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THE GOLD "Accursed thirst for gold! What have you not compelled mortals to do?" Virgil Golden Rule, and the Gold Standard, to name only a few metaphors. Also, we have such commonplace sayings as heart of gold, good as gold, and so on. Perhaps the two best-known proverbs involving gold are "All that glistens is not gold" and "Gold is where you find it." The Egyptians used the perfect of planar geometric figures, the circle, as the symbol for gold, the most perfect and noblest of the metals. The alchemists associated gold with the sun (Sol) or with the Greek sun-god (Apollo) and represented it by the symbol of perfection, the circle with a dot at the centre, or by the circle with a crown of rays to represent the king or Apollo of metals. To the early Hindu philosophers gold was the "mineral light"; to the early Western philosophers the metal was the image of solar light and hence of the divine intelligence of the universe. The desire for gold has markedly influenced history: the cry "gold" has lured men across oceans and continents, over the highest mountain ranges, into Arctic tundras and scorching deserts, and through nearly impenetrable jungles. Its gleam prompted the expeditions and conquests of Jason of Thessaly, Cyrus and Darius of Persia, Alexander of Macedon, Caesar of Rome, Columbus of Genoa, Vasco da Gama and Amerigo Vespucci of Portugal, Cortez and Pizzarro of Spain, Raleigh of England, and many others throughout history. Gold has carried the torch of civilization to the remotest regions of the world; unfortunately the auri sacra fames* has also wrought terrible acts of slavery, war, and bitter contention upon mankind. So it is also with most other materials of this earth. To make gold from baser metals was a major preoccupation of the alchemists (as were also their ceaseless efforts to discover the elixir of life and the fountain of youth). The fruits of their labours gave us the rudiments of modern chemistry. Gold has a widespread occurrence in practically every country of the world and has influenced the exploration and settlement of most. In Africa, Europe, and Asia ancient gold mines are known in Egypt, Spain, France, Great Britain, Yugoslavia, Romania, Greece, Turkey, Saudi Arabia, Iran, India, China, Japan, and the U.S.S.R. Ancient placers have yielded gold from the rivers Tagus, Guadalquivir, Tiber, Po, Rhone, Rhine, Hebrus (Maritsa), Nile, Zambezi, Niger, Senegal, Pactolus (in ancient Lydia), Oxus (Amu Darya that flows through the golden land of Samarkand), Indus, Ganges, Lena, Aldan, Amur, Yangtze, and a multitude of others. The artisans of the earliest civilizations of Anatolia (Catal Hijyilk), Mesopotamia (Sumer), and the Indus Valley (Harappa and Mohenjo-Daro) worked in gold obtained from many sites in the Caucasus and Middle Asia, the Middle East, and the Indian Peninsula. The Egyptians mined gold extensively in eastern Egypt and Sudan (Nubia) as far back as 4,000 years ago. It was from them that the Persians, Greeks, and Romans in turn learned the techniques of gold prospecting, mining, and metallurgy. The Greeks and Romans mined gold ores from the extensive metalliferous regions of their empires. Pliny the Elder (A.D. 23-79) in his Historia naturalist written in the early years of our era, repeatedly mentions the mining and metallurgy of gold, and during the Renaissance Agricola referred to it, as had many others before him during the Middle Ages. Compared with the gold placers and mines of the Old World, those in parts of the New may be as ancient, although it would appear that the aborigines of North and South America placed little emphasis on gold beyond its use in ornaments, jewellery, sacrificial knives, and the like. Columbus of Genoa found the natives of Hispaniola (Haiti) in possession of gold nuggets in 1492, a fact that excited the Spaniards to later pursue their conquests of Mexico and South America, where in 1550 they found their Eldorado in the fabulous placer deposits of Colombia. In Brazil the Portuguese sought gold during the last half of the sixteenth century, but the deposits found were small and mined only sporadically during the seventeenth century. In 1693 economic deposits of gold were discovered in Minas Gerais, and for a century thereafter this state was one of the world's major sources of the precious metal. One of these deposits, the famous Morro Velho, has been mined by underground workings for almost a century and a half and is still productive. Although silver has been the most important precious metal of Mexico, gold has also been won from many of the silver-gold deposits, mostly of Tertiary age. The bedrock deposits of the great silver-gold vein system of the Veta Madre at Guanajuato were found in 1550 and exploited almost immediately thereafter. El Oro, one of the premier gold districts but now largely exhausted, was discovered in 1521, developed extensively by 1530, and mined intermittently for nearly 400 years thereafter, producing more than 5 million oz. of gold. Since the beginning of the nineteenth century prospecting for gold has ranged widely over Canada and the United States, resulting in many great placer gold rushes, first to California in 1848, then to British Columbia in 1857, and later to the Klondike in Yukon in 1896 and Nome in Alaska in 1899. After the exhaustion or near-exhaustion of many of the placers, attention turned to bedrock deposits during the last half of the nineteenth century and the first half of the present century. The Mother Lode and Grass Valley in California and the famous Comstock Lode in Nevada were discovered and developed in the 1850s. The gold telluride deposits of Cripple Creek in Colorado were located in 1892, and by 1905 the Tonopah and Goldfield districts in Nevada were under development. The Homestake mine, at Lead, South Dakota, was found in 1876 and brought into production soon thereafter. In eastern Canada lode gold was first worked in Nova Scotia in the late 1850s, followed in 1866 by the first discovery of lode gold in the Canadian Shield near Madoc, Hastings County, Ontario. After the discovery of the native silver deposits at Cobalt, Ontario, in 1903, prospectors ranged widely over the Precambrian areas of Ontario, Quebec, Manitoba, Saskatchewan, and Northwest Territories. In Ontario and Quebec, Abitibi and Larder Lake were discovered in 1906; Porcupine, 1909; Swastika, 1910; Kirkland Lake, 