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GOLD

General Types of Auriferous Deposits

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-fluorite-sulfide 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 Russia, 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.


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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011