Noble Metal Speciations in Hydrothermal Sulphides

A significant part of the primary gold reserves in the world is contained in sulphide ores, many types of which are refractory in gold processing. The deposits of refractory sulphide ores will be the main potential source of gold production in the future. The refractory gold and silver in sulphide ores can be associated with micro- and nano-sized inclusions of Au and Ag minerals as well as isomorphous, adsorbed and other species of noble metals (NM) not thoroughly investigated. For gold and gold-bearing deposits of the Urals, distribution and forms of NM were studied in base metal sulphides by laser ablation-inductively coupled plasma mass spectrometry and by neutron activation analysis. Composition of arsenopyrite and As-pyrite, proper Au and Ag minerals were identified using electron probe microanalysis. The ratio of various forms of invisible gold—which includes nanoparticles and chemically bound gold—in sulphides is discussed. Observations were also performed on about 120 synthetic crystals of NM-doped sphalerite and greenockite. In VMS ores with increasing metamorphism, CAu and CAg in the major sulphides (sphalerite, chalcopyrite, pyrite) generally decrease. A portion of invisible gold also decreases —from ~65–85% to ~35–60% of the total Au. As a result of recrystallisation of ores, the invisible gold is enlarged and passes into the visible state as native gold, Au-Ag tellurides and sulphides. In the gold deposits of the Urals, the portion of invisible gold is usually <30% of the bulk Au.


Introduction
Considering the "visible" gold occurrence in gold deposits, it has long been noted that the association of native gold with sulphides is the most sustainable (e.g., [1,2]). However, gold invisible to optical methods also commonly associates with sulphides, notably pyrite [3][4][5]. The presence of invisible gold is established by chemical and assay analyses of bulk samples as well as by sensitive and relatively local LA-ICPMS analysis (e.g., [6][7][8]). Such invisible gold can be extracted from sulphides by repeated heating (up to 850 • C; cf. during metamorphism of sulphide ore [9]), resulting in enlargement of gold particles [3,10]. The mechanism of this process was unclear, and Bürg [11] introduced the concept of "self-cleaning of the crystal lattice" of pyrite. Quartz-sulphide veins of the Berezovsk deposit (about 55% of total gold reserves) contain 90-95 vol% of quartz, 5-10 vol% of sulphides and average 18-20 g/t Au [64]. Impregnated sulphide ores-hydrothermal-altered granitoid dykes ("beresite")-contain 1-2 vol% of pyrite and 0.1-5 g/t Au (commonly 0.2-1.3 g/t) (Tables 1 and 2).
The Svetlinsk deposit is represented by the system of sulphide-quartz veins, veinlets and large lens-shaped vein-disseminated zones of quartz-pyrite (± pyrrhotite) mineralisation. The deposit is located within the strongly metamorphosed (up to amphibolitic facies) volcano-sedimentary series: metabasite, terrigenous/volcaniclastic sediments and marble (D-C). The average gold content is not high (2-3 g/t). Native gold in sulphide-quartz veins is closely associated with tellurides [65,66].
The Petropavlovsk deposit is located in the Silurian-Devonian island-arc volcanic complexes of the Polar Urals [40]. It tends to the apical part of a large polyphase (with dominating diorite) pluton and is closely related to porphyritic diorite. The ore body is a large isometric stockwork composed of gold-sulphide (low-sulphide) stringer-disseminated ore associated with albitisation zones, intersected by moderately Au-rich, late quartz veins [39,40]. Gold, finely dispersed in pyrite (<0.1 mm), predominates in the ore bodies and is associated with Ag, W, Mo, Cu, As, Te and Bi in geochemical haloes. The Novogodnee-Monto iron-gold-skarn deposit (7 t by-product Au reserves) is located on the east flank of the Petropavlovsk deposit, 0.5 km away. They both probably represent a single ore-magmatic system of porphyry type [40], and their features are compiled in Tables 1, 2 and 4.

VMS Deposits
VMS deposits of the Urals began to play a major role in its gold industry in the middle of the 20th Century. The VMS deposits belong to Uralian (or Cu-Zn-pyritic) type (the major one for the Urals), which can be divided into two subtypes: Cu>>Zn and Zn>>Cu, and two minor types: copper (Dombarovsk type) and gold-barite-copper-zinc or Au-polymetallic (Baimak type). Among the Uralian type, ten deposits contain >100 t Au and/or >1000 t Ag-the largest, Gai, Uchaly, Novo-Uchaly and Uzelga, together amount to 1000 t Au and~13,200 t Ag [57,67,68]. According to the modern genetic model, the formation of these deposits was related to a shallow chamber of acidic magma formed as a result of the differentiation of mantle-derived basalt [69,70].
The study of modes in which gold occurs in sulphides (including invisible gold) covers two large, slightly metamorphosed volcanogenic massive sulphide deposits: Uchaly and Uzelga, as well as the giant intensively deformed Gai VMS deposit (Figures 1-3; Tables 1 and 5). Massive sulphide ores predominate in these deposits with a subordinate contribution of disseminated ores (commonly 5-15 vol%). Au and Ag are relatively uniformly distributed in massive sulphide ores (av. values are 0.5-1.5 g/t Au, 5-50 g/t Ag), but local enrichment occurs (up to 10-90 g/t Au and up to 1000-3000 g/t Ag). The specific feature of Uchaly and Uzelga deposits (Zn-dominated subtype) is the uplifted levels of Au, Ag, Pb, Se, Te, Sb, As, Sn and Cd comparing to most of the other VMS districts of the Urals [71,72]. During 60 years, two hundred million tonnes of ore have been mined from the Gai deposit (Cu-dominated subtype), and~45% of initial reserves contained about 10 million tonnes of non-ferrous metals (Cu:Pb:Zn = 1:0.03:0.49), 520 t Au and 6300 t Ag (Au/Ag = 0.08). The annual output of the underground mine reached 5 Mt of Cu and Cu-Zn ore (>70 Kt Cu) [9,73,74]. The deposit consists of a package of steeply dipping sheet-like bodies, from 40 up to 1300 m down the dip, with a thickness of the large lodes up to 150 m in bulges. Together, the ore bodies comprise a lineal mineral zone (thickness~300 m, up to 800 m). The ore zone extends for 3.7 km along the strike and more than 1.7 km down the dip, remaining not contoured at a depth. The deposit was affected by regional metamorphism (greenschist facies), strike-slip deformations and folding. Therefore, ore structures and textures observed are mostly epigenetic (e.g., [9,74,75]). Massive, breccia-like, impregnated and stringer-impregnated structures are dominant. Gneissose, foliated and banded structures often occur in the outer parts of massive sulphide lodes and are found within narrow zones controlled by later steeply dipping normal and strike-slip faults [57,74].
The Uchaly deposit also demonstrates one of the largest potentials if the Novo-Uchaly deposit located directly to the south of the Uchaly deposit is considered as its separate ore body. Therefore, the total reserves of metals contained in the two ore bodies-the northern one, Uchaly, and the southern one, Novo-Uchaly-will amount to 9.96 million tonnes Cu + Pb + Zn, with Cu:Pb:Zn = 1:0.17:3.12, 344 t Au and 4381 t Ag (Au/Ag = 0.08) [15,36,76]. The Uchaly lode comprises a single subvertical thick (up to 180 m in bulges) lens of solid Cu-Zn ore, approximately 1.2 km in the lateral direction and 1.3 km along the dip. The Novo-Uchaly lode (1250 m × 900 m) reaches 186 m thick and comprises a steeply dipping VMS lens, crumpled into an anticlinal fold. The deposit was affected by regional metamorphism (subgreenschist facies). The ore body reveals complex lenticular contours complicated by pinch and swell areas. The primary ores-brecciated and rhythmically foliated-are preserved only as relics. Gneissose and folded varieties of ore occur along the contacts of the ore body and in zones of postmineral faults.
The Galka deposit is small (Table 5) and comprises gentle VMS lenses (max. 2000 m × 350 m; thickness up to 30 m) of veinlet-impregnated Au-pyrite-polymetal ores [80,81]. Locally semi-massive and massive sulphide ore forms thin (up to 1 m) lenses in interlayers of carboniferous fineclastic sedimentary rocks. The abundant veinlet-impregnated ores are hosted by argillic (illite/smectite-sericite-quartz and kaolinite) alteration, formed after cement of rhyodacitic breccia [82]. The ores are almost unaffected by any metamorphism, so colloform structures, and fine-grained textures of ores, are widespread, and sulphides carry a high proportion of invisible gold [15].
During processing, most of the total amount of trace elements is not extracted (Au, Ag, Pt, Pd, Pb, Se, Te, Sb, As, Bi, Sn, Co, Ni and Hg), and many of them (Pb, Se, Te, Sb, As, Co, Ni and Hg) become pollutants (together with sulphur dioxide and Fe) [72,83]. The increasing volume of processed VMS ores has aggravated the problem of gold recovery: while copper and zinc are taken in concentrates almost completely (75-85%), integrated gold recovery into the copper and zinc concentrates ranges from 20% (Uchaly) up to 50% (Gai). The loss of gold into the pyritic concentrate and tailings exceeds 15 tonnes (up to 20 t) annually, which is three times more than Au recovery from massive sulphide ores of the Urals. Therefore, tailing dumps of ore-processing plants can be compared with large gold deposits: for example, the Gai (110 t Au in the tailing dump) and Uchaly (90 t Au) concentrating mills. gold recovery into the copper and zinc concentrates ranges from 20% (Uchaly) up to 50% (Gai). The loss of gold into the pyritic concentrate and tailings exceeds 15 tonnes (up to 20 t) annually, which is three times more than Au recovery from massive sulphide ores of the Urals. Therefore, tailing dumps of ore-processing plants can be compared with large gold deposits: for example, the Gai (110 t Au in the tailing dump) and Uchaly (90 t Au) concentrating mills.

Figure 2.
Parameters of VMS deposits of the Urals in terms of ore and base metal tonnage and their comparison with large, Au-rich VMS deposits of the world (>100 t Au); red circles-deposits under consideration. Parameters for other deposits were selected from the review in [84]. Parameters of VMS deposits of the Urals in terms of ore and base metal tonnage and their comparison with large, Au-rich VMS deposits of the world (>100 t Au); red circles-deposits under consideration. Parameters for other deposits were selected from the review in [84].

Materials and Methods
The study was based on mapping and sampling of drill core and open pits. Collections of ores include 100-300 samples for each deposit, and 150-500 polished Figure 3. Parameters of VMS deposits of the Urals, in terms of ore and gold tonnage, and their comparison with major Au-bearing VMS deposits of the world; red circles with a red ring-deposits under consideration. Design and parameters for other deposits were adapted from the review in [84].

