Composition Of Native Gold Research Articles

МОРФОЛОГИЧЕСКИЕ ХАРАКТЕРИСТИКИ, ХИМИЧЕСКИЙ СОСТАВ РОССЫПНОГО ЗОЛОТА БАССЕЙНА РЕКИ КЕНГЕВЕЕМ И ПЕРВЫЕ СВЕДЕНИЯ О СОПУТСТВУЮЩЕЙ ПЛАТИНОВОМЕТАЛЛЬНОЙ МИНЕРАЛИЗАЦИИ (МАГАДАНСКАЯ ОБЛАСТЬ)

The paper presents the results of the study of typomorphic characteristics of placer gold of the tributaries of the Kengeveem River, Magadan region. Placer gold was studied by optical and scanning electron microscopy at the Institute of Volcanology and Seismology of the Far East Branch of the Russian Academy of Sciences. As a result, the first information on the morphology, internal structure and chemical composition of placer gold from the southeastern part of the Taigonos Peninsula was obtained. Three types of the gold were determined. The first type is dominant. The grains are chacterized by sub-ore morphology, with weak and medium degrees of roundness. Only Au-Ag is present in the composition of native gold, other macrocomponents were not found. The gold grade is 750–950 ‰. Some gold grains bear signs of hypergene transformation, expressed in the development of high-grade gold rims and intergrain veins. The second type of gold is represented by well-rounded lamellar extractions with a grade of 980–990 ‰. The third type is loose aggregates of secondary high-grade mustard gold. The obtained data allowed us to determine the different drift distances of native gold and to consider the low-sulfide epithermal Au-Ag mineralization and presumably far-distant gabbroids as the primary sources of gold.

Mineral Associations of Concentrates from Gold-Bearing Placers of the Miass Placer Zone (South Urals) and Possible Primary Sources of Gold

Abstract —The placer deposits of the Miass valley in the Southern Urals have been known since the beginning of the 19th century. Over two centuries of mining more than 120 tons of gold have been extracted from alluvial and alluvial–deluvial placers. Despite the industrial significance of this territory, native gold of alluvial deposits is practically unstudied and there is no data on the mineral composition of concentrates. Native gold and heavy fraction minerals from seven placers of the Miass placer zone of deluvial–alluvial and alluvial types were researched. The study of the typomorphic features of native gold and distribution of heavy fraction minerals in loose deposits makes it possible to determine the genetic types of primary sources of placer formation. Chrome-spinel and ilmenite prevail in the concentrates, magnetite, epidote, rutile, titanite, clinopyroxene, hematite, zircon, garnets, and monazite are less common. Chromespinelides of various placers are represented by chromite and subferrichromite, they are characterized by a high average content of Cr2O3 (50.9 wt.%). Chromespinelide chemical composition features show evidence of its penetration into placer sediments from rocks of the ophiolitic association. The composition and morphological features of phosphates of rare earth elements (monazite and xenotime) indicate their metamorphogenic nature and penetration into placers from the Riphean strata. Native gold is characterized by a low degree of roundness. Mineral inclusions in native gold are represented by sulfides, arsenides, bismuthides, cuprous gold, platinoids. Composition of native gold from the Miass River valley placers varies over a wide range. The main admixtures are silver (4.5–23.4 wt.%) and mercury (up to 5.14 wt.%), copper (up to 2.03 wt.%) was noted in several grains. The EBSD study of native gold internal structure showed predominance of primary crystallization structures, a low degree of deformation and growth of gold-rich rims with a fine-grained mosaic structure. Formation of gold-rich rims in gold grains in placers is explained by recrystallization of the most deformed parts of gold during transportation. Placers occurred due to the destruction of primary sources located in the upper reaches of streams or near them, which is confirmed by a low degree of grains’ roundness. The composition of native gold in combination with a set of micro-inclusions indicate the gold–sulfide-quartz type of mineralization of most primary sources.

PHYSICO-CHEMICAL CONDITIONS FOR THE FORMATION OF PRODUCTIVE MINERAL ASSOCIATIONS OF GOLD DEPOSITS OF THE TAIMYR-NORTH EARTH ORGEN

For the first time, based on the data of the study of fluid inclusions, the PTX parameters of the formation of gold mineralization of the Taimyr-Severozemelsky orogen were established. The temperature of formation of gold ores covers the range from 310 to 105°C, the pressure was not lower than 110–96 MPa. The ore-forming fluids were characterized by NaCl + KCl + H2O + CO2 + CH4 + N2 composition and salinity up to 9.7 wt. % in equiv. NaCl. The composition of native gold (fineness varies from 960 to 550‰) and minerals of productive mineral associations has been determined. Ore-forming fluids of gold-sulfide-quartz ore occurrences of the accretionary Schrenk-Fadeevskaya zone (Yasnenskoye and Malinovskoye in North-East Taimyr) are more oxidized and less saline compared to the gold-quartz occurrences of the orogenic type of the Mininsko-Bolshevitskaya myogeoclinal zone (Nizhnelitkenskoye, Vidimoe, Nerpichye).

