Page 114 - Handbook of Gold Exploration and Evaluation

94 Handbook of gold exploration and evaluation

mercury, arsenic, antimony, gold, silver, lead and zinc may occur both inside
and outside the caldera margins and sometimes with diatremes and breccia pipes
(Sillitoe, 1993). Porphyry copper-gold deposition may be associated with deeper
level intrusives. Marine basaltic rift volcanism is a potential source of Cyprus
type copper-pyrite mineralisation with minor associated gold. These deposits
occur as massive sulphide bodies at the sediment-seawater interface and as
subjacent hydrothermally altered stockworks (Rona, 1984).
Submarine felsic and silicious volcanic centres, which host Kuroko-type and
Canadian Archaean base metal ore deposits occur as massive stockwork systems
or base-metal sulphides, in some cases with significant gold mineralisation.
Flood and valley fill non-explosive lava flows in continental basaltic settings,
although not directly associated with the formation of primary gold deposits,
may cover and preserve placer gold concentrations such as the Tertiary deep
leads of California and Australia.

Stratovolcanoes
Stratovolcanoes are built up by eruptions of calc-alkalic magma of andesitic to
basaltic-andesitic origin at subducting plate boundaries. Their settings range
from compressive continental margins to extensional margins associated with
island arc and back-arc systems. They have the potential for the formation of
porphyry-type copper-molybdenum-gold orebodies and higher-level epithermal
gold-silver deposits. Marine stratovolcanoes differ from their continental
counterparts by having a submerged foundation of oceanic or continental crust
and, usually, a subaerial summit region. Prime exploration targets for gold-rich
porphyry copper deposits are well defined in Circum-Pacific and Alpine-
Himalayan regions (Sillitoe, 1993).

Rhyolitic calderas
Multiple eruption points characterise rhyolitic volcanic centres, which occur
typically in clusters of rhyolitic hills associated with shallow level hydrothermal
systems. Epithermal deposits of mercury, arsenic, antimony, gold, silver, lead
and zinc may occur both inside and outside the caldera margins and sometimes
with diatremes and breccia pipes (Sillitoe, 1993). Porphyry copper-gold
deposition may be associated with deeper level intrusives.

Rhyolitic volcanic centres
Submarine caldera types are sites for the formation of Kuroko-type massive
stockwork systems of base-metal sulphides, in some cases with significant gold
mineralisation (Ohmoto, 1978; Ohmoto and Takashati, 1983). Miocene mineral-
isation in the Kuroko area of Japan is associated with rhyolite lava domes, and