Small orogenic gold
deposits have the same deposit-scale geological and geochemical features as
those that characterise giant orogenic gold systems (>16 Moz Au). Therefore,
a broader-scale approach is needed to identify the critical controls on giant
deposits and their disproportionate distribution throughout time and space. Ages
of giant gold systems cluster around well-defined periods of lithospheric growth
at continental margins, and it is orogen-scale processes during these Late
Archaean, Early Proterozoic and Phanerozoic times that determine gold endowment
of a province.

A
critical factor for giant orogenic gold provinces appears to be asthenospheric
upwelling at the time of gold mineralisation to most effectively transfer the
thermal energy required to initiate and sustain high hydrothermal fluid flux.
The extent of this fluid flux should be related to thickness of the lithosphere
beneath a province at the time of gold mineralisation, such that giant gold
deposits are much more likely to develop in orogens with oceanic or thinned
continental lithosphere. A proxy for the latter is likely to be a short
premineralisation crustal history such that thick subcontinental lithospheric
mantle was not developed. Conversely, orogens with protracted premineralisation
crustal histories are more likely to have a thick subcontinental lithospheric
mantle, which is difficult to delaminate and, hence, such provinces will
normally be poorly endowed. The nature of the lithosphere also influences the
intrinsic gold concentrations of potential source rocks, with back-arc basalts,
transitional basalts and basanites enriched in gold relative to other rock
sequences. Thus, segments of orogens with thin lithosphere may be defined by
conjunction of giant-scale fluid flux through gold-enriched
sequences.

Although the nature of
the lithosphere plays the crucial role in dictating which orogenic gold
provinces will contain one or more giant deposits, the precise siting of those
giants depends on the critical conjunction of a number of province-scale factors
that control fluid conduits, traps, and seals in tectonically and
lithospherically suitable terranes within accretionary
orogens.