Page 122 - Handbook of Gold Exploration and Evaluation

102 Handbook of gold exploration and evaluation

However, some deposits are zoned and in these deposits individual zones are
characterised by different assemblages. Huston warns that dynamic modelling
requires both a high degree of computer literacy and a firm understanding of the
basic fundamentals of thermodynamics. The programs, which are written to solve
a particular set of problems, may not be able to cope with the particular geological
program involved. Composition and mineralogical characteristics of the erupted
magma are the end result of a complex history of processes causing chemical and
physical change. Widely different geological histories include the degree of
partial melting of the source rocks and other melting events, the amount of
contamination by wall rock, periodic replenishing, tapping and fractionating of
magmas in a succession of magma chambers as they rise to the surface.

2.3.1 Partial melting
The relevance of partial melting to the differentiation of gold-enriched silicic
magmas has been demonstrated experimentally. It is shown that quartz-rich,
water-rich silicic magmas such as those that produce quartz veins can be
synthesised by partial melting of a mixture of wet sediments, volcanics and other
materials as provided by a subducting plate. The composition of each melt varies
according to the composition of the original mixture and the composition of
individual partial melts produced sequentially at different temperatures and
pressures.
It follows that the formation of similar type magmas may occur at certain
critical depths in a subduction zone under similar conditions of mineralogy,
temperature and pressure. Basaltic magma produced by partial melting of upper
mantle material pushes and stopes its way up through the overlying crust.
Material broken off or melted from the surrounding rock is assimilated into the
rising magma by processes involving magma mixing, immiscibility and thermal
segregation. Localised heat and pressure differences in magma chambers formed
at higher stratigraphic levels provide complex and rapid changes in lithology,
thus resulting in melt-crystallisation relationships producing differentiation of
different rock types. Such rising magmas are less dense than mantle fluids
because of their increasingly silicious nature; hence their tendency to rise
buoyantly as intrusive and extrusive bodies into the arcuate systems of
volcanoes and mountain belts developed at both divergent and convergent plate
settings.
Partial melting of upper mantle sulphides contributes metals to crustal fluids
that rise into the crust. Andesitic varieties are of major importance because of
their common association with gold ores. All known mineralisation in the
southwestern Pacific areas lies within regions of predominantly andesitic rocks
(Liddy, 1972). The andesitic magma forms an array of island arcs that stretch
along a belt of Mesozoic-Cainozoic mountain building in major zones of crustal
instability. Intrusion-associated porphyry copper deposits, epithermal gold and