1911; Matachewan, 1916; Rouyn (Noranda), 1924; and Red Lake, 1925. In Manitoba the Rice Lake district was discovered in 1911, and in Northwest Territories, the deposits in the sediments of the Yellowknife area were discovered in 1933 and those in the greenstones in 1935. The most recent discoveries in the Canadian Shield are the large auriferous orebodies in the Hemlo area, northwestern Ontario, originally indicated in 1869 and extensively developed in the early 1980s. In western Canada the bedrock gold deposits in British Columbia first attracted attention in 1863 during the first great placer gold rushes to the province. Little work was done, however, on most of the discoveries and many were forgotten. The area in the vicinity of the Cariboo Gold Quartz and Island Mountain mines in the Barkerville district was prospected in 1860, and some mining was done in 1876 and a few years thereafter at these mine sites; large scale mining, however, did not commence until 1933 and 1934 respectively at the two mines. In 1897 the Cadwallader gold belt in the Bridge River district, containing the Bralorne and Pioneer deposits, was prospected, but it was not until 1928 that the Pioneer mine was brought into production, followed in 1932 by the Bratorne mine. Rossland in West Kootenay District was located in 1889 and brought into production in 1890, the Premier mine in Stewart District in 1918, and the Zeballos gold belt on the west coast of Vancouver Island was discovered and developed in 1934. The discovery of payable placer gold in Australasia was made first in 1851 near Bathurst, New South Wales, Australia, by Edward Hammond Hargraves. There followed then the discoveries of large eluvial placer and bedrock gold deposits in Australia at Bendigo and Ballarat (1851), Gympie (1867), Charters Towers (1870), Beaconsfield in Tasmania (1876), Mount Morgan (1886), Kimberley (1886), Coolgardie (1892), Kalgoorlie (1893), and Tennant Creek (1932). In Papua New Guinea the rich gold placers of the Morobe field were discovered by the Spaniards in 1528 but brought into extensive production only in 1926. There are few references to gold in the journals of the early explorers of New Zealand, and it was left mainly to the prospectors who had followed the great gold rushes to California and later to Australia to establish the presence of gold in commercial amounts (Salmon, 1963). Alluvial gold was first discovered in 1852 at Coromandel (Hauraki Goldfield) North Island in Driving Creek by Charles Ring; later at Collingwood in Nelson, South Island, in 1857; at Gabriels Gully in Otago by Gabriel Read in 1861; and in a number of sites along the western coastline of the South Island in 1864. Lode gold in the Hauraki Goldfield, the main centers being Waihi, Thames, Karangahake, and Coromandel, was first exploited in the 1860s; the quartz deposits in Otago, South Island, were first worked in the 1870s; and the productive quartz lodes at Reefton produced their first gold in the early 1870s. Gold and gold tellurides are widely distributed in the Fijian Islands, and the presence of these minerals was evidently known to the early Fijians. Baron A. B. de Este is said to have discovered gold in the Tavua area in 1872, but some sixty years were to pass before any serious prospecting was carried out. In 1932 B. Borthwick and J. Sinclair discovered payable gold on Vunisina Creek, a small tributary of the Nasivi River. Further investigation of this prospect, associated with the Tertiary Vatukoula volcanic caldera, led to the development of a number of mines, of which the famous Emperor mine has been in continuous production since about 1935. Elsewhere in the Pacific region gold had been sought and mined long before the Christian era. The gold-silver mines at Radjang Lebong, Indonesia, are said to have been exploited in a desultory manner by the Hindus as early as 900 B.c. and more systematically by others since 1700. The deposits at Bau, Sarawak, likewise have a long history but have received detailed attention only since 1820. In the Philippine Archipelago gold was known to the Chinese and Hindus from the beginning of our era; the Spaniards found gold there as early as 1524, but extensive exploitation of the deposits took place only after Spain ceded the Philippines to the United States in 1898. The Chinese have mined gold for millenia, the first indication of this pursuit being in the artifacts of the Shang dynasty (1765 B.C.). More recently considerable gold mining has been carried out in Archean, Proterozoic, and younger terrains in Shandong, Yunnan, Kansu, Szechuan, and other provinces. Likewise in Korea gold mining is an old technology going back to at least the beginning of the Christian Era in mining districts such as those of Unsan, Nurupi, Sak Ju, and Sen Sen. Similarly in Japan the search for gold was in progress long before the Christian era, judging from archaeological evidence; the methods of searching for and mining the metal were probably introduced from Korea. The Sado mine on Sado Island in the Japan Sea, the largest gold and silver producer in Japan, was discovered in 1542 and has been in production intermittently since that time. India has long been the site of gold mining, first from placers and then in more modern times from the oxidized and primary zones of a variety of auriferous deposits. Pliny, writing at the beginning of our era in his Historia naturalis, mentions the gold of India, and the land of Ophir mentioned in I Kings 10:11 in the Old Testament can, according to some authorities, be equated with India. It is certain that gold placers and the rich oxidized zones of auriferous deposits were worked in India long before the Christian era, as evidenced by archaeological data and written records. Large-scale mining in India began with the Mauryan colonization of the Deccan about the end of the fourth century B.C. The discovery of the Kolar field would seem to date from the beginning of the Christian era, probably coeval with that of the Hutti field to the north. The modern mining of the famous Champion Lode in the Kolar field, rediscovered in 1873, began about 1880 and has continued since that date. The Russia has long been a legendary source of gold. The land of Cotchis drained by the river Phasis (the modern Rioni) in Georgia is reputed to have provided great quantities of gold. Similarly, the Persians are said to have obtained much gold from the Scythians, a polyglot group of tribes