Disseminated Gold–Sulfide Mineralization in Metasomatites of the Khangalas Deposit, Yana–Kolyma Metallogenic Belt (Northeast Russia): Analysis of the Texture, Geochemistry, and S Isotopic Composition of Pyrite and Arsenopyrite

: At the orogenic gold deposits of the Yana–Kolyma metallogenic belt (northeast Russia) both Au–quartz ‐ sulfide mineralization with native gold and disseminated sulfide mineralization with invisible Au developed. The textural and mineralogical ‐ geochemical features, isotope ‐ geo ‐ chemical characteristics of gold ‐ bearing sulfides from proximal metasomatites, and possible forms of Au occurrence in pyrite and arsenopyrite have been studied using electron microprobe, atomic absorption, LA ‐ ICP ‐ MS trace element, isotope analysis, and computed microtomography. Four gen ‐ erations of pyrite (Py1, diagenetic; Py2, metamorphic; Py3, metasomatic; Py4, veined) and two gen ‐ erations of arsenopyrite (Apy1, metasomatic; Apy2, veined) have been identified at the Khangalas deposit. In the proximal metasomatites, the most common are Py3 and Apy1. Studying their chem ‐ ical composition makes it possible to identify the features of the distribution patterns of typochem ‐ ical trace elements in pyrite and arsenopyrite, and to establish the nature of the relationship between Au and these elements. In Py3 and Apy1, structurally bound (solid solution) Au + prevails, isomor ‐ phically entering the crystal lattice or its defects. Isotope characteristics of hydrothermal sulfides (   S = –2.0 to –0.6‰) indicate that mantle/magmatic sulfur was involved in the formation of the deposit, though the participation of sulfur from the host rocks of the Verkhoyansk clastic complex cannot be ruled out. The Khangalas deposit has much in common with other gold deposits of the Yana–Kolyma metallogenic belt, and from this point of view, the results obtained will help to better reveal their gold potential and understand their origin.


Introduction
The Late Jurassic-early Early Cretaceous Yana-Kolyma metallogenic belt (YKMB) is located in the central part of the Verkhoyansk-Kolyma folded region, in the boundary zone between the northeastern margin of the Siberian craton and the Kolyma-Omolon superterrane.Within the belt, gold deposits are concentrated in three sectors: Upper Kolyma, Upper Indigirka, and Adycha.The length of the belt is approximately 1000 km and the width is approximately 200 km.This is the richest gold province in northeast Russia, including the large Natalka (1500 t Au) and Degdekan (400 t Au) deposits.It is of global economic importance.Production of gold from the Yana-Kolyma belt since the 1930s was ~3200 t, with current estimated resources of about 5000 t Au [1,2].This is comparable with the output of major Paleozoic-Mesozoic gold provinces of the world (Jiaodong, China; Juneau gold belt, AK, USA; Lachlan fold belt, Australia; Baikal fold belt, Russia; and Southern Tien Shan, Uzbekistan) [3].To date, the areas with the most economic value within the YKMB are orogenic gold deposits with Au-quartz vein and/or Au-sulfidequartz veinlet-disseminated types of mineralization.The origin and conditions of the localization of the first mineralization type have been much studied (e.g., [4][5][6][7][8][9] and many others).Recent research has shown great economic value of the insufficiently studied disseminated gold-bearing pyrite and arsenopyrite mineralization in the YKMB (see [10][11][12][13][14][15] and references therein).Disseminated sulfide mineralization is known both within the gold deposits [10,11,[15][16][17][18][19]] and in regional sulfidation zones [17,[20][21][22] localized in transcrustal faults [12,23].
This work presents the mineralogy, geochemistry, isotopic composition, and gold content of disseminated mineralization in clastic rocks of the Khangalas orogenic gold deposit located in the Upper Indigirka sector of the YKMB [24,25].In early research of the geology and ore content of the deposit, attention was focused on Au-quartz mineralization [26][27][28], its supergene alteration [28][29][30], and placer-to-primary source relationships [31].In recent years, extensive prospecting work has been carried out within the Khangalas ore cluster.A large number of surface and underground mined workings have been completed.This made it possible to obtain new data on the mineralogy and geochemistry of the Khangalas deposit.Despite the fact that the current estimated reserves and resources of gold at the deposit are small (11 tonnes of Au, with a high average grade of 11.2 g/t Au [32]), the presence of gold-bearing pyrite and arsenopyrite in the wall-rock metasomatites is evidence that the total contained tonnage may be much higher.The Khangalas deposit has much in common with other gold deposits of the YKMB.From this standpoint, the findings presented here will help geologists and prospectors evaluate the overall potential gold resources of the region.

Regional Tectonic Framework
The Khangalas deposit is located in the central part of the Yana-Kolyma metallogenic belt, which includes the Kular-Nera and the more easterly Polousny-Debin terranes, as well as the eastern part of the Verkhoyansk fold-and-thrust belt (Figure 1).The Kular-Nera terrane consists of Upper Permian, Triassic, and Lower Jurassic clastic sedimentary rock sequences, metamorphosed to initial stages of greenschist facies, and the Polousny-Debin terrane is composed mainly of Upper Triassic-Upper Jurassic turbidites.The terranes are separated by the Charky-Indigirka and Chai-Yureya faults.To the southwest, the regional-scale Adycha-Taryn fault separates the Kular-Nera terrane from the Verkhoyansk fold-and-thrust belt made of Mesoproterozoic-Lower Carboniferous carbonate-clastic and carbonate rocks and Upper Paleozoic-Mesozoic clastic rocks of the passive continental margin of the Siberian craton.The structural pattern of the Yana-Kolyma metallogenic belt is defined by linear folds and faults of NW strike, manifesting several deformation stages [9,33].
Magmatism in the Upper Indigirka sector is manifested by granitoids of intermediate to felsic intrusions and volcanics of the Tas-Kystabyt magmatic belt (J3-K1).Also present are mafic (162 ± 4 Ma, whole rock, Rb-Sr [34]), intermediate, and felsic dikes of the Nera-Bohapcha complex (151-145 Ma, U-Pb SHRIMP-II, zircons [23]).According to Parfenov et al. [35] and Parfenov and Kuzmin [33], the emplacement of the Late Jurassic granitoids was related to collision events.More recent data indicate that subduction processes were involved in their formation [36,37].Tectonic structures, magmatism, and ore deposits of the YKMB were closely related to the Late Jurassic to earliest Early Cretaceous subduction, accretionary events at the eastern active continental margin of the Siberian craton [33].

