Page 132 - Handbook of Gold Exploration and Evaluation
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Geology of gold ore deposits 111
2.19 Generalised occurrence model showing intrusive and alteration
relationships in and around gold-rich porphyry copper deposits (modified from
Sillitoe and Gappe, 1984).
Carbonates, chlorites, epidote, zeolites and adularia are generally diagnostic in
the immediate vicinity of veins containing quartz and chalcedony. Alteration of
the wall rock adjacent to the gold-bearing veins commonly extends above the
ore zone, occurring extensively in mafic volcanics but much less so in rhyolite.
Chloritisation extends outwards for considerable distances from vein systems,
while seritisation is closely restricted to fractures and mineralisation.
2.4 Gold deposition in volcanic terrain
The entrance of metals into magmas occurs in volcanically active basins. The
composition and mineralogical characteristics of the magmas are affected by a
wide variety of processes. These processes include the nature of the rocks where
melting occurs; the history of previous thermal, metamorphic and melting
events; degree of partial melting; addition of components from wall rocks and
subducting slabs; and the effects of crystal fractionation on the composition of
