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General Types of Auriferous Deposits

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, Russia (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 Geraes, 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 Russia (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 Russia 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 Russia)
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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March, 2011