Palladian Gold: Chemical Composition, Minerals in Association, and Physicochemical Conditions of Formation at Different Types of Gold Deposits

: This paper reviews and summarizes the available information on the composition of pal-ladian gold with various contents and sets of isomorphic impurities (Ag, Cu, Hg) at 50 deposits and ore occurrences with Au-Pd mineralization. It is revealed that Palladian gold is represented by the systems Au–Pd, Au–Pd–Hg, Au–Pd–Cu, and Au–Pd–Ag–Hg, but more frequently corresponds to Au–Pd–Ag, Au–Pd–Ag–Cu, and Au–Pd–Ag–Cu–Hg. Objects with palladian gold belong to different types of gold deposits and to the deposits at which the main components of ores are PGE, Cr, Cu, Ni, V, and Ti. We propose a classification of the types of deposits with palladian gold: (1) PGE ore deposits related to mafic–ultramafic magmatic complexes (two subtypes—(a) low-sulfide-grade (less than 2%–5% sulfides) Alaskan, and (b) high-sulfide-grade (more than 5% sulfides) Norilsk); (2) orogenic gold deposits (OG); (3) epithermal (porphyry) gold–copper deposits (EPGC); (4) iron oxide copper gold deposits (IOCG); (5) ferruginous quartzite deposits; (6) volcanic exhalation; and (7) gold-PGE placers of five subtypes corresponding to the types of 1–5 primary sources. Palladian gold is mainly high-fineness (910‰–990‰), is less frequently medium-fineness, and contains Ag and Cu, but does not contain Hg at the deposits of types 1, 3, and 4. The only exception is the Au-Pd-Hg Itchayvayam ore occurrence (Kamchatka, Russia), for which two varieties of Pd,Hg-bearing native gold (fineness 816‰–960‰ and 580‰–660‰) are determined. Low-fineness palladian gold with the major content of Ag is typical of OGD deposits. Medium-fineness palladian gold occurs at fer-ruginous quartzite deposits and in volcanic exhalations. Hg, Ag, Cu-bearing high-fineness palladian gold is present mainly in placer deposits (type 7). The most common minerals in association with palladian gold are arsenides, stibioarsenides, sulfides, stannides, bismuthides, tellurides, and selenides of Pd and Pt. These are typical of deposit types 1 and 7. The minerals of Au, Ag, and Cu (tetra-auricupride, aurostibite, chalcopyrite,


Methods
Impurity elements in native gold are determined using chemical, spectral, electron probe microanalyses (EPMA), inductively coupled plasma-mass spectrometry (ICP-MS), and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The chemical and spectral methods as well as ICP-MS require a considerable amount of native gold and its purification from other minerals. Systematic studies of the typomorphism of native gold from different territories of Russia and the Commonwealth of Independent States, with the application of semi-quantitative ICP-MS analysis carried out in Russia [14,80], showed the presence of 70 chemical elements in it. The drawback of the chemical, spectral, and ICP-MS methods is the inability to separate isomorphic impurities and mineral microinclusions in native gold.
EPMA is used for quantitative analysis of the elemental composition of native gold at a micrometer scale. The detection sensitivity of many elements in the local electron raster microprobe analysis is limited by concentrations no less than 0.1-0.01 wt.%. A more complex composition of native gold can be detected by LA-ICP-MS, the sensitivity of which is considerably higher than that of EPMA. With the appearance of the LA-ICP-MS technique [81] for determining the composition of microelements in solid samples, data on micro-and trace elements in native gold were obtained [35,[82][83][84][85][86][87][88][89][90]. The laser ablationinductively coupled plasma-mass spectrometer has high sensitivity and low detection levels required for measuring microelements in amounts up to ppb (1 per billion). One of the advantages of this technique is the possibility of cleaning the sample surface. LA-ICP-MS is widely used by foreign scientists to study the composition of gold and has not been used for studying gold in Russia yet. According to Chapman et al. (2023) [91], limits of detection (LODs) for Pd and Cu were typically around 900 ppm and 200 ppm, respectively, whilst the avoidance of spectral interference between the HgMα and AuMβ X-rays necessitated using the HgMβ X-ray line and the associated higher detection limit of 3000 ppm.
In the majority of studies, the chemical composition of native gold was investigated using EPMA, and less frequently, LA-ICP-MS. The concentration of elements Ag, Cu, Pd, and Hg in native gold is commonly measured using ЕРМА. The spectrum accumulation time in electron probe microanalysis is 20 or more-60-80 s. Macro-impurities in native gold are determined using SEM/EDS, which allows one to determine Ag, Cu, Pd, and Hg with detection limits of 100-1000 ppm.
We analyzed the samples of ores and individual gold grains with Pd-bearing gold from the Chudnoe, Volkovskoe, and Ozernoe deposits (Urals, Russia) and from placers and ore occurrences of the mafic-ultramafic Itchayvayam massif (Kamchatka, Russia) and Au-Pd Bleida Far West deposit (Morocco). Chemical analyses of native gold and other minerals were conducted at the Analytical Center for Multi-elemental and Isotope Research in the IGM SB RAS (Novosibirsk, Russia) by electron probe microanalysis (EPMA) using a MIRA 3 LMU scanning electron microscope (Tescan Orsay Holding, Brno, Czech Republic) equipped with an X-ray energy-dispersive spectrometer (EDS) AZtec Energy XMax-50 (Oxford Instruments Nanoanalysis, Oxford, UK) (analysts Dr. N. Karmanov, M. Khlestov). The composition of native gold was studied at the following parameters: accelerating voltage was 20 kV and live spectrum acquisition time was 60 s (total area of spectra ~10 6 counts). The following X-rays were selected: K series for Fe, Cu, and As and L series for Pd, Ag, Sb, Au, and Hg. We used pure metals (Fe, Cu, Pd, Ag, Au) and InAs for As and HgTe for Hg as the standards. The detection limits (in wt.%) were 0.1 Fe, 0.15 Cu, 0.25 Pd, Ag, Sb, 0.3 As, 0.6 Au, and 0.8 Hg. The error in determining the main components with contents higher than 10 wt.% did not exceed 1 relative (rel.) %, and when the content of components ranged from 2 to 10 wt.%, the error was no higher than 6-8 rel. %. Close to the limit of detection, the error was 15-20 rel.%. In some cases, the spectrum acquisition time increased to 120 s, and the lower limits of determined contents and the random error of the analysis decreased about 1.4 times. This review is primarily based on the EPMA results. A significant amount of EPMA data was taken from the published literature.