Materials and Methods
The study was based on mapping and sampling of drill core and open pits. Collections of ores include 100-300 samples for each deposit, and 150-500 polished sections were studied for each deposit. Mineralogical observations, electron probe microanalyses (EPMA) and a study using the scanning electron microscope (SEM) with an EDS were carried out for polished sections as well as for sulphide and heavy concentrates prepared from ores and, in some cases, from tailings. Manually selected sulphide monomineral fractions and sulphides from heavy concentrates by separating dozens of samples for each deposit were studied.
EPMA: The analyses were performed in IGEM RAS on the JXA-8200 Jeol, JEOL Ltd., Tokyo, Japan, electron microprobe equipped with five WD spectrometers and one ED spectrometer. The major constituents were determined at an accelerating voltage of 20 kV, current intensity on the Faraday cup of 20 nA, 10 s counting time and beam diameter of 1 µm. Conditions of analysis are provided in Appendix A Table A1. The AuMα line was chosen for Au analysis in arsenopyrite and pyrite (with 100-200 s counting time for PGE and Au) because it was established that the signal/background ratio of M series lines is better than that of L series lines [18]. These operating conditions and correctly selected background points made it possible to decrease the detection limit (3σ) down to 45 ppm for line AuMα.
SEM/EDS: The JSM-5610LV electron microscope (JEOL Ltd., Tokyo, Japan) equipped with an X-Max 100 energy dispersive spectrometer (IGEM RAS) was used for the studies.
INAA: For instrumental neutron activation analysis, sulphide separates of 20 to 50 mg weight were handpicked under a binocular microscope. The purity of separation was tested with the help of the X-ray powder diffraction technique. The fraction of a sulphide examined in a sample analysed was more than 90%. The grains of sulphides selected for analysis were sealed in polyethylene film. For activation, the polyethylene packages were wrapped in filter paper and aluminium foil. Samples, standardised by the State Geological Survey of the USSR (State Standard Samples 3593-86; 3594; 3595; RUS 1 ÷ 4) and the United States (USGS: BHVO-1; Mag-1; QLO-1; RGM-1, SCo-1, SDC-1, SGR-1), were prepared for irradiation in the same manner. The samples were activated for 15 to 17 h in the IRT reactor at the Moscow Engineering Physical Institute with a neutron flux of 1 × 10 13 s cm −2 . Measurements of induced activity were carried out in IGEM RAS with a gamma spectrometer: the analyser was the 919+GEM45190 ORTEC (AMETEK ORTEC, Oak Ridge, TN, USA)(HPGe coaxial detector; the range of energy measured was from 100 to 1800 KeV, with a resolution of 1.8 KeV at the line of 1332 KeV). The nuclide and the gamma line used for the analyses (KeV) were: 198 Au (412), 122 Sb (564), 124 Sb (602 and 1691), 60 Co (1332), 76 As (559), 110 MAg (657), 59 Fe (1099), 65 Zn (1115), 115 Cd (336 and 528). The measurements were conducted in two stages: in 7 to 10 days after activation, As, Cd and Au concentration were detected, and in 25 to 30 days after activation, Fe, Co, Zn, Ag and Sb were analysed (see details in [12]). INAA examined the contents of NM, base metals and some rare elements in the bulk samples, ultra-heavy concentrates and hand-made mineral concentrates.
LA-ICPMS: The high-sensitivity mass-spectrometer laser ablation method (LA-ICPMS, ThermoXSeries, NewWave 213 device, AMETEK, Berwyn, PA, USA) was chosen as a key analytical method for trace element analysis of sulphides, including determination of noble metals [39]. Primary determination of major components was carried out on EPMA. For the LA-ICPMS method, sulphide reference material MASS-1 [85] was used as an external calibration standard together with in-house pyrrhotite-based standard Fe 0.9 S (20 ppm PGE, gold and silver, synthesised at IGEM RAS using the method from [86]) and calibrated in the LabMaTer at the Université du Québec à Chicoutimi (Chicoutimi, QC, Canada) for to the concentration of Au against a standard prepared by J.H.G. Laflamme. The analysis of sulphide grains was realised using spot and profile ablation. The diameter of the laser beam was 30-60 µm. The laser pulse frequency was 10 Hz, and the energy at the sample surface was 7-8 J/cm 2 . The detection limit for most elements was 0.02-0.05 ppm. Investigations were carried out in IGEM RAS, and a part of LA-ICPMS control analyses was also done in the LabMaTer at the Université du Québec à Chicoutimi. We used the thermochemical method to determine the ionic form of gold according to [87][88][89].
To study the form of invisible Au in sulphides, we and our colleagues synthesised the Au-bearing chalcogenides using different methods (hydrothermal and gas transport methods and salt flux technique [44,45,90]) at contrasting T/f S 2 conditions and addressed the Au local environment using XAS [16,91,92]. High Au, Pt, Pd and other metals' contents intentionally introduced into the synthetic phases allowed using the spectroscopic methods to determine the structural-chemical state of the NM in sulphides using first-principles quantum chemical calculations and Bader charge analysis (e.g., [19]).
The microanalyses of natural sulphides for all deposits under consideration were accompanied by studies on conditions of ore formation based on fluid inclusion data and mineral geothermometry, and they are partly implied but not discussed in detail in this paper. The regime of volatiles was also briefly discussed.  Figure 4) and various As-Sb mineralisation: native As, realgar AsS, orpiment As 2 S 3 , tennantite, tennantite-tetrahedrite, aktashite Cu 6 Hg 3 As 4 S 12 , cinnabar HgS, alabandine MnS, clerite MnSb 2 S 4 , routhierite TlHgAsS 3 , pierrotite Tl 2 (Sb,As) 10 [63]. There are four groups of mineral assemblages in the ore bodies. The later assemblages often overprint the early ones: (1) VMS-like (with low Au content), (2) gold-pyrite-arsenopyrite (gold fineness 910-998), (3) magnetite and epidote-garnet skarn and skarnoid (with low gold content) and (4) gold-pyrite-realgar (gold fineness 680-690) [63].  Both arsenopyrite generations are characterised by zonal structure (Figures 5-8). Apy-1 contains ~28.9 at% As and As/S is 0.89 ± 0.04, while CAu varies from the detection limit to 0.25 wt%. CAu increases to 170 ppm in the lighter zones of the S-rich arsenopyrite ( Figure 7). In contrast, the outer high-arsenic rim of this arsenopyrite crystal (the lightest in BSE zones on the Figure 7a,b) contains no gold (Au < 45 ppm, EPMA).

Noble Metal Distribution and Speciations
Apy-2 has the average ratio As/S = 1.04 ± 0.03, and the Au content in Apy-2 varies from the detection limit to 1.23 wt% (Figure 8). The average gold content is 5-6 ppm. Typomorphic impurities for this arsenopyrite are Te (up to 1.2 wt%, mainly 400-500 ppm) and Tl (up to 43 ppm), and the Au content fluctuations correlate with the thallium variations. Gold distribution in the late arsenopyrite demonstrates a absence of positive Au-As correlation ( Figure 8). Invisible gold in arsenopyrite and As-pyrite: The study of gold modes in arsenopyrite is particularly important and relevant. This mineral is widespread in the dominant-in terms of gold reserves-black-shale-hosted gold deposits. This particular mineral demonstrates peak concentrations of invisible gold, as well as a exceptionally high resistance to endogenous and exogenous epigenetic processes. Simultaneously, arsenopyrite is "refractory" in hydrometallurgy processing ( [51]; cf., [94]). One of the most relevant for the study of invisible gold is the Carlin deposit type [7,50,51,[95][96][97], where gold-bearing arsenopyrite is the predominant host mineral for gold in the ore (i.e., Au-concentrator, along with As-pyrite) with up to sub-wt% level of Au content. In this paper, we conducted a comparative study of the distribution of gold in 'Carlin' and 'pre-Carlin' arsenopyrites on the example of the Vorontsovka gold deposit in the Urals [18,63,80] (Figure 5). Studied arsenopyrite-bearing mineral assemblages were crystallised at the contrast TPX conditions (Table 3). Early ore assemblages of the Vorontsovka gold deposit were formed at 510-240 °C (including magnetite skarn and later arsenopyrite-sulphosalt-polymetallic assemblage), whereas late Carlin-style gold-(Fe, As, Hg)-sulphide-quartz mineralisation was deposited at decreasing temperatures from ~350 to 100 °C [63,80,98]. In general, crystallisation of the arsenopyrite-bearing mineral assemblages occurs as temperature and, especially, f S2 (from 10 -7 to 10 -17 ) decrease [63,80]. Carlin-style gold-bearing mineralisation, Vorontsovka gold deposit: (A) argillised volcaniclastic rock with "spots" and zonal veinlets: argillisite → quartz → carbonate → realgar, (B) carbonated layered volcano-sedimentary rock with thick impregnation of realgar (± orpiment), and disseminated pyrite, arsenopyrite and stibnite along layers. (C) Aggregate of small (less than 20 µm) needle-shaped crystals of arsenopyrite, (D) zonal pyrite crystals: pyrite without As admixture occurs in the central part, the next zone contains 1.8 wt% As. (E,F) Intergrowths of arsenopyrite of different composition and morphology in the gold-pyrite-realgar assemblage: (E) idiomorphic crystals of arsenopyrite and a felt-like aggregate of thin needle-like arsenopyrite and interstitial gold inclusions (Au), (F) aggregates of parallel elongated zonal crystals of As-rich arsenopyrite (Apy-2) overgrowing the prismatic crystal of the earlier S-rich arsenopyrite (Apy-1). (C-F) BSE images.
With more detail, we studied arsenopyrite from the arsenopyrite-sulphosalt-polymetallic assemblage of the skarn group (Apy-1) and arsenopyrite from the later As-löllingitearsenopyrite assemblage (Apy-2) by INAA and EPMA [80]. Moreover, As-rich arsenopyrite (Apy-2) grows orthogonally on the elongated prismatic relics of the S-rich arsenopyrite (Apy-1) that appears to be the earlier arsenopyrite from the previous mineralisation stage (Figure 5d).
Both arsenopyrite generations are characterised by zonal structure (Figures 5-8). Apy-1 contains~28.9 at% As and As/S is 0.89 ± 0.04, while C Au varies from the detection limit to 0.25 wt%. C Au increases to 170 ppm in the lighter zones of the S-rich arsenopyrite ( Figure 7). In contrast, the outer high-arsenic rim of this arsenopyrite crystal (the lightest in BSE zones on the Figure 7a,b) contains no gold (Au < 45 ppm, EPMA). Au is distributed zonally across the profile, as well as Sb and Te, but there is no correlation between the peaks of these element contents across the profile.  Au is distributed zonally across the profile, as well as Sb and Te, but there is no correlation between the peaks of these element contents across the profile.   According to INAA data, pyrite from the sample Vr10-17 of the gold-skarn ore of the Vorontsovka deposit contains: As 4.3 wt%, Co 215 ppm, Sb 259 ppm, Ag 104 ppm and Au 2777 ppm. In this sample, a precision study of the relationship between the contents of Au and the main components of pyrite was performed by the EPMA method under conditions similar to those described in [18]. The correlation coefficient of As and Au in the pyrite crystal is 0.91 (N = 143). Figure 9 clearly shows two groups of impurity contents in the pyrite: one group has low As contents and close to Au detection limit (<0.004 wt%). In contrast, the second group ranges from ~2 wt% As and with increasing its concentration, the Au impurity increases linearly to 0.036 wt%.

The Berezovsk Gold Deposit
The dominant opaque minerals commonly filling the fractures in quartz are pyrite, tetrahedrite, aikinite, galena and chalcopyrite. The sulphide contents range from 2 to 10 vol%. Major Au mineral is native gold (Table 3; Figure 4), bearing 85-100 wt% of its total balance. Early dust-like (<10 μm) gold-I grains are included in pyrite, and gold fineness ranges 863-984. The size of later gold-II grains is larger (commonly 0.05-0.5 mm, up to 1 mm, and rarely more). Gold fineness ranges 729-904 [64]. The contents of Au and Ag in sulphides are as follows: pyrite 0.18-73.5 ppm Au and <0.2-92 ppm Ag; galena 0.15-2.96 Apy-2 has the average ratio As/S = 1.04 ± 0.03, and the Au content in Apy-2 varies from the detection limit to 1.23 wt% (Figure 8). The average gold content is 5-6 ppm. Typomorphic impurities for this arsenopyrite are Te (up to 1.2 wt%, mainly 400-500 ppm) and Tl (up to 43 ppm), and the Au content fluctuations correlate with the thallium variations. Gold distribution in the late arsenopyrite demonstrates a absence of positive Au-As correlation ( Figure 8).
According to INAA data, pyrite from the sample Vr10-17 of the gold-skarn ore of the Vorontsovka deposit contains: As 4.3 wt%, Co 215 ppm, Sb 259 ppm, Ag 104 ppm and Au 2777 ppm. In this sample, a precision study of the relationship between the contents of Au and the main components of pyrite was performed by the EPMA method under conditions similar to those described in [18]. The correlation coefficient of As and Au in the pyrite crystal is 0.91 (N = 143). Figure 9 clearly shows two groups of impurity contents in the pyrite: one group has low As contents and close to Au detection limit (≤0.004 wt%). In contrast, the second group ranges from~2 wt% As and with increasing its concentration, the Au impurity increases linearly to 0.036 wt%.
2777 ppm. In this sample, a precision study of the relationship between the contents of Au and the main components of pyrite was performed by the EPMA method under conditions similar to those described in [18]. The correlation coefficient of As and Au in the pyrite crystal is 0.91 (N = 143). Figure 9 clearly shows two groups of impurity contents in the pyrite: one group has low As contents and close to Au detection limit (<0.004 wt%). In contrast, the second group ranges from ~2 wt% As and with increasing its concentration, the Au impurity increases linearly to 0.036 wt%.

The Berezovsk Gold Deposit
The dominant opaque minerals commonly filling the fractures in quartz are pyrite, tetrahedrite, aikinite, galena and chalcopyrite. The sulphide contents range from 2 to 10 vol%. Major Au mineral is native gold (Table 3; Figure 4), bearing 85-100 wt% of its total balance. Early dust-like (<10 μm) gold-I grains are included in pyrite, and gold fineness ranges 863-984. The size of later gold-II grains is larger (commonly 0.05-0.5 mm, up to 1 mm, and rarely more). Gold fineness ranges 729-904 [64]. The contents of Au and Ag in sulphides are as follows: pyrite 0.18-73.5 ppm Au and <0.2-92 ppm Ag; galena 0.15-2.96

The Berezovsk Gold Deposit
The dominant opaque minerals commonly filling the fractures in quartz are pyrite, tetrahedrite, aikinite, galena and chalcopyrite. The sulphide contents range from 2 to 10 vol%. Major Au mineral is native gold (Table 3; Figure 4), bearing 85-100 wt% of its total balance. Early dust-like (<10 µm) gold-I grains are included in pyrite, and gold fineness ranges 863-984. The size of later gold-II grains is larger (commonly 0.05-0.5 mm, up to 1 mm, and rarely more). Gold fineness ranges 729-904 [64]. In this pyrite, C Au ranges 0.08-0.1 ppm and Au occurs as single peaks of the Ag-Pb-Cu-Sb group.
Au-bearing pyrite (C Au varies from 1 to 22 ppm) was found in two adjacent ladder veins of the Pervopavlovsk dyke [99]. An inhomogeneity was revealed in the pyrite (Figure 11a,c) in the form of irregular dark areas on the BSE images, in which small (from less than 1 to 10 × 1 microns) inclusions of bright (in reflected electrons) Sn-containing mineral phases, presumably stannite, were detected.
Profile analysis of the pyrite grains ( Figure 11) revealed that light in BSE parts of the grains are enriched in As and Au, and the dark ones in BSE-Sn, Cu, Zn, Pb, Cd, In, Ag, Ga, Ge. The high C Au are only partially consistent with the regions of C As peaks, since the invisible gold was found only in pyrite crystals containing sub-millimetre domains with small inclusions (from less than 1 to 10 × 1 microns) of stannite. These areas are Au-poor and rich in Sn, Ag, Bi, Cd, Cu, Ga, In, Pb and Zn ( Figure 12). The Au-bearing variety of pyrite is characterised by a low content of Co (<0.16 ppm) and Ni (<0.3 ppm) and increased As (51-8277, geom. mean 1325 ppm). The point-like increased contents of Ni and Co on the element maps is probably associated with relict inclusions of the pyrite-2. The "synchronous" increased contents of impurity elements (Ag, Cu, Pb, Zn, Bi, Sb, Co, Ni) at the grain edges probably reflects the presence of thin film of their sulphides and sulphosalts on the surface of the pyrite grain.
As (51-8277, geom. mean 1325 ppm). The point-like increased contents of Ni and Co on the element maps is probably associated with relict inclusions of the pyrite-2. The "synchronous" increased contents of impurity elements (Ag, Cu, Pb, Zn, Bi, Sb, Co, Ni) at the grain edges probably reflects the presence of thin film of their sulphides and sulphosalts on the surface of the pyrite grain.  The LA-ICPMS method revealed an inhomogeneous distribution of Au (0.01-0.04 ppm) in galena from the sulphide-quartz veins of the gumbeite alteration (quartz + orthoclase + dolomite-ankerite ± scheelite) of the Shartash granite massif, south of the Berezovsk deposit, with an increase in the crystal periphery to 0.59 ppm ( Figure 13). Together with Au, the Cu (up to 241 ppm, geom. mean 22 ppm) and Sb (up to 680 ppm) contents increase to the grain edges, which may be due to fine inclusions of bournonite CuPbSbS 3 , possibly Au-containing. This pattern is traced in all the grains of the mineral (the total number of ablation test points is 33). The distribution of other elements' impurities is homogeneous within single grain with arithmetic mean values (in ppm): Zn-2.5, As-11, Se   The LA-ICPMS method revealed an inhomogeneous distribution of Au (0.01-0.04 ppm) in galena from the sulphide-quartz veins of the gumbeite alteration (quartz +  The LA-ICPMS method revealed an inhomogeneous distribution of Au (0.01-0.04 ppm) in galena from the sulphide-quartz veins of the gumbeite alteration (quartz + Figure 13. Graphic images of the two probing profiles for the galena crystals from quarts veins of the gumbeites from the Shartash massive, the Berezovsk ore field. Profile length: left-1031 µm, right-728 µm.

