Genesis and Timing of Low-Sulphide Gold–Quartz Mineralization of the Upryamoye Ore Field, Western Chukotka

Abstract

The Upryamoye ore field is located in the Chukotka metallogenic belt in Northeast Russia. The orebodies are hosted within Late Jurassic–Early Cretaceous greenschist-facies metamorphosed rocks and structurally controlled by NW-trending fold-and-thrust dislocations. Based on geological exploration, petrographic, mineralogical, and geochronological studies, new data on the geological structure and composition of gold–quartz mineralization of the Upryamoye ore field are presented. Optical and scanning microscopy were used to study the lithological features of the host rocks and determine the ore textures and the morphology and internal structure of native gold, auriferous pyrite, and arsenopyrite. Qualitative and quantitative characterization of the ore minerals was carried out using SEM-EDS and EPMA. To determine the age of the gold mineralization, Re-Os dating of arsenopyrite and U-Th/He dating of pyrite were performed. The results show that the orebodies comprise carbonate–quartz and sulphide–carbonate–quartz saddle reef veins in both the fold hinge and limbs, as well as mineralized shatter zones and mylonite zones that trace thrust faults. The main ore minerals are arsenopyrite and pyrite, associated with minor amounts of galena, sphalerite, chalcopyrite, tetrahedrite, and bournonite. Native gold is distributed extremely unevenly, forming thin and finely dispersed inclusions in pyrite and arsenopyrite. U-Th/He isotopic analyses of auriferous pyrites suggest that gold mineralization in the Upryamoye ore field occurred at 123 ± 4 Ma. The data obtained by Re–Os dating of auriferous arsenopyrite are inconsistent with direct geological observations but indicate that Os in the arsenopyrite was derived from the crustal source. According to a number of characteristic features of mineralization, the Upryamoye ore field is attributed to a metamorphic genetic type of orogenic low-sulphide gold–quartz deposits. The ore-forming process was long and multi-stage, occurring during the final collisional phase and the beginning of the extensional phase of the Chukotka orogen.

1. Introduction

The Chukotka region is Russia’s largest gold producer, accounting for approximately 10% of the country’s proven gold reserves [1,2]. Most large-scale belt of gold–quartz vein deposits of the Chukotka formed in the post-collisional stage of the region’s development [3,4,5]. The collision of the Chukotka microcontinent with the active margin of Siberia, which concluded during the Hauterivian–Barremian, generated fold-and-thrust structures that subsequently became the principal controls on the distribution of ore zones [6,7]. During the Aptian–Albian, the collision compression regime was superseded by post-collisional extension, accompanied by the formation of granitic–metamorphic domes and intrusions. Magmatic activity occurring between ~117–105 Ma (Albian–Cenomanian) [8,9,10] played a crucial role in the orogenic ore-forming processes [3].

Several types of gold-bearing deposits are recognized in Chukotka (Figure 1): copper–porphyry (Berriasian–Valanginian), associated with the formation of the Oloy volcanic belt (Peschanka, Nakhodka) [11]; gold–quartz (Aptian–Albian), resulting from the emplacement of post-collisional granitoid complexes; and gold–silver epithermal (Cenomanian–Campanian), associated with the Cretaceous Okhotsk–Chukotka continental-margin volcanic belt (Kupol, Valunistoe) [12,13,14]. Among the gold–quartz deposits of the orogenic stage of the Chukotka terrane are low-sulphide gold–quartz deposits (Sovinoe, Dor, Karalveem) [15,16] and gold–rare-metal deposits associated with granitoids (Kekura) [17].

The Upryamoye ore field in Chukotka was studied under the leadership of A.I. Grigoriev, V.A. Zhukov, I.V. Deparma, G.F. Zhuravlev, Yu. M. Telegin, and others. Geological surveys, alongside geochemical and geophysical studies conducted between 1963 and 1992, identified gold–quartz veins with grades of up to 22 ppm of Au [18].

Figure 1. Au–bearing deposits on the tectonic scheme of Northeast Russia. Modified after [19,20,21].

Figure 1. Au–bearing deposits on the tectonic scheme of Northeast Russia. Modified after [19,20,21].

This work aims to identify the structural and genetic features of the Upryamoye ore field. The research objectives include the following: analysis of the lithological and structural features of the host rocks, study of the composition and morphology of gold mineralization, construction of a paragenetic sequence, and determination of the time of ore-forming processes. The results of this study contribute significantly to understanding the metallogenic process conditions of orogenic deposits in Western Chukotka and to the development of regional genetic and exploration models, which are key to assessing the mineral potential of the region and planning further exploration.