that inhabited the region north of the Black Sea, and from various Iranian and other tribes who inhabited the Ural-Uzbek-Altai region. The golden road to Samarkand was known centuries before Christ. With time the monopoly of gold mining became the sole preserve of the Imperial Czars, who pursued extensive placer and lode mining first in the Urals, beginning about 1774, and later in many parts of Siberia, especially in the Altai region where alluvial deposits were exploited as early as 1820. In 1829 the placer deposits of the Lena were first exploited and in 1840 those of the Yenisei Ridge came into production; the placers in the drainage system of the Amur were apparently first worked around 1867, and those in the Far Eastern maritime area appear to have been first exploited around 1870 or earlier. In recent years many more gold districts and deposits, both placer and bedrock (e.g., Muruntau, Uzbek), have been developed and brought into production, making the old Soviet Union the second largest, after South Africa, gold producer in the world. Since the new Rusia abolished the network of forced labour camps of political and war prisoniers, many of these deposits are subeconomical to mine. Gold from West Africa found its way into Europe as early as the tenth century and probably before. Most of this gold came by Sahara caravan to Barbary and thence to Europe, the original sources being the kingdoms of Ghana, Mali, and Songhai. It is said that much of this gold came from a region known as Wangara (a tributary of the Senegal River and noted for its placers), but considering the aurificity of West Africa it seems likely that the gold had a much more widespread source. In any event, one of the motives of the Portuguese voyages inspired by Henry the Navigator was to ascertain and exploit the west African gold. The Portuguese were soon followed by a host of English, French, Dutch, Danish, and Spanish entrepreneurs. It is thought that annually more than a quarter of a million ounces of gold reached Europe during the fifteenth and sixteenth centuries from African sources. Based mainly on native workings, numerous gold deposits, both bedrock and placer, were rediscovered during the latter part of the nineteenth century throughout Senegal, Guinea, Sierra Leone, Ghana, Nigeria, and the other nations of the Gold Coast. The Precambrian auriferous Tarkwa conglomerates of Ghana were developed in a modern way during the period 1876-1882 by Pierre Bonnat, the father of modern gold mining in the Gold Coast. In 1895, Ashanti Goldfields Corporation began work in the Obuasi district of Ghana, developing the Ashanti and other mines, which have produced the largest proportion of gold since 1900 in the countries of the Gold Coast. All of these deposits are of Precambrian age. Natives worked gold deposits in Zimbabwe perhaps as far back as the beginning of our era. Long lines of ancient workings in the oxidized zones followed the strike of many Precambrian (Archean) deposits that were later developed by modern mining methods at the beginning of the present century. The Gaika mine was developed in 1894, the Globe and Phoenix in 1895-1900, the Eldorado in 1905, the Antelope in 1908, the Cam and Motor in 1909, the Shamva in 1909- 1910, and numerous others during the period 1895-1911. In South Africa an event in 1834 has influenced the history of gold ever since: Carel Kruger discovered gold on the Witwatersrand, or White Waters Ridge, while on a hunting expedition north of the Vaal River. However, little attention was paid to the find because the goldfields of Barberton and the DeKaap Valley were the chief focus of gold prospecting through 1885. In 1886 George Harrison, an Australian gold digger, and George Walker, an Englishman, discovered payable gold reef on the Witwatersrand. This discovery soon led to the development of the great reefs that constitute the largest gold deposits known and that made South Africa the foremost gold producer in the world. During a century of mining, some 4,000 million metric tons of ore have been treated from the Witwatersrand deposits, resulting in the recovery of 37 million kg of gold (1,200 million oz). The story of the discovery of the remarkable deposits of the Witwatersrand, of the growth of Johannesburg, and of the men who struggled for control of not only the gold reefs but of what it was South Africa is interesting. When the faboulous deposits became subeconomical, the desendants of these men retourned to London, New York or Tel Aviv, abandoning the South Africa to native population. It is estimated that the total amount of gold won from the earth to the end of 1985 is about 3.85 billion (3.85 x 103) troy oz (120 x 103 g.). Of this amount: 2% was produced prior to 1492, 8% during the period 1492-1800, 20% during the interval 1801-1900, 70% from 1901-1985 (all figures being rough estimates). In volume the total amount of gold won from the earth would occupy 6,300 m3 or an 18.5 m cube, a small volume of metal indeed to have so influenced the toil, trials, tribulations, and destiny of man for 5,000 years. The current annual world production of gold is about 1,338 metric tons. 50% of this production is derived from quartz-pebble conglomerate deposits, 20% from eluvial and alluvial placers, and the remainder from the various vein and disseminated deposits. The general literature on gold reaches back some 5,000 years, almost to the birth of writing. The most ancient accounting tablets of the Sumerians of Mesopotamia, scribed about 3100 B.C., mention the metal as do also the pictographics, phonographics (word-syllabic systems), and hieroglyphics on the tablets and papyri of the most ancient Hittite, Elamite, Egyptian, Cretan, Indian (Harappan), and Chinese civilizations. Initial mention of gold in a geological context appeared about 1320 B.C. on the famous "La carte des mines d'or", a Rameside papyrus and fragments depicting a gold mining region in Egypt. Since that time the geological and geochemical literature on gold has multiplied prodigiously, so that today there are perhaps more references to gold in the literature of the earth sciences than for any other element in the periodic table of the elements. Gold has played a unique and prominent part in the history of the theory of mineral deposits, especially in the theories advanced through the ages to explain the origin of veins and placers. For this reason, and to provide a background, I have