Structures and Host Rocks
The geology and structural-tectonic conditions of the formation and localization of the Khangalas deposit are described in detail in [25].Here we give only a brief summary of the necessary information.Extraction of alluvial gold started in 1933, when the Khangalas Creek placers were first discovered.Later on, in 1947, the Khangalas lode gold deposit was discovered, and production of gold from quartz veins with pyrite, arsenopyrite, galena, sphalerite, and chalcopyrite began.The veins occur mostly at the border between sandy and shaly intervals, due to competency contrast.The deposit is localized in the crest of the Dvoinaya anticline of the Nera (Nera-Omchug) anticlinorium (Figure 1).The core of the anticline is composed of Upper Permian (Lopingian, P3) deposits.In the lower part of the section, with a thickness of more than 350 m, these are mainly massive brownishgray and gray sandstones with thin siltstone interbeds.The upper part is dominated by a more than 450 m thick sequence of dark gray to black siltstones with included pebbles of sedimentary, igneous, and metamorphic rocks.The limbs of the anticline are made of Lower Triassic (T1) deposits (mainly dark gray shales, mudstones, and siltstones with rare interlayers of light gray sandstones with a thickness of 680-750 m) and Middle Triassic (T2a) sediments of the Anisian stage (alternating sandstones and siltstones with a thickness of 700-800 m).The Ladinian strata (T2l) consist of interbedded siltstones and sandstones with a total thickness of 850-950 m.The main ore-controlling tectonic structure is the Khangalas fault with a NW strike.This is represented at the Khangalas deposit by five extensive (up to 1400 m) mineralized ore zones (Severnaya, Promezhutochnaya, Centralnaya, Yuzhnaya, Zimnyaya) with low-sulfidation Au-type mineralization localized in the Dvoinaya anticline crest (Figures 1 and 2A).The ore zones are up to 32 m thick and dip to the SW, S, and SE at 30°-50° to 70°-80° [25].No evidence of magmatic activity is observed within the Khangalas deposit.Geophysical data suggest the presence of a granitoid pluton at depth [38].The mineralization formed as a result of progressive fold-and-thrust deformations in the Verkhoyansk-Kolyma fold belt.These were initiated by orogenic processes in late Late Jurassic-early Early Cretaceous [25].

Mineralization
Gold mineralization in the Khangalas deposit is present as two types: Au-quartzsulfide veins with native gold and sulfide-quartz disseminations with invisible gold hosted in quartz-sericite-carbonate metasomatites (Figure 3).The Au-quartz-sulfide mineralization is characterized as concordant and discordant quartz veins 0.1-1 m thick in swells up to 5 m (Figures 2 and 3B,C).The content of sulfides reaches 1-3 vol.%.The main ore minerals are pyrite and arsenopyrite, with less common galena, sphalerite, and chalcopyrite.Accessory minerals include freibergite, boulangerite, tetrahedrite, Fe-gersdorffite, acanthite, and native gold.The veinlet-disseminated sulfide-quartz mineralization with invisible gold is localized both in the ore zones and in their walls (Figure 3D,E).The sulfide content reaches 3-3.5 vol.%, and pyrite and arsenopyrite predominate.Galena, sphalerite, chalcopyrite, native gold, and freibergite are rare.Gold content in the metasomatites varies from <1 ppm up to 5.29 ppm Au, with an average of 0.73 ppm.At the Khangalas deposit, four stages of mineral formation are distinguished: two pre-ore preparatory (sedimentary and metamorphic), ore (hydrothermal), and post-ore (supergene) (Figure 4).During the sedimentary stage, accumulation of clastic material, a change in redox potential, mobilization of ore matter, and formation of diagenetic sulfide mineralization (Py1) took place.The metamorphic stage was marked by alteration of clastic rocks as a result of regional metamorphism and deformation processes, disseminated sulfide mineralization (Py2), and sericitization and chloritization of clastic rocks.The hydrothermal stage is characterized by four successive paragenetic associations: 1.The metasomatic quartz-pyrite-arsenopyrite association is localized in quartz veinlets within the wall rock with a quartz-carbonate-sericite matrix.The main ore minerals are auriferous pyrite (Py3) and arsenopyrite (Apy1), which occur as individual crystals, intergrowths, small aggregates ranging in size from fractions of a millimeter to 2-3 mm, and veins up to a few mm thick.Also observed are microcrystals and microaggregates of high-Co arsenopyrite (danaite), ranging in size from tens to 100-200 microns.Native gold is present here in an invisible, finely dispersed form.It forms very thin films and nanoinclusions in pyrite and arsenopyrite, and also occurs as an isomorphic impurity.  Sulfates: gypsum, jarosite, mangazeite, amarantite, meta-aluminite, meta-alunogen, tamarugite, pickeringite, unknown hydrous iron sulfate [40]. 2 Arsenates: scorodite, kankite, bukovskiite, unknown Fe-Al arsenate-phosphate-sulfate [41].
2. The vein quartz-pyrite-arsenopyrite association is present mainly in the ore bodies, composed of coarse and medium crystalline anhedral quartz.Pyrite (Py4) and arsenopyrite (Apy2) occur as aggregates up to 3-4 cm in size, veinlets up to 1 mm thick, and individual crystals up to 1-2 mm long (Figure 3A).
3. Au-Qz-polysulfide association is represented by small aggregates and microveinlets of sphalerite, chalcopyrite, and galena, and by native gold segregations (Figure 3B,C).They fill voids in quartz and cracks in pyrite and arsenopyrite of early associations.Native gold is elongated, flattened, and dendritic in form (Figure 3D).The size ranges from small to large (most common fractions are 0.5-0.8mm) and the distribution is extremely uneven.The fineness of native gold averages 820‰-830‰ (min 780‰-max 850‰).
4. The quartz-carbonate-sulfosalt association is localized in cracks and voids of the early ore and vein minerals in the form of aggregates and individual grains.The principal mineral type is carbonate, represented by ankerite and ankerite-dolomite.Sulfosalts are sporadic and mainly include freibergite, tetrahedrite, and boulangerite.
The post-ore stage is characterized by supergene minerals.This is one of the characteristic features of the Khangalas deposit [29,40,42].They are expressed as an oxidation zone with the formation of sulfates, arsenates, iron oxides, clay, and other minerals, extending to a depth of 50-100 m (Figure 2F).The authors have previously identified two unknown minerals from the oxidation zone: hydrous iron sulfate [40] and Fe-Al arsenatephosphate-sulfate [41].

