35
peninsula; Fig-Tree in belt Barberton). Stratiformic gold–iron-sulphide ores
are present in greenstons belts Zimbabwe and Abitiby.
These data bring the conclusion that Archean greenstones belts for the
first time in the geologic history of the Earth had became the large
sources of such important mineral resources as nickel, platinum, gold,
sulphides
of copper, zinc and iron.
In the area of greenstone belts there are reomorphic migmatite
granitoids, which generate predominantly dome structures that frequently
deform the rocks containing them. Besides these there are intrusions of
magmatic granites. As a result of tectonic interplays domes of granites with
adjacent rocks, there is a singular «lace» picture of geological boundaries of
granite-greenstons belts. The displays of gold, copper and rare metal are
generically connected with Archean granitoids of these belts. The
auriferous quartz veins are dated to ekzo- and endocontacts of granite
domes and intrusions breaching greenstones rocks of belts Limpopo,
Barberton and Murchison in Southern Africa. The ingrained cupper-
molybden-gold ores are found in district Timmince of the belt Аbitiby.
Molybdenite porphyry deposit Pellapaxk is present in the greenstone belt
Коlmozero-Voronye on Kola Peninsula. The richest deposits of rare metal
pegmatites spodumene-microcline-albite of type are detected in greenstons
belts Fort-Victoria (Bikita, craton Zimbabwe), Murchison (Gravelot, craton
Kaapvaal), Greenbush in Western Australia, Bearnic-Lake in Canada. These
late Archean fields provide sharp dominance of reserves Rb, Cs, Li, Be, Ta,
Nb in the early Pre-Cambrian (Solodov, 1980).
The overlaided volcanogen-sedimentary basins began to form already in
the late Archean period. So, in the craton Kааpvaal in the South Africa after
completion of generating greenstones belts, the overlaid riftogenic
depression Witwatersrand had formed under continental conditions. They
are located at the intersection of regional faults with subwidth direction of
the belt Barberton with younger faults of the northeast direction.
Two stages
are being selected in the formation of similar continental structures, which
are typical for other tectono-magmatic activation on the Earth, for example,
in Transbaikalia, Mongolia, China, Rocky mountains of Northern America
(Shcheglov, 1994). In the beginning, there was derivate overlaid
volcanogenic depression, filled by bimodal effusives (2,900-2,800 Ma),
which were breached by intrusions of granodiorites. At the second stage of
activation (2,700-2,600 Ma) there were generated (overlapped on effusives)
large (300 х 100 km) graben Witwatersrand, fulfilled by terrigene deposites
of quartzites, conglomerates and shales. In the section of sedimentary
stratas the conglomerates compose 16 independent horizons, for which are
dated the unique golden ores, accompanied with pyrite miniralization and
clastic
minerals of uranium, platinum and diamonds.
The field Witwatersrand has a compound polygenic genesis. There is
rare combination of placer accumulations of gold, uranium, platinum and
diamonds with sedimentary - hydrothermal gold ores or even gold – sulfide
mineralization, forming unique of riches and minerals types field (Ramdohr,
36
1955; Shcheglov, 1994). Jacobina in Brazil and Тarqua in Ghana are
apparently composed of the same type of gold deposits.
II. 3. E a r l y P r o t e r o z o i c s t a g e (2,500 – 2,000 Ma). On
the boundary of late Archean - early Proterozoic there was
a sharp fall of the
geothermal gradient (up to 46
o
C / km), stabilization of lithosphere and
origin conditions for location linearly oriental structures in the earth’s crust.
However, further accumulation of the factual material has shown, that the
significance of the indicated boundary is exaggerated. Many features of
tectonic regime, reference for the early Proterozoic, such as appearance
protoplatforms and plutonic faults, the formation swarms of dikes was
observed already in late Archean. On the other hand, greenstone belts still
existed in the early Proterozoic, which is typical for Archean. On the
whole, the structural plan of the early Proterozoic, principally and
essentially differed from the Archean. As a result of alteration of the
geodynamic regime in the early Proterozoic, the destruction of Archean
continental crust began, which lead to location network of rifts and
protogeosynclines, cracking ancient cratons into separate eocratons (Khain,
Bozhko, 1988).
In early Proterozoic time protoplatforms and protogeosynclinals tectonic
structures began to form, among which the following geodynamic systems
are selected: 1. eocratons, places with plate cover and intracraton
depressions (such as syneclises and avlakogenes); 2. pericratonal
protogeosynclinals; 3 inside and between craton’s rifts.
The appearance of red color terrigene depositions on the plate cover of
early Proterozoic platform’s structures already testifies to the presence of
free oxygen in the atmosphere (2,200-1,900 Ma). The elemental
composition of the hydrosphere has changed even earlier, at what point
mass development of stromatolites in dolomites and appearance evaporates
minerals - anhydrite, gypsum has begun. The presence at the basis of
sediment sections of Proterozoic tillites in South
Africa and Canada testifies
to the ancient icing (2,400 -2,100 Ma) and another geographic location of
continents at that time. Two epochs of icing (early Karelian and Kalevian)
are selected in sedimental sections of early Proterozoic in Northern
Hemisphere. Each of which is replaced by the epochs of evaporation
(Akhmedov, 1996).
Based on the conception of primordially hydridic Earth, the reduction
of exothermic reactions of oxidation and endurable of oxygen to exterior
geosphere of the Earth to the end of Archean has stipulated sharp
temperature falling in the silicate-oxide cover and consolidation the
lithosphere. It has predetermined the possibility of appearance in it the
fields of directional deformations above the zones of emission hydrogen
from mantle and has conditioned for location linearly orient (Pavlovskiy &
Markov, 1963) and concentrically-spirally structures (Ivanov, Krutoyarskiy,
1988). The appearance of the dense network of subparallel oriented basic
dykes and greenstone belts is associated with the beginning of the
expansion of the Earth on the frontier between Archean and Proterozoic.