2. Geological Background

Regional Geology

The Chukotka Terrane is a fragment of a Late Paleozoic–Early Mesozoic passive margin. The basement consists of intensely deformed and metamorphosed Precambrian rocks, transformed under greenschist–amphibolite facies conditions. Lower and Middle Paleozoic deposits are composed mainly of terrigenous and carbonate rocks. Upper Paleozoic deposits are either completely absent or significantly reduced. Thick Triassic turbidites are most extensively exposed at the surface [22]. Volcanogenic–sedimentary sequences of the Upper Jurassic–Lower Cretaceous are widespread in isolated depressions and along the southern periphery of the terrane (Figure 2).

The Chukotka metallogenic belt of gold–quartz vein deposits is located in the central and western parts of the Chukotka Terrane. The Upryamoye gold field occupies a central position in the Gremuchinsky gold node of the Chukotka metallogenic belt. It is linearly elongated in a northeast direction for 3.5 km with a width of up to 1.5 km, with a total area of 12.1 km². A detailed description of the geological structure of the area is provided in our earlier works [23,24]. Structurally, the Upryamoye ore field is associated with the Peristaya anticline, which complicates the structure of the Konevaam syncline of the Rauchua fold zone.

Figure 2. Geological map of the Rauchua River basin (according to [25] with modifications) (A) and simplified geological map of Upryamoye ore field (B). The yellow line in (A) shows the orientation of the Peristaya anticline.

Figure 2. Geological map of the Rauchua River basin (according to [25] with modifications) (A) and simplified geological map of Upryamoye ore field (B). The yellow line in (A) shows the orientation of the Peristaya anticline.

The area of the Upryamoye ore field is composed of Late Jurassic–Early Cretaceous deposits of the Netpneyveem and Pogynden Formations (Figure 2B). The lithological and petrographic composition of the rocks comprising the ore field is diverse, consisting mainly of terrigenous sedimentary (sandstones, siltstones) and, less frequently, tuffaceous sedimentary rocks of Late Jurassic–Early Cretaceous age [23]. The sedimentary formations are intruded by single dikes of microdiorites of the Ichuveem complex of the Late Cretaceous in the central part of the ore field. Geophysical data suggest the presence of a subsurface granitoid intrusion [18].

The orebodies of the Upryamoye ore field are confined to hydrothermal saddle reef veins, as well as to mineralized shatter zones and mylonite zones that trace thrust faults (Figure 3). The largest vein within the ore-bearing zones of the Upryamoye ore field is no more than 100 m long, with a thickness of 0.1–2 m.

Hydrothermal formations within the Upryamoye ore field are distributed regularly: zones of hydrothermal breccia tend to be concentrated in the central and northern parts of the ore field, localized in Pogynden Formation, while quartz, carbonate–quartz and sulphide–carbonate–quartz veins are mainly confined to the southern and southwestern parts of the territory, with the Netpneyveem Formation deposits serving as ore-bearing. Near-ore alterations are represented by sericite–carbonate metasomatites.

3. Sampling and Analytical Methods

In 2021–2024, the Gremuchinsky group of the North-Eastern Production Geological Association carried out a complex of exploration works within the Upryamoye ore field, including trenching and core drilling. As a result of the work, gold-bearing mineralized zones and orebodies with gold–quartz mineralization were identified and a rich collection of samples was collected.

Petrographic studies of terrigenous rocks hosting gold mineralization were conducted using optical microscopy with polarizing microscopes Olympus BX-51(Olympus Corporation, Tokyo, Japan) and PLM-215 (LOMO-MA LLC, Saint Petersburg, Russia). The compositions of individual components were obtained using the Phenom XL G2 scanning electron microscope (Thermo Fisher Scientific, Eindhoven, the Netherlands) with an energy-dispersive spectrometer. Measurements were performed at an accelerating voltage of 15 kV, a current of 10–20 nA, and a measurement time of 60 s.

The gold mineralization of the Upryamoye field was studied on 150 samples and polished sections using optical microscopy with a PLM-215 polarizing microscope, as well as electron microscopy on a Phenom XL G2 scanning electron microscope (Thermo Fisher Scientific, Eindhoven, The Netherlands). Samples with the highest abundance of ore minerals and high gold content were selected for a detailed compositional study. The compositions of auriferous ore minerals and native gold were analyzed in the mineral analysis laboratory of the Institute of Geology of Ore Deposits, Petrography, Mineralogy and Geochemistry Russian Academy of Sciences using the EPMA method on a JXA-8200 electron microprobe (JEOL Ltd., Tokyo, Japan) equipped with five wave dispersion spectrometers. The analysis was performed at an accelerating voltage of 20 kV, a current of 20 nA, and a probe diameter of 3 µm.