Materials and Methods
Samples for mineralogical-geochemical and isotope-geochemical studies were collected from natural outcrops and the walls and dumps of surface and underground mined workings.For mineralogical and geochemical studies of disseminated sulfide mineralization, thick polished sections (40 in total) and epoxy-mounted grains (90 sulfide grains in 8 mounts) were prepared.The textural and structural features of the ores were studied using a Karl Zeiss Axio M1 optical microscope.The qualitative chemical and mineral compositions of the samples were studied with the use of a JEOL JSM-6480LV scanning electron microscope equipped with an Energy 350 Oxford energy dispersive spectrometer (20 kV, 1 nA, beam diameter 1 μm) (analysts S.K. Popova and N.I.Khristoforova, Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk).
The major element compositions of pyrite and arsenopyrite were determined by standard X-ray spectral analysis on a Camebax-Micro microanalyzer (analyst N.V. Khristoforova, Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk).The analytical conditions were as follows: accelerating voltage of 20 kV; beam current of 25 nA; measurement time of 10 s; K series for Fe, Co, Ni, Cu and S; M series for Au and Pb; L series for As and Sb.; and wavelength-dispersive spectrometer (WDS) with LiF, PET, and TAP crystals.The standards used were: FeS2 for Fe and S, FeAsS for As, Fe-Ni-Co alloy for Co, Ni, Au-Ag alloy of fineness for Au and Ag, CuSbS2 for Sb, and PbS for Pb.The detection limits 0.01%.Trace elements in pyrite were studied on 9 grains of pyrite-3 and arsenopyrite-1 using a New Wave Research UP-213 laser ablation system (USA) coupled with an Agilent 7700x quadrupole mass spectrometer (Agilent Technologies, Santa Clara, CA, USA) (analyst D.A. Artemiev, Institute of Mineralogy, Ural Branch of the Russian Academy of Sciences, Miass).The measurements were carried out using a 213 nm Nd:YAG UV laser with fluence set at 1.8-5.5 J/cm 2 (1.8-3.0J/cm 2 for pyrite, 3.0-4.5J/cm 2 arsenopyrite) and a rate of flow of He carrier gas at 0.5-0.65 L/min.Mass spectrometer settings were: RF power 1550 W, carrier gas Ar, flow rate 0.85-0.95L/min, plasma-forming gas (Ar) flow rate 15 L/min, and auxiliary gas (Ar) flow rate 0.9 L/min.Data were acquired by singe spot and line analyses using a laser spot diameter of 25 to 80 μm and a frequency of 5-10 Hz.The analysis time for each sample was 90 s, comprising a 30 s measurement of background (laser off) and a 60 s analysis with laser on.Pre-ablation was performed for 3-4 s before each analysis.Between the analyses, and between analysis and pre-ablation, blowing with gas was done for 60-90 s.
The mass spectrometer was calibrated using calibration multi-element solutions and the NIST SRM-612 reference material.The amount of molecular oxides ( 232 Th 16 O/ 232 Th) was kept below 0.2%.The 238 U/ 232 Th ratio, when adjusted according to NIST SRM-612, was 1:1.External calibration standards USGS MASS-1 [43] and UQAC FeS-1 were used to analyze every 7-13 spots to account for drifting of the laser and mass spectrometer.Mass contents of elements for NIST SRM-612 and USGS MASS-1 were taken from the GeoReM database.Data processing and calculation were carried out using the Iolite software package [44].As internal standard (IS) for pyrite, we used 57 Fe measured by SEM-EMF.In some cases, normalization to 100% of the total components was performed according to standard techniques [45].
The Au and Ag contents were determined on powdery monomineral samples by atomic absorption spectrometry with electrothermal atomization on a Thermo Scientific iCE 3500 spectrometer (analysts A.E. Sannikova, E.L. Naryshkina, and E.I. Mikhailov, Diamond and Precious Metal Geology Institute, Siberian Branch, Russian Academy of Sciences, Yakutsk).Au and Ag content of more than 2 ppm was determined on an Agilent 4200 MP-AES atomic emission spectrometer with microwave-saturated plasma.X-ray computed microtomography was used to study the internal structure of minerals and identify high-density phases in sulfide grains; this non-destructive method makes it possible to visualize in 3D the distribution of phases with different density in the sample [46][47][48][49][50][51][52][53], etc. Microtomographic studies were carried out at the Centre for X-ray Diffraction Studies of the Research Park of St. Petersburg State University (analyst L.Yu.Kryuchkova).Samples of pyrite and arsenopyrite (31 in total) were studied by x-ray computed microtomography.Monomineral fractions 0.25-1.0mm in size were hand-picked under a binocular microscope.The studies were carried out on a SkyScan-1172 microtomography scanner (Bruker microCT, Belgium).To reconstruct the array of shadow images, NRecon software (Bruker microCT) was used, which allows leveling instrument artifacts and setting the range of gray gradations corresponding to the value of x-ray absorption and, accordingly, x-ray density.To analyze the obtained microtomographic data, DataViewer and CTVox software (Bruker microCT) were used.For sulfur isotope analysis we used monosulfide fractions (5 samples) selected by hand and ground into powder.The analysis was performed in the Laboratory of Stable Isotopes of the Far Eastern Geological Institute, Far Eastern Branch of the Russian Academy of Sciences (Vladivostok; analyst T.A. Velivetskaya) using a Flash EA-1112 elemental analyzer (Thermo Scientific, Germany) in the S configuration according to the standard protocol for converting sulfur from sulfide to SO2.The 34 S/ 32 S isotope ratios were measured on a MAT-253 mass spectrometer (Thermo Scientific, Waltham, MA, USA) in continuous He flux mode.The measurements were performed against a standard laboratory gas SО2 calibrated according to international standards IAEA-S-1, IAEA-S-2, IAEA-S-3, and NBS-127.The results of δ 34 S measurements are given in reference to the international VCDT standard.