included an opening chapter on the types and geochemistry of auriferous deposits; I have also included in later chapters pertinent details on the various theories of the origin of mineral deposits and discussed briefly the philosophical, social, and scientific milieu within which these various theories developed. General Types of Auriferous Deposits According to Boyle, R. W.. "El Dorado!" Gold is the most noble of metals, and its geochemistry is conditioned principally by this fact. Compared with other elements in the periodic table the terrestrial abundance of gold (0.005 ppm), is low compared with copper (50 ppm) and silver (0.07 ppm) the accompanying two elements in group IB, and approximately equal to that of platinum (0.005 ppm), the adjacent element in group VIII. Two general types of auriferous deposits are recognized: -lode (vein) deposits -eluvial and alluvial placers. The enigmatic quartz-pebble conglomerate deposits, the largest known auriferous concentrations on earth, have generally been classified as modified paleo-placers, but some geologists have considered them to be of hydrothermal origin and akin to lode deposits. The quartz-pebble conglomerate deposits supply 50% or more of the world's annual gold production. The remaining half is provided by the other types of auriferous deposits, including the vein and disseminated types, eluvial and alluvial placers, and the various by-product sources such as polymetallic veins, lodes, massive sulfide bodies, and stockworks. GENERAL GEOCHEMISTRY OF GOLD Gold is a member of group IB of the periodic table, which includes copper, silver, and gold. In its chemical reactions gold resembles silver in some respects, but its chemical character is markedly nobler. The principal oxidation states of gold are + 1 (aurous) and + 3 (auric). These states are unknown as aquo-ions in solutions, the element being present mainly in complexes of the type [Au(CN)2]- , [Au Cl2]- , [Au(OH)4]- , [Au Cl4]- , and [Au S]- . There is only one naturally occurring isotope of gold: 197 Au. In nature, gold occurs predominantly in the native state or as a major constituent of various alloys containing mainly silver, copper, or platinum metals. Several gold and gold-silver tellurides are known, of which the most common are sylvanite, calaverite, petzite, krennerite, and nagyagite. The antimonide, aurostibite, AuSb2, occurs in some auriferous deposits, and there is also an argentiferous gold selenide, fischesserite, Ag3AuSe2, an argentiferous gold sulfide, uytenbogaardtite, Ag3AuS2, and a bismuthide, maldonite, Au2Bi, which is fairly well differentiated. The principal ore minerals of gold are the native metal, aurostibite, and the various tellurides. The abundance of gold in the upper lithosphere is about 0.005 ppm and the Au/Ag ratio is about 0.07. The average gold content of igneous-type rocks in parts per million is ultramafic, 0.004; gabbro-basalt, 0.007; diorite-andesite, 0.005; and granite-rhyolite, 0.003. The average gold content of sedimentary rocks in parts per million is sandstone and conglomerate, 0.03; normal shale, 0.004; and limestone, 0.003. Certain graphitic shales, sulfide schists, phosphorites, and some types of sandstones and conglomerates may contain up to 2 ppm Au or more. The average gold content of soils is 0.005 ppm, and the average for natural fresh waters is 0.00003 ppm. Sea and ocean waters contain an average of 0.000012 ppm Au. Gold is a trace constituent of many plants and animals. Some coals are slightly enriched in gold, with 0.05 to 0.1 ppm Au in the ash. AURIFEROUS DEPOSITS Gold is won from deposits mined essentially for the metal and as a by-product of the mining and treatment of nickel, copper, zinc, lead, and silver ores. Nine principal types of deposits, exploited mainly for gold, are listed subsequently. The classification of these deposits is suggested in a monograph on gold (Boyle, 1979), revised to include more recent data; it is based essentially on the general morphology and chemical constitution of a deposit-type and on its geological and geochemical setting, particularly the nature of its host rocks. Because of the great diversity of auriferous deposits it is thought that this manner of classification is as factual and as objective as can be devised, and that it is relatively independent of speculative genetic theories. Numerous other classifications have been suggested and are discussed in the papers and monographs listed in the selected bibliographies at the end of this and later chapters. Many of these classifications, particularly those of epigenetic gold deposits, are based on magmatic or hydrothermal concepts and are largely speculative. Concerning classifications of gold deposits one should take heed of the admonition by Maclaren (1908): "Auriferous veins or deposits may be of any form, may occur in any rock, and may have received their gold from various sources. Particular classifications based on obviously adventitious characters, as similarity of form of deposit, or identity of matrix or of associated minerals, can therefore serve no useful purpose, either scientific or economic. Such classifications have been current for many years. Some have certainly been suggestive, but the majority have helped the miner and prospector not a whit, and have proved a source of confusion and embarrassment to the student." The following types of auriferous deposits are distinguished: 1. Auriferous porphyry dykes, sills, and stocks; auriferous pegmatites, coarse-grained granitic bodies, aplites, and albitites The indigenous gold content of these granitic rocks is invariably low, of the order of 0.003 ppm. Certain albitites and quartz-feldspar porphyry dykes and stocks with indigenous pyrite and pyrrhotite may contain up to 0.10 ppm Au and I ppm Ag, principally in the sulfide minerals. Porphyritic, aplitic, and granitic bodies of this type are common in Precambrian, mainly Archean, terrains and in younger rocks throughout the world. Most are of an intrusive nature, probably the anatectic products of deep- seated granitization. None so far are known to be of economic value, although many are probably the sources of the gold, silver, and other metals secondarily concentrated in the fractures, faults, and shear zones in the porphyry and albitic bodies themselves and in their nearby host rocks (see type 7. 1). 