Page 87 - Handbook of Gold Exploration and Evaluation

68 Handbook of gold exploration and evaluation

development of extensive continental margins and depositional basins heralded
development of giant Au-U conglomerate and banded iron formations (Solomon
and Shen, 1997). Major Ni-Cu, Au and base metal mineralisation in the Yilgarn
Block of Western Australia and The Superior Province Canada was associated
with orogenesis between 2.8 and 2.6 Ga (Barley and Groves, 1992). The late
Archaean was a period of rapid growth of continental crust and by its close more
than half of the Earth's continental crust may have formed (Taylor and
McLennan, 1995).

Archaean greenstone belts
Diorite, andesite and eventually coarse-grained mainly granitic sialic rocks rich
in silica and alumina (probably plagiogranite/rhyolite) dominate Archaean
geology. The rocks themselves are of two main types: strongly deformed
gneissic rocks that are characterised by the effects of immensely complex
geological structural history, and groups of homogeneous intrusive igneous
masses. Smaller groups of rocks, which formed on top of the crust (supracrustal
rocks) comprise mixtures having volcanic and sedimentary associations. In some
parts of the crust the rocks, commonly referred to as `greenstones', are only
partly metamorphosed and clearly distinguished volcanic components have a
greenish tinge imparted by hydrous metamorphic minerals.
Greenstone belts are interpreted as remnants of very small island arcs
(probable thin platelets) compressed closely together 3±4 billion years ago. The
volcanic chains of island arcs were initially scattered across a single worldwide
ocean, but through developing plate tectonic processes at divergent and
convergent plate margins were swept together to form the first supercontinental
type assemblages. The process, which probably involved extensive overturning
of the convective cells in the upper mantle and very rapid growth gradually led
to the formation of larger plates composed of continental crust. Extensive
platforms were established with major rifts and passive margins in extensional
settings (Lowe and Ernst, 1992). The gold-rich Witwatersrand Basin is thought
to have formed as a foreland basin during the collision of two continents
between 3.1 and 2.7 billion years ago (Burke et al., 1986).
In addition to the Archaean greenstone belts of Southern Africa, greenstone
belts intruded by granitic magmas and surrounded by extensive areas of granites
and granitic gneisses are well represented in highly mineralised shield areas of
Australia, Canada and North America. Notable examples of primary gold deposits
are found in late Archaean rock belts including Timmins Camp and Albibi Belt,
Canada; Kolar and Kanataka, India; and the Kalgoorlie-Norseman Belt of the
Yilgarn Block, Australia. Typical examples of major mesothermal gold deposits
occur in supracrustal Proterozoic rock belts at Homestake in the USA and Ashanti,
Ghana. However, while mutual relations between supracrustal materials and
granitoids in `low grade' terraines are quite evident, areas in `high grade'