Native Gold of the Chudnoe Gold–Palladium Deposit (Subpolar Urals, Russia)

Abstract ––The Chudnoe gold–palladium deposit, located within the Central Ural uplift, is confined to the axial zone of an anticlinal structure complicated by faults. Veinlet mineralization is localized in fractured and brecciated Late Riphean–Vendian rhyolites. Native gold and palladium minerals are concentrated mainly in veinlets of Cr-containing muscovite (fuchsite) and, more seldom, at local sites of rhyolite metasomatism, almost free of sulfides. Using a representative material, we studied the composition and texture of native gold and the regularities of their spatial variations. Native gold has composition Au–Ag–Cu and contains Pd and Hg impurities. The proportion of elements varies significantly: The content of Au ranges from 65.8 to 92.7 wt.%, the content of Ag varies from 0.4 to 33.8 wt.%, the content of Cu reaches 12.7 wt.%, and the content of Pd is up to 2.9 wt.%. Gold formed as a homogeneous Au–Ag–Cu solid solution at temperatures above 220 °C. The presence of copper in native gold is probably due to the absence of sulfide ion from the ore-forming hydrothermal solution. With decreasing temperature, Au–Ag–Cu solid solution containing more than 1.1–2.5 wt.% Cu disintegrates into two or three phases, forming characteristic plate–lattice and tabular exsolution textures. In the case of two-phase exsolution, the final phases are Au3Cu and Au–Ag solid solution or AuCu and Au–Ag solid solution; and in the case of three-phase exsolution, these are Au3Cu, AuCu, and Au–Ag solid solution. In some cases, the exsolution was accompanied by the recrystallization of gold with the formation of grain intergrowths. The composition of native gold in the deposit varies significantly, showing a discrete character at different sites of the ore zones, which is consistent with the veinlet ore mineralization. At the final stages of mineral formation, native gold was partly replaced by secondary highfineness gold.

The Features of Native Gold in Ore-Bearing Breccias with Realgar-Orpiment Cement of the Vorontsovskoe Deposit (Northern Urals, Russia)

This paper describes native gold in ore-bearing breccias with realgar-orpiment cement from the Vorontsovskoe gold deposit (Northern Urals, Russia). Particular attention is paid to the morphological features of native gold and its relation to other minerals. The latter include both common (orpiment, barite, pyrite, prehnite, realgar) and rare species (Tl and Hg sulfosalts, such as boscardinite, dalnegroite, écrinsite, gillulyite, parapierrotite, routhierite, sicherite, vrbaite, etc.). The general geological and geochemical patterns of the Turyinsk-Auerbakh metallogenic province, including the presence of small non-economic copper porphyry deposits and general trend in change of the composition of native gold (an increase in the fineness of gold from high-temperature skarns to low-temperature realgar-orpiment breccias) confirm that the Vorontsovskoe deposit is an integral part of a large ore-magmatic system genetically associated with the formation of the Auerbakh intrusion.

Native Gold in the Chudnoe Au-Pd-REE Deposit (Subpolar Urals, Russia): Composition, Minerals in Intergrowth and Genesis