To determine the age of the gold mineralization, Re-Os dating of arsenopyrite was performed at the Isotope Research Center of the A.P. Karpinsky All-Russian Research Geological Institute using the method described in [26]. Five samples of arsenopyrite weighing 0.2–0.3 g were selected for analysis. The concentrations of Re and Os and the ¹⁸⁷Re/¹⁸⁸Os ratio were determined by isotopic dilution using a calibrated isotope ¹⁸⁵Re-¹⁹⁰Os indicator, which was added to the sulphide single fraction before the sample decomposition. The osmium isotopic composition was measured on a TritonTI (Thermo Scientific, Waltham, MA, USA) solid-phase multicollector mass spectrometer. The rhenium isotopic composition was measured on a single-collector ICP-mass spectrometer Element-2 (Thermo Scientific, Waltham, MA, USA).

Recent studies of helium behavior in the crystal lattice of pyrite have shown that pyrite retains helium effectively and can be used as a U-Th/He geochronometer [27,28]. U-Th/He dating of auriferous pyrite was also carried out according to the method described in [27,28] at the Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences (Saint-Petersburg, Russia). Preliminarily, we studied the ore structure in polished sections and selected a sample with pyrite without inclusions of minerals other than gold. Pyrite grains were picked from the same sample under the stereo microscope EMZ-13TR (Meiji Techno Co., Ltd, Iruma-gun Saitama, Japan). For each of 3 measurements, 3–10 cubic-shaped grains of pyrite with the average size ranging from ~200 to 500 µm were used. The total sample weights were of 1–2 mg. The ⁴He contents were measured with a high-sensitivity MSUG-01-M mass spectrometer (CJSC Spektron-Analit, Saint Petersburg, Russia). The measurements of the ²³⁵U/²³⁸U and ²³⁰Th/²³²Th isotope ratios were carried on an ELEMENT XR ICP mass spectrometer (Thermo Fisher Scientific, Bremen, Germany). The accuracy of the complete dating procedure (measurement of He, U and Th) was assessed by simultaneous experiments on a Durango apatite, which is the international standard for the U-Th-He method. The Re-Os and U-Th/He ages of the sulphides were calculated using the IsoplotR program (version 6.7) [29].

4. Results

4.1. Features of the Geological Structure of the Upryamoye Ore Field

Approximately eleven ore-bearing zones have been identified within the Upryamoye ore field, all located in its southern part [30]. Their distribution is controlled primarily by the extent of hydrothermal alterations and, secondarily, by the presence of tectonically weakened zones. The most promising ore-bearing zones are usually associated with areas where hydrothermal alterations intersect with tectonically weakened zones, in particular with folded discontinuous faults.

The results of the work carried out have established that the main ore-bearing zones are confined to two types of ore-bearing structures. The first type includes saddle reefs in folds formed by Late Jurassic–Early Cretaceous sediments. The orebodies are composed of carbonate–quartz and sulphide–carbonate–quartz gold-bearing veins and veinlets that are localized both in the hinge and limbs of folds (Figure 4). They extend up to the first hundred meters, and their thickness varies from 2.4 m to 2.9 m [30].

The second type of ore-bearing structures includes thrust zones. Orebodies in mylonite zones with intensely tectonized vein material and mineralized shatter zones extend for the first few hundred meters, less often about 1 km, and their thickness varies from 1.7 m to 7.9 m [30].

4.2. Petrographic Composition of the Host Rocks

Clastic rocks belong to the Netpneyveem and Pogynden Formations of the Late Jurassic–Early Cretaceous age. Despite the deposition of these formations at the same stage of sedimentation, their sections differ significantly [23].

The Netpneyveem Formation is represented by graded rhythms of sandstones, siltstones, and mudstones with lenses of gravels or small-pebble conglomerates and their volcaniclastics analogs. Thin-bedded turbidites predominate in sequence with a thickness of up to 700 m.

The Pogynden Formation is composed of alternations of amalgamated sandstones and packets of rhythmically interlayered sandstones, siltstones, and mudstones, forming thick-bedded turbidites. In the most complete sections, the thickness of the formation is 750–800 m [18].

The average composition of metamorphosed sandstones and siltstones differs in the selected divisions, as described below for the most representative units.

