Page 316 - Handbook of Gold Exploration and Evaluation

Gold exploration 277

Table 5.2 Precambrian time units, principal orogenies and major events (modified
from Goossens, 1983)

Orogeny and other major events

Proterozoic Eon
era I 600±800mya Grenville, in America, Kalangan in
Africa, followed by the Pan-African-
Thermal event
era II 900±1600 mya Hudsonian, in America (also
recorded in Europe, USSR, S. Asia,
Australia and Africa)
era III 1600±2500mya Kenoran, in America; Ebumean in
NW Africa and elsewhere
Archaean Eon
era I 2500±2900mya
era II 2900±3500mya
era III 3500±3800mya 3800Ma oldest rocks recorded by
isotopic age dating
Origin of Earth and Moon 4300±4500mya Growth from accumulation of
planetisimosis (meteorites and
asteroids) after the Big Bang?

belt an overall picture of orogenic mountain building episodes may be developed
from the identification of assemblages such as opiolites, blue schist series and
melange characteristic of former plate margins or suture zones between two
plates. In the youngest, most recent settings an interval of only a few thousand
years could be sufficient for the complete reworking, dissection or exhumation
of much older deposits. The Precambrian time units, principal orogenies and
major events proposed by the International Sciences Subcommission on Pre-
cambrian Stratigraphy (1979) (as modified from Anhaeusser, 1981) place the
main events from Precambrian to the present including the `Origin of Earth and
Moon' in broad historical sequence in Table 5.2.
Phanerozoic geological history dates from about 570 mya at the start of the
Palaeozoic Era and extends through to present times. The Palaeozoic was
characterised by reduced physical changes and abundance of sedimentary rocks;
geological periods were separated on the basis of intervals of continental uplift
followed by submergence and encroachment of oceans onto the land. The
Caledonian Orogeny reached its peak during the Silurian Period (395±435 mya)
and fusion of the continents into a single mass (Pangaea). A gradual fragmenta-
tion took place during the Mesozoic Era (235±280 mya) and the continents
drifted apart, opening the way for new oceans. The Mesozoic featured extensive
sub-tropical weathering and favoured the formation of placers. The Urals were
weathered during the Triassic and the first phases of Alpine and Andean folding
introduced the great mountain ranges of modern times at about 100 mya.