The Svetlinsk Gold-Telluride Deposit
The Svetlinsk gold-telluride deposit contains the gold ores, which can be divided into two types (except for ore in regolith): (1) disseminated pyrite-pyrrhotite in the host rocks (CAu up to 1 g/t), and (2) sulphide-quartz veins and veinlets, superimposed on the disseminated mineralisation (average CAu = 0.8-2.5 g/t). Sulphides are typically about 3-5 vol% (sometimes up to 20 vol%) of the bulk gold-bearing ore. Native gold (fineness 618-964; single value 485) in sulphide-quartz veins forms inclusions in pyrite, tetrahedrite and quartz, as well as is closely associated with tellurides: melonite NiTe2, frohbergite FeTe2, altaite PbTe, tellurantimony Sb2Te3, Ag-and Au-Ag-tellurides (Table 2, Figure 14) [41]. The bulk ore analyses revealed that vein-disseminated gold ores (Au~2-4 ppm) contained (in ppm) 5-17 Sb, 4-7.3 Te and 2.5-4 Se. For the Svetlinsk deposit, native (Au0.48-0.96Ag0.02-0.49) and telluride (calaverite AuTe2, montbrayite ((Au,Sb)2Te3), sylvanite AuAgTe4, krennerite (Au,Ag)Te2, petzite Ag3AuTe2, hessite Ag2Te and γ-hessite Ag1,9Te) forms of manifestation prevail among the Au and Ag minerals ( Table 3). At Au ≤ 10 ppm, the direct correlation between the contents of Au and Ag ( Figure  15a), as well as that of Au and Ag with Te was found for early pyrite. This data may both pyrite varieties. At Au from 0.01 to 0.1 ppm, Au-As correlation possibly suggests structurally bound Au in pyrite. The lower Au and Ag concentrations in the late pyrite are probably related to the deposition of their own mineral forms at this mineral formation stage. In contrast, for the early pyrite, an incorporation of Au into the pyrite structure can be suggested. Cu-rich pyrite (I and II) and Ni-rich Py II contain a detectable value of Pd.  (2) pyrite, (b) nonuniform Au distribution in the pyrite crystal according to ablation profile data: Izone with uniform distribution, probably indicating the entry of Au into the pyrite structure, IIzone with inclusions of Au-Ag-Sb-Te phases and III-zone with gold content below the detection limit.

The Petropavlovsk Gold-Porphyry Deposit
Pyrite from the skarn-magnetite assemblage contains peak concentrations of Co up to 17,141 ppm, Ni 3738 ppm and elevated As content of 1944 ppm according to LA-ICPMS data (see Tables 7 and 8). Pyrite from the gold-sulphide assemblage contains maximum Te up to 650 ppm, Au 80 ppm, Bi 116 ppm and elevated Ag 105 ppm and Pb (up to 838 ppm). The peak contents of Pb up to 4.80 wt%, Zn 8.6 wt%, 0.7 wt%, Ni 0.38 wt%, Se 223 ppm, Ag up to 111 ppm, Sb 10.5 ppm and Sn 4.4 ppm, as well as increased Te up to 137 ppm, Au 66 ppm and Bi 5 ppm are found in pyrite from the gold-telluride assemblage. The presence of high "spot" occurrences of Pb and Zn in some samples is commonly associated with tiny inclusions of galena and sphalerite. The contents of most impurity elements (Au, Ag, Te, Sb, As, Co, Bi) decrease-usually an order of magnitude-in the pyrite of the latest quartz-carbonate assemblage.
According to LA-ICPMS data, a positive linear relationship between the Au and Ag contents in pyrite is observed for the gold-bearing ore: correlation coefficient (r) is 0.99 for  (2) pyrite, (b) nonuniform Au distribution in the pyrite crystal according to ablation profile data: I-zone with uniform distribution, probably indicating the entry of Au into the pyrite structure, II-zone with inclusions of Au-Ag-Sb-Te phases and III-zone with gold content below the detection limit.
At Au ≤ 10 ppm, the direct correlation between the contents of Au and Ag (Figure 15a), as well as that of Au and Ag with Te was found for early pyrite. This data may indicate the presence of nano-scale inclusions of petzite Ag 3 AuTe 2 and hessite Ag 2 Te in pyrite. At higher concentrations (Au > 10 ppm), the gold nanoparticles probably occur in both pyrite varieties. At Au from 0.01 to 0.1 ppm, Au-As correlation possibly suggests structurally bound Au in pyrite. The lower Au and Ag concentrations in the late pyrite are probably related to the deposition of their own mineral forms at this mineral formation stage. In contrast, for the early pyrite, an incorporation of Au into the pyrite structure can be suggested. Cu-rich pyrite (I and II) and Ni-rich Py II contain a detectable value of Pd.

The Petropavlovsk Gold-Porphyry Deposit
Pyrite from the skarn-magnetite assemblage contains peak concentrations of Co up to 17,141 ppm, Ni 3738 ppm and elevated As content of 1944 ppm according to LA-ICPMS data (see Tables 7 and 8). Pyrite from the gold-sulphide assemblage contains maximum Te up to 650 ppm, Au 80 ppm, Bi 116 ppm and elevated Ag 105 ppm and Pb (up to 838 ppm). The peak contents of Pb up to 4.80 wt%, Zn 8.6 wt%, 0.7 wt%, Ni 0.38 wt%, Se 223 ppm, Ag up to 111 ppm, Sb 10.5 ppm and Sn 4.4 ppm, as well as increased Te up to 137 ppm, Au 66 ppm and Bi 5 ppm are found in pyrite from the gold-telluride assemblage. The presence of high "spot" occurrences of Pb and Zn in some samples is commonly associated with tiny inclusions of galena and sphalerite. The contents of most impurity elements (Au, Ag, Te, Sb, As, Co, Bi) decrease-usually an order of magnitude-in the pyrite of the latest quartz-carbonate assemblage.  According to LA-ICPMS data, a positive linear relationship between the Au and Ag contents in pyrite is observed for the gold-bearing ore: correlation coefficient (r) is 0.99 for the skarn-magnetite assemblage, for gold-sulphide, r = 0.89, for gold-telluride, r = 0.9, and for quartz-carbonate, r = 0.84 ( Figure 16). Positive correlation of these elements corresponds to the occurrence of invisible gold, mainly represented by submicroscopic and nano-sized native gold (electrum). idiomorphic pyrite grain from the gold-sulphide assemblage (PP 309/10), which also showed a generally uneven distribution of impurity elements in pyrite. We divided the ablation time-profile into 5 areas: Au, Ag and Te are evenly distributed in all zones. The elements form peaks probably due to submicron inclusions of native gold, küstelite and hessite (Table 9).  A negative correlation is observed for Te with Au and Ag in pyrite for the goldsulphide assemblage (−0.4 and −0.44, respectively) and a positive correlation for Ag/Te (0.46) and Au/Te (0.4) for the gold-telluride assemblage. The presence of a positive connection between Ag and Te can be explained by the occurrence of submicroscopic and nano-scale inclusions of Ag telluride (hessite) and Au-Ag telluride (petzite) in the pyrite of the last assemblage.
There is a correlation between Co and Ni for the gold-sulphide assemblage (r = 0.46). A positive correlation is also observed between Co and As for the following assemblages: skarn-magnetite (r = 0.98) and quartz-carbonate (r = 0.5), and it commonly corresponds to small cobaltite inclusions. A significant correlation between Au and As, but negative (r = -0.6), is observed for the pyrite of the gold-sulphide assemblage. Correlation Au/As is absent for other assemblages.
The examples of the spot analyses of two grains of anhedral and subhedral pyrites (gold-sulphide and gold-telluride assemblages, correspondingly) are demonstrated in Figure 17. Gold concentration ranges from 0.3 to 31 ppm (anhedral pyrite) and from 0.06 to 1 ppm for the subhedral pyrite crystal (profile ablation). The contents of silver and tellurium are from 0.6 to 45 ppm and from 0.3 to 8.3 ppm, respectively (sample PP 308/2). Nano-sized petzite inclusions probably occur in the pyrite from the gold-sulphide assemblage ( Figure 17a).
Tiny isolations of petzite are often observed microscopically in close intergrowths with native gold, native silver and galena in this assemblage. Inclusions in another anhedral pyrite grain (from the gold-telluride assemblage, PP 309/5) are unevenly distributed. They are present in both areas and probably represented by küstelite, electrum and petzite (see Figure 17b).
An on-site study of the same pyrite grain ("mapping" mode of LA-ICPMS analysis) shows that increased concentration of Au, Ag and Te corresponds to one of its marginal parts. Dense clusters of bright points with their increased contents (Figure 18a) create an irregular, "fine-spotted" picture of their distribution, indicating the possible occurrence of nano-sized inclusions of Au-Ag tellurides (mainly petzite?) here. Elevated contents of Ni and Co belong to the edge zone of the pyrite crystal. Analysis reveals average contents (excluding peak areas): Au 0.6 ppm, Ag 2 ppm, Te 1.2 ppm, Co 3.4 ppm, Ni 16 ppm and As 23 ppm (Figure 18a).
Laser ablation of another pyrite grain ( Figure 18b) demonstrates a different picture of the distribution of impurity elements with maxima, ppm: Au 0.43, Ag 0.34, Te 6, Co 118, Ni 117 and As 318. Gold contents do not correlate with Te and Ag, and Te and Ag peaks belong to only one of the crystal margins, which is possibly also associated with the abundance of nano-sized inclusions of silver tellurides (hessite?) here. Elevated Co contents belong to the entire edge zone, and Ni, on the contrary, belongs to the central part of the pyrite crystal. Figure 19 shows an example of a LA-ICPMS profile across an idiomorphic pyrite grain from the gold-sulphide assemblage (PP 309/10), which also showed a generally uneven distribution of impurity elements in pyrite. We divided the ablation time-profile into 5 areas: Au, Ag and Te are evenly distributed in all zones. The elements form peaks probably due to submicron inclusions of native gold, küstelite and hessite (Table 9). Chalcopyrite: Elevated concentrations of Co (143 ppm), Ni (242 ppm), As (80 ppm), Se (194 ppm), Sb (20.4 ppm) and Sn (4 ppm) are found in chalcopyrite-1 and Te (up to 4200 ppm), and those of Au (up to 25 ppm), Ag (up to 7600 ppm) and Bi (11 ppm) in chalcopyrite-2 (Tables 10 and 11). It is probably associated with the capture of nano-scale inclusions of cobaltite (Co, As, Ni), fahlore (Cu, Ag, As, Sb), altaite (Pb, Te), as well as petzite (Ag 3 AuTe 2 ), calaverite (AuTe 2 ), native gold, native silver and hessite (Ag 2 Te).
Generally, the gold content in the sulphides of the deposit is maximum in galena (980 ppm), followed by pyrite (80 ppm) and chalcopyrite (25 ppm). According to the LA-ICPMS method, the pyrite of the early ore assemblages of the Petropavlovsk deposit is characterised by high contents of Co, Te, Au, Ag and Bi, increased Ni, As and Se and noticeable Sb and Sn. For the pyrite of later assemblages, elevated Zn, Pb, As, Ni, Se, Sb and Sn are found. The maximum Au was established in pyrite from the goldtelluride assemblage (up to 80 ppm). Minimal Au concentration was established in the pyrite of the latest quartz-carbonate assemblage (up to 1.7 ppm). Detectable admixtures of tellurium characterise the pyrite of all mineral assemblages. Proper mineral forms of the trace elements are presented, including Te compounds with Au (± Ag) in late mineral assemblages. Figure 17. Composition of pyrite from the gold-telluride assemblage obtained by the LA-ICPMS method for the Petropavlovsk deposit: (a) tiny inclusions of native gold and petzite are evenly distributed, the gold-sulphide assemblage, sample PP 308/2. (b) A few peaks are probably related with inclusions of native gold or küstelite and petzite, the gold-telluride assemblage, sample PP 309/5. Note (here and in Figure 19): the graphs show the main (Fe, S) and some minor (Au, Ag, Te) elements in pyrite. Figure 17. Composition of pyrite from the gold-telluride assemblage obtained by the LA-ICPMS method for the Petropavlovsk deposit: (a) tiny inclusions of native gold and petzite are evenly distributed, the gold-sulphide assemblage, sample PP 308/2. (b) A few peaks are probably related with inclusions of native gold or küstelite and petzite, the gold-telluride assemblage, sample PP 309/5. Note (here and in Figure 19): the graphs show the main (Fe, S) and some minor (Au, Ag, Te) elements in pyrite.
According to the LA-ICPMS analyses, gold in sulphides of the deposit is present mainly in the invisible form (from 0.02 to 80 ppm). It mainly associates with pyrite-2. At least part of such gold probably occurs as nano-scale inclusions of native gold (close in composition to AuAg) as well as Au-and Au-Ag-tellurides [39].
The data obtained suggest that the gold was evenly distributed in pyrite crystals in the early assemblages of the Petropavlovsk deposit. Gold was further enlarged and partly redeposited in pyrite defects at the final stages of mineralisation.
A close relationship is observed between Au and Ag (correlation coefficient r > 0.7), as well as between Ag and Te (r = 0.46) for the gold-telluride assemblage. It can be explained that these elements are present in the deposit in the form of tiny inclusions of native gold, hessite and petzite. Au-As correlation is not traced in pyrite. The uneven distribution of Au, Ag and Te over the area of pyrite grains with "spots" of their peak concentrations probably indicates the presence of clusters of nano-sized inclusions of Au-Ag tellurides.
The concentrations of trace elements in galena and chalcopyrite (as well as in pyrite) change with the evolution of ore formation. The contents of Ni, Co, As, Se and Sb are maximum in the main gold stage. The concentrations of Au, Ag, Te and Bi increase at the end of this stage. In general, the gold content in the sulphides of the deposit reaches highs in galena (up to 980 ppm). Next are pyrite (49 ppm) and chalcopyrite (25 ppm). Figure 17. Composition of pyrite from the gold-telluride assemblage obtained by the LA-ICPMS method for the Petropavlovsk deposit: (a) tiny inclusions of native gold and petzite are evenly distributed, the gold-sulphide assemblage, sample PP 308/2. (b) A few peaks are probably related with inclusions of native gold or küstelite and petzite, the gold-telluride assemblage, sample PP 309/5. Note (here and in Figure 19): the graphs show the main (Fe, S) and some minor (Au, Ag, Te elements in pyrite.   Table 9.  Figure 19. Composition of pyrite of the gold-sulphide assemblage from the Petropavlovsk deposit, LA-ICPMS profile (sample PP 309/10). Submicron inclusions of native gold, küstelite and hessite are probable. For average content of elements in the profile intervals, see Table 9.   Pyrite of the magnetite-pyrite assemblage is characterised by impurity concentrations peaking in As (11,050 ppm), Co (up to 3530 ppm), Ni (774 ppm), Au (12 ppm) and elevated of Te (up to 89 ppm) ( Table 14). Pyrite of the polymetallic assemblage contains the maximum concentrations of Zn (3130 ppm) and Sn (0.4 ppm). Pyrite from the gold-sulphide and gold-telluride assemblages is characterised by the highest values of Cu (4520 ppm), Ag (159 ppm), Te (141 ppm) and Bi (15 ppm). Pyrite from the quartz-carbonate assemblage contains the maxima of Sb (98 ppm) and Se (363 ppm). It is possible that high concentrations of some elements are associated with the capture of nano-scale inclusions of chalcopyrite, sphalerite, hessite (Ag 2 Te), petzite (Ag 3 AuTe 2 ), empressite (AgTe) and cobaltite (Co, As, Ni) (Table 15). It is confirmed by synchronous peaks of individual chemical elements on the profiles of laser ablation or in the form of bright point "inclusions" on bitmaps during analysis in the mapping mode [39,101]. found in pyrite of magnetite-pyrite (0.78) and gold-telluride (0.9) assemblages. A notable relationship between Au and As is observed only for the magnetite-pyrite assemblage (0.7). A positive correlation between the Co and Ni is revealed in pyrite for magnetitepyrite (0.7) and polymetallic (0.7) assemblages. The negative correlation between the Co and Ni is observed in pyrite for the gold-sulphide-magnetite assemblage (−0.6). A positive correlation between Co and As is also found in pyrite for magnetite-pyrite (0.5) and gold-telluride (0.5) assemblages.  The profile ablation of pyrite grains from the polymetallic assemblage by the LA-ICPMS method showed an uneven distribution of the majority of impurity elements in pyrite, with average contents (geom. mean., excluding peak areas): 1.3 ppm Au, 9 ppm Ag, 13.1 ppm Te, 177 ppm Co, 132 ppm Ni and 6 ppm As ( Figure 21). Confined to the marginal parts of the pyrite grain, high Au concentrations correlate with elevated Ag contents but do not correlate with Te. An on-site study of the same pyrite grain in mapping mode shows close for Au and Ag areas of the increased concentrations, gravitated to the edge parts of the pyrite grain, while Te-maxima are localised separately. The increased contents of As and Co belong to the marginal zones of the pyrite grain. Figure 20. Binary diagrams of the contents of Au, Ag, Te and As (ppm) in pyrite of the Novogodnee-Monto Fe-Au-skarn deposit according to the LA-ICPMS data. Mineral assembl in the ores: 1-magnetite-pyrite, 2-polymetallic, 3-gold-sulphide-magnetite in skarns, 4sulphide and gold-telluride, 5-quartz-carbonate. The dashed counter shows probable inclu of petzite and hessite.   Native gold in the ore is associated mainly with pyrite impregnations. It is characterised by high fineness (893) in the gold-sulphide-magnetite assemblage in skarns. The Au contents in pyrite are significantly reduced in the later gold-telluride assemblage. Native gold from this assemblage has a size from 2-5 to 10-20 µm and fineness from 750 to 893. The size of the segregations and fineness of gold increase (up to 40-50 µm and up to 893, respectively) in contact with the late dyke. Pyrrhotite appears in the gold-pyrite-magnetite assemblage. The gold concentrations in pyrite of the early magnetite-pyrite assemblage belong to the limits of possible uniformly distributed gold (for a pyrite crystal with a size of 0.5 mm, the maximum value of uniformly distributed gold detected by the method in [102] is about 12.3 ppm). These researchers evaluated the structural gold contribution as 1-10% of the bulk uniformly distributed gold. The peak Te contents (up to 141 ppm) were established in the pyrite of the gold-telluride assemblage, in some cases probably due to submicron inclusions of calaverite AuTe 2 .

