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GOLD

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. 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 1.0 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).


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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011