Page 115 - Handbook of Gold Exploration and Evaluation

Geology of gold ore deposits 95

volcaniclastic rocks believed to have been emplaced at minimum water depths
of 1000 m (Ohmoto and Takashati, 1983).
Observation of the above modern tectonic settings within which volcanism
occurs provides a reasonably clear understanding of the overall regional geo-
logical framework of the volcanics concerned, combined with a consideration of
the original tectonic controls that allowed volcanism to occur. However, caution
is needed in trying to use modern global settings as analogues for past tectonic
configurations. Ancient volcanic and tectonic settings offer little chance of such
unambiguous clarification because of the effects of repeated deformation and
magmatic intrusion over very long periods of geological time and masking of the
original tectonic context by the chemical overprints of metamorphism and
alteration. Furthermore, in Precambrian terraines where volcanic successions
may have been associated with the eruption of similar type basaltic, andesitic
and rhyolitic lavas as for modern volcanics, some of the larger scale tectonic
controls, processes and settings would probably have been quite different. The
continents have continually grown in extent since their earliest beginnings by the
accumulation of felsic rock produced in subduction zones, and critical
evaluation may be lacking of whether all the essential dynamically important
tectonic elements of the modern analogue can be found in ancient configurations
(Cas and Wright, 1995).

2.2.4 Minerals distribution through time

Metallogenic gold provinces usually contain clusters of similar type deposits or
areas of host rock associations in which mineralisation has been active at one or
more intervals of geological time. In summarising the distribution of minerals
through time, Barley and Groves (1992) propose a relatively simple explanation
for the uneven distribution of a wide range of deposit styles. They suggest that this
could be provided by tectonic cyclicity resulting from the interaction between
large continents, such as those that merged together in the formation of Pangaea,
and mantle convection, combined with decreasing global heat flow. Since
cyclicity in orogenic and anorogenic activity has important implications for
eustacy, seawater chemistry and biotic activity (Worsley et al., 1985), and for the
generation and distribution of continental crust both the spatial and temporal
distribution of metal may be affected. Figure 2.16 plots the distribution in
geological time of some important types of mineral deposit from Meyer (1981,
1985), Lambert and Donnelly (1992) and Barley and Groves (1992). Meyer relates
the uneven temporal distribution of metallic mineralisation to three major factors:
· evolution of the hydrosphere-atmosphere, secular decreases in global heat
flow, and long-term tectonic trends
· evolution of the hydrosphere-atmosphere as the cause of restricted distri-
bution of deposit styles (e.g., banded iron formations of the Precambrian) for