Composition of native gold and minerals in intergrowth of the Chudnoe Au-Pd-REE deposit (Subpolar Urals, Russia) was studied using optical microscopy, scanning electron microscopy, and electron microprobe analysis. Five varieties of native gold have been identified, based on the set of impurity elements and their quantities, and on intergrown minerals. Native gold in rhyolites from the Ludnaya ore zone is homogeneous and contains only Ag (fineness 720‰, type I). It is in intergrowth with fuchsite or allanite and mertieite-II. In rhyolites from the Slavnaya ore zone, native gold is heterogeneous, has a higher fineness, different sets and contents of elements: Ag, Cu, 840–860‰ (type II); Ag, Cu, Pd, 830–890‰ (III); Ag, Pd, Cu, Hg, 840–870‰ (IV). It occurs in intergrowth with fuchsite, albite, and mertieite-II (type II), or albite, quartz, and atheneite (III), or quartz, albite, K-feldspar, and mertieite-II (IV). High-fineness gold (930–1000‰, type V) with low contents of Ag, Cu, and Pd or their absence occurs in the form as microveins, fringes and microinclusions in native gold II–IV. Tetra-auricupride (AuCu) is presented as isometric inclusions in native gold II and platelets in the decay structures in native gold III and IV. The preliminary data of a fluid inclusions study showed that gold mineralization at the Chudnoe deposit could have been formed by chloride fluids of low and medium salinity at temperatures from 105 to 230 °C and pressures from 5 to 115 MPa. The formation of native gold I is probably related to fuchsitization and allanitization of rhyolites. The formation of native gold II-V is also associated with the same processes, but it is more complicated and occurred later with a significant role of Na-, Si-, and K-metasomatism. The presence of Pd and Cu in the ores and Cr in fuchsite indicates the important role of mafic-ultramafic magmatism.

Gold and Silver Minerals in Sulfide Ore

Abstract—Gold and silver are capable of forming stable natural compounds with different elements. For gold 36 minerals are known: 10 in the class “Native metals, intermetallic compounds” and 26 in the class “Sulfides (selenides, tellurides, arsenides, antimonides, bismuthides).” For silver, 194 minerals are registered in the MMA, presented in 8 classes. Compared to gold in the class “Native metals, intermetallic compounds” there are far fewer silver minerals. They are widely represented in the classes “Sulfides” and “Sulfosalts” (124). Among silver minerals “halides” are also known (fluorides, chlorides, bromides, iodides), seven; simple and complex oxides, two; there is one mineral each in the classes “sulfates,” “phosphates,” “arsenates,” and “vanadates.” Both metals are in the compositions of 12 minerals: native gold, native silver, weishanite, uytenbogaardtite, petrovskaite, fishesserite, penzhinite, petzite, mutmannite, sylvanite, krennerite, and bezsmertnovite. Every year, the MMA commission registers four to six gold and silver minerals, so there are prospects for the discovery of new minerals of these precious metals in the near future. The indicator elements (fixers) that form natural compounds with gold and indicate the possible presence of gold minerals in sulfide ores include 13 elements: 7 metals (Ag, Cu, Pd, Hg, Sn, Tl, Fe), 3 chalcogens (Te, S, Se), and 3 metalloids (As, Sb, Bi). For silver, this range is wider and also includes such elements as Mn, Zn, Ge, Cd, V, O, H, F, Cl, Br, I, In, and N. This review presents and summarizes the results of studies on the composition of native gold with copper, mercury, and palladium impurities in different types of gold deposits. The article describes the gold and silver minerals established by the author together with colleagues in a study of productive mineral assemblages in some gold deposits in Russia: the Ulakhan, Yunoe, Krutoe, Dzhulietta, Dorozhnoe (Magadan region), Konechnoe (Taimyr Peninsula), Kupol, Valunistoe (Chukotka), Maletoyvayam (Kamchatka), etc. Data are presented on gold and silver minerals found in sulfide ores and of economic importance. Some of their technological properties are characterized, including solubility and behavior under the influence of various reagents. The presence of refractory minerals (calaverite, sylvanite, krennerite, petzite, aurostibite, maldonite, fishesserite, uytenbogaardtite, petrovskaite, penzhinite, weishanite, yuanjianite, hunchunite, anyuite, krynovite, nadjagite, and bogdanovite, bilibinskite, criddleite, etc.), as well as the chemical composition gold and silver, are important technological factors and should be taken into account when developing a technology for extracting precious metals from ores. The presence of increased concentrations of indicator elements (fixers) of gold and silver minerals in sulfide ores is an important indicator and argument for detailed mineralogical and geochemical studies of the source raw material. Identification of mineral (micro-, nano-) forms of gold and silver in sulfide ores is necessary for developing and improving rational dressing schemes.