Composition, Fineness, and Mineral Associations of Palladian Gold
Palladian gold rarely consists of the binary Au-Pd system; it most frequently contains the ternary Au-Pd-Ag, Au-Pd-Cu, and Au-Pd-Hg, and even more complex systems Au-Pd-Ag-Cu, Au-Pd-Ag-Hg, and Au-Pd-Ag-Cu-Hg. Several varieties of palladian gold were found at many deposits. Along with palladian gold, Ag-bearing gold and other varieties of native gold are present at these deposits. Below, we present data on the composition of palladian gold in the binary (Au-Pd), ternary (Au-Pd-Ag, Au-Pd-Cu, Au-Pd-Hg), quaternary (Au-Pd-Ag-Hg, Au-Pd-Ag-Cu), and five-element (Au-Pd-Ag-Cu-Hg) systems, as well as minerals in association and examples of deposits.
According to the "50 mole % rule" for the mineral nomenclature recommended by the Commission on New Minerals and Mineral Names (CNMMN) of the International Mineralogical Association (IMA) [92], the formula composition of palladian gold can be represented as Au1-xPdx, where x < 0.5 atomic part, when calculating the formula for 1 atomic unit. Au-Pd solid solutions with Ag, Cu, and Hg impurities can be present as Au1х-y-z-dPdхAgyCuzHgd, where 0 < x (Pd atomic part in native gold) < 0.5; y, z, d in total ≤0.5 − x atomic parts; and 1 − x − y − z − d > x, y, z, d. The fineness of native gold is calculated by the equation (1000*Au/(Au + Pd + Me1 + Me2 + and so on), where Au, Pd, and other metals (Ag, Cu, Hg, Pt, Ni, Fe) are expressed as wt.%).
The set of minerals in association with palladian gold from the Uderei deposit (Russia) and placer ore occurrences of Lammermuir Hills (Scotland) are partially similar in that both objects contain aurostibite, arsenopyrite, galena, and argentian gold. Tetra-auricupride is present in paragenesis with palladian gold at the Krutoe ore occurrence (Russia) and the Lammermuir Hills deposit (Scotland), which suggests not only Au-Pd but also Cu-Pd mineralization.
It is worth noting the high concentrations of Pd (21.3 wt.%, Au2Pd) in gold particles from gray fumaroles and in andesite lavas (Q) of Ebeko volcano (Russia) [43] (Table 1). Au7Pd (928‰), Au3Pd (847‰), and Au2Pd (790‰) phases and Au-bearing palladium are present in the Au-Pd mineralization of the Serra Pelada deposit, Brazil [25]. The Au3Pd phase as small inclusions was identified in chromite from the Cedrolina Chromitite (Goiás State, Brazil) [75]. Pure gold and Au3Cu3Ni alloy were also found at the boundary between chromite and talc or in the silicate matrix of the chromitite ores.
The reported results show that Pd concentrations in gold in the absence of other impurities vary in a wide range from minor (3.3 wt.%, 970‰) up to Au0.50Pd0.50 (35.1 wt.%, 649‰). The compositions of this gold are arranged on the Au-Pd side of the ternary diagram of Au-Pd-Ag ( Figure 2). This diagram also shows the compositions of palladian gold with and without Ag, which are widespread at other deposits (there are 24 of them; see capture in Figure 2). Summarized literature data on the compositions of palladian gold with the presence of Ag at these deposits are presented in Section 4.1.2.

Pd,Ag-Bearing Gold and Au-Pd-Ag System
Palladian gold with Ag impurity (Ag-bearing palladian gold or Pd, Ag-bearing gold) covers the range of solid solutions from Au to Au1-х-yPdхAgy, where y + x ≤ 0.5 atomic fraction, x ≠ 0, y ≠ 0, and 1 -x -y > x, y. As an example, Table 2 contains data on five deposits and occurrences with Ag-bearing palladian gold.
At the Au-Pd Brownstone deposit (England), native gold is present in a vertical quartz-calcite vein hosted by Devonian slates [20] and is represented by two generations (Ag < 0.5 wt.%): high-fineness gold (970‰-994‰) with low content of Pd < 2.4 wt.% (core) and lower-fineness gold (882‰-924‰) with a higher content of Pd up to 11.2 wt.% (border), in association with clausthalite, tiemannite, and eucairite. The low contents of Ag 1.1-4.8 and 0.2-1.9 and the high contents of Pd ≈ 7 wt.% and up to 14.2 wt.% are typical of native gold at the Au-Pd Stillwater (USA) and Hope's Nose (England) deposits (Table 2), respectively. The trace and low contents of Pd < 0.15 and < 2.76 wt.% and high contents of Ag 21.4-22.6 wt.% and 10.9-19.7 are typical of native gold in ferruginous quartzites, accompanying metasomatites, and in the zones of increased sulfidation at the Au-Pd-Pt Lebedinskoye deposit [59] and in the ores of the Fedorova Tundra Pt-metal deposit (Russia) [58], respectively. Table 2. The composition of Ag-bearing palladian gold, its fineness (NAu), and minerals in association at endogenous deposits.