Pyrite and Arsenopyrite Types and Textures
At the Khangalas deposit, four generations of pyrite and two generations of arsenopyrite are established.The metasomatic quartz-pyrite-arsenopyrite association forms the basis of the veindisseminated mineralization type with invisible gold in the wall rock metasomatites.Metasomatic Py3 and Apy1 occur as individual crystals, intergrowths, small aggregates, and veinlets up to a few mm thick in thin quartz veins (Figure 6).Apy1 is characterized by short prismatic to pseudo-pyramidal crystal shapes (Figure 6D,E, insets), while Py3 features complicated cubic shapes up to pyritohedra (Figure 6A, inset).The crystals size to 1-1.5 mm, less often to 2-3 mm.Py3 and Apy1 contain superimposed microinclusions of minerals of polysulfide and sulfosalt-carbonate associations (Figure 6F-H).In ~20% of Py3 and ~12% of Apy1 grains, inclusions of galena and rare inclusions of sphalerite and chalcopyrite (Figure 6F-I) were found.Microinclusions of tetrahedrite and freibergite were recorded in single samples.In only one sample, K-4-17, in interstices between crystals of arsenopyrite Apy1 and pyrite Py3, we found native gold about 15 μm in size with a fineness of 827‰ (Figure 6G).
X-ray computed microtomography (3D) of arsenopyrite (Apy1) and pyrite (Py3) grains from two samples collected from the Khangalas deposit showed that they consist of x-ray contrasting phases (Figure 7).Dense minerals are light to white in color, have isometric, subisometric, and flattened shapes, and are up to 10-15 μm in size in Apy1 (Figure 7A) and from a few to 30-40 μm in Py3 (Figure 7C,D).They form an impregnated texture.One can observe a linear-planar distribution of dense phases (or aggregates), probably confined to the defect and crystal growth zones.Uniform arsenopyrite grains are noted (Figure 7B).Native gold and galena are denser than pyrite and arsenopyrite.The veined quartz-pyrite-arsenopyrite association forms the basis of the vein ore bodies of the deposit (Figure 8).Arsenopyrite Apy2 and pyrite Py4 crystallized simultaneously with quartz, and they occur as scattered small euhedral and anhedral grains and nests up to 1-2 cm in size, less often up to 3-5 cm, as well as veinlets oriented along carbonaceous bands in quartz (Figure 8A,B).Galena, chalcopyrite, sphalerite, and native gold form microinclusions in Apy2, Py4, and quartz crystals, and are confined to their fractures and growth zones (Figure 8C,D).

EPMA Results: Major and Minor Elements CAPS
The results of electron probe micro-analyzer (EPMA) spot analysis of the different varieties of pyrite are given in Table 1.All types of pyrite contain As, Co, Ni, Cu, and Sb impurities, the most abundant of which is As.In Py1 and Py2, the total content of minor elements varies from 0.04 to 0.8 wt.%.The As concentrations are within 0.01-0.31wt.%, making up 30-70% of the total.About 10% of the analyzed pyrite grains refer to the arsenic-free variety because their As values are below the detection limit of the probe.Py1 and Py2 have permanent minor elements of Co (coefficient of variation (CV) = 48%) and Ni (CV = 104%).Their contents range as follows: 0.02-0.2wt.% Co and 0.02-0.33wt.% Ni (Figure 9A,B).The Co/Ni ratio varies from 0.2 to 18.5; 70% of the analyzed grains have СCo > CNi, which are characterized by a strong correlation (r = 0.74).Copper constitutes 5-6% of the total amount of trace elements in Py1 and Py2 (0.02-0.11 wt.% Cu), and its content is variable, even within the same crystal.Another constant but quantitatively insignificant minor element in Py1 and Py2 is Sb (0.03-0.1 wt.% Sb).Correlation analysis revealed a Co-Ni-Pb geochemical association in Py1.The empirical formula of sedimentary and metamorphic pyrite is Fe0.96-1.04Ni0.0-0.01S2.00(Ni is present in 18% of the analyzed grains).In Py3, the total amount of minor elements varies from 0.38 to 3.27%, with the As proportion higher than 75%.The As content in 47% of the analyzed grains is 1-3 wt.%.The amounts Co, Ni, Cu, and Sb in Py3 are most subject to variation, but are generally comparable with those in Py1 and Py2 (Figure 9C).Cu is present in all analyzed grains, and its content varies from 0.02 to 0.2 wt.%, in rare cases ranging as high as 0.6 wt.%.Nickel is present in significant amounts in 60% of the analyzed grains (0.02-0.48 wt.% Ni).The Co/Ni ratio is from 0.2 to 32.5.In Py3, Co prevails over Ni, and their correlation coefficient is moderate (r = 0.6).The Cu content does not exceed 0.12 wt.%, and in 48% of the analyzed grains its concentration is below the detection limit.There is a moderate correlation between Cu and Au (r = 0.48).About 30% of the analyzed grains have Pb with an average content of 0.04 wt.%.The metasomatic Py3 formula is Fe0.98-1.08Ni0.0-0.01Co0.0-0.01S1.95-2.00As0.01-0.05.
In veined Apy2, the contents of Co, Ni, Cu, and Sb are lower than in Apy1 and do not exceed 0.24 wt.% in total (Figure 9F).Cobalt is a constant minor element; its content is two times lower than in Apy1 (0.015-0.067 wt.% Co), but always exceeds the nickel amount (Co/Ni = 1.1-22.5).Sb is steady and the most quantitatively significant minor element in Apy1, accounting for about 60% of the total volume (0.01-0.22 wt.% Sb).Cu concentration above the detection limit was recorded in only 20% of the analyzed grains and did not exceed 0.02 wt.%.
Table 2. Data of LA-ICP-MS trace element analysis of Py3 and Apy1 of Khangalas gold deposit (all values in ppm; bdl, below detection limit; nd, not detected).

Gold Content of Sulfides from Proximal Metasomatites and in Veins
Table 3 shows the results of atomic absorption analysis of proximal metasomatites, their sulfides, and sulfides in veins.The average Au content in Py3 is 12.51 ppm and in Ag is 5.89 ppm.In Apy1, Au varies from 12.3 to 23.8 ppm (17.51 ppm on average), and Ag from 0.43 to 11.83 ppm (6.48 ppm on average).Proximal metasomatites contain Au from 0.001 to 5.29 ppm (0.81 ppm on average), and Ag from 0.007 to 0.14 ppm (0.06 ppm on average).The Au grade in Py4 averages 29.30 ppm and in Ag 6.35 ppm; in Apy2 the Au value amounts to 20.49 ppm, and in Ag 2.06 ppm.

