126
Gold is not a lithophile element, it is more a siderophile than a chalcophile
element. Gold concern to group of chalcophile elements such Cu, Ag, Au, but
on geochemical properties it is closer to such siderophile elements as Fe, Pt. So,
when the molted phase of pure iron is separated from a molted phase of iron
sulfide, gold is found in higher concentration in pure iron (Bache, 1987).
On the diagram of curve nuclear volumes chalcophile elements settle
down in the bottom parts of the left ascending branches, forming the following
three lines of associations: 1) Au, Hg, Tl, Pb, Bi, Po; 2) Pd, Ag, Cd, In, Sn, Sb,
Te; 3) Cu, Zn, Gd, Ge, As, Se. It is necessary to note, that to Au from below
adjoin such siderophile elements as Pt, Ir, Os, to Ag - Pd, and to Cu - Fe. Ni.
Co (Figure 1). In aurous ores similar associations of minerals as polymetals,
tellurides, selenides and platinumides of gold are frequently traced. Some
geochemical types of native gold are allocated: Au-Ag, Au-Ag-Hg, Au-Ag-Cu,
Au-Ag-Cu-Hg, Au-Hg, Au-Sb-Hg, Au-As-Hg, Au-Te and others.
Reprinted with permission from Publishing House “Encyclopedia”, T.10.
Gold is most frequently found in the free state. Gold is often alloyed with
silver. Similarly gold can be alloyed with copper and more rarely with bismuth,
antimony, platinum, palladium, rhenium or iridium. Gold also occurs combined
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with tellurium or selenium. Main auriferous minerals are represented by
electrum Au-Ag, maldonite Au
2
-Bi, calaverite (Au,Ag)Te
2
, krennerite
(Au)Te
2
, sylvanite (Au,Ag)Te
4,
petzite Ag
3
Au
2
Te, nagyagite Pb
5
Au(Te
Sb
4
)S
5.8
, fischesserite Ag
3
AuSe
2
.
Grains of gold are found in the gangue of the lodes and similarly also in
rock walls. In addition, it is very often found as microscopic inclusions in a
large number of sulphides and sulph-arsenides, mainly those of iron, copper,
silver, antimony and arsenic. Arsenopyrite, pyrite, pyrrhotite and grey copper are
the most common sulphide hosts. In contrast, the sulphides and sulpho-salts of
zinc and lead not in general auriferous, although some galenas and sphalerites
show high gold contents.
Microscopic amounts of gold also occur in some native elements,
according to R.W.Boyle, such as arsenic, bismuth, copper, silver and platinum
group elements.
In a hypothermal (500-300
o
C) deposits the test of gold is high (850-
990), in a mesothermal ( 300-200
o
C) - near (800-850) and in epithermal - it is
usual (500-750). In placers, in connection with dissolution of extraneous
impurities into gold, than further from a radical source, higher is the test of
gold.
Processes of migration in a zone of oxidation results into concentration of
gold in a zone of secondary sulphidic enrichment, but the increasing contents of
gold are more often connected to its accumulation within the limits of “an iron
hat” and in a zone of leach “ a baritic loose “. A congestion of high-standard
gold arise also as a result of oxidation of gold tellurides.
2. CLASSIFICATION OF GOLD DEPOSITS
There are a lot classifications of natural deposits of gold. One of the first
has suggested by V.G.Emmons (1937), that based on various depths and
temperatures of precipitation of ore minerals. So, he differentiated among them
a hypothermal, mesothermal and epithermal deposits of gold. Moreover, he
noted skarn and pegmatite sources of gold. Among hydrothermal deposits of
gold he allocated hypothermal gold - quartz, mesothermal gold - sulphidic and
epithermal gold - silver ore formations.
N.V.Petrovskaya (1955) defined the follow three groups of
hydrothermal auriferous ores: quartz - lowsulphidic, moderate - sulphidic and
essential – sulphidic (kolchedanic) ores, connecting them to large massives of
granite, small post-batholith intrusions and dykes of granodiorite and, at last,
with keratophyre.
The classification of auriferous deposits, offered by J.J.Bache (1980), has
based on three aspects: on geostructural context, the nature of the environment
rocks and on mineralogical ore associations. The gold deposits are subdivided
into a three large groups: pre-orogenic submarine volcano-sedimentary
deposits; post-orogenic subterranean plutono-volcanic deposits; and detrital
deposits (placers).
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The first two groups of deposits have a fundamental difference in the
mode of formation an auriferous ores. In the submarine pre-orogenic volcano-
sedimentary group, the main cause of deposition the ore minerals is a sharp fall
of temperature, when the hot hydrothermal solutions arrive into the cold sea
water. In the terranean post-orogenic plutono-volcanic group the main
parameter, whose variation induces deposition gold minerals is the suddenly fall
of pressure: an explosive phenomenon that decompress the solution causing
them to boil. Among the detrital deposits there are old ancient and young gold
placers of eluvial, alluvial, deltaic or coastal beach genesis.
In our opinion, the main reason of formation the auriferous epochs and
deposits are caused by relay of geochemical and geodynamic mode at the
development of the Earth, when on the change of waterless granulite
metamorphism of early Archean has came waterfull amphibolite
metamorphism and granite magmatism, that located in late Archean granite-
green belts or in Phanerozoic geosynclinals and island volcanic arcs at the
geodynamic conditions of pulsating-expanding Earth. The generation of large
and super large deposits rich of gold is connected with processes of tectono-
magmatic activity, rejuvenation, secondary concentration and introducing auric
metals by the gas-fluid hydrothermal solution from a new deep underlying
intrusions of granites and others magmatic rocks.
In that reason we suggest a new “Geodynamic classification of gold
deposits of the Earth”, based on the uniform hypothesis of the primordial
hydridic Earth, designed by V.N.Larin (1980), in which the geodynamic and
tectonic conditions of origin deposits of gold have been considered, as well as
their genetic connection with magmatic, sedimentary and metamorphic
formations; emphasizing mineralogical types of gold ores (Table 2).
Table 2
Geodynamic classification of gold deposits of the Earth
Geodynamic
systems
Continents
Geosynclines
Transitals
Oceans
Thermodynamic regime
Periodic warming up of hydridic core of the Earth, cyclic degasation of protoning
hydrogen throughout the mantle and earth’s crust, periodic tectono-magmatic
activities and formation the hydrosphere of the Earth
Tectonic movements
Elevations
and
depressions,
riftogenes and
tectono-maigmatic
activity,
plutonic faults
Depressions and
extensions,
elevations and
folding, pulling
Depressions and
extensions,
elevations and
pressing, pushing
Depressions,
riftogenes and
extensions,
transform faults
Earth’s crust
Continental
Suboceanic and
continental
Suboceanic and
continental
Oceanic
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