2. Carbonatites and carbonatite-related bodies These bodies are extremely complex magmatic and hydrothermal assemblages of rocks in which four stages can usually be recognized: (1) an ultramafic followed by an alkalic magmatic phase, (2) a magmatic dyke phase, (3) a (magmatic?) carbonatite phase, and (4) a late hydrothermal phase. The third and fourth phases are marked by extensive replacement processes in some complexes. Most carbonatites are zoned, often in a ring pattern. The late hydrothermal stage is commonly marked by sulfide mineralization that occupies late fractured and faulted parts of the rocks of all zones. In some carbonatites, however, sulfides, mainly pyrite, pyrrhotite, chalcopyrite and molybdenite, are widely distributed as (indigenous) disseminations in the fenites and rocks of all zones of the complexes; in the great Palabora deposit in South Africa the chalcopyrite and bornite occur in a disseminated (indigenous) form in a number of rock types, but the main concentrations are in the fractured transgressive carbonate (s6vite) complex and in transgressive carbonate veinlets that cut several rock types. Carbonatites and carbonatite-related bodies (e.g., carbonate-barite-fluoritesulfide veins, dykes, and disseminations) are characterized by a distinctive suite of elements that includes Na, K, Fe, Ba, Sr, rare earths, Ti, Zr, Hf, Nb, Ta, U, Th, Cu, Zn, P, S, F, and more rarely Li, Be, and Pb. Most of the rocks comprising carbonatites are low in gold and silver (0.005 ppm Au and 0.1 ppm Ag). The late stage carbonate-sulfide mineralization, however, commonly contains slightly enriched amounts of both gold and silver. The silver is present mainly in galena, tetrahedrite, and other such minerals; the gold is invariably native and occurs in association with pyrite, pyrrhotite, molybdenite, chalcopyrite, and other copper sulfides. Few if any carbonatites are enriched enough in gold and silver to constitute economic orebodies. However, the fact that the sulfide phases exhibit enrichments in the two precious metals suggests that deposits of this type should be considered as possible gold deposits. The Cu-U-Au deposits at Olympic Dam (Roxby Downs) in South Australia and the auriferous quartz-carbonate counterparts of the rare- earth carbonate deposits at Mountain Pass, San Bernardino County, California, may be carbonatite- related bodies. Gold is a frequent constituent of skarn deposits, in which it is commonly more abundant than the literature would indicate. Most skarn deposits yield gold as a by-product of copper and lead-zinc mining, but many of these deposits are greatly enriched in gold and silver and are mined essentially for the two precious metals. The general features of skarn deposits are well known and need not be described in detail here. Most of the deposits occur in highly metamorphosed terrains, particularly those containing carbonate rocks or carbonate-bearing pelites, and in which there has been much granitization and injection of granitic rocks. Some deposits occur near the contacts of granitic bodies and have long been called contact metamorphic; others are developed in favourable reactive beds or zones some distance from granitic contacts. The deposits contain a characteristic suite of early-developed Ca-Mg-Fe silicate and oxide minerals and a lower-temperature, generally later, suite of silicate, carbonate, sulfide, and arsenide minerals. The gold minerals include native gold and various tellurides. Most of the skarn deposits worked essentially for gold contain much pyrite and/or arsenopyrite. The elements most frequently enriched with gold in skarn deposits are Fe, S, Cu, Ag, Zn, Pb, Mo, As, Bi, and Te. There is commonly a positive correlation between Au and Cu in some skarn deposits. Tungsten is a common trace element in gold-bearing skarn deposits. The element belongs to the early phase of skarnification, whereas gold tends to be precipitated late in the mineralization processes. The two elements may, therefore, be negatively correlated, for it is common to find skarn deposits that are rich in tungsten (scheelite) but practically devoid of gold and vice versa. The Au/Ag ratio of the auriferous skarn-type ores is variable but is commonly greater than 1. Auriferous skarn deposits occur at widespread points in the Canadian Cordillera, particularly in the Hedley district of British Columbia, where the Nickel Plate and French mines worked arsenopyrite-pyrite orebodies in skarn developed in Triassic limestone and limy argillites. In the Canadian Shield, auriferous skarn deposits occur mainly in the Grenville Province, examples being the lead-zinc-silver-gold ores of the Tetreault mine near Quebec and the New Calumet mine northwest of Ottawa. Elsewhere in the world auriferous skarn deposits have a widespread distribution, especially in belts of carbonate rocks invaded by diorites, monzonites, granodiorites, and granites. Here belong the auriferous skarn deposits at Cable in Montana, the La Luz and Rosita mines in Nicaragua, a number of mines in the Altai-Sayan of USSR, and the skarn deposits of Bau, Sarawak, and the Suian district of Korea. 4. Gold-silver and silver-gold veins, stockworks, lodes, mineralized pipes and irregular silicified bodies in fractures, faults, shear zones, sheeted zones and breccia zones essentially in volcanic terrains Representatives of this type of deposit are widespread throughout the folded and relatively flat-lying volcanic terrains of the earth. The deposits occur in rocks of all ages, but the largest numbers occur in those of Precambrian and Tertiary age. The favourable host rocks are commonly basalts, andesites, latites, trachytes, and rhyolites. In Precambrian rocks, such assemblages are usually referred to as greenstones. Many deposits of Precambrian age occur in tuffs, agglomerates, and sediments interbedded with the volcanic flows, particularly in banded ironformations. In the older terrains, the rocks are generally regionally metamorphosed and have the characteristic regional metamorphic facies outward from igneous or granitized centres. The younger rocks generally show the effects of chloritization, carbonatization, hydration, and pyritization (propylitization) over broad zones, but locally some of the andesites and rhyolites may be relatively fresh. In the older rocks, the deposits are veins, lodes, stockworks, pipes, and irregular mineralized masses generally in extensive fracture and shear-zone systems. Some occur in drag folds. The deposits in the younger rocks are usually confined