Native Gold and Unique Gold–Brannerite Nuggets from the Placer of the Kamenny Stream, Ozerninsky Ore Cluster (Western Transbakalia, Russia) and Possible Sources

Abstract

We carried out a comprehensive study of native gold (morphology, composition, intergrowths, and microinclusions) from alluvial deposits of the Kamenny stream (Ozerninsky ore cluster, Western Transbaikalia, Russia). The study showed that there were four types of native gold, which differed significantly in their characteristics and probably had different primary sources from which placers were formed: gold–quartz, oxidized gold–sulfide, gold–silver, and zones of listvenites with copper–gold and gold–brannerite (Elkon-type). Particular attention was paid to the study of unique, both in size and in composition, gold–brannerite nuggets of the Kamenny stream. It was established that the gold in the gold–brannerite nuggets (GBNs) had wide variations in chemical composition and mineral features. According to them, there were five different fineness types of native gold: 750–800‰; 850–880‰; 880–920‰; 930–960‰; and 980–1000‰. The data obtained indicated a multistage, possibly polygenic, and probably polychronous formation of GBN gold–uranium mineralization. The first stage was the formation of early quartz–nasturanium–gold–W–rutile–magnetite association (Middle–Late Paleozoic age). The second was the crystallization of brannerite and the replacement of an earlier pitchblende with brannerite (Late Triassic (T3)–Early Jurassic (J1) age). The third was the formation of the hematite–barite–rutile–gold association as a result of deformation–hydrothermal processes, which was associated with the appearance of zones of alteration in brannerite in contact with native gold with 8–15 wt.% Ag. The fourth was hypergene or the low-temperature hydrothermal alteration of minerals of early stages with the development of iron hydroxides (goethite) with impurities of manganese, tellurium, arsenic, phosphorus, and other elements. The carbon isotopic composition of an organic substance indicates the involvement of a biogenic carbon source. In the OOC area, there were signs that the composition of the GBNs and the quartz–chlorite–K–feldspar-containing rocks corresponded to Elkon-type deposits.

1. Introduction

The causes and timing of the appearance of ore clusters, which often include large deposits of various minerals, are of great interest to researchers. A striking example is the Ozerninsky ore cluster (OOC), with an area of more than 200 km² and located in Western Transbaikalia (Russia) as part of the Uda–Vitim structural formation zone, which includes 12 deposits of Pb, Zn, Fe, Cu, baryte, and gold (including one of the largest in Russia, Ozernoe pyrite–polymetallic) and 23 ore occurrences [1,2]. A detailed geological study of the Ozerninsky ore cluster began in 1960 due to iron ore finds. For a short period of time from 1961 to 1964, a series of deposits was discovered in the OOC area: Ozernoe and Ulzutuyskoe polymetallic, Hematitovoe, Magnetitovoe, Gurvunurskoe, Arishinskoe iron ore deposits, Turkulskoe, and Gunduyskoe iron–copper–barite deposits. The geological study of the OOC continued for a long time, but despite this, a consensus on the structure, deep structure, tectonics, magmatism, age, mineralogical–geochemical, and physical–chemical conditions for the formation of deposits and the entire OOC has not yet been developed.

The problem of the gold content of the OOC ores has not been solved to date, despite the fact that many deposits and ore occurrences have encountered ores with elevated gold grades (from 0.2 to the first ppm). In addition, gold grains were found in the alluvial deposits of the Ulzutui, Izvestkovy, and Pravy Surkhebt streams, which have not been sufficiently studied so far. To date, the Nazarovskoe gold deposit (up to 23 ppm Au) has been discovered, as well as a number of gold mineralization points among magnetite, sulfide–magnetite, copper–barite, pyrite–sphalerite, pyrite–quartz ores of most deposits, and the OOC ore occurrences. In 2005, an industrial placer was discovered and exploitation began at the Kamenny stream, with an average content of alluvial gold per layer of –830 mg/m³. At the beginning of the 21st century, industrial gold placers were discovered virtually throughout the entire area of the OOC, indicating a wider distribution of gold ore than previously thought.

The first studies, using modern analytical methods, revealed a wide variety of typomorphic features of alluvial gold in the OOC area [3,4,5]. The purpose of this work was a comprehensive study of placer gold [4,5] from the placer of the Kamenny stream (the right tributary of the Levyi Surkhebt stream), located in the central part of the OOC. Studying the Kamenny stream placer was of the greatest interest due to its location in the central part of the OOC, representing the maximum variety of native gold (NG) types and finds of gold–brannerite nuggets [6,7]. Particular attention was paid to the study of gold–brannerite nuggets, unique both in size (up to 200 g) and in mineralogical and geochemical features. We also carried out a comparative analysis of the typomorphic features of native gold, in particular the morphology and composition of both gold itself and the minerals associated with it.

1.1. Brief Description of the Geological Structure of the Ozerninsky Ore Cluster

The studied region is characterized by a highly complex geological structure and the long history of the formation of its structural and material complexes. The region is located in the zone of development of the world’s largest Angara–Vitim granitoid batholith. The data on the geological structure of the OOC have been published in a number of papers [8,9,10,11,12], in the last of which many problematic issues were discussed [13,14,15,16,17]. In tectonic terms, this territory is considered as the Paleozoic Uda–Vitim island arc system, which includes the large Eravna volcano tectonic structure. According to the popular view, the structural block containing the OOC is a “remnant” or roof sag (20 × 10 km²) of the Angara–Vitim batholith, composed of island arc Lower Paleozoic volcanic, volcanic–sedimentary, and sedimentary rocks of the Oldynda Formation, as well as intrusive and subvolcanic bodies of the Permian age. The formation of the OOC sulfide–iron–oxide mineralization is associated with Lower Cambrian volcanism and hydrothermal–sedimentary ore deposition [11]. But, according to drilling data, no granites have been found within the OOC area at a depth of 1700 m, despite the geophysical forecast. According to the results of gravity studies, the OOC has been described as the “Ozerninskaya group of stratovolcanoes” [18]. The data of magnetotelluric sounding do not confirm the assumptions about the presence of ore-bearing rocks of the Lower Paleozoic age in the roof sag of the Angara–Vitim batholith [19]. The appearance of young datings of intrusive complexes of the Angara–Vitim batholith suggests that the OOC is not sag of the roof of granitoids, but an independent tectonic block injected with younger igneous formations. The main part of the OOC is composed of stratified Paleozoic terrigenous-carbonate-volcanogenic formations intruded by a large number of intrusive (granitoids, syenites) and subvolcanic rocks (dolerites, diorite porphyry, syenite porphyry, etc.) [20]. The age of the material complexes developed in the OOC varies within 530–270 Ma [16,21,22].

A number of researchers substantiate the hydrothermal-metasomatic origin of the ores of the Ozernoe deposit at the late stages of tectonomagmatic activation [23]. At the same time, starting from the Paleozoic, five major stages of magmatism are observed at the OOC [5]. In the Cambrian and Ordovician, island-arc magmatism was intensely manifested; Caledonian tectonic movements ended with powerful granitoid magmatism, which was preceded by mafic intrusions. In the Middle and Late Paleozoic (D-P), two stages of tectonic-magmatic activation occurred due to the initiation and development of large intracontinental rift-like structures, which are associated with felsic volcanism and granitoid magmatism. In the Jurassic and Cretaceous, large outpourings of basaltic lavas occurred, timed to coincide with tectonic depressions. The latest stage of magmatism ended in the Mesozoic after the main stages of granite formation, which may indicate the polychronicity of the volcanotectonic structure and the existence of a magma conduit penetrating the crust and reaching the mantle [24]. According to the latest generalizations, the deposits of the Kurba–Eravninsky ore region, including the OOC, are associated with the formation of the Uda–Vitim island-arc system of the Vendian–Cambrian ensialic type (similar to the Kuril–Kamchatka basalt–andesite–dacite–rhyolitic association with mature island-arc systems) and subsequent processing of its structures in the Middle and Late Paleozoic [20,25].

1.2. Geological Structure of the Kamenny Stream Placer Deposit

Explored placers of gold are located in the central and southeastern parts of the Ozerninsky ore cluster (Figure 1). They are confined to the deposits of river valleys draining the northwestern slope of the Eravna depression. The placers were found in the valley of the Levyi Surkhebt stream, its right tributary the Kamenny stream, the Ulzutui stream, its right tributaries the Hematitovy and Nezametny streams, and the Gunduy-Kholoi stream. As it was said, the Kamenny stream placer is of the greatest interest due to its location in the central part of the OOC, with the maximum variety of native gold types and finds of gold-brannerite nuggets, which are described below. The measured length of the placer is 3.4 km, with an average width of 72 m. The average thickness of the sands is 1.8 m, with an average overburden–1.6 m. The placer is located in the height range of 1130–1070 m and is not contoured in the upper and lower ends along the stream. The gold contents reached 2318 mg/m³, averaging 724 mg/m³. The section of loose deposits of the valley of the Kamenny stream contains (top-down):

• Soil–vegetation layer with blocks, boulders, pebbles, crushed stone, gruss of volcanic rocks and granites (thickness—0–0.4 m);
• Boulder–block deposits with pebbles, crushed stone, gruss and gravel, cemented by dark brown, brown, reddish brown loam soil (thickness—1.0–2.5 m);
• Blocks, weakly rounded boulders, crushed stone in brown loam soil (thickness—0–2.5 m);
• Sand–gravel–pebble deposits in brown and dark brown loam soil, representing a productive layer (thickness—0.4–3.6 m).