Pyrite and Arsenopyrite Types and Textures
The formation of pyrites took place for a long time, under changing physicochemical conditions, which is reflected in the morphology of crystals and the place of their formation.The morphology becomes more complex: from framboid and cubic forms to pentagonal dodecahedra.Rounded (framboid) microaggregates of Py1 were formed during the sedimentary-diagenetic stage of the clastic strata deposition subsequent to a change in the redox potential of the sedimentation environment.Py1 accumulations are often arranged parallel to the rock bedding.Some framboids have a zonal structure with fragments of rocks and minerals present in the central part.In part, early pyrites served as a substrate/nucleus for metamorphic Py2 formed during recrystallization.Accumulations of finely crystalline Py2 are confined to microfractures and foliation planes formed in the process of dislocation-metamorphic transformation of rocks.Py2 underwent cataclasis under the effect of late deformation processes.Py3 was developed in the wall rock zones; it has cubic and pentagonal-dodecahedral forms.The complication of crystal forms is accompanied by a change in the type of conductivity and the appearance of defects in the crystal lattice, which is a positive factor for isomorphic Au substitution into pyrite.Py3 is also characterized by a zonal or block structure, which provides the possibility for gold adsorbtion at interphase boundaries and crystal growth planes [54].In quartz veins, Py4 occurs as intergrowths and aggregates, less often as individual grains altered in the course of cataclasis and corrosion processes.They contain numerous inclusions of late sulfides and native gold.
In the zone of wall rock metasomatism, along with Py3, euhedral short prismatic Apy1 is developed.It forms disseminated grains and intergrowths, as well as aggregates and microveinlets.Macroinclusions are most often represented by silicate minerals entrapped during the crystal growth.Late sulfides, sulfosalts, and native gold are recorded as microinclusions, which are confined to the crystal growth zones.From the results of AFM analysis, Vorobiev and Kozyrev [54] came to the conclusion that Au (III) is sorbed to a greater extent on the FeAsS surface, being reduced to Au (0).Thus, it can be assumed that the gold content of metamorphic arsenopyrite is partially related to the adsorption of gold nanoparticles on the crystal growth surfaces.Vein Apy2 forms nests and clusters, often in association with Py4.In some Apy2 grains, zoning is observed, which is expressed by microtextures of replacement of the central part of the crystals by late sulfides and native gold.
The As/S atomic ratio is an indicator of the temperature in the sulfur-buffer group, i.e., it increases with increasing temperature (see [55][56][57], etc.).Apy1 from the Khangalas deposit has As/S < 1 (0.81 to 0.93).For Apy2, this ratio is higher: 30% of the analyzed grains have As/S > 1 (1.01 to 1.08).Kovalchuk et al. [55] studied the composition of arsenopyrite at the Vorontsovskoye deposit (Northern Urals, Russia) and showed that in arsenopyrite enriched in sulfur and depleted in gold, the ratio is As/S < 1, and in arsenopyrite depleted in sulfur and enriched in gold it is As/S > 1.The crystallization temperature and fugacity of sulfur in Apy1 are higher than in Apy2.These data suggest that the Apy2 vein of the Khangalas deposit crystallized at a higher temperature and S fugacity than Apy1.Arsenopyrite from the Khangalas deposit is characterized by negative correlation coefficients between As and S (r = -0.65 for Apy1 (Figure 10A) and r = -0.85 for Apy2), which reflects their pairwise replacement during crystallization [56,[58][59][60][61].
Microprobe analysis of the chemical composition of pyrite and arsenopyrite revealed a range of stable minor and trace elements in them.The main indicator element in pyrite is As.Pre-ore pyrite has As ≤ 0.3 wt.%, while syn-ore pyrite exhibits As ≥ 0.3 wt.%.In diagenetic and metamorphic pyrites, As makes up 30-35% of the total volume of trace elements; in the ore-stage metasomatic and vein pyrite, the proportion of As reaches 85-91% (max 3.19 wt.% As).The distribution of As in pyrite is uneven even within one grain.For Py1 and Py2, increased As content in the center of grains is often observed; for Py3 and Py4, blocks of maximum As content are characteristic.
Other typochemical minor elements (Co, Ni, Sb, Cu, Pb) are present in all generations of pyrite and arsenopyrite, and their proportion decreases from the pre-ore to the hydrothermal stage (Figure 11).Pyrite of early generations (Py1, Py2) is persistent in Cu content.In hydrothermalmetasomatic Py3 and Py4, the Cu content is reduced, and in Apy1 and Apy2, Cu concentration above the detection limit is found in only few grains.Most likely, this is due to the isomorphic Cu  Au substitution in sulfides.
Sb is a permanent trace element in pyrites and arsenopyrites.It can enter the pyrite structure isomorphically by replacing S to form [S-Sb] 3-or [Sb-Sb] 4-dumbbells [62].The negative correlation (r = 0.3-0.6) between antimony and iron indicates the possibility of isomorphic Fe → Sb substitution.
The presence of As, Co, Ni, Sb, Cu, and Au in Py1 is related to muds enriched in organic matter laid down by the water of marginal oceanic basins or inland seas [63,64].They were isomorphically incorporated into Py1.During regional dislocation-metamorphic events, the processes of recrystallization of Py1 and redistribution of trace elements in it took place.For example, in Py2, a drop in Co and Sb content and a slight increase in Ni value are observed.Under the influence of hydrothermal-metamorphic solutions, pyrite gets freed of early trace elements and becomes enriched in As, Sb, and Au.Thus, according to the results of microprobe analysis, we observe regular changes in the range and content of trace elements in pyrite and arsenopyrite during their formation and transformation.This is evidenced by the concentration of elements and the degree of supersaturation of the solution during crystallization and recrystallization of sulfides [65].

