply gradient conditions are created with high initial temperatures and a rapid cooling of the ore-forming system, which generally prevents the differentiation of natural gold-silver compounds (high content of silver in gold and, accordingly, its reduced fineness). In this unstable environment with a limited cooling time of the system, no conditions are created for the growth of large gold individuals—more often it is finegrained. Therefore, the first two types are placer-forming (large gold, which quickly precipitates and accumulates in a water stream), and volcanic—although it carries fine and fine gold with a water stream during the destruction of ores, it precipitates more slowly, and in most cases, does not give industrial accumulations.

The scope of mineralization is determined mainly by the depth of deposit formation and the largest has an "orogenic" GIT (many hundreds of meters)—volcanogenic, 50–100 m—but in the case of localization of mineralization in the vent facies, it can reach 300–400 m.

At an early stage of geological prospecting at ore deposits, we do not consider the size of native gold as an indicator property, since the amount of gold particles found in specimens and lump ore samples is not representative enough for sieve analysis. However, in the study of placer gold, this typomorphic feature can be of great importance, in combination with roundness and flatness.

To determine the GIT at an early stage of prospecting and exploration at ore gold deposits, we considered the following typomorphic features of native gold: microimpurities, average fineness and fineness distribution of gold on histograms in combination with fineness dispersion value, as well as mineral intergrowths and internal structures of native gold. The given descriptions and examples of typomorphic features of native gold deposits for various GIT showed characteristic differences for each type (Table 5).


**Table 5.** Main typomorphic features of native gold for various GITs.

Note: \* for the Shkolnoye deposit, the dispersion values are determined horizontally (see Table 3).

For gold–arsenic-sulfide GIT in black shale strata there is relatively coarse Au and high fineness with low dispersion of this index, polygonal-grained structures with simple twinning and a constant microimpurity of As in the composition; for the gold–quartzporphyry type in granitoids—coarse gold predominates, fineness decreases slightly and its dispersion increases, heterogeneities are noted in the structures, and granitogenic elements Bi, Te, W, Sn act as microimpurities; for the epithermal gold–silver adularia GIT is characterized by fine, relatively low-grade Au with a high dispersion of this indicator, up to native Ag, a constant increased admixture of Sb, Cu and Hg, zonal internal structures complicated by heterogeneities during thermometamorphism of ores.

The study of the typomorphic features of native gold, tied to the geological environment and named by N.P. Yushkin [28] with topomineralogy, is a rational way to display the features of native gold. It makes it possible to establish the spatial patterns of the distribution of these features in connection with the geological structure and morphostructure of the territory.

Here, we give an example of such a study for the Maldyak deposit (gold–arsenicsulfide in black shale strata GIT). It was the indicator properties of native gold that made it possible to speak about the existence of a granitoid pluton under the deposit, as well as about the dome-ring structural position of the Maldyak ore field (Figure 14). The conclusion is justified by the concentric-zonal arrangement of typomorphic features of native gold associated with the radial and concentric orientation of ore bodies. The ubiquitous presence of Bi impurities and the appearance of Sn and W impurities suggest that they were introduced by an undiscovered intrusion that formed a dome-shaped structure. All this made it possible to predict the different positions of gold-bearing veins (ore-bearing cracks), both radial—steeply dipping, and concentric—consistent with the schistosity of sedimentary rocks, crumpled into folds. Thanks to this approach, one of the rich ore bodies, previously unknown, was identified during exploration work at the deposit in 2001 by geologist, who called it the "Sedlovidnya vein", due to its confinement to a radial crack and a sharp bend (fold) in the black shale sedimentary sequence.

**Figure 14.** Topomineralogical map of typomorphic features of native gold in the Maldyak ore field. The image was constructed by the authors of the present paper using data from [10] with additions. 1–3—sedimentary deposits of the Jurassic age: 1—lower mudstone-siltstone sequence; 2—middle argillite sequence; 3—upper predominantly sandstone strata; 4—quaternary deposits; 5—residential zones; 6—mineralized dikes; 7—on the pie chart along the periphery shows the content of impurity elements Pb, As, Fe, Cu, Sb (the sector is 100%), in the center—the degree of hypogene transformations in native gold; 8—other microimpurities found in native gold; 9—mineral associations of native gold: a—gold–quartz, b—gold–arsenopyrite, c—gold-galena-sphalerite, d—gold-sulfoantimonite; 10—sample of native gold (in ppm): numerator—variations, denominator—average value.