to fissures, fractures, faults, and brecciated zones that cut the volcanic rocks of calderas and generally have a limited horizontal and vertical extent. Others, however, are associated with fracture and fault systems that extend for many kilometres across volcanic sequences and their associated intrusive granitoids. A few deposits in young volcanic terrains occur in or near the throats of extinct (or present day) hot springs and/or in siliceous hot spring aprons. The mineralization of these particular deposits is characterized essentially by quartz; carbonate minerals, pyrite, arsenopyrite, base-metal sulfide minerals, and a variety of sulfosalt minerals. The principal gold minerals are the native metal and various tellurides; aurostibite occurs in some deposits. Characteristic types of wall rock alteration are generally developed adjacent to and in the vicinity of nearly all deposits in this class. In the old Precambrian rocks, the most common types of alteration are chloritization, carbonatization, sericitization, pyritization, arsenopyritization, and silicification. In the younger rocks, propylitization (chloritization and pyritization) is especially characteristic, and there may also be a development of adularization, silicification, kaolinization, sericitization, and more rarely alunitization. The elements commonly concentrated in this class of deposits include Cu, Ag, Zn, Cd, Hg, B, TI, Pb, As, Sb, Bi, V, Se, Te, S, Mo, W, Mn, Fe, C02 and SiO2; less commonly Ba, Sr, U, Th, Sn, Cr, Co, Ni, and E Hg and Sb are particularly characteristic of the younger deposits. The Au/Ag ratio of the ores is generally greater than I in most Precambrian and in some younger deposits; in many Tertiary deposits the ratio is less than 1. Deposits of this type are widespread in the Precambrian greenstone belts of the world; examples include Yellowknife, Northwest Territories, Canada; Red Lake and Timmins, Ontario, Canada; Kolar goldfield, India; Kalgoorlie goldfield, western Australia; and the Cam and Motor, Dalny, and other similar mines in.Zimbabwe. Younger representatives are the Mother Lode system of California (Mesozoic); Comstock Lode, Nevada (Tertiary); Goldfield, Nevada (Tertiary); Cripple Creek, Colorado (Tertiary); Sdcdrimb (Nagydg), Romania (Tertiary); Coromandel gold belt, New Zealand (Tertiary); Emperor mine, Fiji (Tertiary); Lebong and other auriferous districts, Indonesia (Tertiary); Lepanto mine, Philippines (Tertiary); Kasuga mine, Japan (Tertiary), and the Belaya Gora and other similar deposits in the far eastern USSR (Tertiary). 5. Auriferous veins, lodes, sheeted zones, and saddle reefs in faults, fractures, bedding- plane discontinuities and shears, dragfolds, crushed zones, and openings on anticlines essentially in sedimentary terrains; also replacement tabular and irregular bodies developed near faults and fractures in chemically favourable beds These deposits are widespread throughout the world and have produced a large amount of gold and silver; they are often referred to as "Bendigo type". The deposits are developed predominantly in sequences of shale, sandstone, and greywacke dominantly of marine origin. Such sequences are invariably folded, generally in a complex manner, metamorphosed, granitized, and invaded by granitic rocks, forming extensive areas of slate, argillite, quartzite, greywacke, and their metamorphic equivalents. Near the granitic bodies, various types (kyanite, andalusite, cordierite) of quartz-mica schists and hornfels are developed and grade imperceptibly into relatively unmetamorphosed slates, argillites, quartzites and greywacke marked by the development of sericite, chlorite and other low- grade metamorphic minerals. Most of the gold deposits are developed in the lower-grade facies. A few economic deposits occur in the granitic batholiths and stocks that invade the greywacke-slate sequences. The principal gangue mineral in these deposits is quartz; feldspar, mica, chlorite, and minerals such as rutile are subordinate. Among the metallic minerals, pyrite and arsenopyrite are most common, but galena, chalcopyrite, sphalerite, and pyrrhotite also occur. Molybdenite, bismuth minerals, and tungsten minerals are local. Stibnite occurs in abundance in a few deposits, but is relatively rare in most deposits. Acanthite, tetrahedrite-tennantite, and other sulfosalts are not common in these deposits. Carbonate minerals, mainly calcite and ankerite, are common but not abundant. The valuable ore minerals are native gold, generally low in silver, auriferous pyrite, and auriferous arsenopyrite. Telluride minerals are relatively rare, and aurostibite is an uncommon mineral in these deposits. A few deposits in this category are tabular or irregular replacement (disseminated) bodies developed in carbonate rocks or calcareous argillites and shales. The principal minerals in these deposits are quartz, fluorite, pyrrhotite, pyrite, arsenopyrite, sphaterite, galena, chalcopyrite, and stibnite. As a general rule, wall rock alteration associated with these deposits is minimal, and the quartz veins, saddle reefs, and irregular masses are frozen against the slate, argillite or greywacke wall rocks. In places, thin zones of mild chloritization, sericitization, and carbonatization are present. Some veins are marked by thick black zones (up to 15 cm wide) of tourmatinized rock. Disseminated pyrite and arsenopyrite are common in the wall rocks of most of these deposits. This pyrite and arsenopyrite is usually auriferous. The elements exhibiting a high frequency of occurrence in this type of gold deposit include Cu, Ag, Mg, Ca, Zn, Cd, (Hg), B, (In), (TI), Si, Pb, As, Sb, (Bi), S, (Se), (Te), (Mo), W, (F), Mn, Fe, (Co), and (Ni). Elements in parentheses have a low to very low frequency of occurrence. The Au/Ag ratio in the ores is generally greater than 1. Deposits in essentially sedimentary terrains are widespread throughout the world. In Canada, examples occur in the Archean Yellowknife supergroup in the Yellowknife district, Northwest Territories (Ptarmigan, Thompson-Lundmark and Camlaren mines); in the Paleozoic Cariboo group at Wells, British Columbia (Cariboo Gold Quartz mine); and widespread in the Ordovician Meguma group of Nova Scotia. Elsewhere in the world deposits of this type occur in the auriferous Appalachian "Slate Belt" of the United States (Paleozoic); Salsigne mine, Montagne Noire, France (Paleozoic); Sovetskoe deposit, Yenisey region, USSR (Proterozoic); Muruntau deposit, Uzbec SSR (Paleozoic); Bendigo deposits, Victoria, Australia (Pateozoic); and the Pilgrims Rest and Sabie goldfields in the Transvaal System, South Africa (Proterozoic?). 