{"word_count": 2075, "character_count": 14136}
1. IntroductionThe major part of the Au in the planets and meteorites of our Solar system is found in high temperature solid solutions – metallic Fe-Ni and monosulfides Fe-Ni (Mss) and Fe-Cu (Iss). Au leaves them under fluid reworking. As a result, Au minerals are formed. They are mainly developed in hydrothermal deposits of the upper part of Earth’s continental Earth. Au is the noblest chemical element. Thus, the most part of Au in deposits appears as native gold – Au-Ag, Au-Ag-Hg, Au-Cu, Au-Pd, and Au-Pt system minerals. The most important feature of native gold feature is its fineness, i.e. the Au content in the mineral specimen, expressed in ‰. Chalcogenides of Au – sulfides and selenides, and, similar to them, tellurides, plumbotellurides, antimonides, bismuthides – are not so widespread. Much Au is found as the finest impurity in hydrothermal nonstoichiometric As-bearing pyrite Fe(S,As)2-x and arsenopyrite FeAs1-yS1+y and bitumen-like Au-organic compounds.2. Au – Ag system mineralsThe main part of native gold appears as cubic solid solutions – Au-Ag system minerals. According to the results of thousands of precision analyses (Brauns, 1906; Vernadsky, 1914; Lindgreen, 1933; Smolin, 1970; Editorial Committee, 1970; Petrovskaya, 1973; Bermanet al., 1978; Nedra, 1984-1990; Oberthür and Saager, 1986; Shikazono and Shimizu, 1987; Amuzinskyet al., 1992; Nekrasovet al., 1994; Spiridonov, 1995; Soet al., 1995; Frimmel and Gartz, 1997; Hytőnen, 1999; Knight and Leitch, 2001, etc.) it was established that the series Au-Ag is uninterrupted in composition. These series are fixed in the limits of the series (Vernadsky, 1914; Petrovskaya, 1973): gold (0-30 wt% Ag; fineness 1000-700); electrum (30-70 wt% Ag; fineness 700-300); kuestelite (70-90 wt% Ag; fineness 300-100); gold-containing silver (90-100 wt% Ag; fineness 100-0). The confirmed mineral species are noted in bold. On observations of the authors, the phenomena of disintegration of solid solution in Au-Ag minerals are not shown. The size of gold crystals varies from colloidal (&#60; 0.1 microns) up to several cm, usually around a fraction of mm. Colloidal gold colours a chalcedony-like quartz in green colour (Baley, Transbaykalia). The size of the granulated intergrowths of gold (nuggets) is up to one metre. As Al-Biruni (1963) noted, the largest nugget of gold discovered weighed ~ 2500 kg (Zaruban, Afganistan). Figure 1: (See colour plate) Gold (fineness 885) in quartz vein. 81 mm. Bestube, Northern Kazakhstan. Collection and photography – E.S. Perhaps this deposit was one of the gold sources for the well known ancient Scythian jewellery.Figure 1 : (Voir planche couleur) Or (titre 885) dans le filon de quartz. Bestube, nord du Kazakistan. Collection et photographie – E.S. Peut-être ce gisement a été une source d’or, exploité pour la production d’orfèvrerie Scythe. High-standard gold (Fig. 1) is typical for plutonogenic hydrothermal gold-quartz deposits. These formations are the main source for placer Au deposits. Electrum, kuestelite and Au-bearing silver, their dendrites, skeletal and wire-shaped crystals (Fig. 2) are typical for volcanogenic hydrothermal Au-Ag deposits (Goldschmidt, 1918; Lindgreen, 1933; Petrovskaya, 1973). Widespread in these deposits, electrum corresponds to the famous ‘electron’ of the ancient Greeks, and the favourite ‘white gold’ of the Incas. Gold crystals of primary endogenic deposits are zonal: later zones are enriched by Ag and Hg (usually insignificantly). In