Gold in Minerals and the Composition of Native Gold Gold in Minerals and the Composition of Native Gold By Robert S. Jones and Michael Fleischer GEOLOGICAL SURVEY CIRCULAR 612 Washington 1969 United States Department of the Interior WAl.TfR J. HICKEL, Secretary Geological Survey William T. Pecora, Director Free on application to the U.S. Geological Survey, Washington, D.C. 20242 CONTENTS Page Abstract -----------------------------------------------~----------------- 1 Introduction -------------------------------------------------------------- 1 General geochenrlcal considerations ----------------------------------------- 1 Gold in minerals ------------------------------------------------------ _ 2 Composition and the fineness of gold ---------------------------- ___ 13 References cited --------·-------_____________________ ______________ ____ _ 15 TABLES Page 'fABLE 1. Major gold-bearing minerals ----------------------------------- __ 2 2. Analyses of precious metals in minerals made before 1955 _______ _ . _ 3 3. Analyses of gold in minerals made since 1954 --------------------- 10 4. Variation in fineness of gold with depth, Lily mine, Transvaal, South Jlfrica -------------------------------------------------------- 14 5. Fineness of mill bullion prior to 1882 at the Homestake nrlne, South Dakota -------------------------------------------------------- 15 Ill GOLD IN MINERALS AND THE COMPOSITION OF NATIVE GOLD By ROBERT S. JONES and MICHAEL FLEISCHER ABSTRACT much lower concentrations in the sulfiie phase, Gold occurs in nature mainly as the metal and as and occurs in much lesser amount~ in the various alloys. It forms complete series of solid solu­ silicate phase. Gold occurs in natur~ mainly tions with silver, copper, nickel, palladium, and as the metal and as various alloys, especially platinum. In association with the platinum metals, gold with silver, and as intermetallic co:'llpounds. occurs as free gold as well as in solid solution. Laboratory studies show that gold can form The native elements contain the most gold, followed by the sulfide minerals. Several gold tellurides are complete series of solid solutions with silver, known, but no gold selenides have been reported, and copper, nickel, platinum, and palla:dium. Gold only one sulfide, the telluride-sulfide mineral nagyagite, is commonly present in association with plati­ ii:; known. num metals; most microscopic stuc"ies have The nonmetallic minerals carry the least gold, and shown that free gold is present in platinum, the light-colored minerals generally contain less gold than the dark minerals. but a recent electron-probe analysis of ferro­ Some conclusions in the literature are conflicting in platinum shows uniform distributioi' of gold regard to the relation of fineness of native gold to its ( Ottemann and Augustithis, 1967). This distri­ position laterally and vertically within a lode, the bution indicates that gold and platinum aTe nature of the country rocks, and the location and size present in solid solution. of nuggets in a streambed, as well as to the variation of fineness within an individual nugget. Several gold tellurides are known (table 1), but no gold selenides have been repC'...-ted, and INTRODUCTION the only minera~ in which gold is certainly combined with sulfur is the telluride-sulfide, This report on the occurrence of gold in nagyagite. Gold commonly occurs in sulfide minerals and on the fineness of native gold minerals, but largely, if not entirely, as the was prepared as background material for the free metal; it is uncertain whether any gold Heavy Metals program of the U.S. Geological occurs in these minerals in true isomorphous Survey, an intensified program of research substitution. on new sources of heavy metals, including gold. It is even less likely that gold is present in ionic substitution in silicate miner·als. Man­ GENERAL GEOCHEMICAL CONSIDERATIONS tei and Brownlow (1967) state, "Tl'e concen­ tration of gold in the various minerals is proba­ Gold belongs to group Ib of the periodi.c bly due to an inclusion or entrappinr-- phenom­ table, as do silver and copper. Its atomic num­ ena rather than to ionic substitutior. Because ber is 79, and atomic weight is 197.0; it con­ of its oxidation potenti.al, it would 1~<~ difficult sists of a single isotope. Its metallic radius for gold to become oxidized and thus be able is 1.44A., univalent ionic radius 1.37A., and to take part tn ionic substitution. Krauskopf trivalent ionic radius 0.85A. (1951) states that simple ionic goli can not Gold is strongly siderophilic and somewhat exist in geological environments, alth('mgh com­ chalcophilic; that is, it tends to be co,ncen­ plex ions containing gold may form. Ringwood trated in the metallic phase of meteorites, with (1955) points out that Au+, because of its 1 large electronegativity, would form a very been made by more sensitive methods, mostly weak covalent bond, and one which would pre­ by neutron activation, than the older ones, fer not to form. Thus the gold of a crystallizing and values as low as 0.0003 ppi1. (part per magma tends to concentrate in the residual million) are reported. However, comparatively fluids. The factors which would control the few neutron activation analyses . of rock-form­ amount of gold entrapped in a given mineral ing minerals have been reportei in recent would be the concentration of gold in the years. The older analyses tend to be significant­ magma at the time of crystallization and the ly higher than more recent analyses of the type of crystal structure formed by the min­ same minerals. The highest gold values listed eral." in tables 2 and 3 are reported for the native Helgeson and Garrels (1968), on the basis elements; next highest are for the sulfide min­ of thermodynamic calculations, think that all erals. but marginal or low-grade hydrothermal native TABLE 1.