The Au-Pd-Ag diagram ( Figure 2) shows the compositions of Ag-bearing palladian gold, and other minerals and phases of this ternary system found in the ores of some deposits.
Ag-bearing palladian gold of the Au-Pd-Ag system depending on the Pd and Ag contents (Figures 2 and 3) can be divided into three varieties: (1) Au-Pd solid solution in the absence of impurities (fineness > 649‰, LOD < Pd < 50 at.%). Such gold occurs at the following deposits: Uderei, Norilsk-1, Talnakh

Pd,Cu-Bearing Gold and Au-Pd-Cu System
Palladian gold with Cu impurity (Cu-bearing palladian gold or Pd,Cu-bearing gold) has a composition covering the range of solid solutions from Au to Au1-х-zPdхCuz, where x + z ≤ 0.5 atomic part, z ≠ 0, x ≠ 0, and 1 − x − z > x, z. As an example, Table 3 shows data for two objects with native gold containing Pd and Cu: the Au-Pd-Cu ore occurrence of the Skaergaard massif (Greenland, Denmark) [73] and the placer deposit of the Ambositra region (Madagascar) [70]. Table 3. The composition of Cu-bearing palladian gold, its fineness (NAu), and minerals in association at endogenous and exogenous deposits.

Name of Deposit (Location)
NAu Phases compositionally similar to Pd-Au3Cu and to Pd-tetra-auricupride in association with skaergaardite, bornite, chalcocite, chalcopyrite, kotulskite, palladoarsenide, vasilseverginite, and vysokýite occur at the Skaergaard layered ultramafic massif (Greenland, Denmark) [73]. In the Au-Cu-Pd-phases, the presence of Pt and Fe was observed (Table 3). Compounds close to stoichiometric (Au,Pd)3Cu are attributed to the Au3Cu phase containing on average 24.3 at.% Cu and 15.8 at.% Pd (n = 76). A more numerous (n = 132) group, referred to as (Au,Cu,Pd) alloys containing 31 at.% Cu and 8 at.% Pd, is closely related to the above group. In the grains with the decomposition structure of the solid solution, the matrix contains 32.8 at.% Cu and 8.3 at.% Pd, and lamellas (AuCu phase) contain 44.1 at.% Cu and 8.5 at.% Pd (n = 13) [73].
The Cu content in palladian gold from the placers in the Ambositra region (Madagaskar) is low and varies from 0.6 to 2.8 wt.%, and the Pd content ranges from 1.1 to 7.9 wt.%. Palladium was not detected in tetra-auricupride. In association with tetra-auricupride and Cu-bearing palladian gold, Ba-Kfsp, Ca-REE phosphates were found [70].
Cu-bearing palladian gold can also contain Ag and (or) Hg. Examples of such deposits are described in Section 4.1.6.
Native gold with Pd and Hg impurities in the absence of Ag is quite rare. Table 4 shows data for two objects with Pd,Hg-bearing gold in the absence and presence of Ag in the Itchaivayam mafic-ultramafic massif (Russia) and Mataganya-Siguiri Zone (Guinea). In the weathering crusts of the Mataganya-Siguiri Zone (Guinea), native gold is represented by two varieties: (1) Hg-bearing high-fineness palladian gold with low contents of Pd~2.7 wt.% and Hg~0.1 wt.%, sometimes Pt up to 0.09 wt.%; (2) Ag,Hg-bearing middlefineness palladian gold with a high content of Ag~5.5-12.5 and low contents of Pd~1.1-1.8 and Hg up to 0.24 wt.% [69]. Palladian gold from the placers that occur in the layered Itchayvayam platinum-bearing mafic-ultramafic massif (northern part of the Koryak-Kamchatka platinum-bearing belt, Russia), according to preliminary data, has varying compositions of Pd,Hg-bearing gold [42]. Au-Pd-Cu ore occurrences of this massif were found to contain high-fineness argentian gold and Au-Hg-Pd phases, which form thin veins in bornite and are in paragenesis with platinum group minerals [24,96].
Hg-bearing palladian gold of both varieties-low Pd, Hg (1) and high Pd, Hg (2) contents were found at the Itchayvayam deposit. Native gold in the Mataganya-Siguiri Zone (Guinea) placer is an example of ore occurrence with high-fineness Hg-bearing palladian gold (with low Pd and Hg contents) (1). At some deposits, Pd,Hg-bearing gold has minor contents of Ag. As an example, in Table 4, we provide data on the composition of Pd,Hg,Ag-bearing gold from two alluvial placers related to sediments (conglomerates) and volcanic rocks-Zimnik Creek (Poland) and river Dart (England). The Hg content varies in the range of 0.07 to 13.9 wt.%, and the Ag content is lower at 2.2 wt.%. Native gold with Pd and Hg impurities and higher Ag contents is spread at many other deposits-Serra Pelada (Brazil) [97], Gongo Soco (Brazil) [78,98], Mayat river and Bol'shaya Kuonamka river (Russia) [47], and Dziwiszow (Poland) [65].