The Lukoganskoe Au–Cu Skarn Deposit (Eastern Transbaikalia): Mineral Composition, Age, and Formation Conditions

Abstract —The Lugokanskoe deposit is located in southeastern Transbaikalia and has been studied for a long time by many researchers. However, the type of its formation is still debatable. In this paper we study the mineral composition of ores by modern methods, recognize and describe the main gold mineral assemblages, and present detailed data on the chemical composition of native gold and sulfide minerals and their isotope composition. We have established that gold–pyrite–chalcopyrite–arsenopyrite and gold–bismuth parageneses localized in skarn deposits are the main productive assemblages. Study of the sulfur isotope composition of sulfide minerals has shown an endogenous source of sulfur of the ore minerals. The carbon and oxygen isotope compositions of carbonates of ore-bearing veins indicate the participation of a magmatic fluid. The established age of the gold mineralization and igneous rocks of the Shakhtama complex, together with direct geological observations, points to their spatial, temporal, and genetic relationships. According to their petrochemical and geochemical characteristics, the igneous rocks of the Shakhtama complex are I-type ilmenite (reduced) granitoids. Study of fluid inclusions by heating and cooling and Raman spectroscopy has shown that the mineral formation was accompanied by a gradual decrease in the content of salts in the ore-forming fluids and by a decrease in their homogenization temperatures. Optical observations demonstrate that the fluid was heterogeneous at the early stages of the mineral formation. The evolution of the ore system was accompanied by a change in the gas phase composition of fluid inclusions from predominantly nitrogen–carbon dioxide to essentially aqueous, with carbon dioxide impurity (H2O + CO2 ± N2 → H2O ± CO2). The research data testify to the magmatic nature of fluids and the participation of meteoric waters at the late stages of the ore-forming process. The data obtained have led to the conclusion that the Lugokanskoe gold deposit is related to reduced intrusions formed at a shallow depth.

Geology, mineral associations, and composition of native gold in mineral deposits of Taimyr–Severnaya Zemlya Orogen

Diverse grain sizes and morphologies, chemical composition, and productive mineral associations of native Au from new mineralizations within two mineragenic areas (Mininsk–Bolshevik miogeocline, Shrenk–Faddeev accretionary) in Western and Central Taymyr, are indicative of a complex, multi-component composition of ore-forming systems and differences in the physical and chemical conditions of formation of gold ore mineralization. The obtained data on the characteristics of native Au, combined with peculiarities of the geological structure of the objects, suggest a significant gold ore potential of central sector of the Russian Arctic.

The Geology, Mineral Associations, and Composition of Native Gold in the Mineral Deposits of the Taimyr–Severnaya Zemlya Orogen

The variety of sizes and morphology of grains and chemical composition of native Au, and the diversity of mineral productive associations in the new ore occurrences of the Western and Central Taimyr (the Minina–Bolshevik miogeocline and the Shrenk–Faddeevskaya accretionary mineragenic zones) indicate the complex multicomponent composition of ore-forming systems and different physicochemical conditions of the formation of gold ore mineralization. The native Au characteristics and specific features of the geological structure of the objects point to the significant gold ore potential of the Central Sector of the Russian Arctic.

Contrasting Surficial Composition of Native Gold from Two Different Types of Gold Ore Deposits

Native gold grains sampled at two different gold ore deposits in Eastern Russia have been studied by the techniques of electron spectroscopy (X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES)), electron microprobe analysis (EMPA), and scanning electron microscopy with energy dispersive X-ray spectrometry (SEM-EDX). The high-fineness gold of the deposit hosted by relatively high temperature gold-quartz-sulfide mesothermal ore formation contains no oxidized Au constituents on grain surfaces, whereas the less fine gold of the epithermal Au-Ag deposit contains gold oxidized to the Au (I) state, or higher, in half of the cases. At this deposit the surface of native Au consists of a thin layer (~15 nm) with elevated Ag and S contents and an underlying SiO2–containing layer ~30–60 nm thick. Such a composite coating can perform a protective function and prevent the gold-silver sulfides in surficial parts of AuAg grains from oxidation. The sulfur-enriched marginal parts of native gold particles do not always correlate with the stoichiometry of well-known binary AuAg-sulfides and have a variable composition. This may be due to the existence of solid solutions, Ag2−xAuxS, if there is enough S or S adsorption-stabilized cluster agglomerates, AgnAumS, under sulfur deficit. The effect of the formation of nano-scale surficial zonality on the surface of native gold is quite common in nature and applicable to geological exploration and technology of gold-ore processing. It can facilitate establishing the geochemical environment and genetic type of Au mineralization.

Up-dated ore composition data (Central ore-field, Kuznetsk Alatau)

Applying scanning electron microscope (SEM), energy-dispersive microanalyzer and X-ray ray fluorescence microscope the ore mineral composition in Central gold-ore field ore field (Kuznetsk Alatau) was investigated. Eleven new minerals were detected in this ore field. The differentiated behavior of mineral formation stages in veinsand near-veined beresites was determined. The composition of native gold was studied, as well as the distribution of trace elements in pyrite.

