Page 133 - Handbook of Gold Exploration and Evaluation

112 Handbook of gold exploration and evaluation

ore components in the magmas. Trace concentrations of gold in volcanically
hosted metalliferous sulphides may be chemically dissolved and re-precipitated,
or otherwise modified in a near-surface, oxidising environment.
Deposition occurs by replacement and filling in dilated openings in the
country rocks when temperature, pressure and other depositional relationships
reach certain critical levels. The development of a complex vein system is due to
a combination of displacement parallel to and opening at a high angle to the
length of individual shears as the result of bulk homogeneous flattening. This is
demonstrated by Cox et al. (1991) in Fig. 2.20(a and b) in respect of the gold
deposits of the Lachlan fold belt in Victoria, Australia. Figure 2.20(a) shows the
development of a complex vein system consisting of non-parallel shear veins;
(b) the pattern of intersecting limb thrusts and vein-filled dilated shear zones and
fold hinges.
Orebodies are most favourably distributed if they shed gold into an alluvial
setting from all around the catchment area. A cluster of small apophyses is more
likely to provide wide-ranging hydrothermal plumbing systems than a single
large pluton. The total amount of gold in the system and its distribution in an

2.20 (a) Diagrammatic representation of development of a complex vein
system consisting of non-parallel shear veins as the result of bulk homogeneous
flattening. (b) Pattern of intersecting limb thrusts and vein-filled dilated shear
zones and fold hinges.