metamorphosed deposits, grains of gold are azonal, and intergrowths of different composition are observed (Frimmel and Gartz, 1997). In placers, grains of gold are usually enclosed by margins of porous high-standard gold; silver from these margins is leached. In placers, native gold often is found together with minerals of platinum group elements (ferroplatinum Pt3Fe, alloys Os-Ir-Ru, etc.).3. Au (Ag) – Hg system minerals After Ag, Hg is the second most typical element impurity in native gold. In ores formed under low f S2 conditions, there are cubic a-amalgams: mercurian gold (Au, Hg) (Nazmova and Spiridonov, 1979; Oberthür and Saager, 1986; Shikazono and Shimizu, 1988; Amuzinsky et al., 1992); mercurian electrum (Au, Ag, Hg), mercurian kuestelite (Ag, Au, Hg), gold-containing mercurian silver (Spiridonov and Pletnev, 2002). Mercurian gold contains up to 18 wt% Hg, and mercurian silver up to 26% Hg. Au amalgams enriched by Hg are much more rare. They usually associate with native Hg: hexagonal weishanite(Au,Ag)3Hg2 (Li et al., 1984; Wu, 1993); cubic gold amalgama g-Au2Hg3 (Berman and Harcourt, 1938). Almost all Au amalgams found in placer deposits are technogenic formations (Wu, 1993). 4. Au – Cu system mineralsCu is the third most typical element impurity in native gold, after Ag and Hg. Au-Cu phases poor in silver and Au-Ag phases poor in copper are formed at temperatures below 350 °C (Lyakischev, 1996). Au-Cu minerals (cuprian gold) are found in hydrothermal Au deposits, among listvenitized peridotites, komatiites and rodingites (Lozechkin, 1939; Spiridonov and Pletnev, 2002). These last minerals replace native copper. Cubic auricupride AuCu3 is known (Ramdohr, 1967). Metastable cuproaurideAuCu corresponds to the low ordered a-(Au, Cu) solid solution (Lozechkin, 1939; Chvilevaet al., 1988). The mineral corresponding to the Au3Cu cubic modification (Knipe and Fleet, 1997; Spiridonov and Pletnev, 2002) does not have an authorized name. Below, it is referred to as mineralAu3Cu. The products of cuproauride solid phase transformation are tetragonal tetraauricuprideAuCu (Chenet al., 1982). Tetraauricupride can contain up to 17 wt% Pd (Spiridonovet al., 2003). Figure 2: (See colour plate) Golden wire in calcite. 4 mm. Tyrny-Auz, Northern Caucasus. Collection of V. Andreenko.Figure 2 : (Voir planche couleur) Fil d’or dans de la calcite. 4 mm. Tyrny-Auz, nord du Caucase. Collection de V. Andreenko.Photography – M. Bogomolov.Photographie M. Bogomolov.Tetraauricupride – hongshiite CuPt solid solutions are known (Kwitkoet al., 2002). After annealing, non-stoichiometric cuproauride Au1+xCu shows two types of thin lattice solid solution disintegration structures: Au + tetraauricupride and tetraauricupride + mineral Au3Cu. The disintegration structures are known, where the matrix is Au and lamellas are mineral Au3Cu; probably they are products of the non-stoichiometric mineral Au3+xCu’s disintegration. The standard roentgenometric data that are required for the determination of Au-Cu system minerals are provided in Chvileva et al. (1988).5. Au – Fe system minerals It was established by a combination of magnetometric and mineralogical methods that the high-magnetic ferrous gold with 4-5 wt% Fe described earlier shows fine crystal intergrowths of gold with 0-0.2 wt% Fe, and magnetite octahedrons 0.01-0.2 microns in size (Yacubovskaya et al., 1989). 