Page 100 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 81

apply the same tectonic principles to explanations of some aspects of Pre-
cambrian geology, particularly the genesis of greenstone belts, but this is a
contentious issue.

2.2.1 Crustal deformation

Crustal movements are of two main types, orogenic and epeirogenic. Orogenic
movements are spasmodic in nature due to such causes as the buckling of
continental crust imposed by plate movement or telescoping of continental shelf
strata during the collision of two continental plates. Mineralisation is associated
with the sub-surface volcanic activity of the subducting plates. Important deposit
types are distinguished according to geological setting, host rock type,
associated minerals and depths of emplacement. The sporadic nature of the
movements is due to the strength of the massive lithospheric layer, which resists
uplift until at some critical time the pressures build up sufficiently to fracture or
fold the overlying rock. Epeirogeny appears largely to influence the
modification of existing landscapes and the re-working of pre-existing alluvial
gold concentrations. Epeirogenic movements are slow, pulsatory mass move-
ments of continental shield rocks, which produce extensive uplift or sinking of
broad areas of crust in a vertical or radial direction. It is important to note that
whilst orogeny engenders hydrothermal activity and is associated with the
formation of primary gold ore bodies, epeirogeny appears largely to influence
the modification of existing landscapes and the re-working of pre-existing
alluvial gold concentrations.

Orogenesis
Orogenesis refers to major episodes of crustal deformation and uplift associated
with tectonic activity at lithospheric plate boundaries. Colliding plates give rise
to both compressional and tensional forces acting tangential to the Earth's sur-
face. Compressional forces buckle the overriding plate into an elongated series
of folded mountain ranges along the continental margins. The emplacement of
large bodies of igneous rock enhances orogenesis and in metallogenic regions,
granitic plutons may be associated with the deposition of gold in hydrothermal
vein systems. Tensional tectonic activity occurs at divergent plate boundaries
where oceanic plates are separating or continents are drifting apart. Vertical
tension cracks that appear in the upper parts of anticlines formed by uplift of the
overriding plate provide passages through which outpourings of lava build up to
form volcanoes of the andesitic type. Orogenic belts, exposed at various levels
of erosion, make up much of the world's continental area, either at the surface or
under a sedimentary cover. With the possible exception of very old shield areas,
most of the cratonic crust of the Asian region consists of orogenic belts of late
Precambrian to Mesozoic age.