Page 151 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 129

arc, fore-arc region and trench and, in some cases, back-arc basins. Back-arc
basins are underlaid by oceanic crust and lithosphere between island arcs and
continental margins or between two island arcs (refer to Chapter 1). Folded
mountain chains and sedimentary basins are created either by the extrusion of
magma as in island arc orogeny, tectonic uplift of continental crust at subducting
plate boundaries, or by a combination of both. Deposits of economic
significance in these belts are found mainly in stratovolcanoes and rhyolitic
centres in continental and marine volcanic centres.
The active life of stratovolcanoes and rhyolitic volcanic centres is made up of
short-lived eruptive periods in which enormous volumes of rocks are erupted,
and much longer repose periods between eruptions when some of the erupted
material is eroded away. However, the total time of all of the periods of
quiescence far exceeds the total time of all of the eruptions during the active life
of the volcano and surface processes operate at very high rates. Consequently,
when volcanic activity ceases, erosion is capable of eliminating the majority of
volcanic surface piles within a geologically short period of time.
Repose periods between eruptions occupy the bulk of its active lifetime.
Large volcanoes such as stratovolcanoes and those of rhyolitic centres are most
affected. In general, stratovolcanoes have active lives of less than 100,000 years,
but overlapping and closely spaced stratovolcanoes of a polygenetic complex
may be developed over time periods of 10 Ma and more (Cas and Wright, 1995).
For example, Martinique has at least six volcanic centres; K-Ar isotope dating
indicates their development over a period of at least 20 Ma (Briden et al., 1979).
Individual cycles in the history of the Guatemalan volcanic chain appear to be
marked by four distinct phases of activity (Vessell and Davies, 1981):
1. an inter-eruption phase of 80 to 100 years characterised by low rates of
sediment deposition, erosional incision of meander rivers and reworking of
deltas
2. an eruptional phase lasting less than one year
3. a fan building phase dominated by debris flows and coarse fluvial
sedimentation lasting up to two years
4. the influx of large volumes of sediment into stream systems, transforming
them from incised meandering into flood-prone braided systems; this phase
lasts from 20 to 30 years.
Similarly in other volcanic environments, all measurements and observations
(Kuenzi et al., 1979; Mills, 1976; Francis, 1983) show that the duration of
epiclastic sedimentary processes is typically far greater than the eruptive phases
and that they are rejuvenated by renewed volcanic activity. Based upon global
geological evidence, rates of recent tectonic uplift vary globally from about 4±
12 m/1,000 y in mountain belts above active subduction zones or along
lithospheric sutures (Strahler and Strahler, 1992). The authors suggest that by
assuming an average rate of uplift of 6 m/1,000 y, a mountain top might rise to