Sandstones of the Netpneyveem Formation, in terms of the ratio of rock-forming components [31], belong to the feldspar–quartz grauwacke group. They are composed of quartz (40%–60%), plagioclase (10%–15%), rock fragments (27%–50%), and mica mineral flakes (up to 1%). Quartz is predominantly monocrystalline of volcanic origin. Feldspars are represented by tabular-shaped crystal clasts of albite-type plagioclase with polysynthetic twinning; altered, more rounded fragments are also found (Figure 5C). Among the micaceous minerals, almost completely altered elongated biotite flakes, as well as less altered muscovite, have been diagnosed.

Among the lithoclasts, fragments of acidic volcanic rocks predominate (25%–45% of the total number of rock-forming components). They are represented by quartz–feldspar aggregates, often with a felsite structure, and quartz and feldspar phenocrysts are diagnosed in large fragments. The rock also contains volcanic glass of a more basic composition and singular fragments of andesite. Fragments of siliciclastic rocks (2%–5%) are represented by high-carbon argillites, less frequently by siltstones, which are often aligned parallelly to more competent grains (Figure 5D). The rock also contains singular fragments of quartz–mica schists and quartz–feldspar intergrowths (granitic fragments).

Sandstones of the Pogynden Formation, according to classification [31], belong to arkoses. In sandstone varieties, the content of fragments larger than 0.1 mm ranges from 50 to 80%. The rock-forming components are quartz (45%–49%), feldspars (45%–51%), rock fragments (6%–8%), and micaceous minerals (1%–2%).

Monocrystalline quartz predominates, often containing rutile inclusions. Plagioclase occurs in two types: slightly altered, sub-rounded, twinned, euhedral tabular crystals (albite); and more altered, often more rounded crystals with more basic compositions. Muscovite predominates among the micaceous minerals; chloritized biotite flakes are also present.

Among the lithoclasts, volcanic rock fragments predominate (5%), with smaller amounts of quartz–mica and quartz–chlorite schist fragments (1%–2%) and quartz–feldspar intergrowths (1%–2%). The volcanic rock fragments consist of a felsic bulk mass and relics of volcanic glass, which are almost completely replaced.

Secondary alterations in the Netpneyveem and Pogynden Formations deposits are associated with diagenesis, metagenesis, and metasomatism. The following diagenetic and metagenetic transformations can be distinguished.

Plagioclase and the matrix are altered, forming a saussurite aggregate. Sericite occurs along the cleavage planes, contributing to schistosity (foliation) at grain margins, and is also developed within the matrix, feldspars, and rock fragments. Chlorite forms within the matrix and after biotite, and almost completely replaces volcanic glass. It also occurs as larger, newly formed silt-sized flakes, often in packets with hydromica. Two types of secondary carbonates are observed: calcite and higher-iron carbonate. Carbonates form island-type cement and develop along rock fragments, mainly feldspars. Diagenetic sulphide minerals are also present in the rocks, forming microgranular clusters. Primary framboidal pyrite has been identified in high-carbon fine-grained rocks.

The newly formed minerals are represented by kaolinite, quartz, and highly ferruginous carbonate. Kaolinite crystallizes in large pores and quartz–kaolinite–carbonate veins as fan-shaped aggregates and smaller flakes. Newly formed quartz occurs at grain margins and is observed at the contacts between quartz grains and acidic volcanic rocks. Additionally, in rocks of the Netpneyveem Formation, metamorphic nodular rounded aggregates composed of limonitized, high-iron microgranular carbonate with a diameter of 0.05 to 0.2 mm are scattered.

It should be emphasized that mudstones and siltstones are characterized by an increased carbon content, which gives the rocks a characteristic dark gray color.

Siliciclastic rocks are intensely deformed. They clearly show two generations of cleavage: first-generation intergranular cleavage (S1), by pressure dissolution during metagenesis under lithostatic pressure, and a later foliation cleavage (S2), superimposed on the intergranular cleavage after quartzification (Figure 6).

During the final stage of planar microtexture formation, the rock deformation was overprinted by regional greenschist-facies metamorphism of the sericit–chlorite subfacies. Sericite plates grew along the developed microtextures, enhancing the schistosity.