Figure 1. Schematic geological map of the area of the Ozerninsky pyrite-polymetallic deposit. The star marks the location where the nuggets were found. Compiled using materials [1,4].

Figure 1. Schematic geological map of the area of the Ozerninsky pyrite-polymetallic deposit. The star marks the location where the nuggets were found. Compiled using materials [1,4].

The total thickness of loose deposits varies from 2 m in the sides to 4.4 m in the central part of the valley. Productive deposits contain pebble material for 3%–5%, rarely–up to 10%, pebble size–2–3 cm. The content of loam soil in the lower part of the layer reaches 20%–30%. The main part of the layer is made up of uneven-grained sand and slightly rounded gravel. The clastic component of productive deposits, in addition to fragments of host rocks (granitoids and volcanic rocks), contains quartz, magnetite, hematite, limonite, and red jasper. The heavy concentrate fraction is dominated by limonite, hematite, magnetite, and gold; ilmenite, rutile, zircon, leucoxene, and mafic minerals are present in a subordinate amount.

The productive layer is located in the lowest part of the floodplain, sometimes reaching the sides of the valley. The placer bedrock is composed of rocks of the Surkhebt sequence, granitoids and dark gray volcanic rocks of basic composition, containing gold from traces to 0.6 g/t (average for 25 samples 0.034 g/t). The structure of the shallow placer of the Kamenny stream along the drilling line BL-58 is shown in Figure 2. In the studied section, the productive layer of sand–gravel–pebble deposits has a thickness of 0.4 to 5.6 m (about 1 m on average) and is overlain by “peat” with a thickness of 0.4–1.2 m; there are mostly volcanic rocks of basic composition.

2. Materials and Methods

2.1. Sampling

Native gold and gold–brannerite nuggets were obtained during the mining of the placer of the Kamenny stream, the right tributary of the Levyi Surkhebt stream (OOC) using machinery and equipment for gravity extraction of gold, as well as from drilling wells. Further, the gold grains were selected using binocular microscopes. A comprehensive mineralogical and geochemical study on separate populations of several hundred grains of native gold and seven gold–brannerite nuggets was carried out using various methods.

2.2. Optical and Scanning Electron Microscopy

The chemical composition, morphology and spatial relationships of minerals were studied using scanning electron microscopes (SEM) with energy dispersive spectrometers (EDS): LEO 1430VP (Carl Zeiss, Oberkochen, Germany) with the INCA Energy 350 microanalysis system (Oxford Instruments Nanoanalysis Ltd., High Wycombe, UK) at the Geological Institute, Siberian Branch, Russian Academy of Sciences, Ulan Ude; MIRA 3 LMU (Tescan Orsay Holding, Brno, Czech Republic) with Aztec Energy XMax 50 microanalysis system (Oxford Instruments Nanoanalysis, UK), as well as optical transmission and ore microscopy (AxioScope. A1, Carl Zeiss Microlmaging GmbH, Germany) at the Analytical Center for Multi-Elemental and Isotope Research, Siberian Branch, Russian Academy of Sciences, Novosibirsk. Scanning electron microscope studies were performed at an accelerating voltage of 20 kV and an electron beam current of 0.2–0.3 nA (LEO 1430) and 1.4 nA (MIRA 3). Live spectrum acquisition times were 50 s (LEO 1430) and 20 to 40 s (MIRA 3). In this case, the detection limits of impurities are tenths of a percent and depend, in addition to the acquisition time of the spectra, on the type of the analyzed matrix, on the intensity of the X-ray series used as an analytical signal, and also on the presence of spectral overlaps. The analysis was performed in the mode with the exclusion of the results of analysis below the level of 3 σ background variation, so all the data presented in the work exceed the detection limit. In some cases, to reduce the limits of detection of impurities, the time was increased to 300 s (MIRA 3). Possible spectral overlaps are automatically taken into account by the software of microanalysis systems by means of deconvolution of the spectra of chemical elements when processing energy-dispersive spectra. Both microanalysis systems used the same set of standards—simple synthetic and natural compounds of stoichiometric composition (Al₂O₃, SiO₂, FeS₂, ThO₂, UO₂, CaMgSi₂O₆, HgTe, PbTe, BaF₂, phosphates of rare earth elements, etc.), as well as pure metals (Au, Ag, Ti, Fe, Ni, Cu, etc.). In accordance with the recommendation for INCA Energy and AZtec Energy microanalysis systems, the analytical signal was normalized to the intensity of the K-series of Co in the periodically acquired spectra of cobalt metal. Matrix corrections were taken into account by the XPP method implemented in the software of INCA Energy and AZtec Energy microanalysis systems. The analysis was performed in the “All elements analyzed” mode without normalizing the analytical total to 100%. In the final report for oxide minerals, the results were presented as component concentrations with the addition of oxygen concentration, taking into account the valence of cations. It should be noted that the accuracy of the direct determination the oxygen content by the SEM EDS method is high enough to reliably distinguish iron minerals wustite, magnetite, hematite, goethite, siderite by the ratio of Fe and O atomic concentrations (Fe/O ≈ 1:1, 3:4, 2:3, 1:2, 1:3). Similarly, SEM EDS oxygen can be used to estimate the degree of oxidation of uranium or to detect the hydration of uranium-bearing minerals. To reduce the influence of the sample microrelief on the analysis accuracy, most analyzes were performed by scanning mineral areas up to 100 µm² in size; fine phases with linear sizes less than 3–5 µm were analyzed in the point mode.

2.3. Raman Spectroscopy

Micro-Raman spectra were recorded on a Horiba Jobin Yvon LabRAM HR800 spectrometer with a 1024 pixel liquid Nitrogen cooled Charge Coupled Device (LN/CCD) detector using for excitation the wavelengths of 532 nm from a Nd:YAG laser. Raman spectra were collected in a backscattering geometry using an Olympus BX41 microscope. The microscope with an Olympus 50× objective lens of WD = 0.37 mm with 0.75 numerical aperture produces a focal spot diameter of ~2 μm. The power of the laser radiation used was set to about 0.5 mW on the sample to avoid sample heating. The absence of sample heating was tested by comparison with the Raman spectra recorded at 7 and 0.3 mW power of the incident laser beam at the sample. The majority of the spectra were recorded using a neutral density filter, D = 1. The spectrometer was wavenumber calibrated with a silicon standard [26].

2.4. Measurement of δ¹³C Signatures

The sample was washed with HCl (3 mol/L) to remove carbonates for isotopic analysis. The carbonate-free residues were combusted in a Thermo elemental analyzer (Flash EA 1112) integrated with a Thermo MAT253 isotope ratio mass spectrometer at the Analytical Center for Multi-Elemental and Isotope Research, Siberian Branch, Russian Academy of Sciences, Novosibirsk. The analysis was calibrated using Standard Reference Materials NBS-22 (δ¹³C_VPDB = −29.74‰). The δ¹³C results are expressed in the conventional delta (δ) notation as the per mil (‰) deviation from the Vienna Peedee Belemnite (V-PDB). Standard deviation for the measured δ¹³C values is better than ±0.1‰.

3. Results

3.1. Typomorphic Features of Native Gold

Morphological characteristics of native gold (NG) from the Kamenny stream placer vary widely [4]. The size of the studied gold particles ranges from 0.04 to 2.5 mm. The granulometric composition of native gold grains (NG) is represented by the classes: +1 mm–34.8 wt%; 1 + 0.5 mm–28.1 wt%; 0.5 + 0.25 mm–16.1 wt%; −0.25 mm–21.0 wt%. The NG shape is lumpy, lamellar, dendritic, club-shaped, irregular shape, fissured-veinlet, and less often intergrowths of crystals (Figure 3). The roundness of the gold is weak, rarely to medium. A distinctive feature of most gold particles is the presence of supergene iron hydroxides on their surface. Among mineral inclusions in NG, only quartz and, rarely, barite are reliably identified macroscopically. Under an optical microscope, several varieties of gold are quite confidently distinguished by color: bright yellow, greenish yellow, grayish greenish white, reddish yellow with reddish rims (Figure 4).