Invisible Gold and Its Relationship with Other Elements in Py3 and Apy1 According to LA-ICP-MS Data
Trace elements revealed by the LA-ICP-MS method in Py3 and Apy1 can be divided into several independent groups according to the degree of their connection.A stable paragenetic association with strong correlations (r = 0.8-0.9) in both pyrite and arsenopyrite is formed by the trace elements of the first group: Ti, V, W, Sn, Ga, Ba, Ag, Pt, and Pd.Py3 also includes Cr and Au, with a moderate degree of correlation (r = 0.5-0.6).They do not form isomorphic substitutions in pyrite and arsenopyrite; they are present as nanoinclusions.The enrichment of sulfides with trace elements of the first group could partly be due to the entrapment of microinclusions of early minerals during crystal growth.The second stable geochemical association, Cu-Zn-Au, is connected with a polysulfide vein association of the hydrothermal stage of mineral formation.The minerals of these elements (chalcopyrite, sphalerite, and native gold) are present in Py3 and Apy1 in the form of nano-and microinclusions.Both pyrite and arsenopyrite are characterized by strong Sb-Bi and Se-Te correlations.In Apy1, these elements can be combined into one geochemical association, which also includes Co and Ni.For Py3, the Sb-Bi pair is supplemented by Pb, and one more stable Co-Mn-Mo-Se-Te geochemical association is observed.It is possible that the enrichment with these elements occurred at later stages of mineral formation, due to a deeper source.In general, these data confirm the conclusions of many researchers regarding different depth sources for fluids at the gold ore objects of the YKMP [66,67].
LA-ICP-MS trace element analysis of metasomatic Py3 and Apy1 made it possible not only to determine a range of trace elements, but also to analyze the relationship between Au and various components of the system.Trace elements, including Au, can be found in pyrite in various forms: solid solution, nanoparticles, and microinclusions [15,17,61,68,69].
Using a scanning electron microscope, microinclusions of native gold about 15 m in size were detected in only one sample out of about 200 examined Py3 and Apy1 grains (Figure 6G).All other inclusions are galena, sphalerite, and chalcopyrite, and less often tetrahedrite and freibergite.This allows us to conclude that the elevated Au content in Py3 and Apy1 may be due to the presence of micro-and nanoinclusions of native gold.Low content is typical for gold incorporated in the crystal lattice of pyrite or its defects [17,60,68,[70][71][72].
Based on EPMA, LA-ICP-MS, SIMS, and l-PIXE analyses, Deditius and co-authors [71] studied the mechanism of Au and As incorporation and the solubility of gold in pyrite from various types of deposits.In addition to the Carlin and epithermal types, pyrites from porphyry Cu, Cu-Au, orogenic (OGD), volcanic-massive sulfide (VMS), iron-oxide copper-gold (IOCG), Witwatersrand Au, and coal deposits were studied.Deditius et al. [71] found that on the Au-As diagram, the analysis of pyrite from gold deposits showed that they form a wedge-shaped zone and most of the data points fall below the Au saturation line of the solution determined by Reich et al. [60] (Figure 12E).They show that Au + is the dominant form of gold in the arsenian pyrite of the studied deposits.Analytical data [60] indicate that the Au solubility limit in arsenian pyrite of epithermal deposits is defined by an Au/As ratio of ~0.02.The solubility limit of Au in pyrite of orogenic deposits is lower (~0.004)[71].Our LA-ICP-MS results for Py3 are in good agreement with those of Reich et al. [60] and Deditius et al. [71].Figure 13F shows that, in the Au-As (ppm, log) coordinates, all samples of the studied Py3 from the Khangalas deposit fall into the field of structurally bound gold (Au + ).These results are confirmed by the rather low Au content in the analyzed Py3: in most samples, Au does not exceed 2.5 ppm (Table 2).Earlier, Tauson et al. [76] showed that the content of the Au + structural form in the studied pyrite samples from deposits of different genetic types in Russia (large Natalka and Degdekan orogenic goldquartz deposits, Dukat volcanogenic-plutonogenic Au-Ag deposit, Dalnee and Oroch volcanic Au-Ag deposits, Sukhoi Log giant deposit with a debated genesis, Pokrovskoye epithermal Au-Ag deposit, Amur Dikes deposit with an unconventional type of mineralization, and Zun-Kholbinskoye deposit with a controversial genesis) and Uzbekistan (Kochbulak and Kyzylalmasay epithermal Au-Ag deposits) does not exceed ~5 ppm.Similar results were obtained by Deditius et al. [71] for pyrite from orogenic gold deposits, which, according to their data, contains less than 100 ppm Au.The higher Au content is mainly due to the presence of nano-and microparticles of native gold [78].The occurrence of native superficially bound Au 0 in sulfides of metasomatites is reported from deposits of various genetic types [60,68,76,79].Keith et al. [70] applied the Au solubility line [60] to the Te-As system, which made it possible to distinguish between Te in a solid solution and Au-tellurides in inclusions in pyrite.As shown in Figure 12G, the Te-As values are below the Au solubility line, which indicates that Te is incorporated in the Py3 lattice.This is confirmed by the fact that no Au-telluride microinclusions were found in Py3 grains under a scanning electron microscope.There is a weak positive correlation between As and Te (r = 0.24), while the relationship between As + Te and S in Py3 is negative (Figure 12H); nevertheless, it is known that As significantly affects the appearance of Te nano-and microinclusions in pyrite due to structural distortions [70].
One of the criteria for discriminating between sedimentary and hydrothermal-metasomatic disseminated orogenic pyrite in sedimentary rocks is the Ag/Au ratio.According to Large and Maslennikov [68] and Gregory et al. [80], disseminated arsenian pyrite from deposits with Au-As associations (Mt.Olympus, Macraes) has small Ag/Au values (up to 1), while sedimentary pyrite features a higher Ag/Au ratio (up to 1000).The Ag/Au ratio in Py3 from the Khangalas deposit ranges from 0.01 to 1.7 (0.5 on average).Figure 12F shows the average Ag/Au ratio for Py3 of the Khangalas deposit, which falls within the field of orogenic deposits and is consistent with the results of [68].