4.3. Vein Paragenetic Associations

Hydrothermal formations are represented by quartz, carbonate–quartz, chlorite–carbonate–quartz, and sulphide–carbonate–quartz veins and veinlets. The following vein paragenetic evolution has been established:

• Quartz veins formed by pressure dissolution and redeposition of material in decompression zones (e.g., quartz vein zones, newly formed quartz in rocks, and quartz replacement veins formed during the late stages of cleavage development). They consist predominantly of milky, fine-grained quartz, with iron hydroxides often developing along microcracks. They are barren;
• Pre-ore chlorite–carbonate–quartz veins. The vein material is coarse-crystalline, milky, white, translucent quartz (70%–80%). Calcite content is variable (5%–20%), and chlorite is present in a finely dispersed state. These veins are unproductive for gold mineralization;
• Carbonate–quartz veins and veinlets. They are composed of 80%–85% milky white coarse- to fine-crystalline quartz and 5%–15% medium- to coarse-crystalline ferruginous, magnesian carbonates, and calcite (Figure 7B). The ferruginous carbonate is cream to brown, with a wedge-shaped habit and optical properties corresponding to ankerite. The large ore-bearing zones of the Upryamoye ore field are associated with these hydrothermal formations;
• Sulphide–carbonate–quartz veins and veinlets. The vein material consists of milky quartz and rock crystal (80%–90% on average, up to 95%). Yellow- and buff-colored carbonates (ankerite occur as small nests, comprising 5%–8%). Sulphide mineralization averages 0.5%–3% and forms nodular, veinlet, and disseminated textures. These are the most productive formations for gold mineralization (Figure 7C).
• Post-ore chlorite–carbonate–quartz veins and veinlets are observed as rare, isolated paragenetic associations that crosscut earlier, productive, and tectonized carbonate–quartz and sulphide–carbonate–quartz veins in mylonite zones (Figure 7D). They are unpromising for gold mineralization.

Microstructures resulting from subsequent deformation are observed throughout the veins, including wavy extinction, deformation lamellae, migrating grain boundaries, and newly formed subgrains. Quartz in ore-bearing veins often contains elongated strips of host rock, marking multiple stages of fracture opening and infill. Such inclusion trails and small lenses of host rock are typically elongated along the fracture walls, forming suture seams within the vein (Figure 8).

4.4. Mineralogical Features of Ores

The study of ore mineralization in sulphide–carbonate–quartz and carbonate–quartz veins and host rocks shows that the ores have predominantly vein and vein-disseminated textures. Sulphides are distributed extremely unevenly. The suite of ore minerals in productive hydrothermal paragenetic associations is diverse: pyrite, arsenopyrite, chalcopyrite, galena, and sphalerite are the most common, while covellite, tetrahedrite, bornite, and native gold are less abundant. Supergene mineralization is represented mainly by iron hydroxides, with subordinate amounts of symplesite, scorodite, covellite, and plumbojarosite.

Mineralogical studies have identified seven sequentially formed mineral associations, described below.