The chemical composition of NG is characterized by a limited set of impurity elements, among which only Ag and Cu were found; Te with a content of up to 1.02 wt% was identified in a single grain of NG. Hg is rarely detected from 0.07 to 0.51 wt% Hg. Very significant variations in the concentrations of Ag (0–55.48 wt.%) and Cu (0–22.83 wt.%) have been identified, which is reflected in changes in the color of the gold. The NG placer of the Kamenny stream is characterized by the distribution of very high-grade (960–1000‰) gold in the form of veinlets along the boundaries of individual grains and on the surface of gold particles (Figure 5). The thickness of the veinlets varies from 0.5 to 10, rarely 50 µm. All gold grains were subjected to the process of secondary enrichment under supergene conditions, as evidenced by the cracks and rims of the development of very high-grade gold. Four types of NG are found in the placer of the Kamenny stream. The first type is moderately high-fineness, the second is low-fineness gold, the third is very low-fineness (electrum) and the fourth type is cuprous gold (Figure 6,

Table 1. Typomorphic features of alluvial placer gold.

Type	I	II	III	IV
number of investigated grains	20	15	5	5
fineness (min–max)	765–945	650–764	445–648	769–946
average	890	697	590	919
Cu (min–max)	0–3.78	0–2.06	0–1.14	3.3–22.83
average	0.38	0.19	0.28	4.88
size (mm)	0.1–2.5	0.5–2	0.5–1	0.5–3
color	bright yellow	greenish yellow	grayish greenish white	reddish yellow
morphology	lamellar, amoeba-shaped, elongated	lamellar	flattened, isometric	ellipsoid-flattened, lamellar
roundness	low	weak to medium	medium	weak to medium

). The most common is bright yellow moderately high-fineness gold, with an average fineness of 890‰. It occurs either in the form of small grains of a lamellar, amoeboid, elongated shape. The NG roundness is weak. The main impurities are Ag and Cu, which are present in 50% of the NG, reaching 3.78 wt% (average 0.38 wt%). Low-fineness greenish-yellow gold makes up more than 20% of the placer gold. Its average fineness is 697‰. The size of the NGs ranges from 0.5 to 20 mm, their morphology is predominantly lamellar. The degree of roundness is weak. Cu is present in 50% of the gold grains, reaching 2.06 wt%. The placer also contains (about 10%) very low-grade gold (electrum) (the third type) with an average fineness of 646‰. The size of electrum structures is about 0.5 mm, less often up to 1 mm. The shape of the grains is flattened to isometric, the degree of roundness is medium. This is the most rounded gold of all the studied types of the placer of the Kamenny stream.

There is also copper gold (about 10% of all types) of reddish-yellow and copper-red color. The morphology of such gold is ellipsoid-flattened to lamellar, the size is from 0.5–3 mm. In an optical microscope, copper gold particles have a reddish tint and significant inhomogeneity. The average fineness of the gold varies from 769 to 946‰, with Cu contents from 3.3 to 22.83 wt%.

3.2. Morphologic Features of Gold-Brannerite Nuggets

Finds of gold–brannerite nuggets (GBN) in the placer of the Kamenny stream are of particular interest. The weight of the nuggets ranges from 1–2 to 200 g. Outwardly, these are golden-brown aggregates, and more than 50% of their surface is composed of bright yellow native gold. The rest is represented by a dark brown to black slightly fractured mineral—brannerite (Figure 7). The sections show that the bulk of the nuggets is occupied by brannerite and associated minerals, and the amount of gold does not exceed 15–40 vol.%. In nuggets, gold is located in brannerite in association with hematite (±magnetite), rutile, barite, and less often with muscovite. Sometimes there are quartz, siderite, goethite, nano- and micro-isolations of uraninite, Pb (native or Pb oxide) and tellurides Au, Ag, Bi, Pb (petzite, tellurobismuthite, altaite), and single grains of chalcopyrite. The gamma-spectrometric analysis data testify to the essentially uranium nature of radioactivity, with contents of 14–26 wt% U and 1–6 wt% Th. Based on the study of GBNs, mineralization with different cataclastic textures is observed in the sections. The textures are defined by different morphology and spatial ratios of brannerite, native gold, hematite, rutile and other minerals (Figure 7 and Figure 8). In particular, there are 1—stockworked or veinlet-disseminated texture, which is a network of multidirectional microcracks in brannerite, made by gold; 2—spotted texture represented by(i) gold-hematite aggregate, which includes “relics” of brannerite, and (ii) isometric grains of rutile in gold, slightly elongated with “corroded” edges; 3—“microbrecciaed” texture represented by crushed hematite or rutile cemented with gold (Figure 9).

3.2.1. Gold

Gold in GBNs occurs in the form of spherical micro- and nanoparticles scattered in brannerite, less often as chains of gold microparticles in brannerite linearly located (Figure 9e) or crumpled into isoclinal micro-folds. Most often, gold fills cracks (thickness from a few microns to mm) in brannerite, and both multidirectional “bush-like” crack systems with gold (Figure 9d) and oriented intersecting at an angle close to 90° are common (see Figure 9a–c). Taking into account that brannerite does not have cleavage, this nature of cracks should be related to brittle deformation of brannerite. That is, gold in brannerite is represented to the maximum extent by fractured and veinlet types (Figure 9d,e,g,h). In areas where rutile is replaced by hematite, gold is definitely associated with a fine-grained rutile aggregate, framing it or surrounding individual rutile grains (Figure 10). In muscovite, gold is confined to cleavage planes (Figure 10e). Near relatively large cracks, accompanied by crushing and brecciation, gold acts as a “cementing” mineral, while its dimensions in the polished section reach hundreds of microns. The largest NG structures (up to mm) are observed at the intersections of cracks with gold (Figure 9d–e), rarely unrelated to them.

The composition of NG is determined by Ag impurity and varies from 73 to 100 wt% Au (Figure 11,

Table 2. Chemical composition of gold in GBNs.

Cu	Ag	Au	Total	Au, ‰
0	5.62	92.38	98	943
0	5.86	93.84	99.7	941
0	21.87	79.6	101.47	784
0	21.8	78	99.8	782
0	22.36	77.42	99.78	776
0	11.89	89.2	101.09	882
0	5.9	93.82	99.72	941
0.25	5.93	94.04	100.22	938
0.3	5.83	93.88	100.01	939
0	9.74	87.49	97.23	900
0	13.09	85.41	98.5	867
0	11.14	86.95	98.09	886
0	1.05	99.21	100.26	990
0	1.74	99.32	101.06	983
0	21.88	76.73	98.61	778
0	22.25	76.44	98.69	775
0	22.11	77.59	99.7	778
0	11.58	86.28	97.86	882
0	13.31	84.14	97.45	863
0	3.23	94.62	97.85	967
0	11.85	87.77	99.62	881
0	4.87	93.64	98.51	951
0	0.38	100.05	100.43	996
0	4.63	94.39	99.02	953

). The distribution of Au and Ag in NGs is polymodal, several groups with different modes being definitely distinguished (in ‰): 960–1000 (995); 930–950 (943); 860–925 (885); 820–860 (850) and 750–820 (775). The polymodality of NG fineness is determined by its variation between samples.

Thus, the composition of NGs corresponds to a very high-fineness, high-fineness, moderate-high-fineness and low-fineness, according to the classification of fineness of native gold according to N.V. Petrovskaya [27]. There is definitely a dependence of the composition of NGs on the host mineral. Moderate fineness gold (86 wt% Au) was found in cracks in brannerite, in association with hematite—87–88 wt% Au and with magnetite—88–89 wt% Au. Gold in association with W-containing rutile, as well as with muscovite, quartz and barite, has a composition of 90–95 wt% Au. Uraninite is found in gold with a composition of 83.34–84.91 wt% Au. Particularly high-grade gold is found in the form of veins, lenses, and isometric micro-sections among NGs with a composition of 85 wt% Au.

3.2.2. Brannerite

Uranium mineralization is represented primarily by brannerite ((U⁴⁺, Ca, Th, Y) [(Ti, Fe)₂O₆]·nH₂O) with submicron inclusions of uraninite. Grains of brannerite are 1–10 mm in size; for some nuggets, brannerite is a matrix containing other minerals. The structure of brannerite is heterogeneous at the macro and micro levels. Brannerite grains are usually cut by a dense network of veinlets filled with native gold and/or brannerite decay products (Figure 12,

Table 3. Elemental composition of brannerite and zones of its alteration according to SEM-EDX data, wt %.