6. Au – Pd, Pt, Os, Ir system minerals Au-Pd and Au-Pt system minerals are typical rare minerals of telethermal Au deposits formed at low f S2 and at heightened f O2. They are cubic palladian gold (porpezite) (Au, Pd). (Vernadsky 1914; Kwitko et al., 2002); platinian gold (Au, Pt) (Levin et al., 1986); intermetallides with compositions similar to Pt3Au and Pt(Au,Pd). Even a low impurity of Pd (of the order of a few percent) results in the steel-grey colour of this type of native gold. 7. Au – Bi, Sb, Pb, Sn system minerals Intermetallides of gold –maldonite Au2Bi (Hytőnen, 1999) and aurostibiteAuSb2 (Graham and Caiman, 1952; Spiridonov, 1995) are rather rare minerals of hydrothermal ores. Maldonite contains ~ 65 wt% Au, and aurostibite ~ 45 wt% Au. Aurostibite pseudomorphs after maldonite are rarely encountered; such aurostibite contains up to 9 wt% Bi. These minerals are formed under low f S2. Maldonite usually replaces native bismuth. Aurostibite usually replaces native gold. The following three intermetallides: anyuiite AuPb2 (Razin and Sidorenko, 1989), hunchuniteAu2Pb (Shangquan et al., 1992), yuanjiangiteAuSn (Chen Lichang, 1994) are found only in placers. Perhaps these three intermetallides are technogenic products.8. Au – Ag (Cu) – Te system minerals Au and Ag tellurides are typical minerals of hydrothermal Au deposits. In a series of cases, they are the main Au and Ag bearing minerals in ores. Hg and Cu are typical elemental impurities in these tellurides. Triclinic montbrayite Au2(Te,Sb,Pb,Bi)3 (Peacock and Thompson, 1946; Chvileva et al., 1988) is a rare telluride, and it contains ~ 50 wt% Au and up to 5 wt% Sb. Monoclinic calaverite AuTe2 (Stillwell, 1931; Markham, 1960) contains ~ 45 wt% Au, and usually &#60; 0.5 wt% Ag. In some deposits (Kalgoorly, Australia), there are calaverite aggregates reaching 1m3 in size. Rhombic krennerite Au3(Au,Ag,Cu)Te8 (Markham, 1960; Editorial Committee, 1970; Chvileva et al., 1988) contains ~ 40 wt% Au, and 0.5-6.5 wt% Ag; its composition varies from AuTe2 to Au3AgTe8. Monoclinic silvanite Au(Ag,Au,Cu)Te4 (Stillwell, 1931; Markham, 1960; Editorial Committee 1970; Chvileva et al., 1988) contains ~ 25 wt% Au. Kostovite Au(Cu,Ag,Au)Te4 (Terziev, 1966), similar to silvanite, is a typical reactionary mineral of volcanogenic Au ores. Mutmannite AuAgTe2 (Spiridonov and Chvileva, 1985), developed in the cementation zone of the gold-telluride deposits, is an example of mimicry in the mineral kingdom, because it is indistinguishable from petzite optically and on spectra of reflection light, and from calaverite according to the X-ray pattern. Cubic petzite AuAg3Te2 (Markham, 1960; Chvileva et al., 1988) with garnet-like structure is the most widespread Au-Ag telluride; it contains ~ 25 wt% Au. Cubic solid solution (Ag,Au)2Te is rarely encountered as homogeneous phase; usually it is transformed into petzite and hessite intergrowths. 9. Au – Ag – Se, S system minerals Cubic fischesserite AuAg3Se2 (Johan et al., 1971), cubic uytenbogaardtite AuAg3S2 (Barton et al., 1978), monoclinic petrovskaite AuAgS - AuAg(S,Se) (Nesterenko et al., 1984; Nekrasov et al., 1988), hexagonal penzhinite AuAg4(S,Se)4 (Bochek et al., 1984), cubic solid solution (Ag,Au)2Se (Spiridonov et al., 2009), cubic solid solution (Ag,Au)2S (Samusikov et al., 2002) are typical minerals of Au volcanogenic hydrothermal deposits, from low sulfide to rich sulfide ones. Petrovskaite is also developed in crusts of weathering of sulfide rich deposits, in association with jarosite and native sulphur. 