The compositions of palladian gold with Hg and Ag impurities cover the range of solid solutions from Au to Au1-x-y-dPdxAgyHgd, where x, y, d in total ≤ 0.5 atomic part, x ≠ 0, y ≠ 0, d ≠ 0, and 1 − x − y − d > x, y, d. Hg,Ag-bearing palladian gold is more widely distributed in nature than Pd,Hg-bearing gold (see Section 4.1.6).

Pd,Cu,Ag-Bearing Gold and Au-Pd-Ag-Cu System
Palladian gold with Cu and Ag impurities (Cu,Ag-bearing palladian gold or Pd,Cu,Ag-bearing gold) covers the range of solid solutions from Au to Au1-х-yPdхAgyCuz, where x, y, z in total ≤0.5 atomic part, x ≠ 0, y ≠ 0, z ≠ 0, and 1 − x − y − z > x, y, z. The group of deposits with Cu,Ag-bearing palladian gold is the most numerous group compared to other groups (Tables 1-4) and includes 18 objects among which there is only one placer deposit-Kuoyka river, basin of the Anabar river (Russia) ( Table 5).
Native gold in copper-sulfide ores of the Serebryansky Kamen from gabbro of the Serebryansky massif is heterogeneous and corresponds to Au-Ag (fineness 760‰-970‰) and Au-Cu-Pd-Ag solid solutions with high Pd and lower contents of Cu and Ag (fineness 780‰) ( Table 5) [54]. It forms inclusions in bornite and chalcopyrite and frequently occurs in association with Pd sulfides, arsenides, and tellurides.
Au-Cu alloys with significant amounts of Pt, Pd, and Ag impurities were reported from platinum placers associated with the Konder alkaline-ultramafic complex, where abundant Au-Cu alloys intergrow with PGM (particularly Pt-Fe alloy) and show a range of compositions close to Au3Cu and AuCu along with a single Cu3Au grain [44,45,110]. The Pt and Pd concentrations in these alloys reach 11.8 and 10.3 wt.%.

Name of Deposit (Location) NAu ‰/Impurity wt.% Minerals in Association
The Chudnoe deposit is the recharge source for the placer of the Al'kes-Vozh creek and, partly, for other placers downstream of the Balaban-Yu river. The presence of native gold with impurities of Pd (to 2 wt.%), Cu (to 5 wt.%), and Hg (to 0.6 wt.%) in the Al'kes-Vozh placer, as well as the presence of Pd minerals (mertieite, atheneite, stibiopalladinite) in gold grains, served as the basis for identifying a placer-forming type of gold mineralization in the Urals prior to the discovery of its endogenous source [52]. The occurrence of palladian gold in the Al'kes-Vozh placer is 81% and it gradually decreases to 13% in the Balban-Yu placers located below [115].
Ores from the Au-Pd-Pt Serra Pelada deposit (Brazil) contain up to 1 cm coarsegrained gold aggregates that occur in powdery, earthy weathered material [25]. They exhibit a delicate arborescent fabric and are coated by goethite. Four compositional types of palladian gold are recognized at this deposit: (1) "Au7Pd", the most abundant Au-Pd alloy, hosts Pd arsenides ("guanglinite" and Sb-bearing "guanglinite"), Pd-Pt-Se and Pd-Se phases, sudovikovite, and palladseite; (2) Au-Pd-Hg alloy, characteristically with atheneite and rarely observed Pb-bearing Pd-Hg-Se and Pd-Bi-Se phases; (3) Pd-poor goldisomertieite-Mn-Ba oxide assemblage; and (4) pure gold with trace Pd (>993‰) in goethite assemblage ( Table 6). The Ag content in these types of palladian gold is lower than 0.4 wt.%. The Pd,Ag-poor gold assemblage possibly formed later than the Au-Pd-Hg-Ag alloy. In the ores of the Fe-oxide-Cu-Au Gongo Soco deposit (Brazil), palladian gold with Ag, Cu, and Hg impurities occurs in the form of nuggets, grains, and their aggregates in association with hematite. Gold grains have dark-color coatings composed of Pd-O particles and iron hydroxide. The palladian gold is characterized by a variable Pd content reaching up to 6 wt.% and sporadically observed Hg content up to 1 wt.% [97]. Pd,Hgbearing gold at these deposits can also contain Cu and Ag impurities to 3 and more wt.% [78]. Palladian gold contains inclusions of platinum group minerals (PGMs): isomertieite, mertieite-II, chrisstanleyite, selenides Pd5(Hg,Sb,Ag)2Se6, and (Pd,Sb,Ag,Hg)5Se4.

Types of Deposits with Palladian Gold
Native gold occurs in a wide range of gold deposit types and settings [1,2,[5][6][7][8]. Impurity elements in native gold and minerals in association are indicators of geochemical and mineral types of deposits and regional geochemical conditions [13]. Petrovskaya (1973) [1] reported that the number and concentrations of impurity metals in native gold tend to increase at the deposits with a multistage formation of mineralization or with features of hybridism, and local and uneven changes in the composition of ore bodies with superimposed hydrothermal transformations.
The mechanism of formation of native gold is complex and depends on many factors. A wide range of impurity elements is associated with different geochemical environments in which the mobilization, transportation, and deposition of gold ore mineralization take place. The content of elements in native gold depends on their amounts in hydrothermal solutions, which are controlled by temperature, the presence of ligand elements-Cl, S, and Se and Te to a lesser degree-and also the pH of solutions and oxidation-reduction conditions [20,117]. Metals are largely derived from the mantle or crust by partial melting and fluid-related leaching. Ligands can be provided from the same sources, or from the atmosphere, hydrosphere, and biosphere. Gold and impurity elements can be transported not only by hydrothermal solutions and supercritical fluids, but also by gas mixtures, as well as by sulfide, silicate, and carbonate melts. The spectrum of element impurities and mineral microinclusions often depends on the formational affiliation of the gold deposit, on its connection with any magmatic complex and post-ore processes, and also on the metallogenic features of the gold-bearing provinces.