Name	Si	Ti	Al	Fe	Mn	Mg	Ca	Ba	V	Ce	Y	Th	U	W	Pb	P	O	Total
brn-1	0.61	20.54	0.1	3.34			1.55		0.22	0	0.19	1.07	43.96		1.45		28.16	101.19
brn-2	0.74	20.62	0	3.2			1.4		0.22	0	0.29	1.12	43.27		1.42		27.85	100.13
brn-3	0.71	20.6	0.1	3.34			1.5		0.2	0	0.25	1.35	42.68		1.29		27.9	99.92
brn-4	0.43	20.4		2.8	0		1.48		0.2		0.2	0.51	46.14		1.58		27.47	101.21
brn-5	0.85	20		2.98			1.78		0		0.19	0.52	45.14		1.7		27.97	101.13
brn-6	0.84	20.44		3.13			1.72		0		0.27	0.53	44.26	0	1.55		28.35	101.09
brn-7	0.73	20.6	0.13	3.14			1.56		0.21		0	0	44.88		1.54		28.25	101.04
brn-8	0.73	20.68		3.43			1.57		0		0	0.63	43.72		1.39		28.43	100.58
brn-9	0.27	20.72		3			1.09		0.25		0	0.59	46.52		1.48		27.49	101.41
brn-10	0.66	20.12		3.15	0.16		1.74		0		0	0.68	45.27		1.45		27.53	100.76
brn-11	0.7	20.53		3.09			1.81		0		0	0.68	44.41		1.46		28.4	101.08
brn-12	0.56	20.46	0.11	3.08			1.67		0.23		0.19	0.67	44.59		1.48		27.99	101.03
brn-13	0.57	20.06		3.3			1.53		0.18		0.23	0.5	45.36		1.39		28.14	101.26
U-(Ti)-1	0.59	8.25		0.31			1.19	1.99	0.16	0.24	0.09	0.32	55.95		0.2		24.04	93.33
U-(Ti)-2	0.59	8.36		0.39	0.09		1.26	2.06	0.16	0.39	0	0.42	56.23		0.32		23.94	93.66
U-(Ti)-3	0.49	10.7		0.66			1.15	2.37				0.55	49.76		0.62		30.38	96.68
U-(Ti)-4	0.53	9.44		0.64	0		1.14	1.44	0.1	0.21		0.53	51.69		0.31		24.34	89.45
U-(Ti)-5	0.35	10.51	0.06	1.13			1.13	1.81	0.13	0.19		0.3	49.45	1.72	0.34		22.59	88.88
U-(Ti)-6	0.62	8.78		0.34			1.1	2.44	0.1	0.19		0.47	52.24	1.61	0.28	0.18	24.82	90.67
U-(Ti)-7	0.58	8.82	0.13	0.56			1.31	2.2	0			1.61	47.61	1.13	0	0.24	27.85	89.45
U-(Ti)-8	0.29	9.05		0.61			1.49		0		0	0	57.38		0		28.54	94.71
U-Ti-1	0.76	18	0.17	2.89			1.37		0		0	1.39	35.62	1.32	1.25		34.56	97.33
U-Ti-2	0.54	18.03		2.7			0.88		0		0	0.68	40.35		3.25		34.51	100.94
U-Ti-3	0.63	17.3	0.11	2.5			1.5		0		0	0.73	35.94				34.31	93.02
U-Ti-4	0.72	18.27		2.7			0.7		0			0.46	39.4		3.04		34.55	99.84
U-Ti-5	0.75	17.44		2.36			0.82		0			0.67	38.42		1.4		35.76	97.62
U-Ti-6	0.74	21.42		2.72	0.16		1.2		0		0	0.6	35.08		3.55		33.08	98.55
U-Ti-7	0.55	19.14	0.11	4.58	0.62	0.19	0.88	1.78	0.27		0	0.89	30.39	0	1.35		33.7	94.45
U-Ti-8	0.66	21.46		6.21	0.98	0.33	0.96	1.27			0	0.85	25.63		1.69		36.32	96.36
U-Ti-9	0.69	21.43		4.48	0.29	0.27	1.01	1.91	0.28				27.01		1.64		34.29	93.3
U-Ti-Pb-1	1.05	33.28		4.15	0.18		0.58		0			1.04	14.61		4.19		37.61	96.69
U-Ti-Pb-2	0.63	38.96		4.77	0.18		0.51					1.07	11.48		2.47		39.54	99.61
U-Ti-Pb-3	2.94	29.02	0.11	5.17	0.56		0.86	1.49	0.25	0.57		0.84	5.87		9.12	0.44	39.63	96.87
U-Ti-Pb-4	1.35	24.48		7.31	0.99	0.21	0.88					0.79	1.21		4.68	0.27	35.39	77.56
U-Ti-Pb-5	2.62	31.79	0.08	6.78	0.83		1.22		0	0.78		0.7	3.03		6.24	0.39	39.95	94.41
U-Ti-Pb-6	3.66	29.06	0.1	4.35	0.45		1.13	1.9	0.27	0.85		2.76	6.02		7.69	0.59	36.23	95.06
Fe-Ti	0.51	16.28	0.71	34.93	0.24	0	0.13		0.35				2.35	0	0.68	0.34	33.9	90.42
Ti-Fe	0.45	41.1	0.28	11.69	0.2		0.09	0	0.37			1.28	2.76	1.01	0.79	0.28	40.72	101.02
Ti-Fe	1.18	32.53	0.13	11.92	1.41	0.3	1.62		0.2	0.47		2.11	0.82		2.6	0.22	39.62	95.13

Note: brn—original brannerite; U-(Ti)—rounded segregations in brannerite with a higher content of U and a lower content of Ti; U-Ti и U-Ti-Pb–zone 1 and zone 2 of brannerite alteration with a lower content of U and a higher content of O, zone 2 is additionally characterized by a higher content of Ti and Pb; Fe-Ti and Ti-Fe are zones of change with a higher content of Fe or Ti.

). Brannerite is characterized by the presence of rounded cavities 10–20 µm in size. One part of these cavities is free of substances and the other part is filled with either barite (Figure 12f,g) or U-Ti gel (Figure 12c) with a higher content of U and lower Ti and Pb than in brannerite (see Table 3, group “U-(Ti)”). At the micro level, brannerite shows an inhomogeneous block structure, which can be observed as darker and lighter areas when the BSE-image is viewed in detail. In some cases, lighter blocks appear patchy and emulsified. At a high magnification of the BSE-image of brannerite, numerous lighter inclusions with a size of fractions of a micron are visible. It is impossible to unambiguously determine the composition of such small inclusions, but judging by the change in the composition of brannerite, they represent two groups: more enriched in U or more enriched in Pb. Data on the composition of brannerite are given in

Table 4. Statistics of recalculated composition (wt%) and formula of brannerite (apfu), n = 880.

	Min	Median	Max	Mean	S.D.
Wt.%
UO2	46.41	49.55	54.09	49.52	1.05
TiO2	32.02	34.14	36.42	34.19	0.55
CaO	1.56	2.10	3.12	2.14	0.19
SrO	0.00	0.00	0.61	0.00	0.03
MnO	0.00	0.00	0.29	0.00	0.02
MgO	0.00	0.00	0.26	0.00	0.02
PbO	1.12	1.70	2.57	1.73	0.19
ThO2	0.00	1.06	2.56	1.04	0.52
Y2O3	0.00	0.00	0.74	0.10	0.17
Ce2O3	0.00	0.00	0.59	0.01	0.06
Al2O3	0.00	0.00	0.61	0.03	0.09
Fe2O3*	3.42	4.67	5.79	4.62	0.34
SiO2	0.18	1.44	2.85	1.44	0.32
V2O5	0.00	0.00	0.60	0.06	0.14
ZrO2	0.00	0.00	0.57	0.00	0.02
H2O*	1.76	5.24	7.09	5.13	0.73
Apfu (6 O)
U	0.68	0.74	0.80	0.74	0.02
Ca	0.11	0.15	0.22	0.15	0.01
Sr	0.00	0.00	0.02	0.00	0.00
Mn	0.00	0.00	0.02	0.00	0.00
Mg	0.00	0.00	0.03	0.00	0.00
Pb	0.02	0.03	0.05	0.03	0.00
Th	0.00	0.02	0.04	0.02	0.01
Y	0.00	0.00	0.03	0.00	0.01
Ce	0.00	0.00	0.01	0.00	0.00
Sum(A)	0.87	0.94	1.03	0.94	0.02
(excl. U)	0.16	0.20	0.26	0.20	0.01
Ti	1.64	1.72	1.80	1.72	0.03
Al	0.00	0.00	0.05	0.00	0.01
Fe	0.17	0.24	0.29	0.23	0.02
Si	0.01	0.10	0.19	0.10	0.02
V	0.00	0.00	0.03	0.00	0.01
Zr	0.00	0.00	0.02	0.00	0.00
Sum(B)	1.97	2.06	2.13	2.06	0.02
(excl. Ti)	0.24	0.33	0.44	0.33	0.03
Av. U valency	4.70	4.82	4.95	4.82	0.04
U4+	0.36	0.44	0.51	0.44	0.02
U6+	0.26	0.30	0.35	0.30	0.01
n(H2O)	0.38	1.17	1.58	1.15	0.17

Note: H₂O recalculated for excess oxygen; S.D.—standard deviation.

. The calculation of the main and impurity components in formula units was carried out for six oxygen atoms and three cations following Colella et al. [28] for a formula without water—AB₂O₆. The water content was estimated from excess oxygen. Distribution of the main and impurity components of brannerite in formula units is shown in Figure 13. For all sections of brannerite analyses, the calculated formula coefficients were deficient in U and Ti, and on average they were 0.74 and 1.72 apfu (atom per formula unit).

