Page 86 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 67

source of overlying sediment for true open-ocean oceanic crust is of calcareous
and/or volcanic pelagic origin. Pelagic (marine) sedimentation is dominant
along the edges of median valleys, on ridges and abyssal plains at mid-ocean
spreading centres.

2.1.2 Late Archaean (2.9±2.6Ga)
With few exceptions only those parts of the Precambrian geological cycle
relating to Archaean continental crust are preserved and the possibility of small
relics of Archaean oceanic crust still being in existence is by no means certain.
The mafic parent (Komatite Suite) generated oceanic lithosphere in Archaean
volcanic arc systems that are quite different to those operating today. Rocks
older than 2,500 Ga exist only because they are geologically stable and have
survived episodes of faulting, uplift and erosion that have occurred over the past
billion years. The oldest known rocks, of which the last unit has been dated from
studies of isotropic systematics at around 3,827 Ga, occur in the Amitsoq
Gneisses, Greenland. Judging from these deposits and similar occurrences in
Canada, China and the USA, and of occurrences of detrital zircons aged 4.1±
4.2 Ga in Western Australia the crust, atmosphere and hydrosphere must be
several hundred million years older than rocks of the Isua Supercrustal Belt
(Solomon and Shen, 1997). Surviving remnants of Proterozoic mobile belts may
be cores of old mountain chains formed as a result of continental drift and plate
tectonic processes similar to those that became widespread in the Phanerozoic.
In the view of some authors the overall uniformity of both chemical composition
and range of types of sedimentary and igneous rock from 3.87 Ga points strongly
to the continuity of tectonic processes broadly similar to those of recent Earth
(Lowe and Ernst, 1992; Windley, 1984).
The growth and stabilisation of continental crust, combined with high heat
flow led to the formation of cordilleran and island arc volcanism and calc-
alkaline magmatism, accretion of volcanic arcs in greenstone belts and develop-
ment of cratonic and craton margin sequences (Taylor and McLennan, 1995;
Solomon and Shen, 1997). Initially these movements and subduction would
have taken place under the influence of shallow convection due to the hotter
mantle. Deep penetration of the mantle by subducted material was probably
inhibited by the shallow depth of melting so that a few crustal blocks could have
formed to reach diameters of >1000 km in virtual isolation (McCulloch, 1993).
Large volumes of crust, both ultramafic and felsic were periodically formed,
recycled and reprocessed in the crustal environment to about 3.7 Ga through
lithosphere subduction, meteorite impact, and crustal remelting. Extensive
platforms were established with major rifts and passive margins in extensional
settings and foreland basins in convergent settings (Lowe and Ernst, 1992).
With gradual cooling of the mantle, a reduction of melting of the subducted
oceanic crust led to changes in the overall composition of the upper crust. The