In this review, based on the literature data and our own studies of the composition of palladian gold from numerous deposits with Au-Pd mineralization, we suggest the following classification of types ( Table 7). The studied objects with palladian gold belong not only to gold deposits of different types but to deposits at which the main ore components are PGE, Cr, Cu, Ni, V, and Ti. In the proposed systematization, we do not claim innovations but report them based on materials from various authors for the convenience of presenting results. It includes the main types of PGE-deposits in mafic-ultramafic magmatic complexes [118] within one type with two subtypes-low-sulfide-grade (less than 2%-5% sulfides) Alaskan (1a) and high-sulfide-grade (more than 5% sulfides) Norilsk (1b) (base on Naldrett, 2010 [119]), and gold deposits of 2-4 types: orogenic gold (OG) (type 2a,b), epithermal (porphyry) gold-copper (EPGC) (type 3), and iron oxide-copper-gold (IOCG) (type 4) [119][120][121][122][123] (Table 7). To these types, we added ferruginous quartzites (type 5), which were identified by Dill (2010) [118] within the frames of an independent type of PGE-deposits, and a new non-industrial type of volcanic exhalations (Kamchatka) (type 6), in which gold with Pd impurity was found (Table 1). An important role in the studied objects belongs to gold from placer alluvial deposits and occurrences, which we subdivided into four subtypes (7-1 … 7-4) in accordance with the profile of their primary sources and a special subtype of placers associated with horizons of conglomerates and weathering crusts in the cover of large cratons (7-5). Table 7. Types and subtypes of deposits with palladian gold, its fineness, and impurities.

Type
Subtype (Numbers) Deposits (Country) Fineness (Impurities) 1. PGE ore deposits related to mafic-ultramafic magmatic complexes According to Figure 1, deposits with palladian gold occur in different geological and dynamic settings-terranes and orogenic belts and shields. Table 8 illustrates that palladian gold is found in the deposits related to different types of magmatism (but mainly with mafic-ultramafic) and in different periods. It is worth noting that the lacuna is more than a billion years old, coinciding with the so-called "dead billion" in the history of the Earth (1.7-0.7 Gya), when the stability conditions existed throughout the planet and a limited number of deposits formed, among which there were virtually no ore-bearing deposits (except IOCG type) [124].
Thus, the above-mentioned facts suggest that Au-Pd mineralization formed in different conditions and from different sources, from mantle riftogenic (oceanic), active margins, and island arcs (subduction) to orogenic accretion-collision and post orogenic (riftogenic).

Physicochemical Conditions for the Formation of Palladian Gold at Deposits of Different Types
Scarce data are available on the physicochemical conditions of the formation of palladian gold. We summarize the published information on the РТХ-parameters of the formation of Au-Pd mineralization known for some types of deposits in Table 9.
Type 1-PGE-deposits in mafic-ultramafic magmatic complexes. In Au-Pd ores at the Skaergaard massif, the formation of Au-Cu-Pd melts and Pd-tetra-auricupride is due to the decomposition of Au-Cu-Pd solid solutions at temperatures of 1200-1000 °С [73]. Au-Pd mineralization of the Volkovskoe and Baronskoe deposits has a hydrothermal-metasomatic genesis, was formed in the range of temperatures of 600-400 °С [55,106,139], and is related to superimposed metamorphic and metasomatic processes (occurrence of amphibole, serpentine, and chlorite) on olivine clinopyroxenites [107,108].
Data on fluid inclusion studies show that the crystallization of palladian gold in Cu-Ni-PGE ores of the Norilsk deposit proceeds with the participation of reduced hydrothermal fluids (CH4 ± C2H6, C2H2, and C3H8) at temperatures of 270-140 °С and salinities of 23.3-13.6 wt.% eq. NaCl [27]. Table 9. T,P,X-conditions for the formation of Au-Pd mineralization with Pd-bearing gold according to the results of the study of fluid inclusions (FIs) (reference data).
PGE ore deposits related to mafic-ultramafic magmatic complexes Type 2-orogenic gold deposits. Au-Pd mineralization at the Chudnoe deposit (Russia) could have been formed by chloride fluids of low and medium salinity at temperatures from 105 to 230 °С and pressures from 0.05 to 1.15 kbar. The salinity of fluids varies from 20.1 to 0.2 wt.% NaCl eq. [29] (Table 9). Au-Pd and Au deposits of the Permian-Triassic basins in SW England (Thorverton) formed in the temperature range from 60 to 155 °С, and the salinity of fluids varied from 2 to 30 wt.% NaCl eq. [140,141]. In the ores from the Au-Pd Zechstein deposit (Poland), an association of palladian gold with chalcosine, digenite, and djurleite was detected, which limits its formation temperature to a range of 93 to 103 °С [63,142,148] (Table 9).
Type 3-epithermal (porphyry) gold-copper deposits. Hematite-quartz veins of the Au-Pd Bleida Far West deposit (Anti-Atlas, Morocco) formed at temperatures from 145 to 325 °С and fluid salinities from 6 to 33 wt.% NaCl eq. The gas phase of fluids included CO2, CH4, and N2 [68,143]. The Au-Pd mineralization at this deposit was formed in the temperature range from 80 to 132 °С with the participation of fluids with high salinity ranging from 18.8 to 29.6 wt.% eq. NaCl. The composition of fluids is dominated by NaCl and CaCl2. Quartz-hematite veins were formed at pressures from 0.76 to 31 kbar; during the formation of quartz veins with Cu-mineralization, the pressure changed from 0.63 to 0.51 kbar. EPMA analysis showed that the mineral-forming fluids contained Cl, Na, Ca, K, Mn, Ba, Sr, Fe, Cr, and S [32].