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In general, gold of the early assemblages was evenly distributed in pyrite crystals at the Novogodnee-Monto deposit, and that at the Petropavlovsk deposit. Later, at the final stages of mineral formation, invisible gold (submicron native gold isolations, nano-scale domains and isomorphic gold in pyrite) was enlarged and redeposited in pyrite defects.
It is possible that high concentrations of some elements are associated with the capture of nano-scale inclusions of chalcopyrite, sphalerite, hessite (Ag 2 Te), petzite (Ag 3 AuTe 2 ), muthmannite (AuAgTe 2 ), sylvanite ((Au,Ag) 2 Te 4 ), krennerite (Au 3 AgTe 8 ), empressite (AgTe) and cobaltite ((Co,Ni)AsS) during alalysis. It is confirmed by the appearance of synchronous peaks of individual chemical elements on the laser ablation profiles or in the form of bright dot-like "inclusions" on bitmaps during analysis in the mapping mode [40]. It is also confirmed by strong (r > 0.7) bonds between Au and Te (0.8), Au and As (0.7), Co and Ni (0.7) and Ag and Te (0.8) for the magnetite-pyrite, Ag and Te for the goldtelluride (0.9), Au and Ag (0.9) for the gold-sulphide-magnetite and Co and Ni (0.7) for the polymetallic assemblages. The uneven distribution of Au, Ag and Te over the pyrite grain area with "spots" of their peak concentrations probably indicates, as in the Petropavlovsk deposit, the presence of dense clusters of nano-scale inclusions of Au-Ag tellurides, as well as cobaltite. Moreover, the Novogodnee-Monto deposit is characterised by increased concentrations of Au and Ag. These concentrations are consistent with each other, while Te is localised separately.
Concentrations of trace elements in chalcopyrite also change along with the ore formation. The contents of Ni, Co, As, Se, Au and Bi reach their maximum values in the earlier polymetallic assemblages. The concentrations of Te, Sn, Ag, Zn, Sb and Pb increase for chalcopyrite of the gold-sulphide-magnetite assemblage in skarn. In general, the gold concentration in chalcopyrite is slightly lower (geom. mean 0.22 ppm) than in pyrite (geom. mean 0.6 ppm). The gold concentration in magnetite does not exceed tenths of a ppm (up to 0.2 ppm, geom. mean 0.11 ppm).
We will consider the mineral composition of ores and the behaviour of precious metals in their minerals on the example of the largest deposits of the Urals: Gai (Cu-dominant subtype), Uchaly and Uzelga (Zn-dominant subtype).

The Uchaly Deposit
In the Uchaly deposit, the bulk of Au and Ag occurs in pyrite (Figure 23), average contents ~1.6 ppm Au and ~13 ppm Ag (Tables 19 and 20; Figure 23). Most gold (approximately 85%) is represented by dispersed and finely impregnated A pyrite, while free gold constitutes only 16% [15,88,89]. Gold occurs mainly in the copper and pyrite concentrates (4.0, 2.5 and 1. In the massive ores, visible gold was found in near-contact zones with a ga diorite dyke (up to 20 m thick) or in areas that experienced local dynamometamorp and tectonic flow. The gold aggregates (Figure 26a) have a fineness of 610-640 and 735 and are usually found in intergrowth with galena, chalcopyrite and tennantite As a rule, this mineral association cements recrystallised coarse-grained pyrite, cataclased.
The mineral forms of silver (Table 18) are represented by Ag-containing tetrah

The Uzelga Deposit
Pyrite is the dominant mineral of the ores (40-90 vol%). Chalcopyrite and sphalerite are the major economic minerals (1-10, up to 15 vol% for chalcopyrite and up to 60 vol% for sphalerite). Fahlore (usually tennantite) is a common mineral in the lower-level ore bodies and is a widespread one (0.5-5 vol%) (Table 20) in the upper ore level predominating above chalcopyrite. Galena also occurs widely but in smaller quantities (up to 0.5 vol%). Pyrrhotite is abundant in the axial zone of the southern part of the largest ore body of the deposit (body 4, see [77]). Hessite, altaite, coloradoite and other Te minerals [25,78] are rare. Gold occurs predominantly in pyrite and chalcopyrite (0.1-20 ppm Au in the mode of "invisible" gold) (Tables 19 and 20; Figure 23). Au enrichment in reniform pyrite (5.5-19.6 ppm) also exists [25]. The euhedral pyrite associated with this variety is characterised by an order of magnitude lower concentrations of Au, Ag and As. Ag content in pyrite ranges 0.3-204 ppm. Gold contents in chalcopyrite average 1.5-3 ppm (total ranges 0.001-19 ppm Au and 0.5-511 ppm Ag) (Table 21; Figure 24).
Sphalerite that contains emulsion-like inclusions of chalcopyrite contains 1.8 to 11 ppm Au (Table 22; Figure 25). The gold content in pure grains of sphalerite is 0.02 to 5.5 ppm. Therefore, gold-bearing chalcopyrite inclusions in sphalerite are considered to be responsible for an essential part of bulk gold. Galena contains various combinations of Au 0.05-0.41 wt%, Ag 0.01-0.34 wt%, Se 0.1-0.2 wt% and Te 0.1-0.14 wt% (EPMA). Additionally, Au is present as a trace element in some tellurides, such as altaite (0.02-5.2 wt%) and hessite (0.02-0.63 wt%).
Fahlore is the main concentrator of Ag. Its dominant variety is a tennantite-(Zn). Tetrahedrite grains are commonly inhomogeneous with internal mosaic texture or regular growth-zoning. The Ag content in tennantite from the upper-level ores ranges from 0.1 to 0.6 wt% (average 0.4 wt%), whereas tennantite from the lower ore level commonly contains <0.2 wt% Ag. The largest concentration of Ag (0.2-0.5 wt%) and Hg (up to 1-2 wt%) in fahlore of the lower ore bodies (bodies 3 and 4) was found near contacts with mafic dykes and near the obtuse east end of the ore body 4.

The Uchaly Deposit
In the Uchaly deposit, the bulk of Au and Ag occurs in pyrite ( Figure 23), with average contents~1.6 ppm Au and~13 ppm Ag (Tables 19 and 20; Figure 23). Most of the gold (approximately 85%) is represented by dispersed and finely impregnated Au in pyrite, while free gold constitutes only 16% [15,88,89]. Gold occurs mainly in the zinc, copper and pyrite concentrates (4.0, 2.5 and 1. In the massive ores, visible gold was found in near-contact zones with a gabbro-diorite dyke (up to 20 m thick) or in areas that experienced local dynamometamorphism and tectonic flow. The gold aggregates (Figure 26a) have a fineness of 610-640 and 724-735 and are usually found in intergrowth with galena, chalcopyrite and tennantite-(Zn). As a rule, this mineral association cements recrystallised coarse-grained pyrite, often cataclased.  The mineral forms of silver (Table 18) are represented by Ag-containing tetrahedrite and tennantite (Figure 26b), and rarer electrum, silver tellurides (hessite, empressite), sulphides (acanthite, uytenbogaardtite, petrovskaite) and sulphosalts (freibergite, polybasite and pearceite; Figure 27). Silver impurity in pyrite (up to 400 ppm, EPMA) was also recorded. A strong positive correlation of silver and antimony was found for the fahlore (Figure 27).