For the Shkolnoye deposit (gold–quartz veins in granitoids GIT), we analyzed the average fineness value of native gold from well cores from six ore horizons. It turned out that changes in the average fineness value change with depth in waves, but the dispersion of this indicator naturally falls from the upper horizon to the lower one, indicating an increase in a more stable situation in this direction, as well as a consistent differentiation of gold-silver from the lower horizons to the upper ones (Figure 6a–f, Table 3).

It should be noted that the metallogeny of the region can influence the formation of various GITs. In this article, we considered the typomorphism of native gold for the Northeastern region—the Yano-Kolyma orogenic belt with its specific As, Sn, W metallogeny and block terrane composition with an extended covering complex of the Okhotsk-Chukotka volcanogenic belt, as well as the Kedon volcanic belt in a rigid block—Omolon craton terrane with Au–Ag specialization. The main axis of the orogenic gold mineralization here is the Ayan-Yuryakhsky anticlinorium with the Tenkinsky deep fault extending along it.

At the same time, the Verkhoyansk fold system adjacent to the Northeast from the west has an independent collisional terrane structure of the Verkhoyansk-Chukotka thrust belt and its own metallogenic Sb-Hg and Ag-Pb-Zn specialization. Here, along the Adycha-Taryn deep fault, gold deposits related to the gold–arsenic-sulfide (gold– antimony–mercury) GIT prevail—the Sarylakh, Kyuchus and other deposits. The study

of the typomorphic features of native gold from this province showed the presence in its composition of high concentrations of As, Sb, especially Hg. Native gold may contain from 5 to 15 wt.% Hg (Figure 15). In intergrowths, along with arsenopyrite, aurostibite, antimonite, and berthierite are often found; a higher sulfide content of ores (up to 15%) is typical. For this GIT, the appearance of native gold is significantly different (Figure 15b), the most common spongy gold with a fineness of 940–970‰ intergrown with antimonite [53,54]. In the north-east of Russia, gold–antimony–mercury GIT deposits have not yet been found.

**Figure 15.** Typomorphic features of native gold (gold–antimony–mercury) GIT: (**a**) Ag-Hg-Au diagram of native gold of the Kyuchus deposit. The image was constructed by the authors of the present paper using data from [54]; (**b**) spongy gold intergrown with antimonite, Sarylakh deposit.

#### **6. Conclusions**

Summing up our study, we can state that native gold is an indicator mineral for various GITs of gold deposits. In a comprehensive study, it allows us to obtain the physical and chemical parameters of the main useful component of ores, to determine the deposit GIT, as well as genetic information about a particular object. All this makes it possible to preliminarily assess the scope of mineralization and outline schemes for processing ores, as well as to plan exploration work in order to identify placers.

On the example of different GITs and geological formational types of gold deposits in the north-east of Russia (gold–arsenic-sulfide—Natalka, Degdekan, Karalveem, Maldyak; gold– quartz veins—Dorozhnoye, Butarnoye, Shkolnoye, Maltan; gold–silver adularia—Kubaka, Olcha, Burgali, Kupol, Primorskoye, Dalnee) a comparative description of the typomorphism of native gold was carried out and it was shown that the reliability of geological interpretation is directly related to a comprehensive study of this mineral, including fineness variations, a spectrum of trace elements, internal structure and mineral intergrowths.

**Author Contributions:** Conceptualization, N.E.S., R.G.K. and G.A.P.; methodology, N.E.S., R.G.K., G.S.A. and G.A.P.; formal analysis, N.E.S., R.G.K. and G.S.A.; investigation, N.E.S., R.G.K., G.S.A. and G.A.P.; resources, G.S.A.; writing original draft preparation, N.E.S. and R.G.K.; writing—review and editing, R.G.K. and G.A.P.; project administration, N.E.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded with financial support from the Russian Foundation of Basic Research (project No 20-05-00393, 20-05-00142) and by within the framework of the state assignment of Shilo North-East Interdisciplinary Scientific Research Institute of Russian Academy of Sciences, Vinogradov Institute of Geochemistry of Russian Academy of Sciences, Diamond and Precious Metal Geology Institute of Russian Academy of Sciences, Sobolev Institute of Geology and Mineralogy of Russian Academy of Sciences.

**Acknowledgments:** The authors thank the analysts E.M. Goryacheva and N.S. Averchenko (North-East Common Use Center of the SVKNII FEB RAS, Magadan) for help in carrying out work on a scanning electron microscope, an X-ray spectral microanalyzer and a spectrograph.

**Conflicts of Interest:** The authors declare no conflict of interest.