Invisible Gold in Apy1
The relationship between Au and Fe indicates the form of gold occurrence in arsenopyrite (see [55,58], etc.). Figure 13A shows an inverse correlation between Au and Fe in Apy1 of the Khangalas deposit, which is strong (r = -0.9)at Au > 2 ppm, and weak at Au < 2 ppm (r = -0.18).The strong inverse correlation between Au and Fe is due to isomorphic Au  Fe substitution in the arsenopyrite structure at Au > 2 ppm [55].
No direct Au vs. S or Au vs.As correlations are identified (Figure 13B,C), which is probably due to the heterogeneous composition of Apy1.As shown in Figure 13C, the Au/As values fall into the zone below the Au saturation line [60], which indicates the predominance of the Au + structurally bound form in Apy1.This is confirmed by the low Au content (<6.1 ppm) in Apy1.At the same time, the results of atomic absorption analysis revealed high Au content in the bulk samples of Apy1 (Table 2), which indicates the presence of nano-and microparticles of native gold.
Correlation plots for Au concentrations with Ag, Pb, Cu, Ni, Co, Bi, Sb, and Te in Apy1 are shown in Figure 14A-H.Gold and silver do not show a significant correlation (Figure 14A), but gold has a noticeable positive correlation with Cu and Zn.A close positive correlation between Cu, Zn, and Pb elements and Au is recorded for arsenopyrite from layered quartz veins of the Samgwang deposit (Korea) [58].In the Au vs. Pb correlation diagram (Figure 14B), a weak negative relationship (r = -0.39) is observed for Apy1 from the Khangalas deposit.At the same time, Figure 14K shows that Au-Pb-Cu have a highly positive relationship in the Apy1 crystal.Qualitative line scanning with the use of LA-ICP-MS analysis has proved to be effective in recognizing variations in trace element concentrations in zoned or altered sulfide grains (Py, Apy) [58,[81][82][83].Using this method, we found that concentrations of invisible Au, Pb, and Cu tend to decrease from the rim to the core of the grains, while the contents of Co and Ni are noticeably higher in the central zones of the crystals (Figure 14K).A similar variation in the amount of trace elements within arsenopyrite of the Samgwang orogenic deposit (Korea) was shown by Lee et al. [58], who attributed this fact to later hydrothermal alteration.For the Khangalas deposit, the increased Au, Pb, and Cu in the rim of the Apy1 grain is associated with the input of a portion of fluids enriched in Au-polysulfide association.The rest of the trace elements do not show any clear regularity in their distribution within the Apy1 grain.
Apy1 from the Khangalas deposit shows a noticeable positive correlation of Au with Ni and Co (Figure 14D,E).This may indicate [58] the involvement of basic and ultrabasic sources in ore formation at the Khangalas deposit.The lack of correlation between Au and Bi, Sb, and Ag in Apy1 (Figure 14A,F,G) and the marked positive correlation between Ag and Pb (Figure 14I) indicate the contribution of Ag-, Bi-, and Sb-containing fluids to the formation of deposits, which is typical for the metallogenic specialization of the region [8,84,85].
Thus, LA-ICP-MS trace element analysis data show that Au in Apy1 and Py3 of the Khangalas deposit is present in the form of both invisible gold (solid solution in the crystal lattice/nanoparticles with a size of <100 nm) and native gold.The predominant form is the solid solution Au + in the Py3 and Apy1 crystal lattices.

Gold Content of Proximal Metasomatites and Their Sulfides
The results of atomic absorption analysis of the proximal metasomatites and their sulfides at the Khangalas deposit showed that average Au content in Py3 (12.51 ppm) and Apy1 (17.51 ppm) is lower than in veined Py4 (29.30 ppm) and Apy2 (20.49ppm, one measurement).For the proximal metasomatites, average Au content with a highly uneven distribution (CV = 225%) was 0.81 ppm.Taking the average Au content at 0.5 ppm, the length of the ore body at 1400 m, and the thickness of the proximal metasomatites at 50 m, the gold resources of Khangalas deposit to a depth of 100 m would amount to 9.1 t Au.A significant contribution of disseminated mineralization to gold reserves is also reported for other orogenic deposits of the Yana-Kolyma metallogenic belt (Natalka [10,11], Degdekan [11], Drazhnoe [86], etc.).For example, for the Drazhnoe deposit (Upper Indigirka sector of the YKMB), with reserves of 49.8 tonnes of gold with an average grade of 2.86 ppm, the contribution of disseminated sulfide ores with invisible gold is estimated at 75% [86,87].High concentrations of Ni in sulfides suggest, according to Lee et al. [58], that mafic and ultramafic components introduced into hydrothermal fluids were involved in the precipitation of sulfides (maximum 2230 ppm for Apy1, 1620 ppm for Apy2, 4830 ppm for Py3).Negative correlations between Co and Fe (r = -0.6)(Figure 15B) and Ni and Fe (r = -0.1, Figure 15C) in Py3 indicate the presence of Ni and Co in the crystal lattice through isomorphic substitution for Fe [61].