• Pyrite association. Diagenetic pyrite-I forms of cubic and pentagonal dodecahedral metacrystals in siltstones and sandstones, ranging in size from 1 × 1 mm to 5 × 5 mm (Figure 9A). These metacrystals and intergrowths are surrounded by quartz–carbonate rims, characteristic of sulphides in greenschist-facies metamorphic rocks. Rare, small inclusions of sphalerite and chalcopyrite (up to 100 µm) are found within pyrite crystals.
• Chalcopyrite–sphalerite–galena–pyrite association (Figure 9B). Pyrite-II forms granular masses as aggregates, lenses, and interlayers up to 0.3 mm, as well as veins up to 2 cm in siltstones. Chalcopyrite and sphalerite occur as small inclusions in pyrite aggregates. In the intergranular space of boudinaged pyrite-II nests, xenomorphic, vein-like inclusions of chalcopyrite, sphalerite, and galena (up to 100 µm) are observed.
• Gold–pyrite–arsenopyrite association. Pyrite-IIIa forms large cataclastic aggregates up to 0.5 cm, sometimes containing relict arsenopyrite-I crystals up to 250 µm (Figure 9C). Smaller pyrite-IIIb crystals of pentagonal dodecahedral habit are also present, not exceeding 300 µm. Arsenopyrite is non-stoichiometric, a sulphur-rich variety with an As/S ratio of 0.77 to 0.89. It contains Ni (0.01–0.05 wt.%), Co (0.02–0.08 wt.%), Cu (up to 0.05 wt.%), and Sb (0.02–0.25 wt.%) impurities. Pyrite is nearly stoichiometric, containing As (0.56–1.14 wt.%) and Co (up to 0.06 wt.%) impurities, with low concentrations of Cu, Ni, and Sb. Native gold occurs as inclusions (5–25 µm) in both pyrite-III types and in arsenopyrite-I, with a fineness of 876–945‰ (Figure 10A).
• Gold–pyrite association with galena–tetrahedrite–sphalerite–chalcopyrite–arsenopyrite association. Pyrite-IVa forms large subhedral cubic crystals up to 800 µm in size, containing vein-like and droplet-shaped inclusions of native gold up to 150 µm, with a fineness of 776–850‰. This pyrite is stoichiometric, containing up to 1.74 wt.% As and up to 0.99 wt.% Co. Notably, the arsenic-bearing pyrite has an elevated gold content. Highly fractured, wedge-shaped, dipyramidal arsenopyrite-II crystals (200–500 µm) are also observed (Figure 9D). This arsenopyrite is non-stoichiometric and sulphur-rich (As/S = 0.74–0.86), with Ni (0.01–0.73 wt.%), Co (<0.25 wt.%), and Sb (up to 0.26 wt.%). Cu and Zn concentrations are low. Xenomorphic inclusions of chalcopyrite (up to 500 µm), sphalerite, and tetrahedrite (up to 250 µm) are also present.
• Gold–pyrite–arsenopyrite association. Arsenopyrite-III occurs as dipyramidal and short prismatic crystals up to 0.4 mm, and as xenomorphic grains, intergrowths, and veins up to 8 mm thick (Figure 9E). Pyrite-IVa forms subhedral cubic crystals up to 0.2 mm and fine pentagonal dodecahedral inclusions (pyrite-IVb). Native gold occurs as single, finely dispersed segregations, droplet-like inclusions, and metacrystals in arsenopyrite-III, not exceeding 50 µm (Figure 10). Gold fineness is 944–946‰.
• Pyrite–tetrahedrite–chalcopyrite–galena–sphalerite association (Figure 9F). Sphalerite forms large aggregates and nests up to 2 mm, containing emulsion inclusions of chalcopyrite. Galena forms xenomorphic grains up to 500 µm and larger aggregates, veins up to 2 mm. Chalcopyrite forms xenomorphic inclusions up to 250 µm in contact with galena. Tetrahedrite occurs on the periphery of sphalerite aggregates as xenomorphic aggregates up to 200 µm. Pyrite is subordinate, forming xenomorphic grains (50–100 µm, rarely up to 700 µm), often associated with sphalerite. Bournonite is present as relic xenomorphic grains ~300 µm in size.
• Supergene mineral association, consisting of iron hydroxides, hydroarsenates, covellite, and other minerals that replace primary and secondary sulphides.

The analyzed compositions of ore minerals and native gold are presented in Tables S1–S3. The content of typomorphic impurities in pyrite and arsenopyrite analyzed by EPMA is shown in Figure 11.

The results of studying the structural characteristics and mineral associations in the ores have enabled the identification of three stages of the mineral-forming process, divided into six phases (Figure 12).

4.5. Results of Ore Mineralization Dating

To determine the age of gold mineralization, we conducted isotope dating of auriferous pyrite-III and arsenopyrite-II from the productive gold–pyrite–arsenopyrite association.

The U-Th/He isotope system was studied in three analytical samples of pyrite. The helium content in pyrite ranges from 2.83 to 5.19 cm³/g, Th/U ranges from 2.35 to 6.43 (

Table 1. ⁴He, U, Th concentrations and U–Th/He isotope age of pyrite-III from Upryamoye ore field.

ID	He, 1010 at	1σ	U, 1010 at	1σ	Th, 1010 at	1σ	Th/U	U-Th/He, Ma	±(2σ)
pyr 1495	51.9	1.1	129.5	3.0	832.8	14.3	6.43	125	6
pyr 1499	28.3	0.5	116.1	3.5	273.4	6.8	2.35	122	6
pyr 1565	38.8	0.9	139.1	5.0	432.5	26.7	3.11	125	8

). Based on the results of pyrite-III dating, a U-Th/He age of 123 ± 4 Ma was obtained (Figure 13A).

The Re and Os contents in the analyzed arsenopyrite-II grains vary significantly. Re concentration ranges from 2.8 to 8.1 ppb, while Os concentration reaches 1229.1 ppb (minimum 299.8 ppb) (

Table 2. Re–Os concentrations and isotope compositions of arsenopyrite-II from Upryamoye ore field.

Sample No.	Re, ppb	Os, ppb	187Re/188Os	187Os/188Os	Rho
K4-593.4	8.06	0.05	1229.09	4.97	0.01
K4-805	2.82	0.04	601.90	4.77	0.01
K8-233	4.97	0.09	339.57	2.41	0.004
C20-117.4	5.16	0.07	458.98	2.27	0.003
C22-74.5	2.87	0.06	299.88	2.58	0.002

). The Re-Os isotope data did not define a single isochron (Figure 13B). The calculated age of 159 ± 29 Ma with the initial isotopic ratio (¹⁸⁷Os/¹⁸⁸Os)₀ = 1.61 ± 0.23 is inconsistent with direct geological observations of the structural and textural features of orebodies, as it is older than the Late Jurassic–Early Cretaceous host rocks (133–147 Ma) [32].