There is a slight shift in the amount of cations included in positions A and B in favor of position B: on average, the sum of cations of position A is 0.94 apfu, and 2.06 apfu for position B. Position A along with U (46–54 wt% UO₂) includes constant impurities of Ca (1.6–3.1 wt% CaO) and Pb (1.1–2.6 wt% PbO); in addition, most analyses of brannerite are characterized by an impurity of Th (0–2.6 wt% ThO₂). Position B, along with Ti (32–36 wt% TiO₂), constantly contains admixtures of Fe (3.4–5.8 wt% Fe₂O₃) and Si (0.2–2.9 wt% SiO₂). In the case of long spectrum acquisitions, the appearance of Y in brannerite (max 0.74 wt% Y₂O₃), rarer Mn (max 0.29 wt% MnO) entering A position and V (max 0.60 wt% V₂O₅) belonging to B position is typical. Single analyses of the composition of brannerite show the presence of Ce (max 0.59 wt% Ce₂O₃) and Sr (max 0.61 wt% SrO) belonging to position A, and Al (max 0.61 wt% Al₂O₃) and Zr (max 0.57 wt% ZrO₂) related to position B. In all analyses of brannerite, when calculating the formulas, there is excess oxygen content (0.4–1.6 apfu). If we assume that all excess oxygen is explained by the incorporation of water into the composition of brannerite, then its content can be estimated in the range of 1.8–7.1 wt%. The average uranium valence was estimated as 4.82, which leads to the calculation of the U⁴⁺ and U⁶⁺ contents in brannerite as 0.44 and 0.30 apfu, respectively. The formula for the average composition of brannerite according to the SEM-EDX data can be written as (U⁴⁺_0.44U⁶⁺_0.30Ca_0.15Pb_0.03Th_0.02)_∑0.94(Ti_1.72Fe_0.23Si_0.10)_∑2.06O₆·1.15H₂O.

In brannerite, there are often areas of alteration, among which there are zones “U-Ti”, “U-Ti-Pb”, and “Ti-Fe”. In most cases, alteration zones with a thickness of 2–3 to 10 µm are recorded in brannerite along the boundary between gold and brannerite (see Figure 12d–f). Depending on the intensity of alteration, there are zones of moderate (zone 1) and strong alteration (zone 2) of brannerite, which are well distinguished both in BSE images and in chemical composition (see Table 3). The areas of moderate alteration (zone 1) are usually located inside brannerite grains along cracks or along the boundary with gold. The elemental composition of this zone is characterized by a lower content of U and a higher content of O compared to the original brannerite (see Table 3, group “U-Ti”). The areas of strong alteration (zone 2), on the one hand, inherit the features of the location and composition characteristic of zone 1, and on the other hand, receive new features. Such areas of strong alteration occur either in veinlets inside grains, usually closer to the periphery, or form rims around grains. The elemental composition of zone 2 is characterized by the lowest content of U and the highest content of O compared with the original brannerite and zone 1. In addition, the content of Ti, Pb, and Fe increases (see Table 3, group “U-Ti-Pb”). Another type or direction of change in the original brannerite substance, possibly reflecting its replacement by oxides or hydroxides of Fe and Ti, is detected both inside brannerite grains and rims (see Table 3, groups “Fe-Ti” and “Ti-Fe”). Thus, in the environment of brannerite, three or more types of alteration sites can be distinguished, differing both in the direction of removal or concentration, and in the list of elements that change the content (U, V, Ca, Ti, Si, Fe, Mn, Ba, Pb, Y, P, O) (Figure 14).

3.2.3. Uraninite

Uraninite (UO₂) is rare in the studied GBN samples, with the exception of uraninite structures in brannerite in the form of accumulations of nano- and microparticles (from tens and hundreds of nanometers, up to 1–2 μm), as well as dendritic crystals (see Figure 12h,i). Grains of uraninite occur as inclusions in brannerite or in native gold. Uraninite is represented by two varieties (Figure 15). Some uraninite grains correspond in composition to a mixture of UO₂ and UO₃ (Figure 15a). In addition to U and O, Pb, Th, REE, Ca, Ti, Si, and sometimes Fe are constantly present in the composition (

Table 5. Elemental composition of uraninite by SEM-EDX data, in wt.%.

Name	Si	Ti	Fe	Ca	K	Ce	La	Dy	Y	Th	U	Pb	O	Total
u1-1	0.15	0.34	0	0.35		0.44	0.49	0	0.27	0.78	76.78	3.77	14.18	97.55
u1-2	0.13	0.34	0	0.4		0.5	0.49	0	0.24	1.27	76.41	3.62	14.65	98.05
u1-3	0.13	0.36	0	0.52		0.39	0.44	0	0.31	2.79	74.49	3.67	14.49	97.59
u1-4	0.14	0.34	0.07	0.53		0.4	0.44	0.18	0.35	0.49	76.68	3.68	14.8	98.1
u1-5	0.14	0.36	0.1	0.46		0.35	0.41	0	0.29	0.78	76.82	3.78	14.45	97.94
u1-6	0.13	0.35	0	0.13		0.36	0.41	0	0.3	0.47	77.88	3.76	14.13	97.92
u1-7	0.14	0.32	0	0.42		0.44	0.43	0	0.26	3.74	74.74	3.73	14.09	98.31
u1-8	0.13	0.34	0	0.32		0.44	0.45	0	0.27	3.64	75.58	3.77	14.58	99.52
u1-9	0.14	0.34	0	0.23		0.39	0.41	0	0.27	0.62	78	3.66	14.84	98.9
u1-10	0.12	0.32	0.08	0.2		0.36	0.45	0	0.29	0.57	78.35	3.78	14.55	99.07
u1-11	0.11	0.36	0	0.18		0.31	0.46	0	0.31	0.49	78.33	3.81	14.4	98.76
u1-12	0.16	0.35	0	0.07		0.41	0.49	0	0.25	2.08	77.06	3.69	14.13	98.69
u1-13	0.13	0.36	0.08	0.59		0.35	0.35	0.17	0.53	2.41	74.67	3.67	14.51	97.82
u1-14	0.12	0.35	0.07	0.42		0.43	0.44	0	0.46	2.23	76.04	3.72	14.75	99.03
u1-15	0.19	0.35	0.09	0.05		0.36	0.43	0	0.27	2.05	77.08	3.73	14.13	98.73
u2-1	0.1	0.49	0.1		1.67	0.33				1.55	68.88	0	22.86	95.98
u2-2		0.43	0.08			0.27				1.35	71	0	21.2	94.33
u2-3	0.11	0.48	0.09			0.32				1.37	70.67	0	26.58	99.62
u2-4	0.07	0.41	0.1		1.92	0.29				1.19	72.89	0	22.42	99.29
u2-5	0.07	0.35	0		1.88	0.26				1	70.31	0.13	20.09	94.09

Note: u1—original uraninite; u2—nasturan.

). Judging by the fact that no micro- and nano-inclusions were found in such uraninite, it can be assumed that all the Pb in it is a product of the radioactive decay of uranium. And another part of the uraninite grains, according to the analysis, contains a significant amount of excess oxygen (Figure 15c,d). Such oxidized or hydrated uraninite can be attributed to nasturan (pitchblende, oxidized uraninite) [29]. Morphologically, nasturan is presented in two varieties. The first is individual rounded, isometric or slightly elongated “ovoids” of nasturan (from 2.6 × 3.6 µm to 3.7 × 4.7 µm) with an inhomogeneous internal structure. The second is crystals resembling ordinary uraninite. In some cases, both varieties of uraninite can be observed in the same grain. In this case, oxidized uraninite forms rims around ordinary uraninite (Figure 15b). A notable difference of this hydrated uraninite is the almost complete disappearance of Pb from its composition and the appearance of K in some analyses.

3.2.4. Rutile

Another important GBN mineral is rutile (see Figure 10), containing in its composition iron (from 0–0.5 to 4.95 wt.% FeO_tot) and tungsten (max 10.51 wt.% WO₃). Rutile occurs as xenomorphic grains in gold or as elongated gold–rutile intergrowth with signs of their replacement by hematite (see Figure 10b–d and Figure 12g). The association of rutile with gold is observed in various samples, both at the level of large aggregates and in the form of microinclusions of gold less than 1 µm in size in rutile grains. According to the ratio of elements, two types of rutiles are distinguished: without W and W-containing, in the latter, W, together with Fe, replaces Ti (Figure 16). High-fineness gold is associated with W-containing rutile (from 6 to 11 wt.% WO₃). Rutile with lower W contents (from 0 to 3 wt WO₃) is associated with gold, having a composition of 85–85 wt.% Au. High-contrast SEM images (Figure 17) clearly show that rutile with low tungsten content replaces W-containing rutile, primarily in the peripheral parts of grains, along cracks, along cleavage, and also grows on it. Grains with a decomposition structure (Figure 17a), were also found, with lamellae from 70 to 250 nm thick of W-bearing rutile in rutile with low contents of W, confined to the cleavage directions of the mineral according to the “sagenite” type [30]. In isolated cases, V is detected in rutile up to 0.88 wt.% V₂O₃.