The physicochemical formation conditions of Pd-bearing gold in the ores of deposits of type 1 include two temperature ranges-magmatic high-temperature and hydrothermal low-temperature. In the former case, the formation of Pd-bearing gold takes place as a result of the decomposition of Au-Cu-Pd solid solutions at 1200-1000 °С. In the latter, Au-Pd alloys form with the participation of low-temperature hydrothermal fluids (270-140 °С) of medium salinity (13.6-23.3 wt.% NaCl eq.) ( Figure 11A). The physicochemical conditions of formation of palladian gold in the ores of deposits of types 2-4 are similar and correspond to the conditions of low-temperature hydrothermal ore formation ( Figure 11B). At these deposits, Pd-bearing gold formed in the temperature interval of 300-60 °С, and the salinity of hydrothermal fluids ranges widely from 30 to 0.2 wt.% NaCl eq. ( Figure 11A). Estimation of the ore formation pressure at Au-Pd deposits of types 2-4 is based on the results of studies of fluid inclusions in the minerals of early stages that preceded the formation of Pd-bearing gold, and the pressure varies from 1.7 to 0.05 kbar ( Figure 11B).

Impurities, Fineness of Palladian Gold, and Minerals in the Association as Indicators of Deposit Types
The content of Ag, Cu, and Hg in palladian gold and minerals in association with it from deposits of various types has been studied by many researchers. Each of the deposits (Figure 1, Tables 1-7) for which the information was collected and analyzed is unique and has a specific composition of palladian gold and a set of minerals in the association. The heterogeneity of palladian gold forms either during primary crystallization or results from further modification under the effect of chemical and physical factors during subsequent residence in hypogene or surficial environments. Figure 12a shows data on the palladian gold with various sets of impurities and other co-occurring phases of the Au-Pd-Ag-Cu-Hg system. The Ag impurity was found in palladian gold at 36, Cu-at 32, and Hg-at 20 deposits. The palladian gold is represented by the systems Au-Pd, Au-Pd-Hg, Au-Pd-Cu, and Au-Pd-Ag-Hg, but more frequently corresponds to the Au-Pd-Ag, Au-Pd-Ag-Cu, and Au-Pd-Ag-Cu-Hg. The variations in Ag content in Au-Pd-Ag-Cu solid solutions are considerable and cover the widest range of fineness from 160 to 993. At some deposits, the Pd content exceeds that of Ag and Cu, whereas at others, Cu dominates over Pd and Ag or Ag dominates over Pd and Cu. Lowfineness palladian gold is related to elevated contents of Pd, Ag, Cu, and Hg. At the Marathon deposit (Canada), the fineness of palladian gold is 663-835, and the content of Cu (and Ag) is higher than that of Pd (Table 6). At the Zechstein deposit (Poland), the fineness of palladian gold is low (576‰-795‰) and the content of Ag is higher than that of Pd, Cu, and Hg. The content of Pd is higher than those of Hg and Ag in native gold from the placers of the Dart river (England) (630‰-972‰). Talnakh ores contain palladian silver (160‰-210‰) ( Table 5). Wide variations in the concentrations of all four elements and fineness (556‰-931‰) were revealed for palladian gold from placers of the Similkameen river (Canada) (Pd,Ag,Cu,Hg) (Tables 6 and 7). The low fineness of palladian gold is due to the high concentrations of Pd (Chorokh river, Turkey); or Pd and Hg (Itchayvayam (Russia); River Dart (England)); or Pd, Ag, and Cu (Norilsk-1, Talnakh, Ozernoe (Russia); Marathon (Canada); Skaergaard (Denmark)); or Pd, Ag, Cu, and Hg (Zechstein (Poland); Similkameen river (Canada)). Figure 12b shows the variations in the fineness and impurity elements of palladian gold from deposits of various types (and subtypes) summarized in Table 7. Palladian gold most commonly occurs in the PGE ore deposits related to mafic-ultramafic magmatic complexes (Table 7, type 1) and contains Ag and Cu impurities; occasionally minor Pt, Fe, and Ni; and does not contain Hg. This is mostly high-fineness gold, is less often medium-, and is rarely low-fineness. The only exception is the Au-Pd-Hg Itchayvayam ore occurrence (Kamchatka, Russia), for which two varieties of Pd,Hg-bearing gold-high-(low content of Pd and Hg) and low-fineness (high content of Pd and Hg) gold-were determined [33,42]. This type is most likely typical of territories with elevated background levels of Hg. It is in the northern part of Kamchatka that a great number of deposits and ore occurrences of the argillisite formation were discovered [149,150]. Ore objects are related to island arc terrigenous and volcanic-siliceous deposits of the Early and Late Cretaceous (terranes of the Olyutorskaya island arc in Figure 1). The Itchayvayam ore occurrence occurs in the deep fault zones and frequently accompanies the massifs of mafic-ultramafic magmatic complexes. The elevated background levels of Hg seem to be because oceanic sediments contain high Hg concentrations compared to clarke. Enrichment of oceanic sediments with Hg could be a result of volcanic activity [150].