-Major gold-bearing minerals gold deposits form above 175°C and, at ele­ vated temperatures, most hydrothermal solu­ Gold Au. Cubic, sp gr 19.3 (pure Au), decreasing with ircreasing con­ tions are probably distinctly acid. They believe tent of Ag. Form!;; a complete that gold is present primarily in the form series of solid solutions with silver of aurous chloride complexes in contrast to (see electrum and silver, below); the low-temperature considerations of Kraus­ commonly contains 10-15 percent Ag. Also reported in percent: Cu kopf (1951) and Cloke and Kelly (1964) who (ma..."X: 20.4), Fe (ma.x 0.1), rarely suggest that the aqueous species AuC17 is Bi (max 2.9), Sn (max 0.3), Ph the principal form of dissolved gold in hydro­ (ma..."X: 0.2), Zn (max 0.8), AI (max thermal solutions. 0.10), Mn (max 0.002). See sec­ Goni, Guiiiemin, and Sarcia (1967) have tion "Composition and Fineness of Gold." investigated the stability of colloidal suspen­ Varieties: sions of gold and the formation of nuggets. Electrum, (Au,Ag), argentian geld with >20 Stable colloidal suspensions of ionic and even percent Ag. metaiiic gold can form which can be floccu­ Porpezite, (Au,Pd), palladian go1d with 5-10 lated to form nuggets. Textures common in percent Pd. gold deposits can be reproduced in gold films Rhsxlite, (Au,Rh), rhodian gold(?) with 34-43 percent Rh. formed by the diffusion of gold solutions Auricupride (cuproauride) has generally been through silica gel. considered to be a solid solution of gold in Gold occurs in notable amounts in hydro­ copper, near AuCua in. composWon. Ramdohr thermal veins and in placer deposits, and to (1967) states, however, that study of "red gold" from Laksia, Cyprus, has shown the a much lesser extent in pegmatites and con­ presence of three· distinct phases Au-Cu solid tact metamorphic deposits. Commo.n minerals solutions, the compound AuCua with a _associated with gold in veins are quartz and characteristic violet color, and the compound pyrite. Some other common minerals associ­ Au Cu. ated with gold (Lincoln, 1911; Schwartz, 1944) Aurosmirid, aurosmiridium ( =aurian osmiri­ are pyrrhotite, arsenopyrite, chalcopyrite, sphal­ dium) Au 19.3 percent. ProbabJy a mixture containing gold. erite, galena, molybdenite, tellurides, selenides, (Palache and others, 1944, p. 111). magnetite, scheelite, feldspar, sericite, biotite, Silver (Ag,Au). Aurian silver, with 0-50 chlorite, amphiboles, garnet, tourmaline, car­ percent Au. bonates, and fluorite. Kiistelite=aurian silver. Gold-amalgam A112Hga(?) GOLD IN MINERALS Au 34.2-41.6 percent. Maldonite Au2Bi. Cubic. Minerals that have been reported to contain Au 64.5-65.1 percert; Bi 34.9- major amounts of gold are listed on table 35.5 percent. 1. Many analyses have been made for some Aurostibite AuS~. Cubic, pyrite tYf<:'\. of the precious metals in minerals that con­ Au 43.5-50.9 percent. Krennerite AuTe2. Orthorhombic. tain little gold;: those made before 1955 are Au 30.7-43.9 percent. listed in table 2 and those made since 1954 M iillerine = Krennerite are in table 3. The more recent analyses have Speculite = Krennerite (?) 2 TABLE l._JMajor gold-bearing minerals-Continued TABLE 1.-Major gold--bearing m.inerals-Concluded Calaverite AuTe2. Monoclinic. Hessite Ag2Te. Monoclinic, pseudocubic. Au 39.2-42.8 percent. Reported to contain as much as Coolgardite =a mixture of calaverite, coloradoite, 4.7 percent Au. and sylvanite. Montbrayite Au2Tea. Triclinic. Sylvanite (Au,Ag) Te2, with Au :Ag usually Au 38.6-44.3 percent. nearly 1:1, that is, AgAuTe4. Nagyagite PbsAu(Te,Sb)4Ss-s( ?) • Monoclinic. Monoclinic ( ?) . Au 24.25-29.9 percent. Au 7.4-10.2 percent. Goldschmidtite= Sylvanite. Silberphyllinglanz = N agyagite. Kostovite CuAuTe4 Blatterine = N agyagite. Au 25.2 percent (Terziev, 1966). Aurobismuthinite (Bi, Au,Ag) sSs ( ?) Petzite AgaAuT~ Au 12.3 percent; Ag 2.3 percent. Au 19.0-25.2' percent. Probably a mixture (Palache and Antamokite=a mixture of petzite and altaite. others, 1944 p. 278). TABLE 2.-Analyses of precious metals in minerals made before 1955 [Minerals containing higher amounts of gold are listed in table l. N. D., not determined] Gold Silver Platinum Mineral and locality (ppm) (ppm) (ppm) Remarks Reference Elements Arsenic, As Germany, Andreasberg, Harz ___ _ 150 >1,000 20 One sample __ Noddack and No·ldack (1931).