3.2.5. Barite

Quite often, GBN contains barite in the form of crystals (see Figure 10c,d and Figure 12f,g), rims along the boundary of brannerite and gold, microveins and clusters in gold, less often as isometric inclusions (up to 10 μm) in brannerite. The appearance of barite in brannerite is accompanied by a concentrically zoned rim (zone) of brannerite alteration and its transition to leucoxene (Figure 12g). Barite does not contain impurities, and Sr (less than 1 wt.% SrO) is detected only in barite inclusions in brannerite. At the same time, the material in the zonal rim is enriched in Pb (up to 10.9–19.8 wt.% PbO), Te (up to 6.3–12.2 wt.% TeO₃), and P (up to 1.2–5.9 wt.% P₂O₅).

3.2.6. Hematite–Magnetite

The mineral composition of GBN contains iron oxides in various forms, which make up to 5%–10% of the volume of individual nuggets. The Fe to O ratio suggests the presence of hematite, magnetite, goethite, and siderite in GBN. Hematite is the most common iron oxide in GBN, judging by the tabular forms of crystals, grouped in parallel or in the form of radially radiant sometimes curved aggregates, color and reflectivity under a microscope (Figure 9). Moreover, hematite is of great importance in the localization of gold, judging by their mutual intergrowths (Figure 9f and Figure 10a,c,f). In many cases, altered brannerite is replaced by hematite (see Figure 12e). According to Raman spectroscopy, hematite is the most abundant in GBN. Raman spectra of the hematite are shown in Figure 18a (curve 1–3). The characteristic bands for identification of hematite are 225, 290–300, 412, 612 and first- and second-order longitudinal optical (LO) bands at ~660 and ~1320, respectively [31,32,33,34,35,36]. Some Raman spectra exhibit the intensive band of magnetite at 667 cm⁻¹ along with an increase in hematite bands of widths (Figure 18, curve 1). Magnetite is characterized by having a main broad band ~668 cm⁻¹ and the bands at 540 cm⁻¹ and 300 cm⁻¹ [32,34]. This bands are present in spectrum, the band at ~300 cm⁻¹ as the shoulder (Figure 18, curve 1).

The SEM-EDX data also indicate the presence of magnetite in GBN. Ti magnetites stand out (from 1.56 to 4.94 wt.% Ti), with an insignificant Ti impurity (from 0.11 to 0.72 wt.% Ti) and without it. Approximately one third of the analyzed magnetites contain up to 0.36 wt.% Al. In isolated cases, magnetites contain Co (0.3 wt.%) and W (up to 1.38 wt.%). In hematites, only Ti was found as an impurity from below the detection threshold to 1.61 wt.% Ti. Siderites constantly contain Mn (1.25–6.15 wt.%), Si (0.83–2.25 wt.%), Ca (0.12–0.52 wt.%), P (0.15–0.52 wt.%), as well as Mg and Al in concentrations less than 1 wt.%.

3.2.7. Goethite

Iron hydroxide is represented by goethite and occurs primarily on the surface of GBN and in large (more than 1 mm wide) cracks in weakly altered brannerite (Figure 7a,b,h,i and Figure 10d). The cracks are filled with highly altered aggregates brannerite grains, hematite, rutile, organic matter, goethite, and are cemented with gold. The microtexture inside such cracks can be characterized as porphyroblastic. Brannerite grains have rounded edges and a large number of gas cavities around which leucoxene is formed. Goethite develops after leucoxenized brannerite and rutile. A characteristic feature of iron hydroxides is the constant presence of Mn from 0.5 to 5.2 wt.%, Si from 0.8 to 1.5 wt.%, Ca 0.1 to 0.37 wt.%, P from 0.2 to 0.64 wt.%, Al up to 0.71 wt.%, and Mg up to 0.57 wt.%. In addition, in some locations, Cu up to 2.19 wt.%, Zn up to 0.45 wt.%, and Pb up to 0.75 wt.% are found in the composition of iron hydroxides. The connection of gold with iron hydroxides can be called “relict” inherited from substituted minerals.

3.2.8. K-Feldspar, Quartz and Chlorite

At the edge of one of the GBNs (C6), a contact was found made by ferruginous chlorite (chamosite) with a silicate rock. The rock consists of xenomorphic grains, about 100 microns in size, quartz and K-feldspar and chamosite, which fill the intergranular space (Figure 17c). Gold nanoinclusions are observed among the chlorite. The composition of feldspar fully corresponds to K-feldspar with low contents of Na₂O, FeO_tot and BaO (

Table 6. Chemical composition (wt%) of K-feldspar and quartz from the silicate rock on the contact with GBN.

Sample	Point	SiO2	Al2O3	FeOtot	Na2O	K2O	BaO	Total
C6-04	317	65.49	18.71	0.58		16.15	0.56	101.48
C6-04	322	68.01	17.72	0.26	0.19	15.92		102.10
C6-04	325	66.37	17.04	0.26	0.19	15.37		101.23
C6-04	315	99.03	1.21	1.25		0,23		101,72
C6-04	326	100.06	0.70	0.41		0.23		101.40

Note: no data—below detection limit.

and

Table 7. Chemical composition of chlorites from the silicate rock on the contact with GBN, at the rate of 12%-OH.

S_P	SiO2	TiO2	Al2O3	FeOtot	MnO	MgO	CaO	K2O	ZnO	P2O5
C6_313	26.02	0.22	17.77	35.61	0.26	2.58	0.2	4.04	0.62	0.69
C6_316	29.48	0.3	19.27	33.54	0.42	1.02	0.7	2	0.31	0.86
C6_318	31.42	0.29	18.31	25.14	0.37	8.11	0.3	2.97	0.68	0.29
C6_323	30.45		17.7	26.47	0.45	10.83	0.22	0.97	0.49	0.32
C6_323	30.9		18.38	24.44	0.44	12.12	0.24	1.11	0.27

Note: S_P—sample_point.

).

3.2.9. Organic Matter

In one of the nuggets, there is a crack 0.5 to 1 mm wide, more than 10 mm long, crossing the entire volume of the sample. The crack with “torn” edges is made by a mineral aggregate cemented with native gold and consisting of fragments of leucoxenized brannerite, columnar crystals of hematite, rutile, goethite, and isometric structures (up to 1 mm) of organic matter. Raman spectra confirming the presence of organic matter in leucoxenized brannerite and in normal brannerite (in a small amount) are shown in Figure 19 [37,38,39]. The broad shoulders around the 1320 cm⁻¹ band at ~1140 cm⁻¹ and ~1520 cm⁻¹ are also assigned to an organic matter in the hematite Raman spectra (Figure 18) [37]. According to isotopic analysis, it was determined that the organic matter has δ¹³C −23.6‰.

4. Discussion

As a result of studies in the alluvial placer of the Kamenny stream of the OOC, four types of primary gold were identified, which differ significantly in their characteristics and, probably, have different primary sources (

Table 8. Possible sources of various types of alluvial placer gold.

Type	I	II	III	IV
fineness	798–945	676–764	445–648	885–946
Possible source	Nazarovskoe gold-zinc skarn deposit		Veins of chalcedony quartz with gold-silver mineralization	Serpentinization zones of basic and ultrabasic rocks

). The most common composition of gold corresponds to fineness 676–774‰. This is fully comparable with the fineness of gold (600–880‰), which prevails at the Nazarovsky skarn gold–zinc deposit. [40]. The Nazarovskoe deposit has a powerful oxidation zone, where gold is constantly present. Ores similar to those found at the Nazarovskoe OOC deposit are the primary source of high-grade and relatively low-grade gold in the placer of the Kamenny stream, which is confirmed by some features of the ore (sphalerite inclusions, composition, morphological relics) [5]. Low-depth gold–silver mineralization can be the root sources of low-grade gold, as evidenced by the significant heterogeneity of the composition of gold and the predominance of silver in it. Alloy heterogeneity is present as microfabrics formed either during primary mineralization or by modification of pre-existing alloys by chemical and physical drivers during subsequent residence in either hypogene or surficial environments [41,42]. Veins of chalcedony quartz with finely interspersed pyrite and gold–silver mineralization were established by G.I. Doroshkevich [43] in the zone of tectonic contact of the intrusion of eruptive breccia’s of trachyriolites with Lower Cambrian limestones in the northwestern frame of the Ozerninsky ore cluster. The cuprous gold allow us to speak about the possibility of the existence of another primary source, not related to those mentioned above, which are characterized by native gold without copper impurities. The spatial and genetic relationship of cuprous gold with zones of secondary metasomatic alteration of basic and ultrabasic rocks is known [44,45]. Similar formations on the territory of the OOC are still unknown.