For orogenic gold (subtype 2a), epithermal (porphyry) gold-copper (type 3), and iron oxide copper gold (type 4) deposits, palladian gold is mainly high-fineness and contains Ag impurities or Cu too. Low-fineness palladian gold with a major content of Ag is typical of the Zechstein deposit (Poland) (subtype 2b). Palladian gold of medium fineness (780‰-790‰) with Ag, Pd, Pt, Ni, and Fe impurities occurs at the Au-Pd-Pt Lebedinskoye deposit (Russia) (type 5), and was also found in volcanic fumaroles of Ebeko volcano (Russia) (only Pd impurity) (type 6). High-fineness gold (910‰-970‰) with low contents of Pd, Ag, Cu, and Hg is spread over a great part of gold-PGE placers (type 7). Pd,Hg-rich low-fineness gold (630‰-770‰) has been found in some placers of the Chorokh river (Turkey) [60], the Dart river (England) [20], and weathering crusts at the Corrego Bom Sucesso (Brazil) [76]. Tables 1-6 show the minerals that occur in association with palladian gold at the deposits with various types of Au-Pd mineralization. The number of minerals is about 90, apart from unnamed phases. Table 10 contains data on minerals in association with palladian gold from deposits of different types. Minerals (bold) are the most frequent phases. Mineral groups are presented by native metals and intermetallic compounds, oxides, hydroxides, and chalcogenides. Pd, Ag, Cu, and Hg metals that occur in native gold frequently form their minerals and are present in association with palladian gold. In Table  10, these minerals are listed separately by group. Palladium minerals make up the largest group. Most common among them are arsenides, stibioarsenides, sulfides, stannides, bismuthides, tellurides, selenides, etc. (Table  10). In association with them, Pt minerals-sperrylite, braggite, sudovikovite, palladoarsenide, isoferroplatinum and tetraferroplatinum-are present. Pd and Pt minerals are typical of the deposits of types 1 and 7 (Table 7), which indicates their paragenetic relationship.
Gold and silver minerals frequently occur in association with palladian gold-these are Au-Ag-Cu solid solutions and Ag-Cu intermetallic compounds (tetra-auricupride, auricupride)-and more rare minerals are aurostibite, anyuiite, sylvanite, fischesserite, hessite, chlorargyrite, bohdanowiczite, naumannite, and eucairite. Au-Cu intermetallic compounds were found at more than ten deposits. Tetra-auricupride is spread at the deposits belonging to groups 1 and 7. Aurostibite is in association with palladian gold in the ores of the Uderei deposit (Russia) (type 2a) [49] and placer ore occurrences of Lammermuir Hills (Scotland) and River Dart (England) (type 7a,b) [20].
The above-listed minerals of Au, Pd, Cu, Ag, Hg, Pt, Fe, and Pb are formed together with palladian gold and influence its composition and redistribution of metals between fluids and solid phases. On the whole, minerals in association with palladian gold reflect the mineralogy of ores, which points out their use as indicators of the type of mineralization. PGE at elevated temperatures are typical chalcophiles, and have a high affinity with S, As, Te, Sb, and Bi. After post-magmatic fluid processing, a significant part of the native metals leave the high-temperature solid solutions, and various minerals of PGE, Au, and Ag are formed: native, sulfides, arsenides, tellurides, antimonides, bismuthides, etc.
An important area of research is works focused on studying the mechanism of the mobilization, transfer, and deposition of noble metals. It was proved that during crystallization, hot water fluids are generated, which migrate in the crust [151]. Metals can be transported by brines, altered meteoric water, and metamorphic and magmatogenic fluids. Their deposition from hydrothermal solutions takes place at temperatures from 500 to 100 °С. They can form their own minerals or occur in other minerals as isomorphic impurities. The ability of particular metals to occur in gold depends on their amount in the ore-forming system and the content of other elements with which they can form stable minerals in ore-forming conditions. The question of the influence of the activity of elements (S, Te, Sb, As, Bi, etc.) that bind palladium into its own minerals on its ability to occur in native gold has not been resolved so far. The solution to this problem is complicated by the fact that hydrothermal fluids that formed Au-Pd mineralization simultaneously contain several of these elements, and native gold is found in parageneses with many minerals of Pd. The Baronskoe in the Middle Urals is an example of deposits at which palladian gold is in paragenesis with Pd sulfides, tellurides, sulfotellurides, arsenides, stibnides, and bismuthides. For this type of deposit, we managed to trace the variations in the contents of Pd in Ag-bearing gold with sulfides (vysotskite), tellurides (kotulskite, telluropalladinite), arsenides Pd and Pt, arsenotellurides (keitconite, As-keitconite), and As-Sb-Te minerals (Te-arsenopalladinite, Teguanglinite, isomertieite). It was revealed that the occurrence of Pd, as well as Ag, is controlled mainly by the activity of S and Te, and the effect of the activity of As and Sb is negligible ( Figure 13). What do the Pd impurity and level of its concentrations in native gold indicate? What other isomorphic impurities along with Pd can be present in native gold? Which minerals occur in the intergrowth with palladian gold? At what deposits are Pd-bearing gold spread? What is the source of Pd and other accompanying impurities in native gold? What are the mechanisms and processes of mobilization, transfer, and joint deposition of these noble metals? In what physicochemical conditions and environments does Pd-bearing gold form? We tried to answer these and other questions in this paper. We will make attempts to answer many questions in the future. Impurities are a kind of fingerprint for minerals that make it possible to determine the composition and conditions of ore-forming medium. The composition of native gold is an important typomorphic characteristic of this indicator mineral, bearing fingerprints of formation conditions specific to each deposit. The knowledge of impurity elements in native gold and their content is important for identifying the ore formation to which gold mineralization belongs, for determining the source of placer ore occurrences, developing efficient criteria for forecasting, and searching gold and gold-bearing deposits. The minerals in association with native gold and the sequence of their formation reflecting the specific features of ore formations are the basis for ore-formational analysis [152]. The distribution of these associations determines the contours of deposits. Analysis of data on the deposits with palladian gold showed specific compositional features of the productive mineral associations from various types of deposits located on different continents and in different countries.