10. Complex chalcogenides Rhombic nagyágite AuPb5Te4-x(Sb,As)xS6 (Spiridonov, 1991a; Simonet al., 1994) –AuPb5Te2(Sb,Bi)S6 (Johan et al., 1994), monoclinic museumite AuPb5SbTe2S12 (Bindi and Cipriani, 2004), rhombic buckhornite AuPb2BiTe2S3 (Franciset al., 1992; Johanet al., 1994), monoclinic criddleite Au3Ag2TlSb10S10 (Harriset al., 1988), monoclinic jonassonite AuBi5S4 (Paaret al., 2006) are typical formations of Au and Au-Ag volcanogenic hydrothermal deposits. Nagyágite – ‘grey foliated gold ore’– is the most widespread among them. 11. Plumbotellurides and stibio-plumbo-tellurides of the Au-Cu-Fe systemThese hypergenic minerals are developed in the cementation zone of gold-telluride deposits. They are monoclinic (?) bilibinskite Au5Cu3(Te,Pb,)5 and bilibinskite-(Sb) Au6Cu2(Te,Pb,Sb)5 (Spiridonov, 1991b), rhombic (?) bogdanovite-(Cu) – Au5Cu3(Te,Pb)2 and bogdanovite-(Fe) – Au5CuFe2(Te,Pb)2 (Spiridonov, 2008), monoclinic (?) bezsmertnovite Au4Cu(Te,Pb) (Spiridonov, 1991b). Bilibinskite, bogdanovite and bezsmertnovite contain ~ 50, 65 and 75 wt% Au, respectively. Figure 3: (See colour plate) Bogdanovite (pseudomorph after kostovite) in gold-quartz vein. 2 mm. Bogdanovite with strong coloured bireflection. Polished section in reflected light. Aginskoe, Kamchatka. Figure 3 : (Voir planche couleur) Bogdanovite (pseudomorphe de kostovite) dans le filon de quartz aurifère. Bogdanovite avec biréflectione fortement colorée. Section polie sous lumière réfléchie. Aginskoe, Kamchatka.Collection and photography – E.S.Collection et photographie – E.S.Macroscopically, they have the appearance of bornite. In reflected light, they are very specific, being characterized by bright colour double-reflection (Fig. 3) and anisotropy. Bilibinskite, bogdanovite and bezsmertnovite replace kostovite, krennerite, silvanite, and nagyágite. In the oxidation zone, plumbotellurides of gold are substituted by finely scalloped gold and tellurides of Cu, Pb, and Fe.12. ConclusionBrief data on the 44 gold mineral species known today are provided above. Each of them has an individual chemical composition, crystal structure, X-ray pattern, unit cell parameters, physical properties (colour, hardness, spectra of reflection light, etc.). For many of these minerals, the most important feature is their fineness, i.e. Au content expressed in ‰.A main new feature – new characteristic of gold minerals – could probably be related to the area sizes of X-radiation coherent dissipation, i.e. the crystallite size. For the majority of crystal matters, the crystallites’ size is nano-dimensional, ranging from a few nm up to several hundreds of nm. The description of the crystallites size can be found in a paper by D. Yanakieva and co-authors in this issue of ArcheoSciences. The first interesting data on the nano-dimensions of crystallites is obtained. High-standard gold from large mesothermal deposits in Northern Kazakhstan, with an age of 445 ± 4 Ma, is studied. These deposits are of a different depth facies – hypabyssal Au-Sb Bestube, mesoabyssal Au Stepnyak, abyssal Au-telluride (Spiridonov, 1995). One of the studied specimens from Bestube is shown in Figure 1. The crystallite sizes of tens of studied Bestube gold specimens are 20.0 ± 0.2 nm; for Stepnyak gold, 21.6-21.8 nm; for Aksu gold, 26.0 ± 0.3 nm (investigator D. Yanakieva). Thus, for this group of deposits, the direct correlation between the size of crystallites and the depth of formed gold ores is established. The size of the crystallites of hypergenic gold from Zana-Tube is quite different: 31.3-31.8 nm.This research was supported by grant no. 07-05-00057 of the Russian Foundation for Basic Research. 