Docstoc

THE GOLD

Document Sample
THE GOLD Powered By Docstoc
					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.

				
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
views:63
posted:8/15/2011
language:English
pages:12