Page 113 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 93

of valley morphology, and the transport and deposition of sediment. Settings are
provided in sequence from the generation of mafic igneous rock at oceanic
divergent plate margins to intermediate and felsic rocks by fractional melting in a
subduction zone.
As a starting point for gaining an understanding of the global tectonic
framework, Cas and Wright (1995) divide areas of modern volcanic activity into
the following tectonic settings:
· mid-ocean spreading ridge volcanism
· marginal sea-back-arc basin spreading volcanism behind oceanic island arcs
such as the Marianas Trough
· intra-plate continental or flood volcanism, e.g. Cainozoic volcanism of
eastern Australia
· continental rift volcanism such as the East Africa Rift zone and the Basin and
Range Province of western USA
· young island arc volcanism associated with oceanic trench subduction zones
as in Marianas, Aleutians and Tonga-Kermadic arcs
· micro-continental arc volcanism associated with oceanic trench subduction
zones, e.g. New Zealand, Japan and Indonesia
· continental margin arc volcanism associated with oceanic trench subduction
zones, e.g. Andes and Cascade volcanic belt, western USA.
Eruptions of calc-alkalic magma of andesitic to basaltic-andesitic origin at a
subducting plate boundary range variably from compressive continental margins
to extensional margins associated with island arc and back-arc systems. Marine
stratovolcanoes differ from their continental counterparts by having a sub-
merged foundation of oceanic or continental crust and, usually, a subaerial
summit region (Cas and Wright, 1995). They have the potential for the
formation of porphyry-type, copper-molybdenum-gold orebodies and higher-
level epithermal gold-silver deposits. Prime exploration targets for gold-rich
porphyry copper deposits are well defined in Circum-Pacific and Alpine-
Himalayan regions (Sillitoe, 1993).
Ore zones located within the igneous and metamorphic rocks of Phanerozoic
volcanic settings occur at various depths ranging from near surface (epithermal)
vein systems, to deep-seated (mesothermal) deposits at depths of 10 km and
more. Gold occurring in trace concentrations in volcanic hosted metalliferous
sulphides may be chemically dissolved and re-precipitated or otherwise modi-
fied in a near-surface, oxidising environment (see Chapter 3). The following
settings are of particular relevance to the genesis of gold-bearing ores.

Continental silicic volcanoes
Continental silicic type volcanoes occur typically in clusters of rhyolitic hills
associated with shallow level hydrothermal systems. Epithermal deposits of