Two exogenous events were established in the OOC area, which influenced the formation of rims of high-fineness gold on NG. One of these events was the formation of an areal weathering crust, which was especially fully manifested in the form of an oxidation zone at the Ozernoye pyrite–polymetallic deposit. The second is the alluvial transport of gold and the formation of a placer. Placer gold is characterized by a wide distribution of high-fineness rims with contrasting boundaries and a rare presence of high-fineness rims with diffusion boundaries, which may indicate a significant duration of gold residence in weathering crusts compared to its presence in alluvium [46]. The association of gold and uranium is well known, rather widespread and characteristic of several types of gold-uranium mineralization [47,48,49,50,51]: iron-oxide-copper-gold-uranium (IOCGU)-Ag deposits and most notably the Olympic Dam in South Australia, associated with hematite breccias, 2.57 to 1.0 billion years old; deposits of the “unconformity” type, Paleo-Meso-Proterozoic age (Alligator Rivers, etc.); Au-U deposits in zones of potassium metasomatism located in areas of tectonic-magmatic activation (TMA) of the Precambrian shields (Elkonian type); and deposits in zones of low-temperature Na-metasomatosis in the TMA structures of folded areas (Shinkolobve and others) (

Table 9. Types of Au-U mineralization (geological conditions of formation, mineral associations and metasomatic alterations).

Deposit/Ore Mineralization Type	Age, Geochemical Association	Geologic Setting	Secondary Alteration	Mineral Type of the Ores	Deposits, Ore District
Unconformity related deposits: (a)-Fracture controlled; (b)-Clay bounded; (c) endogenic, in zones of structural-stratigraphic unconformities; (d) Proterozoic base overlain by Mesozoic cover	Paleo-, Mezo-, Proterozoic (1700–1600 million years; 1600–900 million years) U, Au, Ni, Cu, Ag, As, Co, Pt, Pb	Paleoproterozoic metamorphic base (pelites, arcoses, metamorphosed in amphibolite facies carbonates); gneisses with graphite, often with weathering crust; overlain by Mesoproterozoic sediments; rocks deformed and brecciated: (a) ores in basement; (b) in overlying deposits	Chloritization, argillitization, seritization, carbonatization	Ore mineralization in breccias, veinlike breccias, disseminated in shales: Uraninite (Urn), Nasturan, rare coffinite (Cof), brannerite (Bnr), organic-uran minerals, gold, pyrite (Py), chalcopyrite (Ccp), graphite (Gr).	Athabasca Basin: McArthur River, Rabbit Lake, Eagle Point, McClean Lake, Dominique-Peter in Canada; Jabiluka, Ranger, Nabarlek, Koongarra (the Alligator River region) in Australia; Kuranah
«Brecciated» Hematite breccia complex deposits (IOCGU): (a) Granite-rich breccias; (b) Hematite-rich breccias	From ~2570 to 1000 million years. U, Au, Cu, Ag, REE, Fe.	They are found in a number of different tectonic settings (rift, subduction zones, basin collapse)	Sericitization, chloritization, hematitization	U-Au-Cu granite-derived matrix; hematite-quartz breccias, hematite breccias; Au, nasturan, bornite (Bn), chalcopyrite (Ccp)	Olympic Dam South Australia; Prominent Hill, Emest Henry, Starra, Osborne in Australia; Candelaria, Salobo, Sossego in South America; Michelin, Sue-Dianne in Canada, Igarapé Bahia Deposit (Brazil)
Deposit/ore mineralization type	Age, geochemical association	Geologic setting	Secondary alteration	Mineral type of the ores	Deposits, ore district
Endogenic in areas of tectonic-magmatic activity of the Precambrian craton Metasomatic U deposits: (a) alkaline metasomatites of sodium series; (b) potassium (elkonites) series; (c) albitization in black shales (shungites)	a–1900–1600 million years: U, Au, Ag, Th, Mo; b–155–123 million years: U, Au; c-Au, PGE, U, V?	a-stockworks and lenses in granites, migmatites and pegmatites; b–tabular-,veinlike-, lenslike-form body in crystalline schists, migmatites, granites; c–albitization zones in shungites	a-albitization, amphibolization, pyroxenitization (aceites); b–kalishpatization (elconites); c-albitization	a–U, Au, Ag, Th-U brannerite-uraninite, coffinite-brannerite; b–Au-U-brannerite, rare Au-U-uraninite	a–Yuzhnoye, Lozovatskoye (Ukraine), Rossing (Namibia), Itatiaia (Brasil), Kokchetavskiy massif; b-Druzhnoye, Kurung, Snezhnoye (Elconskiy ore district); c–Sr.Padma (Onezhskiy trough Baltic craton)
Endogenic in areas of tectonic-magmatic activity of the folded regions. Vein deposits (granite related deposits)	U, Au, Mo, Th, Zr, P2O5	Column-shaped, lens- and veinlike deposits in sandstones, carbon shales, diabases, granites and limstones controlled by faulded and crushed zones	Low-temperature Na-metasomatism zones; lightening in black shales	U, P-U, Mo-U; arshinovite-molybdenite-brannerite-nasturan; F-apatite-uraninite	Manybayiskoe, Zaozernoe (Kazakhstan), Shinkolobwe (DR Congo)
Deposits in multiple phase alkaline intrusions Intrusive deposits	Au, U, Cu, Nb, REE, Fe	Disseminated uranothorianite occurs in cupriferous carbonatite complexes.	Sulfidization	REE, U, Th, Au	Palabora (South Africa)
Au-U conglomerates Quartz pebble conglomerate deposits	3070–2200; 1900 million years Au, U, REE	Superimposed troughs in the marginal parts of ancient platforms; oligomictic quartz-pebble conglomerate horizons (termed reef), from 0.5 to >3.5 m thick	Sericitization, chloritization, pyritization	Au-U, Au, U-uraninite, PGM, uranthorite, uranthorianite, monazite, and xenotime are prevailing	Witwatersrand Basin, Jacobina (Brasil), Tarkva (Gana), Blind-River (Canada).

). In the ores of the listed types of deposits, brannerite is one of the main minerals of uranium, together with uraninite and coffinite. The mineral assemblages of native gold with brannerite are known in such deposits as the Witwatersrand, Blind River and smaller gold–uranium deposits [52,53,54,55]. In Russia, a similar gold–brannerite association composes the unique Central Aldan gold–uranium ore region [56]. Here, within the Elkon horst, two types of complex ores have been identified: gold-bearing brannerite mineralization of the Elkon type and brannerite–silver–gold mineralization of the Fedorovsky type [49,50,56]. Gold–brannerite intergrowths are extremely rare and have been found only in ores of the Witwatersrand (South Africa) and Richardson (Canada) in the form of small crystals and grains [54,55]. P. Ramdor (1962) [57] gives a photograph of a polished section (see Figure 288 in p. 332 from [57]) from the Bou-Azzer deposit (Southern Morocco), which shows “cataclastic brannerite is cemented by much gold with a little galena and quartz”. The formation of the gold-brannerite association at the Bou Azzer deposit (the polymetallic Bou Azzer Co–As–Fe–Ni(±Ag ± Au) district) occurred at the early pre-arsenide gold-bearing stage of the hydrothermal system [58]. Also gold–brannerite assemblage is an intermittent feature of gold from various localities [59]. Because of their size, the gold–brannerite nuggets found in the placer of the Kamenny brook are unique in comparison with all previously encountered and described ones [6,7]. GBN is generally characterized by cataclastic structures with a wide variety of microstructures: porphyroclastic, cementitious, reticulate, lattice, zonal, lattice, spotted, radially radiant, and others. The native gold found in GBN has wide variations in composition (from 750 to 1000‰), and at least five types are distinguished: 750–800‰; 850–880‰; 880–920‰; 930–960‰, and 980–1000‰ (Figure 11). NG is found in association with various minerals: uraninite, rutile, brannerite, hematite, barite, carbonaceous matter, goethite. The study of the relationship between the composition of native gold and the associations surrounding it, as well as the identification of the physicochemical conditions of formation, require further research.

Turani et al. [60] presented data on the composition of brannerite from five hydrothermal and five igneous (pegmatite) ore deposits, and some of these deposits have gold-uranium mineralization (Bou Azzer deposit and Mont Chemin mines). A comparison of these data [60] with the compositions of normal brannerite from GBN OOC is shown in Figure 20. It is noticeable that brannerite from GBN samples are characterized by the lowest values in terms of Ti content and are relatively low in Y and, on the contrary, are among the highest in terms of Pb and Fe. According to the content of U, GBN samples are not distinguished with others, and according to the content of Si, an increased scatter of contents is observed.