The Galka Deposit
Along with the usually predominant pyrite, sphalerite, marcasite, chalcopyrite and less often galena, arsenopyrite, pyrrhotite, native gold, native silver, acanthite, freibergite, argentotetrahedrite, pyrargyrite, stephanite, proustite, polybasite and rare Au-Ag minerals petrovskaite, uytenbogaardtite and kurilite appear at the Galka deposit [15]. A special feature of pyrite of the Galka deposit is the increased content of As (0.027-0.878 wt%, avaverage 0.26 wt%, INAA; up to 1.5 wt%, average 0.05 wt%, LA-ICP-MS, Table 19) and Sb (7.4-873.4 ppm, average 184.2 ppm). The Sb content directly correlates with Au in pyrite (0.44-9.59 ppm, average 4.16 ppm), and Au is also directly related to Te (2.4-453.9 ppm, average 70.05 ppm) and Ag (2.15-711.7 ppm, average 137.0 ppm). PGE shows extremely low concentrations in the sulphides of VMS ores (Table 23), although ores and industrial products contain a noticeable amount of PGE [24,25,28]. In the common VMS ores (Au 0.2-3 ppm) of the Urals, essential parts (47-87%, Figures 28a  and 29) of gold are incorporated in the sulphides in the form of invisible gold. Thus, Au fails to tailings with pyrite and partly with other sulphides [15,72]. The fraction of native gold as an Au-concentrator ranges 13-90%, but this number decreases to 13-53% if the technological bulk probes enriched in Au (7.9-21.2 ppm) are excluded (Figure 28a). Native gold and other gold minerals occur in ore samples with bulk Au content higher than 3 g/t [37,67,104,105].

The Galka Deposit
Along with the usually predominant pyrite, sphalerite, marcasite, chalcopyrite and less often galena, arsenopyrite, pyrrhotite, native gold, native silver, acanthite, freibergite, argentotetrahedrite, pyrargyrite, stephanite, proustite, polybasite and rare Au-Ag minerals petrovskaite, uytenbogaardtite and kurilite appear at the Galka deposit [15]. A special feature of pyrite of the Galka deposit is the increased content of As (0.027-0.878 wt%, avaverage 0.26 wt%, INAA; up to 1.5 wt%, average 0.05 wt%, LA-ICP-MS, Table 19) and Sb  (Table 23), although ores and industrial products contain a noticeable amount of PGE [24,25,28]. In the common VMS ores (Au 0.2-3 ppm) of the Urals, essential parts (47-87%, Figures 28a and 29) of gold are incorporated in the sulphides in the form of invisible gold. Thus, Au fails to tailings with pyrite and partly with other sulphides [15,72]. The fraction of native gold as an Au-concentrator ranges 13-90%, but this number decreases to 13-53% if the technological bulk probes enriched in Au (7.9-21.2 ppm) are excluded (Figure 28a). Native gold and other gold minerals occur in ore samples with bulk Au content higher than 3 g/t [37,67,104,105].   In the VMS deposits of the Urals, native gold forms grains and aggregates with size of about 1-50 μm and up to 150 μm. Larger grains are rare [103]. The fineness of gold ranges from 340 to 900 for slightly transformed deposits and from 500 to 980 for highly metamorphosed deposits (Figures 28b and 30). Ag content varies from 11.88 to 39.45 wt%, and admixtures (wt%) of Pt up to 2.23, Pd up to 0.85, Te up to 1.17, Hg up to 0.89, Fe up to 0.5 and Se up to 0.49 are found. Hg-bearing native gold (8-11 wt% Hg) rarely occurs. According to the high-resolution transmission electron microscopy (HR-TEM) data, nanosized particles of native gold (1-50 nm) occur in pyrite of the VMS ores [15].
Characterising native Au-Ag solid solution, we adhere to the terminology adopted in early publications [106,107]: native silver, Ag1.0Au0.0-Ag0.94Au0.06 (0-100‰); küstelite,   In the VMS deposits of the Urals, native gold forms gra of about 1-50 μm and up to 150 μm. Larger grains are rar ranges from 340 to 900 for slightly transformed deposits an metamorphosed deposits (Figures 28b and 30  In the VMS deposits of the Urals, native gold forms grains and aggregates with size of about 1-50 µm and up to 150 µm. Larger grains are rare [103]. The fineness of gold ranges from 340 to 900 for slightly transformed deposits and from 500 to 980 for highly metamorphosed deposits (Figures 28b and 30). Ag content varies from 11.88 to 39.45 wt%, and admixtures (wt%) of Pt up to 2.23, Pd up to 0.85, Te up to 1.17, Hg up to 0.89, Fe up to 0.5 and Se up to 0.49 are found. Hg-bearing native gold (8-11 wt% Hg) rarely occurs. According to the high-resolution transmission electron microscopy (HR-TEM) data, nano-sized particles of native gold (1-50 nm) occur in pyrite of the VMS ores [15].

Geochemistry of Au in Sulphides
An experimental study of concentration mechanisms of NM in base metal su was conducted in the frame of the Russian Scientific Foundation grant No. 14-17 "Distribution and structural-chemical state of noble metals in sulphides through deposits from magmatic to hydrothermal as an indicator of the conditi mineralisation" (2014-2018), led by the first author. As a result of work on the sou synchrotron radiation measurement, X-ray absorption spectra (XANES/EXAF interpretation of the resulting data, and using X-ray photoelectron spectrosco position of gold in the structure of sulphides was revealed. Main results were pu in a series of papers devoted to Au in covellite CuS [16,17], in Fe-S and Fe-As-S m [19,52], in sphalerite (e.g., [92]), Au, Ag, Pt and Pd in pyrite and pyrrhotite [91], Pt in [22] and pyrrhotite [23].
Gold can form a solid solution with Fe-S and Cu-Fe-S minerals. When p heated, chemically bound Au, even in the presence of liquid sulphur, is releas metal, while in copper minerals, heating, on the contrary, promotes the transition metal into a "chemically bound" form. In general, results [17,19,22,52,91,92] sugg Au and other NM (PGE, Ag) can occur in the chemically bound state in the Cu-Fe a Zn-Fe sulphide ores.
Below, we will focus on zinc sulphide and its close crystal-chemical "re greenockite. Both minerals-unlike pyrite and arsenopyrite (see, e.g., [19,52,9 references within)-are much less studied experimentally (e.g., [108]). Zinc su occurs in nature in two polytypes-sphalerite and würtzite-or their mixture phases have different chemical elements' solubility limits. Both forms of Z important in mineralogy as many minerals crystallise in the same structures. For ex there are würtzite and sphalerite forms of AgI (iodargyrite and miersite) an (greenockite and hawleyite) [109]. Most of our gas vapour transport and sa experiments led to the formation of pure cubic sphalerite. The existence of minor am of würtzite in sphalerite crystals, probably arising due to the changes in the s Characterising native Au-Ag solid solution, we adhere to the terminology adopted in early publications [106,107] (Figure 30), but more low-fineness gold is found in some VMS deposits of the Urals, the Galka, for example (fineness~640-720).

Geochemistry of Au in Sulphides
An experimental study of concentration mechanisms of NM in base metal sulphides was conducted in the frame of the Russian Scientific Foundation grant No. 14-17-00693 "Distribution and structural-chemical state of noble metals in sulphides through the ore deposits from magmatic to hydrothermal as an indicator of the conditions of mineralisation" (2014-2018), led by the first author. As a result of work on the sources of synchrotron radiation measurement, X-ray absorption spectra (XANES/EXAFS) and interpretation of the resulting data, and using X-ray photoelectron spectroscopy, the position of gold in the structure of sulphides was revealed. Main results were published in a series of papers devoted to Au in covellite CuS [16,17], in Fe-S and Fe-As-S minerals [19,52], in sphalerite (e.g., [92]), Au, Ag, Pt and Pd in pyrite and pyrrhotite [91], Pt in pyrite [22] and pyrrhotite [23].
Gold can form a solid solution with Fe-S and Cu-Fe-S minerals. When pyrite is heated, chemically bound Au, even in the presence of liquid sulphur, is released as a metal, while in copper minerals, heating, on the contrary, promotes the transition of the metal into a "chemically bound" form. In general, results [17,19,22,52,91,92] suggest that Au and other NM (PGE, Ag) can occur in the chemically bound state in the Cu-Fe and Cu-Zn-Fe sulphide ores.
Below, we will focus on zinc sulphide and its close crystal-chemical "relative" greenockite. Both minerals-unlike pyrite and arsenopyrite (see, e.g., [19,52,91] and references within)-are much less studied experimentally (e.g., [108]). Zinc sulphide occurs in nature in two polytypes-sphalerite and würtzite-or their mixture. These phases have different chemical elements' solubility limits. Both forms of ZnS are important in mineralogy as many minerals crystallise in the same structures. For example, there are würtzite and spha-lerite forms of AgI (iodargyrite and miersite) and CdS (greenockite and hawleyite) [109]. Most of our gas vapour transport and salt flux experiments led to the formation of pure cubic sphalerite. The existence of minor amounts of würtzite in sphalerite crystals, probably arising due to the changes in the sulphur fugacity in the system, contributed to the formation of low-Au-saturated ZnS (<50 ppm Au) with inhomogeneity distribution of all the admixtures. However, we never obtained pure würtzite using the mentioned methods. To supplement missing data on Au solubility, we used synthetic crystals of greenockite as a model compound for the würtzite-type structure.

Noble Metal Speciations in Synthetic Sphalerite and Greenockite
For synthesis experiments, the starting mixtures were pure ZnS (würtzite) or CdS, and several milligrams of In 2 S 3 , FeS and MnS for sphalerite or greenockite. The initial phases were powdered in the agate mortar and then loaded into silica glass ampoules (10-11 mm outer diameter, 8 mm inner diameter and~110 mm length) together with Au metal wire and transport agent or salt mixture. We used mainly the eutectic mixture of NaCl/KCl in the salt flux experiments, and the amount of salt flux melt was approximately 50-65% of the ampoule volume. We used both I 2 and NH 4 Cl as transport agents in chemical vapour transport experiments. The total quantity of the obtained crystals was higher when we used NH 4 Cl compared to I 2 . It is important to note that we control the activity of gold by the presence of pure metal wire inside the ampoule. Therefore, the concentration of gold in sphalerite represents the maximum possible value for the given parameters. In one series of chemical vapour transport experiments, a tiny piece of sulphur was added before sealing. Then, we performed a synthesis without adding any additional sulphur. To understand the influence of f S 2 on the concentration of gold in the final crystals, different amounts (from 0 to 0.035 g) of sulphur were added to the ampoules of the chemical vapour transport suite. The loaded ampoules were evacuated up to 10 −4 bar, sealed with an oxygen-gas burner and placed into a horizontal tube furnace that was then heated to the synthesis temperature over 2-3 h and then kept at this temperature during 16-30 days. The temperature gradient in the furnace was 50-100 • C, and the measured temperature at the hot end of the ampoules was 850 • C. At the end of the experiment, the ampoules were quenched in cold water. Sphalerite crystals were found in the cold end of the ampoules (Figures 31-33). It is important to note that the attempts to synthesise Au-bearing sphalerite at the lower temperatures using other salt mixtures (e.g., CsCl/NaCl/KCl mixture at 645 and 555 • C at the hot and cold end of the ampoule respectively, and LiCl/RbCl mixture at 470 and 340 • C, respectively) led to the formation of tiny crystals of sphalerite with the gold content < 5 ppm. These crystals cannot be used for the XAFS study due to the difficulties in measuring low amounts of the impurities. Therefore, we did not use these samples in our further observations. Only hightemperature synthesis (≥850 • C in the hot end of the ampoule) led to the crystallisation of Au-rich crystals. Table 24. Concentration of In (C In ) and Au (C Au ) in the crystals grown at 850 • C using gas transport method and NH 4 Cl as a transport agent (LA-ICPMS data, ppm ± 2σ).