6. Gold-silver and silver-gold veins, lodes, stockworks, and silicified zones in a complex geological environment, comprising sedimentary, volcanic, and various igneous intrusive and granitized rocks Deposits in this category combine nearly all the epigenetic features described in categories 4 and 5. Quartz is a predominant gangue, and some deposits are marked by moderate developments of carbonates. The orebodies constitute principally quartz veins, lodes, and silicified and carbonated zones. The gold is commonly free but may be present as tellurides and disseminated in pyrite and arsenopyrite. The Au/Ag ratio of the ores is variable depending upon the district and often upon the deposit. The deposits have a widespread distribution throughout the world in rocks ranging in age from Precambrian (Archean) to Tertiary. Examples in Canada include the Precambrian (Archean) deposits in the Kirkland Lake and Little Long Lac-Sturgeon River districts of Ontario and in the Jurassic volcanics of the Rossland gold-copper camp in the West Kootenay district of British Columbia. Elsewhere, gold is won from deposits of this type in Alaska at the Alaska Juneau mine (Mesozoic); Grass Valley and Nevada City auriferous districts, California (Mesozoic); and the Central City district, Colorado (Tertiary). 7. Disseminated and stockwork gold-silver deposits in igneous, volcanic, and sedimentary rocks Three general categories can be recognized within this type: 1. Disseminated and stockwork gold-silver deposits in igneous bodies 2. Disseminated gold-silver and silver-gold occurrences in volcanic flows and associated volcaniclastic rocks 3. Disseminated gold-silver deposits in volcaniclastic and sedimentary beds: (1) deposits in tuffaceous rocks and iron formations (2) deposits in chemically favourable sedimentary beds. The principal economic element in these deposits is gold, with small amounts of silver. A few deposits yield the base metals, but they are generally not known as base-metal deposits. The grade of the deposits is highly variable. Most are relatively low grade (generally less than 15 g Au/ton), but are characterized by large tonnages. The elements commonly concentrated in these deposits (omitting those in the common gangue minerals such as quartz, silicate, and carbonate minerals) are: Cu, Ag, Au, (Ba), (Sr), Zn, Cd, Hg, B, (Sn), Pb, As, Sb, Bi, V, S, Se, Te, Mo, W, (F), Fe, Co, and Ni. Elements in parentheses are infrequent or occur only in certain deposits. The Au/Ag ratio of most deposits is greater than 1. The deposits in the first category occur in igneous plugs, stocks, dykes, and sills that have been intensively fractured or shattered and infiltrated by quartz, pyrite, arsenopyrite, gold, and other minerals. Most of the deposits are stockworks or diffuse irregular impregnations. The alteration processes vary with the types of host rock. In granitic (felsic) rocks, sericitization, silicification, feldspathization (development of albite, adularia, etc.), and pyritization are predominant; in intermediate and mafic rocks, carbonatization, sericitization, serpentinization, and pyritization prevail. Alunitization may occur in both felsic and mafic rocks in places. The Au/Ag ratio in most deposits is greater than 1. Deposits of this type are common in Canada, particularly in the Canadian Shield and Cordillera. Examples are the Howey and Hasaga mines at Red Lake, Ontario, in an Archean quartz porphyry dyke; the Matchewan Consolidated and Young Davidson mines in Archean syenite plugs and dykes in the Matachewan district of Ontario; and the Camflo mine in an Archean porphyritic monzonite stock near Malartic, Quebec. Elsewhere typical examples occur in the Beresovsk auriferous district, Urals, USSR (Paleozoic); Twangitza mine, Kivu Province, Zaire (Precambrian); and the Morning Star mine, Woods Point, Victoria, Australia (Paleozoic). The disseminated gold-silver occurrences in volcanic flows and associated volcaniclastic rocks in the second category are relatively common, but commercial deposits of this type have not been worked. Most occurrences are very low grade, commonly less than 0.01 oz Au/ton (0.3 ppm). Silver contents are higher in places, averaging in some cases 3.5 oz Ag/ton (120 ppm). The disseminated occurrences in the second category are in reality large irregular and diffuse zones of alteration manifest mainly in rhyolites, andesites, basalts, and their associated volcaniclastic rocks. These zones of alteration constitute silicification, sericitization, epidotization, argillization, or alunitization, commonly associated with pyritization and carbonatization. In the mafic and intermediate rocks, the effects are commonly collectively called propylitization. Large volumes of the volcanic country rocks are affected, giving them a bleached and altered aspect. Locally diffuse silicified zones, quartz veins, alunite veins, and pyrite veins and segregations ramify through the altered rocks. Occurrences of this type are generally enriched in S, Ba, B, Hg, Sb, As, Pb, Zn, W, Mo, Se, Te, and Ag. The Au/Ag ratio is variable, but most occurrences exhibit values less than 1. The disseminated gold-silver deposits in volcaniclastic and sedimentary beds in the third category are usually conformable with the sedimentary and volcaniclastic beds, although in some cases their limits may infringe irregularly on overlying or underlying rocks. Some are large and of relatively high grade; others are commonly too low grade or not have sufficient tonnage to merit attention. Two general subcategories of these deposits can be recognized: (1) those developed in tuffaceous rocks and iron-formations within volcanic and sedimentary terrains and (2) those resulting from extensive infiltration or replacement of chemically favourable beds, particularly carbonate rocks or calcareous pelites. The first is especially common in Precambrian terrains, although there is no reason why they should not occur in rocks of younger age; however, examples of the latter are rare to date. The second can apparently occur in rocks of any age. Gold deposits in tuffaceous and other volcaniclastic rocks and in ironformations in volcanic and sedimentary terrains are particularly common in the Archean greenstone