The data obtained on the composition and relationships of GBN minerals in the OOC area indicate a complex history of their occurrence, which can be represented as at least three stages. The first is the formation of early quartz–nasturan–gold (composition 94–95 wt.% Au)–W-rutile–magnetite assemblage. There are preliminary data on determining the uranium–lead age of uraninite microcrystals by the chemical method [61], indicating its Middle-Late Paleozoic age. The study of impurity elements in rutile enables distinction of rutile in metamorphic-hydrothermal and hydrothermal deposits from rutile in magmatic-hydrothermal deposits and magmatic environments [62]. Rutile from mesothermal and related gold deposits invariably contains W, and in some of the larger and more important deposits, rutile also contains Sb and/or V. Tungsten-bearing detrital rutile grains in the Witwatersrand deposit suggest that there was probably a paleo placer with a mesothermal gold source. Cu-Au deposits tend to contain elevated W and Cu (and sometimes V). The Olympic Dam Au-Cu-U deposit hosts rutile that is enriched in W, Sn and Cu [63]. The second is the crystallization of brannerite and the replacement of an earlier pitchblende by brannerite. The age of brannerite, determined from the data of chemical analysis of the WDX and EDX with a point probe and EDX in a small raster, varies in the range of 200–235 Ma, i.e., corresponds to the Late Triassic (T₃)–Early Jurassic (J₁) time period [7]. The third is the formation of a hematite–barite–rutile–gold association as a result of deformation-hydrothermal processes, which is associated with the appearance of zones of alteration (leucoxenization) in brannerite at contact with native gold with 8–15 wt.% Ag. Fluid separation probably occurred in parallel, which also led to the replacement of brannerite by Fe and Ti hydroxides. The fourth is hypergene or low-temperature hydrothermal alteration of minerals of early stages with the development of iron hydroxides (goethite) with impurities of manganese, tellurium, arsenic, phosphorus and other elements. This stage is associated with the appearance of microveins of very high-fineness gold in lower-fineness gold. Thus, the data obtained indicate a multistage, possibly polygenic, and probably polychronous formation of GBN gold–uranium mineralization.

The relationship of native gold with brannerite shows that it is localized not only along cracks that cross brannerite, but also forms structures of joint precipitation from solutions with it. Moreover, there is no zone of brannerite alteration around isometric micrograins of such gold, in contrast to gold located in cracks, which may indicate different ways of “inflow” of gold into brannerite. The presence of idiomorphic uraninite in native gold makes it possible to consider the possibility of joint transfer of gold and uranium. Intensive dissolution and redeposition of the material of experimental ampoules—gold was found during experimental studies of the Au-UO₂-TiO₂-water fluid system, which were carried out in order to determine the solubility of uraninite in hydrochloric acid solutions at 500 °C, 1000 bar. Analysis and modeling of the discovered phenomenon showed that the redox couple UO_2+x/UO₂ oxidizes Au(cr) to Au+(aq), which is then reduced under the action of stronger reducing agents [64,65].

The δ¹³C of organic matter from a crack in GBN with an aggregate of leucoxenization brannerite, hematite, and rutile is −23.6‰, which indicates the participation of a biogenic carbon source. It is known that the presence of organic compounds in mineral assemblages and mineral-forming fluids is one of the characteristic features of hydrothermal ore deposits, which indicates the fundamental possibility of participation of organic matter in the transport and deposition of ore matter [66]. Carbonaceous compounds (graphite, anthraxolite, kerite, and others) have been found in many uranium and rare metal deposits, including in association with brannerite and gold. The formation of solid carbonaceous substances occurred from metal-bearing fluids at the end of the main stage of ore formation [67]. The presence of traces of magnetite in hematite from GBN, detected by Raman spectroscopy, i.e., the replacement of magnetite with hematite, may indicate an increase in the redox potential in the system. This may indicate the possibility of high-temperature oxidation of organic matter in the hydrothermal system, which could be accompanied by gold precipitation. The alteration brannerite at the contact with native gold, barite, and numerous “bubbles” of gas-in-fluid inclusions was probably due to an increase in the redox potential.

The results of mineralogical and geochemical studies and the age of GBN from OOC [7] make it possible to carry out a comparative analysis with gold–uranium mineralization of the Mesozoic age of the Central Aldan ore region [48,68] and to predict the development of the described mineralization not only within the OOC, but also the Kurbino–Eravninsky ore district. From the available literature data [69], it is possible to compare normal and altered brannerite in GBN from OOC and uranium–titanium-metagel (UTM) mineralization from Elkon ore district. According to Aleshin et al. [69], in addition to brannerite, the unique reserves of uranium in the Au-U deposits of the Elkon ore district are explained by polyphaser UTM (different groups of UTMs were designated as UTM-1, UTM-2/1, UTM-2/2). Moreover, according to these authors, brannerite, uranium oxides, and silicates make up an insignificant part of the ores. Our study also found that in GBN, along with ordinary brannerite and accessory uraninite, there is a significant proportion of altered brannerite developed along rims or a dense network of veinlets. On the Ti-U diagram, it is noticeable that the zone of weak brannerite alteration “U-Ti” described in this study can be correlated with UTM-1 and the zone of strong brannerite alteration “U-Ti-Pb” can be correlated with UTM-2/1 and UTM-2/2 (Figure 21).

In particular, the Yuzhnoe deposit in the Elkon gold–uranium ore region is controlled by a tectonic zone. The zone is formed by one or several dikes of metadiorites alternating with blastomylonites and blastocataclasites surrounded by quartz–K-feldspar metasomatites, chlorite-epidote propylites and beresites. Ores are represented by the gold–brannerite mineral type [47,70]. The GBN brannerite of the Kamenny stream of the OOC is comparable in composition to the brannerite of the Elkon region in composition, which are characterized by low thorium content and an admixture of rare earth elements. In these parameters, brannerite from the OOC and Elkon region differ sharply from brannerite from albitites of Ukraine and pegmatites and conglomerates of the Witwatersrand (South Africa). The results of the study of the GBN of the Kamenny stream of the OOC indicate a wide manifestation of cataclasis processes, with the appearance of a characteristic network and porphyroclastic, cataclastic textures. The discovered fragment of the host quartz–K-feldspar rock with chlorite (chamosite) at the contact with GBN is well correlated with quartz–microcline metasomatites of the Yuzhnoye deposit in the Elkon region.

In addition, another reason for predicting gold–uranium mineralization to a depth is the wide distribution of Mesozoic and younger uranium mineralization, manifested in the OOC area (Eravninsky potential uranium-ore region) and to the north of it (Vitimsky uranium-ore region). The mineralization described in this paper can also be considered as evidence of the existence of “multi-level” gold–uranium mineralization of various ages. The probability of the existence of multilevel uranium (gold–uranium) mineralization in the Vitim–Amalat ore region of Transbaikalia was indicated in the regional forecasting of large-scale mineralization of the “Olympic Dam”-type gold–uranium ores in hematite breccias [71].

An analysis of the composition of GBN rutile indicates the presence of rutiles typical of ore zones (Figure 22) [72,73].

The identified type of uranium ores can be considered as one of the possible sources of the formation of hydrogenic uranium ores in the supergenes zone of the Western Transbaikalia (Khiagda group of hydrogenic deposits). An important conclusion is also the fact that the processes of ore formation within the Ozerninsky ore cluster were diverse and continued for a long time from the Early Paleozoic to the Mesozoic, and the formation of placers continued in the Quaternary.

5. Conclusions

A comprehensive mineralogical and geochemical study of NG and GBN was carried out in the alluvial placer of the Kamenny stream of the OOC (Western Transbaikalia). The obtained data on the morphology, composition and relationships of minerals in NG and GBN testify to the complex history of their formation and various primary sources of gold. Separate areas of the placer, where predominantly unrounded and semi-rounded gold is present, can be considered as eluvial placers.

Based on a set of features, four types of primary NG and GBN were identified, which differ significantly in their characteristics, for which primary sources are predicted. The most common high-fineness and relatively low-fineness gold types are well compared with the gold of the Nazarovsky skarn gold–zinc deposit located in the OOC. For low-fineness gold, shallow gold–silver ores are assumed to be the primary source.

The presence of cuprous and copper-bearing gold in placers suggests the existence of ultramafic complexes with gold-bearing zones of listvenites and rodingites in the OOC territory.

In the OOC area, two exogenous events have been identified that influenced the formation of very high-grade NG: the formation of an areal weathering crust and the alluvial transport of gold and the formation of a placer.

The GBN found in the Kamenny Brook placer is unique in its size, variety of microtextures, and mineralogical features in comparison with all previously encountered descriptions of the co-occurrence of gold and brannerite.

The obtained data on the composition and relationships of GBN minerals in the OOC area indicate a complex history of their occurrence, which can be represented as at least three or four stages: 1—formation of early quartz–nasturan–gold–W-rutile–magnetite(?) assemblage; 2—replacement of earlier pitchblende by brannerite; 3—as a result of deformation-hydrothermal processes, the formation of a hematite–barite–rutile–gold assemblage, which is associated with the appearance of zones of alteration (leucoxenization) in brannerite at contact with gold; 4—hypergene or low-temperature hydrothermal alteration of minerals of early stages with the development of iron hydroxides (goethite) with impurities of manganese, tellurium, arsenic, phosphorus and other elements.

In the OOC area, there are indications that the composition of GBN and host quartz–chlorite–K-feldspar rocks corresponds to the Elkon-type deposits.