Page 304 - Handbook of Gold Exploration and Evaluation

268 Handbook of gold exploration and evaluation

5.1 Geological investigations

Observations of patterns and conditions of volcanism associated with different
tectonic settings provide a reasonably clear understanding of the modern global
tectonic framework; but uncertainties of the geothermal gradient and scale of
tectonic systems attached to the events of ancient times have yet to be resolved.
There is a general lack of evidence in Precambrian times of the state of the
Earth's surface in relation to the distribution and nature of climatically
controlled morphogenic regions or to the effects of ancient base level and/or
climatic change. Few of the older mountain chains remain, and the association
of residual gold ores is mainly with Phanerozoic mountain ranges, sometimes as
the result of patterns and conditions of volcanism associated with variable
tectonic movements and periods of erosion in which the sediment has been
reworked and the gold reconstituted. Archaean (>2500 Ma) high-grade gneissic
regions host only a few mineral deposits; most known occurrences are in the
volcanic and volcano-sedimentary sequences called `greenstone belts'. There
may have been deep-seated fractures and rifts in the earlier cratonised rocks of
Archaean times but few of the older mountain chains remain. The Archaean
greenstone belt development was still in an embryonic state when it continued
into the Proterozoic. As shown in Rodinian reconstructions of the Earth's crust,
the earliest known rift with plateau basalt is only 1200±1100 Ma old.
Proterozoic Eon (600±2500 Ma) greenstones are characterised by intensive
folding and faulting and large axial and marginal faults with a more diversified
metallogeny. Basins were developed with thick sedimentary accumulations. The
source of the Witwatersrand and similar type gold deposits of South Africa
appear to have been the ultramafic and mafic volcanics of the Kaapvaal craton.
This craton appears to have been richer in gold than its younger Proterozoic
Yilgarn craton counterparts in Western Australia and in the Superior province of
eastern Canada.
Residual gold ores in the Americas are associated mainly with Phanerozoic
mountain ranges as the result of patterns and conditions of volcanism associated
with variable tectonic movements, and periods of erosion during which the
sediments were reworked and the gold reconcentrated. Yeend (1974) has
suggested that more than 40 km depth of sediment was removed from the Sierra
Nevada in California prior to the stabilisation of the drainage and the formation
of the Eocene placers along the Yuba and associated rivers. New sources of
elemental gold are still derived largely from magma at mid-ocean ridges, but
most of the gold mineralisation in Phanerozoic ores probably derives from the
re-cycling of gold and weathered auriferous material of all ages. Table 5.1 lists
the geological criteria for selecting areas for gold exploration in belts dominated
by volcanic rocks. The distribution of primary sources of gold in Precambrian
and Palaeozoic rocks favourable to the distribution of residual gold is shown in
Fig. 5.1.