Sample 2027 2032
C In 86 ± 5 5400 ± 200 C Au 5 ± 3 84 ± 10 of tiny crystals of sphalerite with the gold content < 5 ppm. These crys for the XAFS study due to the difficulties in measuring low amount Therefore, we did not use these samples in our further observ temperature synthesis (>850 °C in the hot end of the ampoule) led to t Au-rich crystals.  Table 24 for details. Figures 31-33 and 40-Timofey Pashko.   Table 24 for details. Figure 31, Figure 32, Figure 33 and Figure  40-photos prepared by Timofey Pashko.   The synthesis of Ag-bearing sphalerite using the gas transport metho wire led to the formation of Ag-bearing sphalerite with heterogeneous distri dopant in some cases. We also tried to grow crystals of Ag-bearing spha temperatures in an eutectic mixture of LiCl/RbCl at 550 and 460 °C at the ends of the ampoule, respectively. The products of the synthesis contained t The synthesis of Ag-bearing sphalerite using the gas transport method and silver wire led to the formation of Ag-bearing sphalerite with heterogeneous distribution of the dopant in some cases. We also tried to grow crystals of Ag-bearing sphalerite at low temperatures in an eutectic mixture of LiCl/RbCl at 550 and 460 • C at the hot and cold ends of the ampoule, respectively. The products of the synthesis contained the needles of Ag 2 S. According to the phase diagram, the optimal way of Ag-bearing sphalerite synthesis is conducted by the calculated amount of Ag 2 S as a dopant source instead of Ag wire for such experiments.
temperatures in an eutectic mixture of LiCl/RbCl at 550 and 460 °C at the ends of the ampoule, respectively. The products of the synthesis contained Ag2S. According to the phase diagram, the optimal way of Ag-bearing sphal is conducted by the calculated amount of Ag2S as a dopant source instead such experiments. The chemical composition of the final crystals was studied both by E major and minor elements) and by LA-ICPMS (for the trace elements). Exp that in simulated conditions, as in nature (e.g., [15]), sphalerite can incorp in the presence of admixtures of other elements. Natural sphalerite can co Cu, Tl, Hg, Fe, Mn, Cd, In, Ga, Ge, As, Bi, Pb, etc. [8]. Some of these chemica presented in the form of mineral microinclusions (e.g., Pb, Bi, etc.), wh The chemical composition of the final crystals was studied both by EPMA (for the major and minor elements) and by LA-ICPMS (for the trace elements). Experiments show that in simulated conditions, as in nature (e.g., [15]), sphalerite can incorporate more Au in the presence of admixtures of other elements. Natural sphalerite can contain Au, Ag, Cu, Tl, Hg, Fe, Mn, Cd, In, Ga, Ge, As, Bi, Pb, etc. [8]. Some of these chemical elements are presented in the form of mineral microinclusions (e.g., Pb, Bi, etc.), while others can substitute Zn 2+ (e.g., Cd, Mn, Fe, etc.) in the crystal structure of sphalerite [8]. We prepared the sample with admixtures typical for the natural environments: Fe, Mn, Se, In and Cd, together with gold, which were added one by one, in pairs, or simultaneously. The elements are evenly distributed inside the sphalerite (Figures 34 and 35; [110]). In the resulting crystals, we observed an extremely high concentration of gold (up to 3000 ppm) in comparison with the sample of the Fe-bearing sphalerite with Au (up to 250 ppm) (Figures 35-39).
According to chemical analysis, the amount of Au increases instantly with the increase of the sulphur fugacity in cases when In was added to the system (Figures 36 and 37, shaded symbols). In some synthesis experiments, adding more than 2.28 wt% In leads to the formation of intergrowths of sphalerite and sulphospinel phase of the approximate composition ZnIn 2 S 4 (Figure 38a). In the absence of indium, C Au does not exceed 10 ppm (Figure 37, non-shaded symbols). In the synthesis experiments, the close coalescence of sphalerite crystals with native gold dendrites ( Figure 40) indirectly confirms that we are probably dealing with the maximum values of Au impurity that can enter the composition of sphalerite under these conditions. the sample with admixtures typical for the natural environments: Fe, Mn, Se, In and Cd, together with gold, which were added one by one, in pairs, or simultaneously. The elements are evenly distributed inside the sphalerite (Figures 34 and 35; [110]). In the resulting crystals, we observed an extremely high concentration of gold (up to 3000 ppm) in comparison with the sample of the Fe-bearing sphalerite with Au (up to 250 ppm) (Figures 35-39).  Table 26 for details. Table 26. Chemical composition of ZnS sample 1450 (EPMA) synthesised at 850 °C using the gas transport method with I2 as a transport agent.  Table 27 for details.  Table 26 for details. elements are evenly distributed inside the sphalerite (Figures 34 and 35; [110]). In the resulting crystals, we observed an extremely high concentration of gold (up to 3000 ppm) in comparison with the sample of the Fe-bearing sphalerite with Au (up to 250 ppm) (Figures 35-39).  Table 26 for details.   Table 27 for details.  Table 27 for details.  1 Colour on graph ( Figure 35); 2 Synthesis with adding a few mg of additional sulphur in the ampoule.

Figure 36.
Graph showing the relation between the In and Au concentrations in the synthetic crystals of sphalerite. Numbers near the marks correspond to CAu, ppm. GVT (gas vapour transport)-gas transport method. Synthesis procedure is described in [90].
According to chemical analysis, the amount of Au increases instantly with the increase of the sulphur fugacity in cases when In was added to the system (Figures 36 and  37, shaded symbols). In some synthesis experiments, adding more than 2.28 wt% In lead to the formation of intergrowths of sphalerite and sulphospinel phase of the approximate composition ZnIn2S4 (Figure 38a). In the absence of indium, CAu does not exceed 10 ppm (Figure 37, non-shaded symbols). In the synthesis experiments, the close coalescence o sphalerite crystals with native gold dendrites ( Figure 40) indirectly confirms that we are probably dealing with the maximum values of Au impurity that can enter the composition of sphalerite under these conditions.
Our experiments on solubility of Pt, Pd, Os and Ir in sphalerite show that these noble metals cannot penetrate into the structure of zinc sulphide, even in the low amounts. Figure 36. Graph showing the relation between the In and Au concentrations in the synthetic crystals of sphalerite. Numbers near the marks correspond to C Au , ppm. GVT (gas vapour transport)-gas transport method. Synthesis procedure is described in [90]. The presence of In and/or Fe admixtures also affects the concentration of Au in synthetic crystals of greenockite (Table 28; Figure 41 [111]). The concentration of Au in Inand Fe-doped greenockite is ten-fold higher in comparison with pure CdS. The distribution of Au, Ag, In, Cd, Se, Fe and Mn in sphalerite and greenockite is homogeneous according to the LA-ICPMS line mode spectra (Figures 34, 35 and 41).  Figure 39, numbers near the marks show the concentration of Au in ppm. Red asterisks (*) indicate samples whose fugacity is conditionally shown as minimal in these systems.        Our experiments on solubility of Pt, Pd, Os and Ir in sphalerite show that these noble metals cannot penetrate into the structure of zinc sulphide, even in the low amounts.
The presence of In and/or Fe admixtures also affects the concentration of Au in synthetic crystals of greenockite (Table 28; Figure 41 [111]). The concentration of Au in Inand Fe-doped greenockite is ten-fold higher in comparison with pure CdS. The distribution of Au, Ag, In, Cd, Se, Fe and Mn in sphalerite and greenockite is homogeneous according to the LA-ICPMS line mode spectra (Figures 34, 35 and 41).

Discussion
A significant part of the primary gold reserves in Russia and the world are sulphide ores ( [112][113][114] and references cited therein). Many sulphide ores are classified as refractory by technologists. It is the deposits of refractory sulphide ores that are the main potential source of gold production. The refractory gold and silver in sulphide ores can be associated with micro-and nanoinclusions of gold and silver minerals as well as isomorphous, colloidal, surface and adsorbed species of NM. However, the forms of invisible gold and other NM are still insufficiently investigated.
A series of authors' papers [9,15,24,25,39,63,67,80] were devoted to the problem of invisible and microscopic gold in sulphides. Based on the study of the VMS ores of the Urals (e.g., [15,25,67,115]), Rudny Altai [76,116] and modern hydrothermal systems of the ocean floor [76,[117][118][119], it was concluded that gold was primarily manifested in an invisible form, mainly in iron and copper sulphides. Gold enlargement with its release in the form of its own minerals occurred during later epigenetic hydrothermal processes and metamorphism. Native gold and Au-Ag tellurides, sulphides and selenides are found as microinclusions in base metal sulphides, particularly in pyrite, marcasite, chalcopyrite, sphalerite and arsenopyrite sulphosalts [112]. The size and shape of gold particles and their 3D mineral associations within ore samples were established by X-ray tomography [120].
Invisible gold should include gold in the form of a solid solution or an isomorphous impurity that is part of the structure of the Au-Ag-bearing matrix minerals. Invisible gold also includes fullerenes, colloids, clusters and surface-bound gold < 1 nm [15,121,122].
In gold deposits of the Urals, the portion of invisible gold is usually small, for example, only 1-16% in the mesothermal Berezovsk deposit and ~20-30% of the bulk Au in the Vorontsovka Carlin-style gold deposit [9,63,105], but it can also be very high. The data of local analyses (LA-ICPMS, INAA, EPMA, SEM/EDS) allow us to estimate the portion of bound Au in pyrite ~60% of the bulk Au of ores from the Novogodnee-Monto gold-skarn deposit [40], and even more, ~80% of the bulk Au for the Petropavlovsk goldporphyry deposit [39,40].
Based on the composition, pyrites of the Berezovsk deposit can be divided into two groups: (1) Au-bearing, and (2)

Discussion
A significant part of the primary gold reserves in Russia and the world are sulphide ores ( [112][113][114] and references cited therein). Many sulphide ores are classified as refractory by technologists. It is the deposits of refractory sulphide ores that are the main potential source of gold production. The refractory gold and silver in sulphide ores can be associated with micro-and nanoinclusions of gold and silver minerals as well as isomorphous, colloidal, surface and adsorbed species of NM. However, the forms of invisible gold and other NM are still insufficiently investigated.
A series of authors' papers [9,15,24,25,39,63,67,80] were devoted to the problem of invisible and microscopic gold in sulphides. Based on the study of the VMS ores of the Urals (e.g., [15,25,67,115]), Rudny Altai [76,116] and modern hydrothermal systems of the ocean floor [76,[117][118][119], it was concluded that gold was primarily manifested in an invisible form, mainly in iron and copper sulphides. Gold enlargement with its release in the form of its own minerals occurred during later epigenetic hydrothermal processes and metamorphism. Native gold and Au-Ag tellurides, sulphides and selenides are found as microinclusions in base metal sulphides, particularly in pyrite, marcasite, chalcopyrite, sphalerite and arsenopyrite sulphosalts [112]. The size and shape of gold particles and their 3D mineral associations within ore samples were established by X-ray tomography [120].
Invisible gold should include gold in the form of a solid solution or an isomorphous impurity that is part of the structure of the Au-Ag-bearing matrix minerals. Invisible gold also includes fullerenes, colloids, clusters and surface-bound gold < 1 nm [15,121,122].
In gold deposits of the Urals, the portion of invisible gold is usually small, for example, only 1-16% in the mesothermal Berezovsk deposit and~20-30% of the bulk Au in the Vorontsovka Carlin-style gold deposit [9,63,105], but it can also be very high. The data of local analyses (LA-ICPMS, INAA, EPMA, SEM/EDS) allow us to estimate the portion of bound Au in pyrite~60% of the bulk Au of ores from the Novogodnee-Monto gold-skarn deposit [40], and even more,~80% of the bulk Au for the Petropavlovsk gold-porphyry deposit [39,40].
Based on the composition, pyrites of the Berezovsk deposit can be divided into two groups: (1) Au-bearing, and (2) virtually Au-free. Pyrite of the second group prevails; in this pyrite, Au occurs in the amount of 0.08-0.1 ppm in the mode of single peaks of the group of Ag-Pb-Cu-Sb, probably in the form of microinclusions of native gold in pyrite in intergrowths with galena and fahlore. The Au-bearing variety of pyrite of the first group may contain structurally bound Au (C Au up to 73.5 ppm, INAA; 21.8 ppm, LA-ICPMS). It is generally accepted that all gold in the deposit is free, i.e., it is found exclusively in the form of native gold. The new data obtained indicate, although rarely, the presence of finely dispersed, possibly chemically bound gold in pyrite. Such pyrite could be formed during late, relatively low-temperature processes, later than "ordinary" pyrite with a zonal distribution of Co, Ni and As, which carries gold only in the form of native gold inclusions.
In galena from the sulphide-quartz veins of the Shartash granite massif, Berezovsk gold field, there are correlations in the Ag-As-Se-Bi-Sn series (r = 0.8-0.9) and in the Bi-Te (r = 0.84) and Bi-Tl (r = 0.7) pairs. Since the distribution of the elements is uniform, we can assume that they enter the galena isomorphic according to the schemes: (Ag,Tl,Cu) + + (Bi,As,Sb,In) 3+ ↔ 2Pb 2+ , (Sn) 4+ + 2(Ag,Tl,Cu) + ↔ 3Pb 2+ , (Se,Te) 2-↔ S 2-. A high degree of correlation is observed in the Cu-Sb (r = 0.87), Cu-Ag (r = 0.72) and Ag-Sb (r = 0.89) pairs. The inhomogeneous joint distribution of Au with Cu and Sb may be related to the finely dispersed inclusions of bournonite CuPbSbS 3 since the latter is a characteristic common humbeite mineral, and its presence could contribute to the deposition of submicroscopic native gold.
In the Urals, about 25 million tons of VMS ores are processed per year, only 15-40% (for various plants) of the total gold is extracted and losses with pyrite concentrate and in the tailings of enrichment amount to 13-15 tonnes of Au per year [77].
For weakly metamorphosed VMS deposits, the following sequence of decrease in the invisible gold concentration in sulphides is observed: galena (up to 122 ppm, geom. Estimates of the portion of invisible gold in VMS deposits in the Urals vary widely (within 30-90% of the total gold of ores) at a concentration of such fine gold from 0.8 to 5 g/t [9,72]. Higher values (~65-85% of the bulk Au of ores) are typical for the portion of invisible gold of weakly metamorphosed ores [15,36]. With increasing metamorphism [57], the contents of Au and Ag in the main ore-forming sulphides (sphalerite, chalcopyrite, pyrite) generally decrease [25,112]. In most cases, the portion of invisible gold also decreases (~35-60% of the total ore volume), so there is an inverse correlation between the proportion of invisible gold and the increase in the degree of metamorphism of ores [9,57,89]. As a result of the recrystallisation of ores, the invisible gold is enlarged and passes into the visible state [67,94].
For the VMS deposits of the Urals, the Z-shaped variation of native gold composition ( Figure 30) probably reflects the continuous-discrete character of the Au-Ag solid solution. The presence of possible miscibility gaps in this binary system was discussed in [106,123,124]. Independent mineral phases with contrasting compositions were clearly recorded in the Ag-rich zone [123]. In [124], species with Ag < 25 wt% are referred to the zone of stable solid solutions, but at least two phases (Au 3 Ag and AuAg) are believed to be present in the Ag-depleted zone. According to [106], the existence of the Au 2 Ag compound is the most probable in nature. Our data obviously confirm these assumptions, although the occurrence frequency of electrum (AuAg) in the VMS deposits of the Urals is lower than that of Au 3 Ag and Au 2 Ag.
However, only in the last two decades, with the broad involvement of the LA-ICPMS method and other spectroscopic methods ( [6,7,14,17,19,92,[119][120][121][125][126][127][128], etc.), did a breakthrough in studying the forms of noble metal occurrence in sulphides become possible. LA-ICPMS or EPMA correlation analysis of the concentrations in a particular mineral usually gives uncertain conclusions. However, these methods predominate in ore geology (see, e.g., [129] or [130]). X-ray absorption fine structure spectroscopy allows getting more definite results, especially for synthesised sulphides [17,19,52,92], but the beam-time is more expensive and data processing is complicated by concentration levels of NM impurities in studied sulphides (see [15] for a discussion).
Below, we will focus on our data on the noble metal speciation in synthetic sphalerite and greenockite.