Construction of the quantitative genetic models of ore-forming processes at deposits with palladian gold requires thermodynamic data for solid solutions of various compositions. Estimation of the thermodynamic characteristics was performed only for some binary and ternary systems with gold [6,[153][154][155][156], which makes it impossible to carry out this kind of work at this stage. Model calculations were made for the systems containing Au-Ag [4,6,7], Au-Ag-Hg [155], Au-Ag-Pd [156], Au-Ag-Cu [157], and Au-Ag-Cu-Hg [158]. The composition and set of impurities in palladian gold were found to be more diverse than it was thought to be. Most likely, very soon, it will be possible to confidently determine any region from which its samples were obtained from the composition of impurities in native gold.

Conclusions
(1) Depending on the set of impurities, the following varieties of palladian gold are distinguished: Ag-, Cu-, Hg-, Ag,Cu-, Ag,Hg-, Cu,Hg-, and Ag,Cu,Hg-bearing palladian gold. Palladian gold may contain Pt, Fe, Ni, and Cd (in minor quantity). (2) A classification of types of deposits with palladian gold has been proposed: (1) PGE ore deposits related to mafic-ultramafic magmatic complexes (two subtypes-(a) low-sulfide-grade (less than 2%-5% sulfides) Alaskan and (b) high-sulfide-grade (more than 5% sulfides) Norilsk); (2) orogenic gold deposits; (3) epithermal (porphyry) gold-copper deposits; (4) iron oxide copper gold deposit type; (5) ferruginous quartzite deposits; (6) volcanic exhalation; and (7) gold-PGE placers with five subtypes corresponding to the types of 1-5 primary sources. (3) Ag,Cu-bearing palladian gold is mainly high-fineness (910‰-990‰), is less frequently medium-fineness, and is rarely low-fineness and does not contain Hg at the deposits of PGE ore deposits related to mafic-ultramafic magmatic complexes, epithermal (porphyry) gold-copper deposits, and iron oxide copper gold deposits (types 1, 3, 4). The only exception is the Au-Pd-Hg Itchayvayam ore occurrence (Kamchatka, Russia) (type 1d) with two varieties of Pd,Hg-bearing gold (high-fineness 816‰-960‰ and low-fineness 580‰-660‰). Low-fineness palladian gold with the major content of Ag is typical for OGD deposit Zechstein (Poland) (type 2), whereas that with the major content of Pd is typical of the placers of the Chorokh river (Artvin district, Turkey) (type 7). Medium-fineness palladian gold with a high content of Pd occurs at BIF deposits and in volcanic exhalations (types 5, 6). Hg,Ag,Cu-bearing high-fineness palladian gold is mainly present in placer deposits (type 7). (4) The most common minerals in association with palladian gold are Pd and Pt arsenides, stibioarsenides, sulfides, stannides, bismuthides, tellurides, and selenides. They are typical of deposits related to mafic-ultramafic magmatic complexes (types 1, 7). Au, Ag, and Cu minerals (tetra-auricupride, auricupride, aurostibite, chalcopyrite, bornite, sylvanite, hessite, naumannite, eucairite, etc.) occur in association with palladian gold at gold-copper deposits (types 2-4). Cu and Fe oxides (tenorite, hematite, magnetite, (Pd,Cu)O) and Fe and Pd hydroxides (goethite, (Fe,Pd)OOH) are spread at the deposits of 3,4,7 groups and indicate highly oxidizing conditions of ore formation. The most common of Hg minerals is potarite. The main host minerals of palladian gold are quartz, muscovite, including fuchsite (Cr-Ms), chlorite, albite, Kfeldspar, kaolinite, hornblende, and carbonates (calcite, siderite). (5) Palladian gold from many deposits has a heterogeneous composition and occurs in two or more varieties, which suggests unstable deposition conditions, subsequent recrystallization processes, and a long history of formation. Physicochemical conditions of the formation of Pd-bearing gold at some deposits of one type cover two areas-magmatic high-temperature and hydrothermal low-temperature. At the majority of deposits of types 1 and 2, the formation of Pd-bearing gold proceeds with the participation of hydrothermal fluids (300-60 °С) of various salinities (0.2-30 wt.% NaCl eq.) (6) The fineness, impurity metals, and minerals in association with palladian gold reflect the mineralogy of Au-Pd ores and allow them to be used as indicators for types and subtypes of gold deposits.

Future Directions
In the studies of new objects, it is necessary to more thoroughly analyze the chemical composition of palladian gold and minerals in association with it. An important characteristic of palladian gold is the data on the composition of microinclusions obtained by LA-ICP-MS. However, the heterogeneity of its composition and the presence of mineral microinclusions must be taken into account for interpreting the obtained results.
A database of the deposits with palladian gold, as well as argentian, cuprian, and mercury gold must be created. Scarce data are known on the Р,Т,Х-parameters of the formation of native gold of various compositions and sources of both gold and impurity elements (major, minor, and trace). Possible and important directions of further works are a study of different forms of transfer of Au, Pd, Ag, Cu, Hg, and other elements at different Р,Т,Х-parameters of ore-forming conditions, obtaining thermodynamic characteristics of Au-Pd-Ag-Cu-Hg solid solutions and minerals of these elements and minerals in association with the aim to construct the genesis models of palladian gold and other varieties of native gold.