5. Discussion

5.1. The Age and Genesis of Gold–Quartz Mineralization

It is traditionally accepted that orogenic gold–quartz deposits are genetically and spatially associated with transcrustal fault zones and granitic magmatism [33,34,35,36]. Despite the presence of widespread Late Cretaceous Ichuveem complex dikes and a presumed unexposed Early Cretaceous granitoid intrusion in the Upryamoye ore field, a direct connection between mineralization and these magmatic bodies has not been established.

Previous work [30] revealed that all productive ore-bearing zones are located south of the dike swarm, at a considerable distance. Our isotopic studies show a U-Th/He age for auriferous pyrite of 123 ± 4 Ma, corresponding to the Barremian–Aptian boundary. Consequently, gold mineralization in the Upryamoye ore field is not related to the emplacement of Late Cretaceous dikes and post-collisional intrusions.

The Re-Os dating of arsenopyrite-II yielded an isochron age of 159 ± 29 Ma. The ages of the Netpneyveem and Pogynden Formation terrigenous rocks are 147–142 Ma and 140–133 Ma, respectively [32]. Therefore, the ore mineralization age cannot be older than the host sequence. Within error, the youngest possible age is 130 Ma (Hauterivian–Barremian boundary), corresponding to the initial formation of fold–thrust structures in the Chukotka region [6,7].

The initial osmium isotope ratio calculated from the obtained isochron is (¹⁸⁷Os/¹⁸⁸ s)₀ = 1.61 ± 0.23 This value differs significantly from the upper mantle composition (0.1296 [37]) and falls within the range for crustal rocks (>1 [38]), indicating a crustal source for the Os. Consequently, the Re-Os isotopic age lacks direct geochronological significance but does suggest a link between mineralization and tectonic activity in the Chukotka region.

The origin and ore-forming conditions of the ore mineralization can also be inferred from the elemental composition of pyrite. The Co/Ni ratio in pyrite varies from 0.94 to 37, which is consistent with both hydrothermal and magmatic sources (Figure 14A) [39]. Furthermore, according to [40], the contents of As, Co, and Ni in pyrite indicate a mixed magmatic–hydrothermal origin (Figure 14B). Thus, the Re-Os isotopic composition of arsenopyrite and the elemental composition of pyrite suggest that the ore-forming materials were derived from a mixed crust–mantle source, with the dominant contribution from the lower crust. However, more detailed studies are required to fully constrain the genesis of the ore minerals and gold.

5.2. Main Mineralization Stages

The most productive mineralization is localized within the fine-grained deposits of the Netpneyveem Formation, which are characterized by high carbon content, poor to moderate sorting, abundant volcanic rock fragments, volcanic quartz and feldspar crystalloclasts, and a matrix with a component of cryptical pyroclastic material. In contrast, the Pogynden Formation exhibits moderate to good sorting, a predominance of feldspars, the presence of potassium feldspar, and numerous accessory minerals, including large apatite grains. This formation shows brittle deformations, with isolated orebodies represented by quartz breccias.

Based on the study of structural–geological characteristics and mineral associations, three main stages of the ore-forming process, divided into six phases, have been identified (Figure 12).

The earliest stage of mineralization is closely linked to late metamorphic processes in the host rocks. The initial phase involved the sulfidation of siltstones, resulting in abundant pyrite inclusions. The subsequent phase was marked by metamorphic activity, which recrystallized and redeposited the earlier pyrite into sulphide lenses and interlayers within the siltstones. This phase is also characterized by boudinaged sulphide pockets and veins, predominantly of pyrite composition, which replaced the margins of the earlier pyrite. The inter-boudin spaces are filled with sulphide–quartz material.

The second stage represents the main hydrothermal ore-forming process. The fourth phase involved the formation of chlorite–carbonate–quartz veins with weak sulphide mineralization. The fifth phase, which is the most productive for gold–quartz mineralization, saw the emplacement of carbonate–quartz and sulphide–carbonate–quartz veins and veinlets. This was followed by the sixth phase, during which late chlorite–carbonate–quartz veins formed.

During the final supergene stage, the earlier formations underwent weathering, with oxidation being the most widespread process.

Thus, the results demonstrate a prolonged, multi-stage mineralization history within the Upryamoye ore field.

