The geochemistry of gold and its deposits (together with a chapter on geochemical prospecting for the element)
Authors: Boyle R W 
Published: November 2010

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 more noble. The principal oxidation states of gold are
Au(I)(aurous) and Au(III)(auric). These states are unknown as aquo-ions in solution, the element being present mainly in complexes of the type [Au(CN)2]-, [AuCl2]-, [Au(OH)4]-, and [AuCl4]-. There is only one naturally occurring isotope of gold: 197Au.

In nature gold occurs predominantly in the native state or as a major constituent of various alloys containing mainly silver, copper or platinoid 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 as a hypogene mineral in some auriferous deposits, and there is also a selenide, fischesserite, Ag3AuSe2, 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 0.1. The average gold content of igneous-type rocks in parts per million is - ultrabasic (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, sulphide schists, phosphorites and some types of sandstones and conglomerates may contain up to 2.1 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.

Gold is won both from deposits mined essentially for the element and as a byproduct of the mining and treatment of nickel, copper, zinc, lead and silver ores. The following types of primary deposits, exploited mainly for gold, can be distinguished:

1. Auriferous porphyry dykes, sills and stocks; coarse-grained granitic bodies, aplites and pegmatites.
2. Auriferous skarn-type deposits.
3. 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 terranes.
4. Auriferous veins, lodes, sheeted zones and saddle reefs in faults, fractures, bedding plane discontinuities and shears, drag folds, crushed zones and openings on anticlines essentially in sedimentary terranes; also replacement tabular and irregular bodies developed near faults and fractures in
chemically favourable beds.
5. Gold-silver and silver-gold veins, lodes, stockworks, silicified zones, etc. in a complex geological environment, comprising sediments, volcanics and igneous or granitized rocks.
6. Disseminated and stockwork gold-silver deposits in igneous, volcanic and sedimentary rocks.
(a) Disseminated and stockwork gold-silver deposits in igneous bodies.
(b) Disseminated gold-silver occurrences in volcanic flows and associated volcaniclastic rocks.
(c) Disseminated gold-silver deposits in volcaniclastic and sedimentary beds: deposits in tuffaceous rocks and iron formations and deposits in chemically favourable sedimentary beds.
7. Gold deposits in quartz-pebble conglomerates and quartzites.

Placers constitute the principal secondary type of gold deposit.

The quartz-pebble conglomerate deposits provide the bulk of the world's production of gold, some 56 per cent. The other deposits, mainly the vein and disseminated types, eluvial and alluvial placers and the various polymetallic veins, lodes, massive bodies and stockworks (byproduct gold) now
provide the remaining 44 per cent of the production.

The epigenetic vein, lode, stockwork and disseminated types of gold deposits appear to have originated mainly by metamorphic secretion processes, the source rocks of the gold and its associated elements being mainly the enclosing volcanic and/or sedimentary piles. Modern gold placers are of
sedimentary origin, the gold being winnowed into pay streaks as the result of both chemical (accretion) and physical (gravity) processes operating during weathering and subsequent sedimentation. The auriferous quartz-pebble conglomerate deposits appear to have originated as placers, the gold and
many of its associated elements having undergone radical chemical reworking during subsequent diagenetic and metamorphic events.

The oxidation processes in gold deposits are complex and depend essentially on the Eh and pH. Colloidal and coprecipitation phenomena also play a large part. Iron and manganese minerals and carbonates in the gangue and ore greatly influence the reactions that lead to the secondary enrichment of the
element. Gold is not easily solubilized in nature, and its soluble forms are readily reduced to the metal by a great variety of natural materials. The result of this behaviour is that the only common gold mineral found in the oxidized zones of auriferous deposits is the native metal. In this form it
ultimately collects in both eluvial and alluvial placers, which have been exploited throughout the world since time immemorial.

Practically all the geochemical methods of prospecting are applicable in the search for auriferous deposits. The most effective methods appear to be those based on the sampling of stream and lake sediments and soils, analyzing these materials directly or analyzing heavy mineral separates obtained
from them. The most specific indicator (pathfinder) elements for gold are Ag, As, Sb and Te.