Origin of Hydrothermal Sulfides According to Stable Sulfur Isotopes
Examining stable isotopes in geological objects (rocks, mineralizing fluids, vein minerals, and ore deposits) has proven to be a powerful method that allows one to study their genesis in detail [91].The genesis and sources of ore matter in orogenic Au deposits are controversial; first of all, the issue concerns the identification and assessment of the role of host rocks and deep-seated (magmatogenic and/or metamorphic) ore-bearing fluids in the process of ore formation (see [92][93][94] and references therein).The reported δ 34 S values for sulfide minerals from orogenic gold deposits range from -20‰ to + 25‰ [92].As sulfur is an important complexing agent for gold, understanding the S source may be critical in identifying the source areas of gold.A number of researchers came to the conclusion that the δ 34 S composition in Phanerozoic deposits changes depending on the age of the host rock [92,95,96].The sulfur isotopic composition of sulfides from the Khangalas deposit is in good agreement with these results (Figure 15B).
Based on the example of deposits in the Juneau gold belt [97] and on a comprehensive analysis of Jiaodong Province, China [92], it was shown that sulfur entered the composition of ore-forming fluids during the metamorphic conversion of pyrite into pyrrhotite.The sulfur source must have been a disseminated syngenetic/diagenetic pyrite in terranes being devolatilized at depth [95,96].
For gold deposits of the Yilgarn craton, at δ  S ~ 0, it was determined that the oreforming fluid had a felsic magmatic or mantle-level source of sulfur [98].The sulfur isotopic composition (δ 34 S = 0.0 to -3.3‰) in a number of gold-sulfide deposits in Kazakhstan indicates that the ore matter had a mantle-level source of sulfur with some contribution from the crust [99,100].
The sulfur isotopic composition in clastic strata of the Upper Kolyma region varies widely from -23.1 to +5.6‰ (Figure 16A) [19].Tyukova and Voroshin [19] compared the δ  S of accessory sulfides of host rocks with the δ  S of sulfides of gold deposits in the Upper Kolyma gold-bearing region, and suggested the involvement of sulfur mobilized from clastic strata in the hydrothermal process.It is believed that the most probable source of sulfur in sulfides (δ  S -6.3 to + 2.6) from the Natalka orogenic deposit, the largest in the region, is the host rocks of the Verkhoyansk clastic complex [101,102].Sulfur and arsenic were mobilized as a result of phase transformations of iron sulfides from clastic strata during the transformation of pyrite to pyrrhotite in the course of metamorphism.
We studied the S isotopic composition of Py3, Apy1, and Apy2 in five samples from the Khangalas deposit, and obtained a narrow range of negative δ  S values from -2.0‰ to -0.6‰ (Figure 16, Table 4).Similar sulfur isotopic compositions of arsenopyrite and pyrite of ore veins and disseminated mineralization of ore-hosting strata indicate their formation during a single hydrothermal event.The δ  S values of sulfides from the Khangalas deposit are close to those of the well-studied orogenic gold-sulfide deposits: Natalka (Upper Kolyma region) [103]; Suzdalskoe, Zhaima, Bolshevik, and Zherek (Kazakhstan) [99,100]; and deposits of the Adycha-Taryn metallogenic zone [8] (Figure 16A).For the large Nezhdaninskoe orogenic gold deposit in the Allakh-Yun metallogenic zone, deep magma chambers (-5‰ to +1‰ for vein ores) and sulfides of host rocks (Figure 16A) are considered as sulfur sources [104].At the same time, the conclusions about the genesis of the fluid components here are ambiguous, as in the case of the Muruntau deposit [105].Thus, mantle/magmatic sulfur was involved in the formation of the Khangalas deposit, but the participation of sulfur from the host rocks of the Verkhoyansk clastic complex cannot be ruled out.The small volume of the conducted isotope studies does not make it possible to represent the entire range of δ 34 S values for the Khangalas deposit.To obtain more information about the sources of ore matter, a comprehensive analysis of all generations of pyrite and arsenopyrite as well as thermobarometric and microelemental analysis of fluid inclusions are required.[106][107][108].Range of values of sulfur sources after Ohmoto [109] and Kryazhev [94].(B) Variation in δ 34 S values of sulfides in global sediment-hosted orogenic gold deposits after Goldfarb et al. [96].Heavy line and blue error envelope indicate seawater sulfate evolution curve from Claypool et al. [110].
For diagenetic Py1 and metamorphic Py2, the total content of trace elements varies from 0.04 to 0.8 wt.%, and the share of As is from 30% to 70%.The most widespread and commercially significant pyrite with a high content of invisible gold is Py3, which formed in the wall rock metasomatites and occupies up to 3% to 3.5% of their volume.The total content of trace elements in Py3 varies from 0.38% to 3.27%, and the proportion of As is more than 75%.
The results of LA-ICP-MS trace element analysis provide information on the form of occurrence of invisible gold in Py3 and Apy1.Some aspects of Au behavior in sulfides indicate the predominance of structurally bound Au + .A low Au content in sulfides (Py3, avg.1.5 ppm; Apy2, avg.7.6 ppm) is interpreted by some researchers [76] as an indicator of the structurally bound form of Au.In addition, the negative correlation between Au and Fe established for Py3 and Apy1 indicates isomorphic Au  Fe substitution [52,55,58].The close correlation between Au and As indicates that they have a common genesis.Their distribution patterns relative to the line limiting the transition of the solid solution Au + to Au 0 [57,71] also indicate the development of Au + in Apy1 and Py3 from proximal metasomatites (Figure 13F,H).At the same time, a higher Au content was found in sulfides and proximal metasomatites (Py3: 39.32 ppm Au by atomic absorption; Apy1: 39.0 ppm by LA-ICP-MS).This may indicate the presence of micro-and nanoinclusions of native gold in sulfides, which is confirmed by the detection, with the use of a scanning electron microscope, of an Au 0 microinclusion in the Py3 and Apy1 intergrowth in 1 out of about 200 grains studied.Indirect evidence of the presence of native gold inclusions in sulfides may be the large number of dense phases detected by computed microtomography.Isotope characteristics of hydrothermal sulfides (  S = -2.0‰ to -0.6‰) indicate that mantle/magmatic sulfur was involved in the formation of the deposit, but the participation of sulfur from the host rocks of the Verkhoyansk clastic complex cannot be ruled out.
The gold content of sulfides and proximal metasomatites determined during this study indicate that the Khangalas deposit has higher commercial potential than was thought before.It has much in common with other deposits of the Yana-Kolyma metallogenic belt.The results of this work make it possible to re-evaluate the reserves of orogenic gold deposits based on the study of disseminated sulfide mineralization with invisible gold.In these respects, Khangalas provides an exploration model useful in targeting similar gold deposits from the local to the regional scale.

Figure 9 .
Figure 9. Trace element contents by EPMA: (A) Py1; (B) Py2; (C) Py3; (D) Py4; (E) Apy1; (F) Apy2.Box boundaries are first and third quartiles, and line in middle of box is median.Lower border of line shows minimum value, upper shows maximum value, cross shows average value.

Figure 10 .
Figure 10.Ratio of As and S in (A) Apy1 and (B) Py3.

Figure 11 .
Figure 11.Variations in total content of Co, Ni, Sb, Cu, and Pb (wt.%) in sulfides of different generations, Khangalas deposit.Box boundaries are first and third quartiles, and line in middle of box is median.Lower border of line shows minimum value, upper shows maximum value, cross shows average value.

5. 5 . 1 .
Pyrite Genesis as Evidenced by Co/Ni Ratio The Co/Ni ratio reflects the genesis of pyrite [61,88-90].In the studied samples, Co/Ni varied over a wide range (0.2-46.0), but in most analyses CCo > CNi (Co/Ni > 1.0).Increased Ni content (Co/Ni = 0.2-0.8) is characteristic of Py1 and Py2, and is recorded in the central part of zoned Py3 crystals.Variable correlations are observed between Co and Ni: a strong positive correlation (r = 0.64-0.73) in Py1 and Py2, a negative correlation (r = -0.6) in grains with elevated Ni content, and no correlation between Co and Ni in vein Py4 and Apy2.

Table 1 .
Chemical composition of pyrites and arsenopyrites determined by EPMA (all values in wt.%.; nd, not detected).

Table 3 .
Results of atomic absorption analysis of proximal metasomatites, their sulfides, and sulfides in quartz veins.

Table 4 .
Sulfur isotopic composition of sulfides from metasomatites of Khangalas deposit.