5.3. Structural Evolution of the Upryamoye Ore Field

The ore-controlling structures of the Upryamoye ore field most likely formed during the final stage of the collision between the Chukotka microcontinent and the active Siberian margin (Figure 15). Compressive stresses deformed the Late Jurassic–Early Cretaceous rocks, forming NW-trending fold-and-thrust structures [41]. A subsequent change in the tectonic regime from compression to extension established an ore-bearing fault system. During the late syntectonic stage, ore-bearing veins were emplaced into the metamorphosed rocks. The orogenic ore-forming process at Upryamoye ore field therefore occurred prior to the Early Cretaceous granitoid magmatism of the post-collisional stage in Chukotka (117–105 Ma [8]).

The Upryamoye ore field exhibits a complex ore body structure and mineral composition. Its tectonic setting, geological position, structural control, and mineralogical characteristics are similar to many other orogenic gold–quartz deposits, allowing its classification as an orogenic type [14,15,33,35]. This interpretation is supported by the following factors:

• metallogenetic processes occurred in a post-collisional environment (after the Hauterivian–Barremian) [6,7];
• spatial association with a fold structure (the Peristaya anticline);
• structural control by faults (saddle reef veins, shatter zones);
• host rocks that were deformed and subjected to regional greenschist-facies metamorphism, developing schistosity, cleavage, quartz recrystallization, and secondary chlorite and albite;
• emplacement of quartz, carbonate–quartz, and sulphide veins into metamorphosed rocks;
• presence of secondary alterations (sericitization, carbonatization, kaolinitization, sulphidation);
• low ow sulphide mineral content (1%–3%), classifying it as a low-sulphide type;
• extremely uneven distribution of gold within vein zones.

Similar characteristics are observed in numerous orogenic deposits worldwide [33,34,35,36]. A Russian analog is the Sovinoe deposit, located within the Pilkhinkul–Riveemsky gold node. Research [8] indicates that ore deposition at Sovinoe occurred during long-term tectonic movements involving gold-bearing fluids at the stage of orogen activation.

The Reefton gold field in New Zealand [43,44] also displays comparable features. The main mineralization stage at Reefton occurred in the latter stages of greenschist-facies metamorphism, with early structures and textures reflecting a transition from metamorphic to hydrothermal processes alongside a shift from ductile to brittle deformation.

6. Conclusions

The mineralization of the Upryamoye ore field is a typical example of an orogenic, low-sulphide, gold–quartz deposit of metamorphic genetic type. The ore occurrences are confined to the NW-trending Peristaya anticline, which is composed of Late Jurassic–Early Cretaceous deposits of the Netpneyveem and Pogynden Formations. These rocks are intensely deformed, metamorphosed under greenschist-facies conditions, and affected by secondary alterations (carbonatization, sericitization, kaolinitization, sulfidation, carbonization). The ore-bearing structures formed during the transition from a transpressional to a transtensional regime following the collision events of the Chukotka orogeny and comprise systems of thrusts and mylonites containing intensely tectonized hydrothermal vein material.

The orebodies are confined to saddle reef carbonate–quartz veins and mineralized shatter zones. The low sulphide mineral content (not exceeding 3%–5%) and the presence of native gold in quartz allow the mineralization to be classified as a low-sulphide, gold–quartz type of orogenic genesis.

The main ore minerals associated with gold mineralization are arsenopyrite and pyrite, with subordinate galena, sphalerite, chalcopyrite, tetrahedrite, and bournonite. The productive mineral associations are the early gold–pyrite–arsenopyrite, the gold–pyrite with galena–tetrahedrite–sphalerite–chalcopyrite–arsenopyrite, and the late gold–pyrite–arsenopyrite associations. Native gold is the sole concentrator of gold, occurring as inclusions in pyrite and arsenopyrite. The gold forms fine, dispersed inclusions, rendering the ores refractory. The fineness of native gold ranges from 776 to 945‰, with an average of 892‰ based on 26 analyses.

The U-Th/He isotopic age of the gold–quartz mineralization at Upryamoye is 123 ± 4 Ma. This represents the first direct dating of orogenic mineralization in Chukotka that clearly predates the main phase of post-collisional granite intrusion.

Based on the study of structural–textural characteristics and mineral composition, six successive phases of ore-forming process have been identified. The most productive are the third and fourth phases, associated with the emplacement of chlorite–carbonate–quartz, carbonate–quartz, and sulphide–carbonate–quartz veins and veinlets.

In conclusion, this research provides new data on the geological structure, material composition, morphology, and metallogenetic process conditions of the gold mineralization in the Upryamoye ore field. These findings confirm the area’s high prospectivity for further exploration.