Conditions Conducive to the Formation of Au-Saturated Sphalerite
Sphalerite is not considered as a significant source of Au production during the mining and processing of ore deposits [8]. However, a noticeable amount of gold can enter sphalerite in an invisible form [15,25,117], especially in relatively high-temperature conditions realised in medium-and high-temperature types of deposits. The typical contents for natural sphalerite are in the range of 0.1-10 ppm Au [8,15,117,131]. It can contain microinclusions of native gold, especially at the deposits that have undergone syn-metamorphic remobilisation [25,71]. Pure sphalerite, in contrast to bornite and chalcopyrite, is one of the weakest Au absorbers [128]. Using the phase composition correlation principle, Lipko and co-authors calculated the solubility of Au in pure and Fe-bearing sphalerite, and the latter is 3.5 times higher than in pure mineral [128]. Thus, the goal of the first experimental study was to find suitable conditions for the formation of Au-bearing sphalerite. Thus, we also addressed the evaluation of the exact chemical form of Au in synthesised specimens.
In general, the results of our experiments ( [110], this study) clearly show that the concentration of Au is higher in sphalerite containing different impurity components than in its pure crystals. A similar pattern is observed in ore deposits ( [15,25,37], etc.) and modern hydrothermal fields on the oceanic floor ( [118,131,132], etc.). The entry of Au into sphalerite is favoured not so much by the low-temperature conditions of its crystallisation but by the supersaturated nature of the evolved magmatic fluids and the co-deposition with Au of other chemical elements, especially In, Cu and Mn.
For sphalerite, after a few series of synthesis experiments, we conclude that the main element that favours gold to "intrude" in the crystal structure of ZnS is indium [92,108,110]. Iron also affects the concentration of Au but to a lesser degree. We infer that it may be related to the valence state of the elements. Indium is a trivalent element and some recent works of our colleagues suggest that a minor fraction of iron in sphalerite is also trivalent (e.g., [133]). We propose that the same mechanism of coupled substitution as in the case of Cu [134,135] occurs in the Au-bearing-sphalerite: Au + + In 3+ ↔ 2Zn 2+ . Our recent XAFS results demonstrate that at high concentrations (0.03-0.2 wt% In), Au can exist in sphalerite in two forms: primarily as a nano-sized Au 2 S (or AuInS 2 ) cluster and secondarly, in the form of a solid solution (e.g., (Zn,Au)InS 2 ); according to the spectroscopic study, all trivalent In in sphalerite substitutes Zn, without any Au-In clustering [92]. Unfortunately, we were not able to observe the Fe 3+ in the sphalerite using synchrotron methods due to the extremely low concentration of this isotope.
The EPMA and LA-ICPMS data do not provide univocal information regarding the chemical state of Au and In in ZnS. However, the XAFS method is inadequate at concentrations below hundreds of ppm. Therefore, in contrast to arsenian pyrite and arsenopyrite, due to the low C Au in natural sphalerite, it is generally impossible to study the chemical state of NM in the mineral by this method (cf., [134]). EPMA and LA-ICPMS data complement the existing results (Figures 38, 39 and 41; Tables 27 and 28) since they do not contradict the conclusion that at least part of the gold in sphalerite exists in the form of solid solution [136]. This form may predominate at low concentrations of gold in sphalerite, especially in nature.
According to chemical analysis, the amount of Au increases instantly with the increase of the sulphur fugacity in cases when In was added to the system (Figures 38 and 39). This fact proves the mechanism of vacancies formation in cation subcell leading to the accumulation of additional gold and the existence of the following isomorphous scheme: 3Zn 2+ ↔ 2In 3+ + , described in [137]. In our synthesis experiments, adding more than 2.28 wt% indium leads to the formation of intergrowths of sphalerite and sulphospinel ZnIn 2 S 4 phase (Figure 40. In the absence of indium or any other impurities, C Au does not exceed 10 ppm (Figures 37 and 39).
Like natural samples of more chemically pure sphalerite, usually with minimal C Au (e.g., [15,25]), pure synthetic sphalerite is usually unsaturated by Au compared to the specimens with admixtures ( [128], this study). Tauson and co-authors [142] linked this phenomenon to the variation of the chemical bonding parameters due to incorporating of the additional metals in the structure (cf., [128]).
For synthetic crystals of Ag-bearing sphalerite doped by In (or without In), the XAFS data revealed that Ag exists in sphalerite mainly in the form of a solid solution according to the scheme 2Zn 2+ ↔ Ag + + In 3+ , also suggested for natural sphalerite; however, a part of Ag (<5%) exist in the Ag-bearing sphalerite in the sulphide form Ag 2 S ( [108]; cf., [130]). In the absence of In, XANES spectra show Ag mainly in the native element mode (formal oxidation state 0) and sulphide forms. At high sulphur fugacity in the experimental system, sulphide form Ag 2 S predominates [108]. Presence of submicron inclusions of laforêtite AgInS 2 in Ag-, In-bearing sphalerite is probable. This mineral has the same sphalerite structure and may form a solid solution series in ZnS-AgInS 2 system. Occurrence of AuInS 2 in Au-, In-bearing sphalerite can also be assumed. Solid solution ZnS-AuInS 2 is also possible in nature.
Thus, gold and silver both prefer accumulation in In-bearing sphalerite as a solid solution. However, in the absence of In, their dominant forms are Au 2 S, native Ag and Ag 2 S, respectively. This fact corresponds to the low solubility of Au and Ag in sphalerite solid solution [128].

Correlation of As and Au Contents in Pyrite
The positive correlation of the As and Au contents in As-pyrite is noteworthy (Figure 9), which can be considered as evidence of an isomorphic substitution of the Fe position in the As-pyrite lattice (cf., [143,144]). In recent years, this has been interpreted as Au sorption on the growing faces of pyrite crystals [143,145,146]. For example, at sediment-hosted Carlin-type ore, invisible gold in arsenic disulphides represents Au deposited from the metal-bearing fluid by chemisorption at As-rich, Fe-deficient growth surfaces and incorporated into the sulphide crystals in the mode of metastable solid solution [96]. It corresponds to our data for the Vorontsovka Carlin-style deposit ( Figure 9). However, in other cases, there is a weak negative correlation of Au with As according to LA-ICPMS data for pyrite of the Geita Hill, Kumtor and Witwatersrand Carbon Leader Reef giant gold deposits [127]. A similar situation (there is no positive Au/As correlation in pyrite or it is very feeble) is observed for many deposits everywhere [6,8,[31][32][33][34][35]43], as well as for many sulphide deposits of the Urals [36][37][38][39][40][41][42].
When analysing As-pyrite, two aspects should be considered: As and Au impurities in pyrite and their relationship. Thus, the maximum solubility of As in pyrite at 600 • C according to the experimental data of Clark [147] is 0.53 wt%, and As content reaches 9.3 wt% in natural pyrites [96,148], and some researchers detected up to 14 wt% As in pyrite (e.g., [149]). Moreover, most researchers note that the sum of the S and As contents in pyrite remains~66.7 at%, and they interpret low As concentrations (up to~1.2 wt% As) as a solid solution with local clustering of As atoms. However, at higher concentrations of As in pyrite (6-9 wt%), arsenopyrite domains were detected on the HRTEM images [150,151]. Another feature of the As-pyrites that should be noted is the inhomogeneity and, more often, zoning in the distribution of As, which can be caused by changes in the crystallographic orientation of the phase (different interplanar distances and changing parameters of the solid solution) and fluctuations in the contents of not only sulphur but also iron [18,152] as well as other impurities (see [15] for discussion). Perhaps this is consistent with the inhomogeneous type of conductivity in a single crystal [153].
In many cases, there is a linear positive dependence of the increase in the gold content in pyrite on the level of arsenic concentrations (e.g., [143]). Simultaneously, there are two clusters of Au content points in this dependence: in the range of 2-5 wt% As and 6 and above [149]. This fact, as well as in the case of the entry of only As impurities into pyrite [40], can probably be explained by a change in the structural form of the entry of Au into As-pyrite: at relatively low concentrations of As (2-5 wt%), gold enters the solid solution S 2 2− AsS AsS 3− , and at high concentrations of As, gold enters the pyrite and marcasite-structural (arsenopyrite) domains.
Unlike arsenic, the situation with tellurium in pyrite is quite different. Tellurium is also a common isomorphic impurity in pyrite, but its abnormally high contents are due to the occurrence of submicroscopic and nano-scale inclusions of Au-Ag tellurides (mainly hessite and petzite) in this mineral. For the Petropavlovsk gold-porphyry and Novogodnee-Monto Fe-Au-skarn deposits, similarly to in [143], a positive correlation of Au and Ag with Te in pyrite corresponds to the probable occurrence of tiny telluride grains in the smallest defects in the mineral (Figures 16 and 20). It is also proven by the presence of synchronous peaks of Te, Au and Ag on the graphs during laser ablation by the profile sampling mode (Figures 17 and 19) or bright points of their segregations on Te distribution pictures during mapping mode analysis (Figures 18a and 21).

Forms of Gold in Arsenopyrite
Intensive debates about the forms of gold in arsenopyrite of 1988-1989 ([95,154,155], etc.) remain unsettled (see reviews in [19,51], cf., [156,157]). Gold in the chemically bound, structural form commonly reaches its maximum values in this mineral (e.g., [12,128,155]). Simultaneously, there are opposite opinions on the peculiarities of the chemical composition of Au-rich arsenopyrite. In most studies, the ratio As/S>1 is noted in the composition of Au-bearing arsenopyrite [63,96,97,154,158]. However, in other papers, sulphuric (deficient in As) arsenopyrite is richer in Au [12,156,[159][160][161][162]. Some authors reported about As-and S-rich arsenopyrites both rich in gold.
For the Vorontsovka deposit, the data obtained for arsenopyrite crystals in Carlin-style gold-sulphide assemblage indicate the presence of an inverse correlation between the Au and Fe contents and a direct correlation for Au/As (Figure 42; cf., [96]). Besides, this dependence is manifested not only at the local level within a single crystal, but also in the whole deposit, because more arsenic Apy-2, according to the point analysis by the LA-ICPMS method, contains more gold (4-315 ppm, mean geom. 24.3 ppm, [63]). The LA-ICPMS profiles for pyrite showed that high Au concentrations commonly correlate with higher contents of As, Ag, Sb, Se or Tl (Figures 6-8; cf., [63]). consistent with the inhomogeneous type of conductivity in a single crystal [153].
In many cases, there is a linear positive dependence of the increase in the gold content in pyrite on the level of arsenic concentrations (e.g., [143]). Simultaneously, there are two clusters of Au content points in this dependence: in the range of 2-5 wt% As and 6 and above [149]. This fact, as well as in the case of the entry of only As impurities into pyrite [40], can probably be explained by a change in the structural form of the entry of Au into As-pyrite: at relatively low concentrations of As (2-5 wt%), gold enters the solid solution S2 2-AsS AsS 3-, and at high concentrations of As, gold enters the pyrite and marcasitestructural (arsenopyrite) domains.
Unlike arsenic, the situation with tellurium in pyrite is quite different. Tellurium is also a common isomorphic impurity in pyrite, but its abnormally high contents are due to the occurrence of submicroscopic and nano-scale inclusions of Au-Ag tellurides (mainly hessite and petzite) in this mineral. For the Petropavlovsk gold-porphyry and Novogodnee-Monto Fe-Au-skarn deposits, similarly to in [143], a positive correlation of Au and Ag with Te in pyrite corresponds to the probable occurrence of tiny telluride grains in the smallest defects in the mineral (Figures 16 and 20). It is also proven by the presence of synchronous peaks of Te, Au and Ag on the graphs during laser ablation by the profile sampling mode (Figures 17 and 19) or bright points of their segregations on Te distribution pictures during mapping mode analysis (Figures 18a and 21).