TELLURIDE MINERALOGY OF THE GOLDEN MILE DEPOSIT, KALGOORLIE, WESTERN AUSTRALIA

The Golden Mile, Kalgoorlie, Western Australia, is a mesozonal gold deposit unusual because of its enormous size (>1,457 tonnes of gold) and because tellurides are responsible for approximately 20% of gold production. Gold mineralization is hosted primarily by Archean-aged dolerites and basalts that have been metamorphosed to the greenschist facies. This mineralization occurs in hundreds of auriferous and telluride-bearing lodes, which have been classified into three structural types on the basis of orientation: main (trending NNW), caunter (trending NW) and cross lodes (trending NE). Three distinct styles of hydrothermal mineralization are present in the deposit. The early Fimiston-style lodes are characterized by main, caunter and cross lodes. The Oroya-style lodes (V- and Te-rich) occur in later dilational jogs and veinlets within a shear zone. Both of these are overprinted by the younger Mount-Charlotte-style mineralization (quartz-vein stockwork). Nineteen tellurium-bearing minerals occur in the Golden Mile deposit. Calaverite, petzite, coloradoite, altaite and native gold are most common throughout the deposit, whereas tellurantimony, melonite, hessite, stutzite, krennerite and sylvanite are rare in the main and caunter lodes. In addition, hessite and montbrayite are found in trace amounts in the cross and main lodes, respectively. There is no systematic distribution of tellurides within a given lode. Tellurides in Fimiston-style ores formed at ca. 300° to 170°C during cooling of the deposit. Values of f (Te2) and f (S2) determined at 300°C on the basis of the stability of native elements, tellurides and sulfides, are ca. log f (Te2) in the range −11.4 to −6.8 and log f (S2) in the range −12.6 to −5.5. No spatial variations in the composition of native gold, tetrahedrite-group minerals and tellurides occur in the Golden Mile deposit, although there is a weak to moderate decrease in gold fineness with depth. Marked differences in telluride mineralogy in and between individual lodes likely reflect variations in physicochemical conditions during ore formation or mechanisms of precious-metal deposition.

Microchemical Characterization of Alluvial Gold Grains as an Exploration Tool

There is considerable variation in the composition of native gold and the nature of minerals co-existing with it, and this reflects differences in the geological environment and chemistry of ore-forming processes. In areas where gold-bearing mineralization is subject to active fluvial erosion, especially in temperate climatic regimes, any discrete grains of native gold pass into alluvial sediment with little modification. The chemical characteristics of alluvial grains and the nature of preserved mineral inclusions provide a signature which points back to the type of source mineralization. This signature may be established using electron probe microanalysis and scanning electron microscopy and can be interpreted to provide information about the original bedrock mineralization. Identification of the type of source mineralization using the technique at an early stage in regional exploration can help focus attention on targets with the most potential economic importance.

Morphogenesis and composition of native gold in the Chelopech volcanic-hosted Au–Cu epithermal deposit, Srednogorie zone, Bulgaria

Native gold is an important economic component of the complex ores of the Chelopech high-sulphidation volcanic-hosted epithermal Au–Cu deposit (Bulgaria). The ore consists of pyrite, chalcedonic silica, chalcopyrite, enargite, luzonite, tennantite, bornite, sphalerite, galena, and numerous other sulphide, arsenide and telluride minerals. Gold is paragenetically associated with most of the arsenic-bearing and base metal sulphide minerals. The chemistry and morphology of gold grains, which were separated from disintegrated ores, were systematically studied by scanning electron microscopy (SEM) and electron probe microanalysis (EPMA). The gold is characterised by high fineness (~950) and a mean composition of 94.1 wt% Au, 5.27 wt% Ag, 0.53 wt% Cu and 0.10 wt% Fe. Gold grains show variable morphology: subhedral flakes, irregular grains, euhedral isometric crystals, elongated rods, wires and fine-fibrous crystals, {111} twins, fine dendrite-like formations, spongy gold, and polycrystalline grains. The densest faces {111} are morphologically the most important. Morphometric measurements show a pronounced flatness of the gold particles. The mean value of the Corey factor (a measure of flatness from 0 to 1 = spherical) is 0.14 and the mean length to width value is 1.64. It is suggested that crystal growth of gold took place in small voids, fine cracks and intergranular space, the geometry of which have controlled the highly variable grain shapes. No direct correlation between the size and composition of gold grains exists, but larger grains tend to be of higher fineness.