and associated sedimentary belts of the Canadian Shield and in other similar rocks throughout the world. Orebodies in tuffaceous rocks are generally large-tonnage, irregular disseminated bodies containing essentially pyrite, pyrrhotite and arsenopyrite, with much secondary fine-grained quartz and various silicates. Elements exhibiting enrichment in these types of deposits include Cu, Ag, Zn, Cd, B, Pb, As, Sb, Bi, Te, and S. Less common are Ba, Sr, Hg, Sn, V, Mo, W, Co, and Ni. The gold is usually free in the matrix of the rock or present in a finely divided state in the sulfide and arsenide minerals. A typical example of this type of deposit is the Madsen mine in Archean tuff at Red Lake, Ontario. The deposits now being developed at Hemlo, Ontario, may also belong in this category. Auriferous deposits in iron-formations are of two types: (1) Disseminated bodies similar to those just described, and zones of quartz veins or stockworks traversing the constituent rock members of the iron-formations. These bodies and zones contain essentially quartz with pyrite, pyrrhotite, and arsenopyrite; the gold is generally present in the native state, and the enriched elements are similar to those mentioned for the deposits in tuffaceous and other volcaniclastic rocks. Typical examples of deposits in iron-formations are the Central Patricia mine and the Pickle Crow mine in the Archean Crow River greenstone belt of northern Ontario, the Detour Lake mine in north-eastern Ontario, and the Cullaton Lake mine in eastern Northwest Territories. Elsewhere deposits in iron formations include probably the Homestake mine, Lead, South Dakota (Precambrian?); the Morro Velho mine, Minas Gerais, Brazil (Precambrian); and a number of deposits in the Precambrian (Archean) iron-formations of Zimbabwe. (2) Gold deposits resulting from extensive infiltration or replacement of chemically favourable beds (the second subcategory) are developed principally in calcareous and dolomitic pelites and psammites and in thin-bedded carbonate rocks invaded by granitic stocks and porphyry dykes and sills; a few occur in porous sandstones. Most deposits are characterized by one or more of silicification, argillization, pyritization, and arsenopyritization, and introductions of elements such as Au, Ag, Hg, TI, B, Sb, As, Se, Te, and the base metals. The gold is usually disseminated through the altered rocks in a very finely divided form and is generally, although not always, rich in silver. Deposits of this type have a widespread distribution throughout the world and are commonly referred to as "Carlin type" because of their occurrence in Silurian silty limestone and dolomitic siltstone in the Carlin-Gold Acres district of Nevada. Similar deposits have been recognized in British Columbia (Specogna), and in USSR (Kuranakh). 8. Gold deposits in quartz-pebble conglomerates and quartzites These constitute the largest and most productive of the known auriferous deposits, producing some 50% of the annual gold production of the world; some deposits are also economic sources of uranium, thorium, and rare earths. Typical examples are the Witwatersrand deposits in South Africa; other deposits include those in the Tarkwaian System of Ghana and at Jacobina, Bahia, Brazil. The orebodies in the quartz-pebble conglomerate deposits are marked by the presence of abundant pyrite (or hematite) with variable and usually minor to trace amounts of a host of other sulfides, arsenides, sulfosalts, and minerals such as uraninite, thucholite, and brannerite, principally in the matrix of the conglomerates or quartzites. The gold is mainly present as the native metal in a very finely divided form essentially in the matrix of the conglomerates or quartzites; minor amounts of the element also occur in the pyrite and in the various other sulfides, arsenides, sulfosalts, and so forth. Elements concentrated in the quartz-pebble conglomerate type of deposit are variable. Most orebodies report enrichments of Fe, S, As, Au, and Ag; some are marked by above average amounts of U, Th, rare earths, Cu, Zn, Pb, Ni, Co, and platinoids. The average Au/Ag ratio in the ores is 10. 9. Eluvial and alluvial placers These modern placers produce both gold and silver, the latter metal being present usually as a small percentage content of the gold dust and nuggets. Accompanying heavy minerals commonly include variable quantities of monazite, magnetite, ilmenite, cassiterite, wolframite, scheelite, cinnabar, and platinoid minerals. The Au/Ag ratio in placers is generally greater than 1. Fossil (lithified) equivalents of both eluvial and alluvial placers are known, but few are economic. Here we exclude the quartz-pebble conglomerates of the Witwatersrand and other similar deposits already mentioned, which appear to be modified paleo-placers, although other origins have been suggested. Placers have been worked for centuries in most countries of the world. The placers of the Pactolus, a tributary of the Gediz (Sarabat) in Anatolia, Turkey, and of the Maritsa (Hebrus) in Thrace were famous in ancient times; in modern times the placers of Colombia, California, Victoria (Australia), Alaska, Yukon, British Columbia, and the far eastern USSR have produced large amounts of gold. 10. Miscellaneous sources Gold is won from a number of miscellaneous sources, mainly as a by-product from nickel, copper, and other base metal ores as follows: 1. Nickel-copper ores associated with basic intrusives- Sudbury type 2. Massive sulfide deposits containing essentially Fe, Cu, Pb, and Zn sulfides in volcanic and sedimentary terrains 3. Polymetallic vein and lode deposits containing essentially Fe, Cu, Pb, Zn, Ag sulfides in volcanic and sedimentary terrains 4. Kuroko (black ore) sulfide deposits, occurring mainly in Japan, of which some are greatly enriched in both gold and silver 5. Disseminated deposits - porphyry Cu-Mo type (relatively large sources of gold (and silver) especially in the United States, New Guinea, and the USSR) 6. Certain types of uranium (pitchblende) deposits (e.g., Jabiluka, Northern Territory, Australia) In these varied deposits, gold usually occurs as the native metal in a very finely divided state, or as tellurides but can also occur in a finely divided form or be present as a lattice constituent in pyrite, arsenopyrite, chalcopyrite, and other base metal sulfides, arsenides, sulfosalts, and selenides.
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