Forms of Gold in Arsenopyrite
Intensive debates about the forms of gold in arsenopyrite of 1988-1989 ([95,154,155], etc.) remain unsettled (see reviews in [19,51], cf., [156,157]). Gold in the chemically bound, structural form commonly reaches its maximum values in this mineral (e.g., [12,128,155]). Simultaneously, there are opposite opinions on the peculiarities of the chemical composition of Au-rich arsenopyrite. In most studies, the ratio As/S>1 is noted in the composition of Au-bearing arsenopyrite [63,96,97,154,158]. However, in other papers, sulphuric (deficient in As) arsenopyrite is richer in Au [12,156,[159][160][161][162]. Some authors reported about As-and S-rich arsenopyrites both rich in gold.
For the Vorontsovka deposit, the data obtained for arsenopyrite crystals in Carlinstyle gold-sulphide assemblage indicate the presence of an inverse correlation between the Au and Fe contents and a direct correlation for Au/As (Figure 42; cf., [96]). Besides, this dependence is manifested not only at the local level within a single crystal, but also in the whole deposit, because more arsenic Apy-2, according to the point analysis by the LA-ICPMS method, contains more gold (4-315 ppm, mean geom. 24.3 ppm, [63]). The LA-ICPMS profiles for pyrite showed that high Au concentrations commonly correlate with higher contents of As, Ag, Sb, Se or Tl (Figures 6-8; cf., [63]). In general, the set of data obtained for the samples of synthetic [18] and natural ( [63], this study) arsenopyrites shows a weak positive correlation of Au contents with the As/S ratio and a clear negative correlation between Au and Fe. The higher Au contents are typical for the As-rich and close to stoichiometric late arsenopyrite generation, while in S-rich early arsenopyrite, the Au content normally does not exceed 0.02 wt%. However, overall, early generation arsenopyrite (Apy-1) also contains fairly high C Au . Gold is mainly concentrated in the As-rich, low-thickness zones of its prismatic crystals (Figures 7 and 8).
The incorporation of lattice-bound Au into arsenopyrite is resulted from the substitution of Fe by Au as both elements show negative correlation (cf., [5,157,160,161]). Au-bearing arsenopyrite is commonly thinly zonal. Unstable conditions with short-period oscillations of local disequilibrium and some fluctuations in the fluid component fugacities (f S 2 , f Te 2 , f O 2 , f As 2 ) contributed to the crystallisation of the mineral with a less perfect structure, which in turn favoured the entry of gold into the composition of arsenopyrite. Auriferous zones of arsenopyrite can contain nm-sized gold particles: TEM study in FIB foils [162] identified two types of nm-sized gold particles-elongated, rod-like (or discshaped?) Au grains about 35 nm in length and 5 nm in thickness, and roundish gold (or disc-shaped?) grains about 10 nm in diameter.

Crystal-Chemical Basics of Noble Metal Speciation in Sulphides
Several concepts are employed to describe crystal chemistry and to build structural classification schemes of the sulphides. Since the aim of this review is not a detailed analysis of the crystal structures of sulphides, we will only briefly describe the structures of the considered minerals with some preliminary analysis of the possibilities of isomorphic substitution of cations with noble metals (mainly Au, Ag, Pd and Pt). Table 29 lists the main crystallographic features of the studied minerals. With the exception of arsenopyrite FeAsS and chalcopyrite CuFeS 2 , all the studied minerals are binary compounds. Pyrite and galena belong to the structure type of NaCl, marcasite and arsenopyrite to the structure type of TiO 2 -rutile, sphalerite and chalcopyrite to the structure type of sphalerite, greenockite and würtzite to the structure type of ZnO and pyrrhotite to the structure type of NiAs.
Principle schemes of the crystal structures of the considered minerals are shown in Figure 43.
In general, the set of data obtained for the samples of synthetic [18] and natural ( [63], this study) arsenopyrites shows a weak positive correlation of Au contents with the As/S ratio and a clear negative correlation between Au and Fe. The higher Au contents are typical for the As-rich and close to stoichiometric late arsenopyrite generation, while in Srich early arsenopyrite, the Au content normally does not exceed 0.02 wt%. However, overall, early generation arsenopyrite (Apy-1) also contains fairly high CAu. Gold is mainly concentrated in the As-rich, low-thickness zones of its prismatic crystals (Figures 7 and 8).
The incorporation of lattice-bound Au into arsenopyrite is resulted from the substitution of Fe by Au as both elements show negative correlation (cf., [5,157,160,161]). Au-bearing arsenopyrite is commonly thinly zonal. Unstable conditions with short-period oscillations of local disequilibrium and some fluctuations in the fluid component fugacities (f S2, f Te2, f O2, f As2) contributed to the crystallisation of the mineral with a less perfect structure, which in turn favoured the entry of gold into the composition of arsenopyrite. Auriferous zones of arsenopyrite can contain nm-sized gold particles: TEM study in FIB foils [162] identified two types of nm-sized gold particles-elongated, rodlike (or disc-shaped?) Au grains about 35 nm in length and 5 nm in thickness, and roundish gold (or disc-shaped?) grains about 10 nm in diameter.

Crystal-Chemical Basics of Noble Metal Speciation in Sulphides
Several concepts are employed to describe crystal chemistry and to build structural classification schemes of the sulphides. Since the aim of this review is not a detailed analysis of the crystal structures of sulphides, we will only briefly describe the structures of the considered minerals with some preliminary analysis of the possibilities of isomorphic substitution of cations with noble metals (mainly Au, Ag, Pd and Pt). Table 29 lists the main crystallographic features of the studied minerals. With the exception of arsenopyrite FeAsS and chalcopyrite CuFeS2, all the studied minerals are binary compounds. Pyrite and galena belong to the structure type of NaCl, marcasite and arsenopyrite to the structure type of TiO2-rutile, sphalerite and chalcopyrite to the structure type of sphalerite, greenockite and würtzite to the structure type of ZnO and pyrrhotite to the structure type of NiAs.
Principle schemes of the crystal structures of the considered minerals are shown in Figure 43.  The coordination of cations in the structures of sulphides depends on their size, charge and electron configuration [163,164]. Table 30 lists the hybridisation type and coordination numbers of the cations in the structures of sulphides. The coordination polyhedra of cations in the considered minerals are tetrahedra (sphalerite and ZnO structure types, CN = 4) and octahedra (rutile, NaCl and NiAs structure types, CN = 6).  Table 31 lists the main structural characteristics of the sulphides of Au, Ag, Pd and Pt, and corresponding crystal structures are shown in Figure 44. Me-S chains are the main structural elements in the structures of Au 2 S and Ag 2 S. Me-S chains in the structure of Au 2 S form a three-dimensional framework. The whole structure can be considered as an anti-cristobalite structure, i.e., a three-dimensional framework of anion-centered vertexsharing SAu 4 tetrahedra. Ag in the structure of Ag 2 S occupies two positions. AgI forms Ag-S chains along direction b of the unit cell. These chains are linked by AgII ions. Octahedron [179] The coordination polyhedra of AgII is a coplanar triangle (Figure 44d). In terms of anion-centered polyhedra, the structure of α-Ag 2 S is a set of layers composed of vertexshared SAgI 2 AgII 3 tetragonal pyramids. The layers are connected into a three-dimensional framework by AgI atoms. Two types of coordination polyhedra appear in the structures of Pt and Pd sulphides-a planar square in the PtS and PdS structures and an octahedron in the PtS 2 and PdS 2 structures (Figure 44e,g).
Speaking of isomorphic substitution, one must consider a difference in the sizes (ionic or atomic radii) and interatomic distances of host cations and their substitutes. The ionic radii of metals under question with coordination number 6 increases in the following order [180]: Pt < Pd < Ag <Au, which implies that Pt should be the most favourable and, probably, the most abundant admixture in the sulphides, while Au should be hypothetically the less probable isomorphic admixture. In addition, we have to compare Me-S and Me-Me (next nearest cation) distances in the structures of host sulphides and sulphides of the considered noble metals. Me-S distances in the sulphides listed in Table 29 vary between 2.231 Å (arsenopyrite) and 2.903 Å (galena), while Me-S distances in the sulphides of noble metals listed in Table 30 vary between 2.174 Å (Au 2 S) and 2.699 Å (α-Ag 2 S). It is worth noting that Me-Me distances are not correlated with the Me-S distance, i.e., Fe-S distance in pyrite equals~2.26 Å, and the Fe-Fe distance equals~3.83 Å, while Fe-S distance in arsenopyrite equals~2.23 Å and Fe-Fe distance equals~2.73 Å. However, in both base metal sulphides (except for galena) and noble metal sulphides (except for PdS 2 ), larger values of Me-Me distances correspond to smaller values of Me-S distances. The effect is not pronounced but can be detected.   X-ray absorption spectroscopy (XAS) investigations of minerals and their synthetic analogues showed that Au, Ag and Pt could form solid solutions with covellite [17], pyrite [19,22], arsenopyrite [18,19], pyrrhotite [23] and sphalerite [19,92]. However, the concentration of the admixtures is small. Thus, the measured maximum content of Au in natural and synthetic pyrite is~300 and~90 ppm accordingly. The concentration of Au in löllengite is about 800 ppm. The concentration of Ag in sphalerite is no more than 5 wt%. Synthetic pyrite hosts up to 7 wt% of Pt in the solid solution state. The measured concentrations of the considered admixtures in base metal sulphides correlate well with the ionic radii of the admixtures: the smaller the cation is, the higher its content in a host mineral. However, some experimental facts cannot be solely explained by the size factor. For example, the maximum content of Pt (the Pt 2+ -S distance equals 2.31 Å, Table 30) in the solid solution state in pyrite is 7 wt% and is only 0.5 wt% in pyrrhotite [23], while the Fe-S distance is~2.26 Å in pyrite and~2.52 Å in pyrrhotite. Besides, the formation of the solid solution depends on the conditions of synthesis: Au forms a solid solution with pyrite in hydrothermal experiments only, and in the pyrites obtained using the salt flux technique, Au forms metal particles.

Conclusions
(1) For VMS and Carlin-style deposits as well as for gold-porphyry systems, avalanche deposition of sulphides on geochemical barriers in shallow-depth conditions from moderateand high-concentrated solutions contributed to the rapid growth of imperfect and thinzoned crystals, providing the entry of NM in sulphides in finely dispersed form (in the invisible state). Admixtures of metals and metalloids supplied both the increased defectiveness of crystal structure of the minerals (making them chemically more capacious in relation to NM), and the entry of NM impurities into sulphides by the mechanism of heterovalent isomorphism.
(2) For weakly metamorphosed VMS deposits, the following sequence of decrease in invisible gold concentration in sulphides was revealed: galena-chalcopyrite-pyritemarcasite-sphalerite, in entire agreement with the hydrothermal experiments on the solubility of Au in sulphides (galena-bornite-chalcopyrite-pyrrhotite-pyrite-sphalerite). For highly metamorphosed VMS lodes, such sequence is pyrite-chalcopyrite-sphalerite/ arsenopyrite-fahlore-bornite-pyrrhotite-galena, and, in general, the concentration of C invis. gold in sulphides is noticeably lower.
(3) In arsenopyrite crystals, the contents of invisible gold reach their maxima, and Fe is negatively correlated with Au. The ionic form of gold probably prevails in this mineral. Thin compositional zoning reflects short-period oscillations of local disequilibrium during crystal growth and some fluctuations in the fluid component fugacities (f S 2 , f Te 2 , f Se 2 , f O 2 , f As 2 ).
(4) Epigenetic hydrothermal and metamorphic processes, as well as the slower crystallisation of sulphides, favoured the nucleation of gold, coalescence of its clusters and enlargement of nano-sized NM isolations, and supported the formation of relatively large nuggets as a result of collective recrystallisation. Most Au deposits (except for Carlin-type and gold-porphyry deposits) originated at considerable depths, and the crystallisation of sulphides was in relatively stable conditions, providing the formation of more perfect sulphide crystals as well as co-crystallisation of proper NM minerals (except for the strongly scattered PGE), and a small fraction of NM remains in the sulphides in finely dispersed form (in the invisible state).
(5) Proper NM minerals are represented by groups of minerals that are similar, both for VMS and large gold deposits of the Urals: native gold (high-fineness gold and electrum), with sharply subordinated Ag-sulphosalts, Ag-, Au-and Au-Ag-tellurides, and to a lesser extent, native silver, Au-Ag-sulphides (petrovskaite, uytenbogaardtite, acanthite), Auantimonide (aurostibite), Ag-sulphobismuthite (matildite) and Ag-selenotelluride (kurilite). In very sporadic cases, abundant NM-tellurides are comparable as host NM-minerals with native gold, in bulk ore composition (Svetlinsk deposit, etc.). (6) Our experiments showed that admixture of In increases the solubility of Au in sphalerite up to 1 wt%. C Au in sphalerite is higher (up to 1000 times) in samples synthesised at a higher (up to 10 bar) f S 2 in the system. Fe impurity also promotes the incorporation of Au in sphalerite (C Au up to 0.01 wt%). In sphalerite synthesised under the same conditions without In, Fe, etc., C Au does not exceed 0.001 wt% and does not depend on f S 2 . EPMA and LA-ICP-MS analyses, revealing a homogeneous distribution of all studied elements, showed a clear positive correlation between the In, Fe and Au contents in sphalerite, as well as X-ray absorption spectroscopy confirmed the isomorphic entry of these elements according to the following scheme: Au + + In 3+ (Fe 3+ ) ↔ 2 Zn 2+ . A positive effect of f S 2 on the solubility of Au indicates the formation of vacancies in the cation sublattice, proving the existence of the second isomorphic scheme: 3Zn 2+ ↔ 2In 3 + + . A part of Au doped in ZnS In forms Au 2 S clusters, according to the XAFS study. In general, the same pattern was noted for Ag in sphalerite, where Ag 2 S and Ag 0 forms oCCur. Greenockite samples were used as model crystals for the würtzite type of structure, and the results confirm the same behaviour of Au in the presence of In and Fe as in sphalerite. In the samples consisting of both ZnS polytypes, C Au does not exceed 50 g/t, and the distribution of doped elements is inhomogeneous. of LA-ICPMS analyses in Université du Québec à Chicoutimi, Canada. The manuscript was improved through constructive and helpful reviews from the Academic Editor and anonymous reviewers.

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